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The California Arbovirus Surveillance Program was initiated over 50 years ago to track endemic encephalitides and was enhanced in 2000 to include West Nile virus (WNV) infections in humans, mosquitoes, sentinel chickens, dead birds and horses. This comprehensive statewide program is a function of strong partnerships among the California Department of Public Health (CDPH), the University of California, and local vector control and public health agencies. This manuscript summarizes WNV surveillance data in California since WNV was first detected in 2003 in southern California. From 2003 through 2018, 6,909 human cases of WNV disease, inclusive of 326 deaths, were reported to CDPH, as well as 730 asymptomatic WNV infections identified during screening of blood and organ donors. Of these, 4,073 (59.0%) were reported as West Nile neuroinvasive disease. California's WNV disease burden comprised 15% of all cases that were reported to the U.S. Centers for Disease Control and Prevention during this time, more than any other state. Additionally, 1,299 equine WNV cases were identified, along with detections of WNV in 23,322 dead birds, 31,695 mosquito pools, and 7,340 sentinel chickens. Annual enzootic detection of WNV typically preceded detection in humans and prompted enhanced intervention to reduce the risk of WNV transmission. Peak WNV activity occurred from July through October in the Central Valley and southern California. Less than five percent of WNV activity occurred in other regions of the state or outside of this time. WNV continues to be a major threat to public and wild avian health in California, particularly in southern California and the Central Valley during summer and early fall months. Local and state public health partners must continue statewide human and mosquito surveillance and facilitate effective mosquito control and bite prevention measures.

Currently, there are very few studies of avian malaria that investigate relationships among the host-vector-parasite triad concomitantly. In the current study, we experimentally measured the vector competence of several Culex mosquitoes for a newly described avian malaria parasite, Plasmodium homopolare. Song sparrow (Melospiza melodia) blood infected with a low P. homopolare parasitemia was inoculated into a naïve domestic canary (Serinus canaria forma domestica). Within 5 to 10 days post infection (dpi), the canary unexpectedly developed a simultaneous high parasitemic infection of Plasmodium cathemerium (Pcat6) and a low parasitemic infection of P. homopolare, both of which were detected in blood smears. During this infection period, PCR detected Pcat6, but not P. homopolare in the canary. Between 10 and 60 dpi, Pcat6 blood stages were no longer visible and PCR no longer amplified Pcat6 parasite DNA from canary blood. However, P. homopolare blood stages remained visible, albeit still at very low parasitemias, and PCR was able to amplify P. homopolare DNA. This pattern of mixed Pcat6 and P. homopolare infection was repeated in three secondary infected canaries that were injected with blood from the first infected canary. Mosquitoes that blood-fed on the secondary infected canaries developed infections with Pcat6 as well as another P. cathemerium lineage (Pcat8); none developed PCR detectable P. homopolare infections. These observations suggest that the original P. homopolare-infected songbird also had two un-detectable P. cathemerium lineages/strains. The vector and host infectivity trials in this study demonstrated that current molecular assays may significantly underreport the extent of mixed avian malaria infections in vectors and hosts.

To investigate the phenotypic evolution of West Nile virus (WNV) in California, we competed sixteen isolates made during 2007-08 against COAV997-5nt, a genetically marked clone from the founding 2003 California isolate COAV997-2003. Using in vivo fitness competitions in House Finches (HOFI) and Culex tarsalis mosquitoes, we found that the majority of WNV WN02 and SW03 genotype isolates exhibited elevated replicative fitness in both hosts compared to COAV997-5nt. Increased replicative capacity in HOFIs was not associated with increased mortality, indicating that these isolates had not gained avian virulence. One WN02 isolate from Coachella Valley, a region geographically close to the isolation of COAV997, showed neutral fitness in HOFIs and reduced fitness in Cx. tarsalis. Two isolates from Kern County and Sacramento/Yolo County out-competed COAV997-nt in HOFIs, but were transmitted less efficiently by Cx. tarsalis. Competition demonstrated neutral or increased fitness that appeared independent of both WN02 and SW03 genotypes.

St. Louis encephalitis virus infection was detected in summer 2015 in southern California after an 11-year absence, concomitant with an Arizona outbreak. Sequence comparisons showed close identity of California and Arizona isolates with 2005 Argentine isolates, suggesting introduction from South America and underscoring the value of continued arbovirus surveillance.

West Nile virus (WNV) is an important cause of disease in humans and animals. Risk of WNV infection varies seasonally, with the greatest risk during the warmest parts of the year due in part to the accelerated extrinsic incubation rate of the virus in mosquitoes. Rates of extrinsic incubation have been shown in constant-temperature studies to increase as an approximately linear function of temperature, but for other vector-borne pathogens, such as malaria or dengue virus, nonlinear relationships have been demonstrated under cycling temperatures near the thermal limits of pathogen replication. Using typical daily air temperature profiles from three key periods of WNV amplification in a hyperendemic area of WNV activity in California's Central Valley, as well as a fourth temperature profile based on exposures that would result from daily mosquito host-seeking and resting behavior, we explored the impacts of cycling temperatures on WNV transmission by Culex tarsalis Coquillett, one of the principal vectors in the western United States. The daily cycling temperature ranges studied were representative of those that occur across much of California, but they did not significantly alter the extrinsic incubation period of WNV compared with estimates from mean temperatures alone. This suggests that within the relatively broad range we studied, WNV incubation rates are a simple function of mean temperature. Realistic daily temperature patterns that reflected mosquitoes' avoidance of daytime high temperatures during summer reduced transmission over time compared with air temperatures, indicating that adjustment for mosquito exposure temperatures would be prudent for calculating risk.

The emergence of Japanese encephalitis virus (JEV) in the Himalayan highlands is of significant veterinary and public health concern and may be related to climate warming and anthropogenic landscape change, or simply improved surveillance. To investigate this phenomenon, a One Health approach focusing on the phylogeography of JEV, the distribution and abundance of the mosquito vectors, and seroprevalence in humans and animal reservoirs would be useful to understand the epidemiology of Japanese encephalitis in highland areas.

The efficiency of West Nile virus (WNV) transmission by competent mosquito vectors is driven by temperature and defined, in part, by the extrinsic incubation period, which is the time from a mosquito's consumption of an infected bloodmeal until it becomes capable of transmitting the virus to the next vertebrate host. The extrinsic incubation period can be altered by a variety of factors involved in vector-pathogen interactions, and in North America, the WN02 strain of WNV emerged and displaced the founding NY99 strain reportedly because the duration of the extrinsic incubation period in Culex mosquitoes was shortened by a single positively selected mutation. However, recent work has suggested that this change is not universal and may depend on vector species or strain. In the current study, we estimated the extrinsic incubation periods at 22 and 30°C in Culex tarsalis Coquillett. We found that the time to transmission of the original North American WNV strain, NY99, was not different from two more recent California isolates of the WN02 genotype: one of the earliest California isolates from the southeastern deserts, and a more recent 2011 isolate from a hyperendemic region in the Central Valley. We conclude with a model-based assessment of the epidemiological effects of temperature on the duration of mosquitoes' infectious life, which estimated that most mosquitoes have an infectious life of only a few days, but its duration expands markedly at warmer temperatures.

Surveillance systems for West Nile virus (WNV) combine several methods to determine the location and timing of viral amplification. The value of each surveillance method must be measured against its efficiency and costs to optimize integrated vector management and suppress WNV transmission to the human population. Here we extend previous comparisons of WNV surveillance methods by equitably comparing the most common methods after standardization on the basis of spatial sampling density and costs, and by estimating optimal levels of sampling effort for mosquito traps and sentinel chicken flocks. In general, testing for evidence of viral RNA in mosquitoes and public-reported dead birds resulted in detection of WNV approximately 2-5 weeks earlier than serological monitoring of sentinel chickens at equal spatial sampling density. For a fixed cost, testing of dead birds reported by the public was found to be the most cost effective of the methods, yielding the highest number of positive results per $1000. Increased spatial density of mosquito trapping was associated with more precise estimates of WNV infection prevalence in mosquitoes. Our findings also suggested that the most common chicken flock size of 10 birds could be reduced to six to seven without substantial reductions in timeliness or sensitivity. We conclude that a surveillance system that uses the testing of dead birds reported by the public complemented by strategically timed mosquito and chicken sampling as agency resources allow would detect viral activity efficiently in terms of effort and costs, so long as susceptible bird species that experience a high mortality rate from infection with WNV, such as corvids, are present in the area.

BackgroundPathogen transmission by mosquitos is known to be highly sensitive to mosquito bionomic parameters. Mosquito mark-release-recapture (MMRR) experiments are a standard method for estimating such parameters including dispersal, population size and density, survival, blood feeding frequency and blood meal host preferences.MethodsWe assembled a comprehensive database describing adult female MMRR experiments. Bibliographic searches were used to build a digital library of MMRR studies and selected data describing the reported outcomes were extracted.ResultsThe resulting database contained 774 unique adult female MMRR experiments involving 58 vector mosquito species from the three main genera of importance to human health: Aedes, Anopheles and Culex. Crude examination of these data revealed patterns associated with geography as well as mosquito genus, consistent with bionomics varying by species-specific life history and ecological context. Recapture success varied considerably and was significantly different amongst genera, with 8, 4 and 1% of adult females recaptured for Aedes, Anopheles and Culex species, respectively. A large proportion of experiments (59%) investigated dispersal and survival and many allowed disaggregation of the release and recapture data. Geographic coverage was limited to just 143 localities around the world.ConclusionsThis MMRR database is a substantial contribution to the compilation of global data that can be used to better inform basic research and public health interventions, to identify and fill knowledge gaps and to enrich theory and evidence-based ecological and epidemiological studies of mosquito vectors, pathogen transmission and disease prevention. The database revealed limited geographic coverage and a relative scarcity of information for vector species of substantial public health relevance. It represents, however, a wealth of entomological information not previously compiled and of particular interest for mosquito-borne pathogen transmission models.