Independent evaluation of Wolbachia infected male mosquito releases for control of Aedes aegypti in Harris County, Texas, using a Bayesian abundance estimator

Posted by Vector and Vector-borne Disease Committee
December 7, 2022

S Lozano [nkq3@cdc.gov] et al.  PLoS NTD Nov 14, 2022.  https://doi.org/10.1371/journal.pntd.0010907

Abstract [condensed].  We evaluated the effect of releases of Wolbachia infected Ae. aegypti males on populations of wild Ae. aegypti in the metropolitan area of Houston, TX.  Releases were conducted by the company MosquitoMate, Inc. To estimate mosquito population reduction, we used a mosquito abundance Bayesian hierarchical estimator that accounted for inefficient trapping.  In this experiment we found a reduction of 93% after six weeks of continual releases. A similar result was reported by Verily Life Sciences, 96%, in releases made in Fresno, CA.

Note:  This and similar evaluations continue to indicate that genetic methods can show a reduction in Ae. aegypti populations, although none have resulted in extinction.

Comparing Satellite and Ground-Based Measurements of Environmental Suitability for Vector Mosquitoes in an Urban Landscape

Posted by Vector and Vector-borne Disease Committee
December 7, 2022

A McMahon, CMB Franca, MC Wimberly  [email: mcwimberly@ou.edu].  Journal of Medical Entomology 59: 1936–1946, https://doi.org/10.1093/jme/tjac145.

Abstract [condensed]

We investigated how land cover and climate influenced abundances of Ae. albopictus (Skuse)  and Cx. quinquefasciatus (Say) in Norman, Oklahoma. From June–October 2019 and May–October 2020 we sampled mosquitoes along an urban-rural gradient using CO2 baited BG Sentinel traps. We compared statistical models of abundance based on microclimate, satellite, weather station, and land cover data. Mosquitoes were more abundant on trap days with higher temperature and relative humidity. Rainfall 2 wk prior to the trap day negatively affected mosquito abundances. Impervious surface cover was positively associated with Cx. quinquefasciatus and tree cover was negatively associated with Ae. albopictus. Among the data sources, models based on satellite variables and land cover data had the best fits.  

Note:  Methods such as these may be useful for planning surveillance sampling and perhaps even control operations in urban areas.

A new species of tick, Ixodes (Ixodes) mojavensis (Acari: Ixodidae), from the Amargosa Valley of California

Posted by Vector and Vector-borne Disease Committee
December 1, 2022

LH Backus, JE Foley et al.   Ticks and Tick-Borne Diseases 13(6):102020, https://doi.org/10.1016/j.ttbdis.2022.102020

Abstract [condensed]

Ixodes (Ixodes) mojavensis, n. sp. (Acari: Ixodidae), is described from all parasitic stages collected from the endangered vole Microtus californicus scirpensis Bailey, 1900 (Rodentia: Cricetidae), Mus musculus L. 1758 (Rodentia: Muridae), and Reithrodontomys megalotis (Baird; 1857) (Rodentia: Cricetidae) in the Amargosa Valley of California. When first collected in 2014, this tick was tentatively identified as Ixodes minor Neumann, 1902 because the nucleotide similarity between its 16S rDNA sequence and a homologous GenBank sequence from an I. minor from the eastern U.S. was 99.51%. Nevertheless, adults of I. mojavensis differ morphologically from I. minor by hypostomal dentition, absence of a spur on palpal segment I, and punctation patterns; nymphs by the shapes of basis capituli, auriculae, cervical grooves and external files of hypostomal denticles; and larvae by the length of idiosomal setae and hypostomal dentition. DNA sequencing of fragments of 4 different genes shows that the mitochondrial gene sequences are almost identical to the I. minor homologous genes. Phylogenetically, the two species do not cluster in mutually exclusive monophyletic clades.

Note: Ixodes mojavensis has only been collected from the Amargosa and Owens Valleys in California where it was found infected with a bacteria related to the non-pathogenic spirochete Borrelia carolinensis.  The vector potential of I.mojavensis for human disease is currently unknown.

Special Collection: Highlights of Medical, Urban and Veterinary Entomology. Highlights in Medical Entomology, 2021

Posted by Vector and Vector-borne Disease Committee
December 1, 2022

A Gloria-Soria. [email:andrea.gloria-soria@ct.gov].  Journal of Medical Entomology 59: 1853–1860, https://doi.org/10.1093/jme/tjac063

Abstract [condensed]. Here, I reflect on parallels between control of Covid-19 and vector-borne disease control, discuss the advantages and caveats of using new genotyping technologies for the study of invasive species, and proceed to highlight papers that were published between 2020 and 2021 with a focus on those related to mosquito surveillance and population genetics of mosquito vectors.

Note:  In the open access publication, the author thoroughly describes the evolution of genetic methods used to track invasive species and summarizes the immigration of new mosquito species into the USA during the previous few years. 

Interactions between vector competence to chikungunya virus and resistance to deltamethrin in Aedes aegypti laboratory lines?

Posted by Vector and Vector-borne Disease Committee
November 15, 2022

Lanjiao Wang1|  Albin Fontaine2,3|  Pascal Gaborit1|  Amandine Guidez1|Jean Issaly1|  Romain Girod1|  Mirdad Kazanji4|  Dominique Rousset5|Marco Vignuzzi6|  Yanouk Epelboin1|  Isabelle Dusfour1

1Vectopôle Amazonien Emile Abonnenc, Unité de contrôle et adaptation des vecteurs, Institut Pasteur de la Guyane, Cayenne.  2 Unité de Parasitologie et Entomologie, Département des Maladies Infectieuses, Institut de Recherche Biomédicale des Armées, Marseille, France.  3Aix Marseille Université, IRD, AP-HM, SSA, UMR Vecteurs–Infections Tropicales et Méditerranéennes (VITROME), IHU–Méditerranée Infection, Marseille.   5Laboratoire de Virologie, Institut Pasteur de la Guyane, Cayenne cedex, France.  6Unité des Populations Virales et Pathogénèse, Institut Pasteur, Paris cedex 15, France

Medical and Veterinary Entomology 36: 486-495.   https://doi.org/10.1111/mve.12593 

Authors’ Abstract.  The urban mosquito species, Aedes aegypti, is the main vector of arboviruses worldwide. Mosquito control with insecticides is the most prevalent method for preventing transmission in the absence of effective vaccines and available treatments; however, the extensive use of insecticides has led to the development of resistance in mosquito populations throughout the world, and the number of epidemics caused by arboviruses has increased. Three mosquito lines with different resistance profiles to deltamethrin were isolated in French Guiana, including one with the I1016 knock-down resistant allele.  Significant differences were observed in the cumulative proportion of mosquitoes with a disseminated chikungunya virus infection over time across these lines. In addition, some genes related to resistance (CYP6BB2,CYP6N12,GST2,trypsin) were variably over expressed in the midgut at 7 days after an infectious bloodmeal in these three lines. Our work shows that vector competence for chikungunya virus varied between Ae. aegypti laboratory lines with different deltamethrin resistance profiles. More accurate verification of the functional association between insecticide resistance and vector competence remains to be demonstrated.

Note:  This  and  related studies provide some evidence that selection for insecticide resistance also may alter the vector competence of the mosquito.   Although California has been monitoring insecticide resistance, less research has been done on the consequences of this resistance on arbovirus transmission. 

Disease-spreading mosquito could be coming soon

From the Monterey Herald
November 14, 2022

PAJARO VALLEY – From the lip of an overturned bottle cap, an Aedes aegypti mosquito — striped like a zebra — deposits eggs into the tiny puddle within.

The eggs may dry out for up to eight months. They may weather winter nights in the meager shelter of a garage. And yet, as soon as water rehydrates them, they will hatch into adults capable of spreading dengue, Zika, chikungunya and yellow fever.

Though none of these viruses are currently transmitted in California, the mosquito has arrived in Santa Cruz County. Officials still see a path to temporary eradication, but if the experiences of scientists and other counties hold any lessons, A. aegypti may prove impossible to keep out indefinitely.

The mosquito has yet to show up in Monterey County, which recorded its most recent dengue and Zika cases in 2019 and 2017, respectively. But with only four technicians, Monterey’s Mosquito Abatement District would have a tough time checking yards and setting traps for invasions across the county — making prevention paramount.

Read more

Death from West Nile virus reported in Santa Clara County

From Palo Alto Online
November 3, 2022

A Santa Clara County resident has died from West Nile virus after a long illness, public health officials said Wednesday.

The person lives in Santa Clara County but was infected by the virus elsewhere in the greater San Francisco Bay Area, according to the Santa Clara County Department of Public Health.

No other details about the death were released in a brief statement posted Wednesday on the Santa Clara County Department of Public Health website.

As of Oct. 27, seven deaths from West Nile virus had been reported in California this year, according to the California Department of Public Health.

Read more

Invasive mosquitoes could unravel malaria progress in Africa

From ABC News
November 1, 2022

LONDON — Scientists say an invasive mosquito species was likely responsible for a large malaria outbreak in Ethiopia earlier this year, a finding that experts called a worrying sign that progress against the disease is at risk of unraveling.

The mosquito species, known as Anopheles stephensi, has mostly been seen in India and the Persian Gulf. In 2012, it was discovered in Djibouti and it has since been found in Sudan, Somalia, Yemen and Nigeria. The mosquitoes are suspected to be behind a recent rise in malaria in Djibouti, prompting the World Health Organization to try to stop the insects from spreading further in Africa.

On Tuesday, malaria scientist Fitsum Tadesse presented research at a meeting of the American Society of Tropical Medicine in Seattle, suggesting that the invasive mosquitoes were also responsible for an outbreak in Ethiopia.

Read more

Bird species define the relationship between West Nile viremia and infectiousness to Culex pipiens mosquitoes

Posted by Vector and Vector-borne Disease Committee
November 1, 2022

Bird species define the relationship between West Nile viremia and infectiousness to Culex pipiens mosquitoes. 2022.  PLoS Negl Trop Dis 16(10): e0010835. https://doi.org/10.1371/journal.pntd.0010835

Vaughan JA, Newman RA, Turell MJ

Department of Biology, University of North Dakota, Grand Forks, North Dakota. and VectorID LLC, Frederick, Maryland

Author’s Abstract.    The transmission cycle of West Nile virus (WNV) involves multiple species of birds. The relative importance of various bird species to the overall transmission is often inferred from the level and duration of viremia that they experience upon infection. Reports utilizing in vitro feeding techniques suggest that the source and condition of blood in which arboviruses are fed to mosquitoes can significantly alter the infectiousness of arbovirus to mosquitoes. We confirmed this using live hosts. A series of mosquito feedings with Culex pipiens was conducted on WNV-infected American robins and common grackles over a range of viremias. Mosquitoes were assayed individually by plaque assay for WNV at 3 to 7 days after feeding. At equivalent viremia, robins always infected more mosquitoes than did grackles. We conclude that the infectiousness of viremic birds cannot always be deduced from viremia alone. If information concerning the infectiousness of a particular bird species is important, such information is best acquired by feeding mosquitoes directly on experimentally infected individuals of that species.

Note:  Although not presented here, the robins and grackles used in this study were infected by different blood parasites but these apparently did not result in differences in WNV dissemination in Culex pipiens (Vaughan et al. 2021. J. Med. Entoml. 58: 1389).   If confirmed and universal, the data presented here complicate the current models of arboviral host competence. 

 

 

Host selection and forage ratio in West Nile virus–transmitting Culex mosquitoes: Challenges and knowledge gaps

Posted by Vector and Vector-borne Disease Committee
November 1, 2022

Host selection and forage ratio in West Nile virus–transmitting Culex mosquitoes: Challenges and knowledge gaps. 2022. PLoS Negl Trop Dis 16(10): e0010819. https://doi.org/10.1371/journal.pntd.0010819

Riccetti N, Fasano A, Ferraccioli F, Gomez-Ramirez J, Stilianakis NI

European Commission, Joint Research Centre (JRC), Ispra (VA), Italy, Department of Biometry and Epidemiology, University of Erlangen-Nuremberg, Erlangen, Germany

Author’s Abstract [Shortened]

Background.  Whereas mosquitoes’ intrinsic characteristics cause them to favour certain hosts (host preference), absolute selection is impossible in natural settings. Conversely, the selection carried out among available hosts and influenced from hosts’ availability and other ecological/environmental factors is defined as host selection.

Methodology/Principal findings.  In July 2022, we searched PubMed database for original articles exploring host selection among WNV-transmitting Culex mosquitoes, the main WNV vector. We considered only original field studies estimating and reporting forage ratio. This index results from the ratio between the proportion of blood meals taken by mosquitoes on potential host species and the hosts’ relative abundance.  From the originally retrieved 585 articles, 9 matched the inclusion criteria and were included in this review. All but one of the included studies were conducted in the Americas, six in the United States, and one each in Mexico and Colombia. The remaining study was conducted in Italy.  American Robin, Northern Cardinal, and House Finch were the most significantly preferred birds in the Americas, Common Blackbird in Italy.

Conclusions/Significance.  Although ornithophilic, all observed WNV-transmitting mosquitoes presented opportunistic feeding behaviour. All the observed species showed potential to act as bridges for zoonotic diseases, feeding also on humans. All the observed mosquitoes presented host selection patterns and did not feed on hosts as expected by chance alone.   The articles observe different species of mosquitoes in different environments. In addition, the way the relative host abundance was determined differed. Finally, this review is not systematic. Therefore, the translation of our results to different settings should be conducted cautiously.

Note:  This paper reviewed many references on host selection by Culex but seemingly failed to extract any novel conclusions north discussed previously.   An important aspect not discussed here was the significance of female Culex ‘hunting strategies’ and the utilization of flight paths dictated by landscape features which seemed to determine female-host contact.  This concept was originally discussed by Bidlingmayer (Bidlingmayer and Hem 1981) and later investigated for Culex tarsalis (Lothrop and Reisen 2001; Meyer et al. 1989).   Theoretically, if substantiated these flight paths could be targets for control, but operationally difficult to recognize and target effectively.

 

Reference List

Bidlingmayer, W. L., and D. G. Hem. 1981. Mosquito flight paths in relation to the environment. Effect of the forest edge upon trap catches in the field.  Mosq. News 41: 55-59.

Lothrop, H. D., and W. K. Reisen. 2001. Landscape affects the host-seeking patterns of Culex tarsalis (Diptera: Culicidae) in the Coachella Valley of California.  J Med Entomol. 38: 325-332.

Meyer, R. P., W. K. Reisen, and M. M. Milby. 1989. The influence of vegetation on CO2 trap effectiveness.  Proc. Calif. Mosq. Vector Control Assoc. , 57: 80

 

 

Five Years of Surveillance for Tularemia Serovar B (Francisella tularensis holarctica) (Olsufjev) (Thiotrichales: Francisellaceae) Including Two Human Cases at an Endemic Site in San Mateo County, California

Posted by Vector and Vector-borne Disease Committee
October 27, 2022

Tara M Roth, Arielle Crews, Angie Nakano

San Mateo County Mosquito and Vector Control District

Journal of Medical Entomology 59: 1787–1792, https://doi.org/10.1093/jme/tjac096

Author’s Abstract

Tularemia is a highly infectious, potentially fatal disease of humans and animals caused by the gram negative, intracellular bacterium Francisella tularensis. The San Mateo County Mosquito and Vector Control District conducted surveillance for F. tularensis from 2017 to 2021 in Dermacentor occidentalis (Marx) (Ixodida: Ixodidae), D. variabilis (Say) (Ixodida: Ixodidae), and Haemaphysalis leporispalustris (Packard) (Ixodida: Ixodidae) ticks in coastal southwestern San Mateo County, California. A total of 3,021 D. occidentalis and 1,019 D. variabilis were collected. Of those, 25 positive pools of F. tularensis were detected (five ticks per pool, overall minimum infection prevalence: 0.62%). Twenty-two of the 25 positive pools (88%) contained D. occidentalis. Eighty-eight percent (88%) of all positive pools were collected from the western half of the site, nearest to the ocean. We did not detect a seasonal effect on the probability of detecting a positive tick pool. There were two human cases of tularemia during the summers of 2019 and 2021. We conducted rodent surveillance in June of 2019, before the human case report. Twenty-four small mammals were collected, but none of their sera tested positive for F. tularensis. It is clear that tularemia is endemic to this region of San Mateo County, but the extent of its range and its ecology is not currently well understood.

Note:  It is useful for MVCAC districts to remember that there are other tick species that transmit other pathogens of public health concern.  The above study was conducted along hiking trails within a coastal park.

Invasive day-biting mosquitoes discovered in Santa Clara Co.; officials detail potential dangers

From ABC7 News
October 25, 2022

SAN JOSE, Calif. (KGO) — A non-native pest new to Santa Clara County was recently detected at the San Jose and Milpitas border. Traps set by the county’s Vector Control District uncovered two invasive day-biting mosquitoes on Thursday.

Now, an effort is underway to find out whether there are more in the area, and to figure out where they’re coming from.

For the very first time, Aedes aegypti mosquitoes have landed in Santa Clara County. The mosquitoes measure a quarter-inch in diameter with distinct black and white markings on their back and legs.

Aedes aegypti can transmit diseases like Chikungunya, Dengue, Yellow Fever and Zika. The county says none of these diseases are currently found in California.

Read more

Mosquitoes test positive for West Nile virus in Palo Alto and Los Altos

From the Mountain View Voice
October 19, 2022

Mosquitoes have tested positive for West Nile virus in a small area of Palo Alto and Los Altos, the Santa Clara County Vector Control District said on Wednesday.

The areas, which include ZIP codes 94304, 94306 and 94022 will be treated on Thursday starting at 10 p.m. to reduce adult mosquito populations, weather permitting. Truck-mounted equipment will apply the treatment over the area for approximately four hours.

A notice is being sent directly to the public in the treatment ZIP codes through AlertSCC and to those who subscribe to Nextdoor neighborhood networks. A general notice is being provided on various social media platforms — including Facebook, Instagram and Twitter — and to those subscribed to the district’s mosquito treatment notifications.

Read more

Zika virus-carrying mosquito species detected for first time in Sacramento County

From the Sacramento Bee
October 19, 2022

An invasive mosquito species has been newly detected in Sacramento County, local officials announced Wednesday, marking the second such species found in the area since 2019.

Aedes albopictus, commonly referred to as the Asian tiger mosquito, was recently located in the backyard of a Carmichael resident who reported being bitten, the Sacramento-Yolo Mosquito and Vector Control District said in a news release.

Field technicians with the district then conducted door-to-door inspections at other homes in the same neighborhood and found additional mosquitoes and larvae, according to the release.

Read more

Are you a mosquito magnet? It could be your smell

From ABC10
October 18, 2022

NEW YORK — A new study finds that some people really are “mosquito magnets” and it probably has to do with the way they smell.

The researchers found that people who are most attractive to mosquitoes produce a lot of certain chemicals on their skin that are tied to smell. And bad news for mosquito magnets: The bloodsuckers stay loyal to their favorites over time.

“If you have high levels of this stuff on your skin, you’re going to be the one at the picnic getting all the bites,” said study author Leslie Vosshall, a neurobiologist at Rockefeller University in New York.

There’s a lot of folklore about who gets bitten more but many claims aren’t backed up with strong evidence, said Vosshall.

Read more

Invasive yellow fever mosquito species found in Manteca for first time

From CBS Sacramento
October 12, 2022

MANTECA — Eggs of the Aedes aegypti mosquito, commonly known as the “yellow fever” mosquito, have been discovered in Manteca for the first time.

The pesky bloodsucker continues to grow in population across San Joaquin County, first detected in West Stockton, Ripon, Escalon, and South Stockton.

They aren’t your average mosquito, and the San Joaquin district says you should call them right away if one bites you.

“This is a very fearless mosquito, and they are very aggressive biters,” said Omar Khweiss, General Manager for San Joaquin County Mosquito and Vector Control District.

Read more

West Nile virus detected in Petaluma for first time in 2022

From the Argus Courier
October 12, 2022

This year’s first detection of West Nile virus was found last week in Petaluma.

The Marin/Sonoma Mosquito and Vector Control District confirmed the finding after collecting a dead American Crow infected with the virus near South McDowell Boulevard and Casa Grande Road.

District staff will continue trapping, testing and monitoring the adult mosquitoes around the area where the positive dead bird was found. Vector control technicians are inspecting and keeping track of all sources of mosquito production and performing control operations as needed, according to the district.

“This positive dead bird is a reminder that West Nile virus is endemic to our region,” Nizza Sequeira, public information officer for the district, said in a news release.

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The impact of transgenesis on mosquito fitness: A review

Posted by Vector and Vector-borne Disease Committee
October 11, 2022

Padukka Vidanelage Desha Dilani1, Ranil Samantha Dassanayake1, Brij Kishore Tyagi2 and Yasanthi Illika Nilmini Silva Gunawardene3

1Department of Chemistry, Faculty of Science, University of Colombo, Colombo, Sri Lanka

2Sponsored Research & Industrial Centre, VIT University, Vellore (TN), India

3Molecular Medicine Unit, Faculty of Medicine, University of Kelaniya, Ragama, Sri Lanka

REVIEW article.  Front. Insect Sci., 30 September 2022.  Sec. Insect Physiology

https://doi.org/10.3389/finsc.2022.957570 

Author’s abstract.  Transgenic mosquitoes developed by genetic manipulation, offer a promising strategy for the sustainable and effective control of mosquito-borne diseases. This strategy relies on the mass release of transgenic mosquitoes into the wild, where their transgene is expected to persist in the natural environment, either permanently or transiently, within the mosquito population. In such circumstances, the fitness of transgenic mosquitoes is an important factor in determining their survival in the wild. The impact of transgene expression, insertional mutagenesis, inbreeding depression related to laboratory adaptation, and the hitchhiking effect involved in developing homozygous mosquito lines can all have an effect on the fitness of transgenic mosquitoes. Therefore, real-time estimation of transgene-associated fitness cost is imperative for modeling and planning transgenic mosquito release programs. This can be achieved by directly comparing fitness parameters in individuals homozygous or hemizygous for the transgene and their wild-type counterparts, or by cage invasion experiments to monitor the frequency of the transgenic allele over multiple generations. Recent advancements such as site-specific integration systems and gene drives, provide platforms to address fitness issues in transgenic mosquitoes. More research on the fitness of transgenic individuals is required to develop transgenic mosquitoes with a low fitness cost.

Note:  This review paper addresses the fitness of released mosquitoes and should be of interest to those agencies that are using releases for invasive Aedes control, even though some of the genetic methods may not be similar to what is being used locally.

14th California Horse Tests Positive for WNV

From The Horse
October 10, 2022

On Oct. 6 the California Department of Food and Agriculture confirmed a 6-year-old Quarter Horse mare in Sacramento County tested positive for West Nile virus (WNV). She began falling and was acutely unable to move beginning Sept. 28 and is currently alive. 

This is the 14th confirmed case of equine WNV in California in 2022. 

Read more

1st dead bird positive for West Nile virus in North Bay this year found in Petaluma

From North Bay News
October 7, 2022

PETALUMA — The first dead bird in the North Bay infected with the West Nile Virus this year was found in Sonoma County, officials said Friday.

The dead American crow was found in the area of South McDowell Blvd. and Casa Grande Rd. in Petaluma, according to the Marin/Sonoma Mosquito and Vector Control District.

“This positive dead bird is a reminder that West Nile virus is endemic to our region,” said district spokesperson Nizza Sequeira in a press statement. “As we head into fall, we urge the public to continue to look for and eliminate standing water in their yards, stock permanent ponds with mosquitofish, and protect themselves from mosquito bites.”

The district said staffers will continue to trap, test, and monitor the abundance of adult mosquitoes in the area where the positive dead bird was found. The district is also reviewing all documented sources of mosquito production and searching for new sources.

Read more

St. Louis Encephalitis Virus in the Southwestern United States: A Phylogeographic Case for a Multi-Variant Introduction Event

Posted by Vector and Vector-borne Disease Committee
October 1, 2022

Chase L. Ridenour 1,2, Jill Cocking 1,2, Samuel Poidmore 2, Daryn Erickson 2, Breezy Brock 2, Michael Valentine 3, Chandler C. Roe 1,2, Steven J. Young 4, Jennifer A. Henke 5, Kim Y. Hung 5, Jeremy Wittie 5, Elene Stefanakos 6, Chris Sumner 6, Martha Ruedas 6, Vivek Raman 7, Nicole Seaton 7, William Bendik 7, Heidie M. Hornstra O’Neill 2, Krystal Sheridan 2,3, Heather Centner 3, Darrin Lemmer 3, Viacheslav Fofanov 1,2, Kirk Smith 4, James Will 4, John Townsend 4, Jeffrey T. Foster 2, Paul S. Keim 2,3, David M. Engelthaler 3 and Crystal M. Hepp 1,2*

1 School of Informatics, Computing, and Cyber Systems, Northern Arizona University, Flagstaff, AZ, United States, 2 The Pathogen and Microbiome Institute, Northern Arizona University, Flagstaff, AZ, United States, 3 Translational Genomics Research Institute, Flagstaff, AZ, United States, 4 Vector Control Division, Maricopa County Environmental Services Department, Phoenix, AZ, United States, 5 Coachella Valley Mosquito and Vector Control District, Indio, CA, United States, 6 Yuma County Pest Abatement District, Yuma, AZ, United States, 7 Southern Nevada Health District, Las Vegas, NV, United States

  1. Frontiers in Genetics. 12: 667805 [doi: 10.3389/fgene.2021.667895]

Author’s abstract.  Since the reemergence of St. Louis Encephalitis (SLE) Virus (SLEV) in the Southwest United States, identified during the 2015 outbreak in Arizona, SLEV has been seasonally detected within Culex spp. populations throughout the Southwest United States. Previous work revealed the 2015 outbreak was caused by an importation of SLEV genotype III, which had only been detected previously in Argentina. However, little is known about when the importation occurred or the transmission and genetic dynamics since its arrival into the Southwest. In this study, we sought to determine whether the annual detection of SLEV in the Southwest is due to enzootic cycling or new importations. To address this question, we analyzed 174 SLEV genomes (142 sequenced as part of this study) using Bayesian phylogenetic analyses to estimate the date of arrival into the American Southwest and characterize the underlying population structure of SLEV. Phylogenetic clustering showed that SLEV variants circulating in Maricopa and Riverside counties form two distinct populations with little evidence of inter-county transmission since the onset of the outbreak. Alternatively, it appears that in 2019, Yuma and Clark counties experienced annual importations of SLEV that originated in Riverside and Maricopa counties. Finally, the earliest representatives of SLEV genotype III in the Southwest form a polytomy that includes both California and Arizona samples. We propose that the initial outbreak most likely resulted from the importation of a population of SLEV genotype III variants, perhaps in multiple birds, possibly multiple species, migrating north in 2013, rather than a single variant introduced by one bird.

Note:  This interesting study explored several hypotheses concerning the persistence and movement of SLEV in the SW.  Interestingly all recent isolates have been Lineage III previously only known from South America, whereas the historically endemic lineages I and II found only in North America have not been detected since their extinction in 2003 when WNV invaded the SW. 

Invasive “ankle-biter” mosquitos plaguing Southern Californians

From CBS Los Angeles
September 29, 2022

An invasive genus of mosquitos have made their way to Southern California, leaving a trail of irritation and itchy bites in their wake. 

The Aedes mosquito, originally located in tropical climates, have been found on nearly every continent in the world, and are believed to have arrived in California more than 10 years ago via cargo ship. 

This summer, it appears they’ve migrated in mass to the Southland, spreading throughout the San Fernando Valley, Inland Empire and Greater San Diego. 

They’re small and black, covered with white stripes on their legs and body.

Read more

West Nile Virus Detection Continues in Eastern Coachella Valley

From NBC Palm Springs
September 27, 2022

“West Nile virus is and can be a very dangerous disease so we’re telling people right now, just be careful when you go outside,” said Tammy Gordon, Public Information Manager for Coachella Valley Mosquito and Vector Control District. 

At least 100 positive samples of the West Nile virus have been detected in the Coachella Valley so far this year. 

“The current samples that we have are showing the Mecca area, so downtown Mecca. However, we have found samples in La Quinta, South La Quinta,” said Gordon. 

West Nile virus is the leading cause of mosquito-borne disease in the United States, according to the CDC. 

No human cases of the illness have been reported in the Coachella Valley, however, the state of california reports 69 positive cases of the virus this year. 

The virus spreads when a female mosquito bites an infected bird. The carrier mosquito can then transmit the virus to humans. 

Read more

Biology of Vector-borne Diseases six-day course

From The University of Idaho (UI) IHHE

Dear members of the Institute for Health in the Human Ecosystem (IHHE),  

The University of Idaho (UI) IHHE is hosting its annual Biology of Vector-borne Diseases six-day course, Monday through Saturday, June 12-17, 2023, on the UI campus in Moscow, Idaho. This course provides accessible, condensed training and “knowledge networking” for advanced graduate students, postdoctoral fellows, new faculty, and current professionals to ensure competency in basic biology and cutting-edge knowledge for U.S. and global vector-borne diseases of plants, animals, and humans.  

With the support of 30+ internationally recognized faculty, we seek to train the next generation of scientists and professionals to think about vector-borne diseases as interconnected pathosystems and help them to develop effective solutions to current and emerging vector-borne disease threats in complex human ecosystems. 

Applications will be reviewed starting December 1, 2022, and applicants will be notified of their acceptance for the course in spring 2023. Please email chhe@uidaho.edu with any questions 

Please would you/your organization to share our event information 

A link to our event is:  https://www.uidaho.edu/research/entities/ihhe/education/vector-borne-diseases  

Apply here: https://www.uidaho.edu/research/entities/ihhe/education/vector-borne-diseases/application 

 

La Crosse virus is the second-most common virus in the US spread by mosquitoes – and can cause severe neurological damage in rare cases

Posted by Vector and Vector-borne Disease Committee
September 25, 2022

Rebecca Trout Fryxell

Associate Professor of Medical and Veterinary Entomology, University of Tennessee

9 Sep 2022.  The Conversation.  https://theconversation.com/la-crosse-virus-is-the-second-most-common-virus-in-the-us-spread-by-mosquitoes-and-can-cause-severe-neurological-damage-in-rare-cases-184412.

Note:  This recent review describes the impact of La Cross virus (LACV) on human health east of the Mississippi where it is transmitted mostly by Aedes triseriatus and Ae. albopictus.  Unlike West Nile virus this neuroinvasive virus affects children more frequently than the elderly and can result in long lasting sequellae.  LACV is one of several arboviruses within the California encephalitis virus (CEV) serogroup within the family Peribunyaviridae.  Although few cases have been diagnosed and surveillance programs no longer test mosquitoes for these viruses, the type virus for this group, CEV, is endemic to California where it is maintained vertically within Aedes melanimon and Ae. dorsalis populations and amplified by horizontal transmission to rabbits.   Another virus within this serogroup, Jamestown Canyon virus, has been isolated from Culiseta inornata collected in California, but seems to be a human health problem mostly in the upper Midwest. 

One Pasadenan Has Contracted West Nile Virus, And Local Health Officials Are Working To Prevent More Infections

From Pasadena NOW
September 25, 2022

The dangerous West Nile virus (WNV) continues to be detected in mosquito and bird populations in the San  Gabriel Valley and one Pasadena resident is known to have contracted the virus this tear, a city health official said.

Manuel Carmona, Acting Director of the Pasadena Public Health Department, said that as of September 9,  there have been 11 human West Nile vims cases reported in Los Angeles County, including one in  Pasadena. 

Mosquito control experts are advising residents to remove stagnant water due to the early  September rain. The stagnant rainwater sitting in containers can result in a risk of mosquito-borne diseases in the community. 

City officials said that if residents notice any stagnant water issues in their community, they can submit a tip online or call 626-814-9466.  

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‘Drain after rain’ to prevent mosquito breeding

From Fox 40
September 20, 2022

(KTXL) — The Sacramento-Yolo Mosquito and Vector Control District are reminding residents to “drain after the rain” in order to prevent mosquitoes.

According to the Sacramento-Yolo Mosquito and Vector Control District, warm temperatures after rain will create ideal breeding conditions for mosquitoes.

The Sacramento-Yolo Mosquito and Vector Control District said in a news release that mosquito season is not over and the threat of West Nile virus is still being seen in dead birds and mosquito samples across the county. There are 3 confirmed cases of West Nile virus in humans in Yolo County.

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Positive West Nile Virus Mosquitoes Found in Portions of Mountain View, Los Altos Hills and Sunnyvale

From the County of Santa Clara
September 20, 2022

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District has confirmed the presence of West Nile virus-positive mosquitoes in a small area that includes Mountain View, Los Altos Hills and Sunnyvale (ZIP codes 94024, 94040 and 94087). Weather permitting, this area will be treated to reduce adult mosquito populations with the use of truck-mounted equipment on Thursday, Sept. 22, starting around 10 p.m. and concluding a few hours later.
 
The District’s mosquito management program largely focuses on preventing mosquitoes from reaching the adult biting stage by proactively targeting immature stages of mosquitoes found in standing water. When a mosquito with West Nile Virus (WNV) is detected, however, the District takes the added step of conducting adult mosquito control treatment to reduce the mosquito population in the area, which decreases the risk of a WNV-human infection.
 
It is normal to see an increase in West Nile virus during the summer and early fall because mosquitoes thrive in warm weather. Although mosquitoes need water at each stage of life, they are still able to thrive during the drought conditions the state and county are seeing now.

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Positive West Nile Virus Mosquitoes Found in Portions of Milpitas

From the County of Santa Clara
September 19, 2022

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District has confirmed the presence of West Nile virus-positive mosquitoes in a small area of Milpitas (ZIP code 95035). Weather permitting, this area will be treated to reduce adult mosquito populations with the use of truck-mounted equipment on Wednesday, Sept. 21, starting around 10 p.m. and concluding a few hours later.
 
The District’s mosquito management program largely focuses on preventing mosquitoes from reaching the adult biting stage by proactively targeting immature stages of mosquitoes found in standing water. When a mosquito with West Nile Virus (WNV) is detected, however, the District takes the added step of conducting adult mosquito control treatment to reduce the mosquito population in the area, which decreases the risk of a WNV-human infection.
 
It is normal to see an increase in West Nile virus during the summer and early fall because mosquitoes thrive in warm weather. Although mosquitoes need water at each stage of life, they are still able to thrive during the drought conditions the state and county are seeing now.

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Attack Of The Ankle-Biter: Aedes Mosquitos Plague Southern California

From MSN
September 14, 2022

REDONDO BEACH, CA — Residents of Redondo Beach have been pestered by relentless, and frankly annoying, bites from the invasive Aedes mosquito that has made a new home in Southern California.

Atypical to other mosquitos that usually go for just one bite at dawn or dusk, Aedes mosquitos are aggressive and love to bite people during the day according to Aaron Arugay, Executive Director of Los Angeles County West Vector Control.

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First human West Nile virus case of 2022 reported in Shasta County

From the Record Searchlight
September 6, 2022

A Shasta County resident is in the hospital with the West Nile virus, the first reported human case in the county this year, health officials announced Tuesday.

The announcement confirmed the patient is an adult but did not say where they contracted the virus or provide any other identifying information to protect the patient’s privacy.

The first sign of West Nile virus appeared in Redding in early July.

The Shasta Mosquito and Vector Control District said it had found in one of its traps an adult male mosquito that was carrying the disease. 

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Officials confirm 1st human case of West Nile Virus in Madera County this year

From ABC30
September 1, 2022

MADERA COUNTY, Calif. (KFSN) — Health officials have confirmed the first human case of West Nile Virus in Madera County this year.

Officials have not released the person’s condition.

“This case is a reminder of the risks from West Nile Virus-infected mosquito bites. It is important to learn and follow the recommendations to lower your risk of being bitten by mosquitoes,” stated Simon Paul, M.D., Madera County Public Health Officer.

Symptoms include a fever, head or body ache, and joint pain.

But according to the CDC, 8 out of 10 people infected with West Nile won’t develop symptoms.

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Death from West Nile virus reported in Fresno County

From Fresno 26 News
September 1, 2022

FRESNO, Calif. (FOX26) — The Fresno County Department of Public Health (FCDPH) confirmed a human death caused by West Nile Virus (WNV).

The department has reported nine positive human cases of WNV this year, and that number is expected to increase.

There 14 positive cases and one death reported in Fresno County in 2021.

According to the California Department of Public Health (CDPH) in 2021, there were 129 reported WNV cases in the state, including 12 deaths.

Since WNV was first introduced into California in 2003, there have been more than 7,300 human WNV cases and more than 330 WNV-related deaths across the state.

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Invasive species of mosquito detected in Chico

From KRCR 7
August 31, 2022

BUTTE COUNTY, Calif. — The Butte County Mosquito and Vector Control District (MVCD) detected an invasive species of mosquito on Tuesday, Aug. 30, 2022.

Officials said the Yellow Fever Mosquito, (Aedes Mosquito, Ades aegypti) was found in Chico near East Avenue and Manzanita Avenue.

This invasive, non-native species of mosquito has been detected in Butte County for three consecutive years now—in Oroville and now Chico.

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Heat brings longer mosquito season, more West Nile cases to Shasta County

From KRCR 7
August 31, 2022

SHASTA COUNTY, Calif. — As excessive, triple-digit temperatures engulf our area, the number of mosquitoes infected with West Nile Virus (WNV) continues to grow.

On Wednesday this week, the Shasta Mosquito and Vector Control District (Shasta MVCD) said they saw WNV-positive mosquito sample levels above what they have seen in the last six years.

Shasta MVCD officials said the continued record drought and warm temperatures will encourage a longer season for mosquitoes and the diseases they carry. Out of the hundreds of samples taken from adult mosquitoes in the area, as of Aug. 26, 30 were found to be positive with WNV.

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Estimating Aedes aegypti (Diptera: Culicidae) Flight Distance: Meta-Data Analysis

Posted by Vector and Vector-borne Disease Committee
August 30, 2022

Thomas C Moore, Heidi E Brown

Department of Epidemiology and Biostatistics, Mel and Enid Zuckerman College of Public Health, University of Arizona, Tucson, AZ, USA

Journal of Medical Entomology, Volume 59, Issue 4, July 2022, Pages 1164–1170, https://doi.org/10.1093/jme/tjac070

Abstract  [shortened]:  To generate a more robust estimate of Ae. aegypti flight distance, we conducted a meta-data analysis with the aims 1) to provide a flight distance measurement and 2) investigate how mosquito flight range can be affected by study design and climatic factors. Published studies were retrieved from public databases and reviewed for mean distance traveled (MDT) or maximum distance traveled measurements of Ae. aegypti. Linear regression was used to assess potential relationships between Ae. aegypti flight distance and factors pertaining to climate, degrees of urbanization, and study design. MDT estimates were pooled from 27 experiments to calculate a weighted MDT of 105.69 m. This study addresses the average flight distance of Ae. aegypti with the intention of informing vector control programs in Ae. aegypti prevalent regions of the world.

Comment:  This review incorporates published data to provide a global estimate of flight range, a parameter critical in understanding potential virus transmission patterns and establishing protocols for containment around imported cases.  Although this information was focused on females, male dispersal data establishes possible infiltration rates into areas treated with the releases of genetically altered males for control. 

First West Nile virus cases reported in Antelope Valley

From the Antelope Valley Press
August 28, 2022 

The first human cases of West Nile virus, this year, have been reported in the Antelope Valley, Los Angeles County Public Health Department officials reported, Thursday.

The Department reported six cases in Los Angeles County, in the Antelope, San Gabriel and San Fernando valleys.

Most of the six people infected were hospitalized for the illness, in late July and early August, according to the report.

Further information as to where the cases were in the Antelope Valley, or how many were here, was not available.

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LA County Announces First Human Cases Of West Nile Virus

From the Los Angeles Patch
August 27, 2022

LOS ANGELES, CA — Los Angeles County health officials have confirmed the county’s first human cases of West Nile virus this year, saying six cases have been identified since late July.

No specifics about the patients were released on Thursday — but according to the county Department of Public Health, the victims live in the Antelope Valley, San Fernando Valley and San Gabriel Valley.

Most of the patients were hospitalized in late July and early August, and all are recovering, according to the county.

“Mosquitoes thrive in hot weather and residents should follow simple steps to reduce their risk of exposure to mosquito-borne diseases, such as West Nile virus,” county Health Officer Dr. Muntu Davis said in a statement.

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Fresno County health officials report 9 human cases of West Nile Virus

From ABC 30
August 26, 2022

FRESNO, Calif. (KFSN) — You might want to take a closer look at any standing water near your home, from planters to unmaintained swimming pools.

“If you see anything that’s wiggling like this, it’s mosquito larvae, dump it out,” says Katherine Ramirez, the scientific education coordinator with Fresno County Mosquito Control.

She adds: “It doesn’t matter the area. We’re finding the virus in all areas. It could be that your neighbor has an unmaintained swimming pool.”

While these aren’t fully grown mosquitoes, Ramirez says mosquitoes are temperature-dependent, so the hotter the temperature the faster the development.

“That’s why its important to check water sources on a daily basis when we have hot temperatures or at least once a week,” she says.

Read more

 

LA County reports 1st cases of West Nile virus in humans for 2022

From ABC7
August 25, 2022

LOS ANGELES (CNS) — Los Angeles County health officials Thursday confirmed the county’s first human cases of West Nile virus this year, saying six cases have been identified since late July.

No specifics about the patients were released, but according to the county Department of Public Health, they live in the Antelope Valley, San Fernando Valley and San Gabriel Valley. Most of the patients were hospitalized in late July and early August, and all are recovering, according to the county.

“Mosquitoes thrive in hot weather and residents should follow simple steps to reduce their risk of exposure to mosquito-borne diseases, such as West Nile virus,” county Health Officer Dr. Muntu Davis said in a statement.

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West Nile Virus numbers rising in Fresno County

From MSN
August 25, 2022

FRESNO COUNTY, Calif. (KSEE/KGPE) – The California Department of Health (CDPH) reported that nine Fresno County residents have tested positive for West Nile Virus (WNV) infections.

Health officials say three of the cases are asymptomatic and were blood donors.

The cases are the first reported in this season and according to CDPH, the Fresno County mosquito control districts have also collected 216 samples of mosquitoes that tested positive for WNV.

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West Nile Virus Surge Detected In Cupertino

From The Cupertino Patch
August 24, 2022

SANTA CLARA COUNTY, CA — As Santa Clara County faces its driest year to date in the past 128 years, county officials say drought conditions are causing a rise in mosquito populations and West Nile virus activity

The Santa Clara County Vector Control District warned this week of an uptick in West Nile virus-infected mosquitos found in a small portion of the county.

The district recently detected infected mosquitos in Sunnyvale and Cupertino in the 94086, 94087 and 95014 zip codes.

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East Bay fighting aggressive invasive mosquito species

From KRON 4
August 18, 2022

MARTINEZ (KRON) – Contra Costa County has identified its first group of invasive mosquito species.

The Contra Costa County Mosquito and Vector Control District is now doing surveillance and treatment in the area where the mosquitos were found in Martinez to make sure they are eliminated before becoming widespread.

Officials say these mosquitoes are very aggressive day biters that can transmit viruses like Zika and the virus that causes Yellow Fever. The species, Aedes aegypti, is originally from West Africa, and was first detected in California in 2014.

Mosquitoes can hide in vegetation and debris which makes it hard to find and eliminate them, so the control district is setting up traps and going door-to-door.

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Despite drought, mosquitoes carrying West Nile found locally

From the Los Altos Town Crier
August 17, 2022

The drought baking California has rendered water-restricting local lawns and yards arid, but mosquitoes have fared well this summer nonetheless. Santa Clara County Vector Control District surveillance identified mosquitoes with West Nile virus in parts of Cupertino and started spraying insecticide to target the insects last week, with similar spraying last month in parts of Sunnyvale, Santa Clara and San Jose.

The county maintains surveillance throughout local cities including Mountain View and Los Altos using carbon dioxide traps – which attract hungry adult mosquitoes seeking the exhalations that indicate a likely mammalian meal – and gravid traps, boxes of standing water baited with the funky aroma of rotting alfalfa to attract pregnant mosquitoes ready to start a new generation. The district also uses light traps – a green metal device with a light bulb and a jar of captured insects at its bottom – to monitor the ambient abundance of mosquitoes in the area, which fluctuates seasonally and tends to rise with warm summer weather.

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West Nile Virus Update: Six More Areas in County Report Positive Samples

From SCV News
August 16, 2022

The Greater Los Angeles County Vector Control District has confirmed 13 additional mosquito samples that tested positive for West Nile virus.

This brings the total number of positive samples within the District’s service area to 76 this year.

This is the first detection of the virus this year in the following communities: Canoga Park, Downey, Eagle Rock, Elysian Valley, Los Angeles city and Winnetka.

West Nile virus is endemic to Los Angeles County, and warm temperatures can increase virus activity and mosquito populations. Visit VectorSurv Maps or WestNile.ca.gov for a comprehensive look at this year’s West Nile virus activity throughout Los Angeles County and Southern California.

Join Mosquito Watch to access online tools and resources such as the DIY Mosquito Source Checklist or request door hangers to be distributed in their community at MosquitoWatch.org.

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Orange County man tests positive for West Nile virus; 1st reported case in county

From ABC 7
August 13, 2022

ORANGE, Calif. (KABC) — Orange County has its first reported case of West Nile virus after a man tested positive this week.

Details surrounding the man’s condition weren’t immediately released. It’s also unclear of what portion of Orange County the resident resides in.

People over age 50 and those with certain medical conditions are at an increased risk of getting the virus, health officials warn.

“West Nile Virus is endemic in Orange County, recurring every year during the summer months and continuing into the fall,” said Deputy County Health Officer Dr. Matthew Zahn. “The best way to avoid West Nile Virus infection is to take precautionary measures to avoid mosquito bites.”

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‘It’s tick season all year round’: Precaution recommended for Sonoma County residents against Lyme disease carrying bugs

From The Press Democrat
August 13, 2022

Former Bay Area news anchor Leslie Griffith died Aug. 10, according to a report by KTVU Channel 2 News. Family members say Griffith had been suffering from Lyme Disease since she was bitten by a tick while living in Oregon in 2015.

In California, the western black-legged ticks — the species that carries the bacterium that causes Lyme disease — have been found in 56 of the state’s 58 counties, including Sonoma County, according to the California Department of Public Health.

Yet officials at the Marin-Sonoma Mosquito and Vector Control District say residents can employ several precautions to avoid ticks outdoors and around their homes.

“In Sonoma County, ticks are actually active year-round,” said Nizza Sequeira, public information officer for Marin-Sonoma Mosquito and Vector Control District.

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Orange County reports this year’s first human case of West Nile virus

From CBS Los Angeles
August 12, 2022

A man who tested positive for West Nile virus infection is the first human case reported in Orange County this year, public health officials said Friday.

The patient was described only as an adult male, and the Orange County Health Care Agency gave no further information about the case or where the man may have contracted the virus. However, no one in Orange County has ever died of West Nile virus, and last year there were just three reported cases of human infection.

West Nile virus has already been detected throughout Los Angeles County. Three dead crows in North Hills tested positive for the virus, but LA County has not yet reported any human infections.

West Nile virus is spread most often by mosquitos, which can infect humans, birds, horses, and other mammals. Most people who become infected do not become sick, but about 20% will develop flu-like symptoms, including fever, headache, body aches, nausea, fatigue, and sometimes a skin rash. More serious symptoms include neck stiffness, confusion, muscle weakness, and paralysis, but those cases are rare.

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More Mosquitoes Test Positive For West Nile Virus In Contra Costa

From the Martinez, CA Patch
August 12, 2022

CONTRA COSTA COUNTY, CA — More mosquitoes have tested positive for West Nile virus in Contra Costa County, authorities said Friday.

The mosquitoes were collected from a trap in an agricultural area east of Brentwood, according to a news release from the Contra Costa Mosquito & Vector Control District.

A previous group of mosquitoes that tested positive this year for WNV was found in Oakley.

WNV comes from certain birds, mostly crows and jays; mosquitoes become infected after biting an infected bird.

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Epidemiologic and environmental characterization of the re-emergence of St. Louis Encephalitis Virus in California, 2015–2020

Posted by Vector and Vector-borne Disease Committee
August 10, 2022

Mary E. Danforth1, Robert E. Snyder1, Tina Feiszli1, Teal Bullick2, Sharon Messenger2, Carl Hanson2, Kerry Padgett1, Lark L. Coffey3, ChristopherM. Barker3, William K. Reisen3, Vicki L. Kramer1

1 California Department of Public Health, Vector-Borne Disease Section, Richmond and Sacramento, California, 2 California Department of Public Health, Viral and Rickettsial Disease Laboratory, Richmond, California, 3 Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, California, United States of America

PLoS Negl Trop Dis 16(8): e0010664. https://doi.org/10.1371/journal.pntd.0010664

Author summary.   St. Louis encephalitis virus is a mosquito-borne virus that can cause human disease and is found in California, where it was detected every year from 1938 to 2003. However, after

West Nile virus arrived in 2003, St. Louis encephalitis virus was not detected again until 2015, when it re-emerged in Riverside County. From 2015 through 2020, St. Louis encephalitis virus has been detected in mosquito pools and sentinel chicken sera samples in 16 counties and a total of 24 human disease cases have been reported. However, during that same time-period, West Nile virus has been detected in 10 times as many mosquito pools and 60 times as many chicken sera samples across 58 counties and over 2,400 human disease cases have been reported. Although mosquitoes are tested routinely for both viruses, surveillance is not uniform throughout the state, and there has been a steady decline in the use of sentinel chickens. Since St. Louis encephalitis virus patient screening is dependent upon environmental detection, California may be underestimating the incidence of human disease due to this virus.

Note:  Although WNV and SLEV cycle enzootically in basically the same vectors and avian hosts, historically SLEV in California never amplified to the levels documented for WNV.  This may be because SLEV does not elicit elevated viremias in a wide diversity of avian hosts which limit transmission efficiency and therefore amplification.   In addition, recently widespread WNV activity may have ‘immunized’ avian populations against SLEV as well as WNV, because previous infection with WNV produces sterilizing immunity against subsequent infection with SLEV.