BackgroundThe effect of temperature on insect biology is well understood under constant temperature conditions, but less so under more natural, fluctuating conditions. A fluctuating temperature profile around a mean of 26°C can alter Aedes aegypti vector competence for dengue viruses as well as numerous life-history traits, however, the effect of fluctuations on mosquitoes at critical thermal limits is unknown.Methodology/Principal FindingsWe investigated the effects of large and small daily temperature fluctuations at low (16°C) and high (35–37°C) mean temperatures, after we identified these temperatures as being thresholds for immature development and/or adult reproduction under constant temperature conditions. We found that temperature effects on larval development time, larval survival and adult reproduction depend on the combination of mean temperature and magnitude of fluctuations. Importantly, observed degree-day estimates for mosquito development under fluctuating temperature profiles depart significantly (around 10–20%) from that predicted by constant temperatures of the same mean. At low mean temperatures, fluctuations reduce the thermal energy required to reach pupation relative to constant temperature, whereas at high mean temperatures additional thermal energy is required to complete development. A stage-structured model based on these empirical data predicts that fluctuations can significantly affect the intrinsic growth rate of mosquito populations.Conclusions/SignificanceOur results indicate that by using constant temperatures, one could under- or over-estimate values for numerous life-history traits compared to more natural field conditions dependent upon the mean temperature. This complexity may in turn reduce the accuracy of population dynamics modeling and downstream applications for mosquito surveillance and disease prevention.

BackgroundEnvironmental factors such as temperature can alter mosquito vector competence for arboviruses. Results from recent studies indicate that daily fluctuations around an intermediate mean temperature (26°C) reduce vector competence of Aedes aeygpti for dengue viruses (DENV). Theoretical predictions suggest that the mean temperature in combination with the magnitude of the diurnal temperature range (DTR) mediate the direction of these effects.Methodology/Principal FindingsWe tested the effect of temperature fluctuations on Ae. aegypti vector competence for DENV serotype-1 at high and low mean temperatures, and confirmed this theoretical prediction. A small DTR had no effect on vector competence around a high (30°C) mean, but a large DTR at low temperature (20°C) increased the proportion of infected mosquitoes with a disseminated infection by 60% at 21 and 28 days post-exposure compared to a constant 20°C. This effect resulted from a marked shortening of DENV extrinsic incubation period (EIP) in its mosquito vector; i.e., a decrease from 29.6 to 18.9 days under the fluctuating vs. constant temperature treatment.ConclusionsOur results indicate that Ae. aegypti exposed to large fluctuations at low temperatures have a significantly shorter virus EIP than under constant temperature conditions at the same mean, leading to a considerably greater potential for DENV transmission. These results emphasize the value of accounting for daily temperature variation in an effort to more accurately understand and predict the risk of mosquito-borne pathogen transmission, provide a mechanism for sustained DENV transmission in endemic areas during cooler times of the year, and indicate that DENV transmission could be more efficient in temperate regions than previously anticipated.

The strain of West Nile virus (WNV) currently epidemic in North America contains a genetic mutation elevating its virulence in birds, especially species in the family Corvidae. Although dead American Crows (Corvus brachyrhynchos) have been the hallmark of the epidemic, the overall impact of WNV on North America's avifauna remains poorly understood and has not been addressed thoroughly in California. Here, we evaluate variation by species in the effect of WNV on California birds from 2004 to 2007 by using (1) seroprevalence in free-ranging birds, (2) percentage of carcasses of each species reported by the public that tested positive for WNV, (3) mortality determined from experimental infections, and (4) population declines detected by trend analysis of Breeding Bird Survey (BBS) data. Using Bayesian linear models, we extrapolate trends in BBS data from 1980 to . We attribute significant declines from expected abundance trends in areas supporting epiornitics to WNV transmission. We combine risk assessed from each of the four data sets to generate an overall score describing WNV risk by species. The susceptibility of California avifauna to WNV varies widely, with overall risk scores ranging from low for the refractory Rock Pigeon (Columba livia) through high for the susceptible American Crow. Other species at high risk include, in descending order, the House Finch (Carpodacus mexicanus), Black-crowned Night-Heron (Nycticorax nycticorax), Western Scrub-Jay (Aphelocoma californica), and Yellow-billed Magpie (Pica nuttalli). Our analyses emphasize the importance of multiple data sources in assessing the effect of an invading pathogen.

Abstract. Diurnal temperature fluctuations can fundamentally alter mosquito biology and mosquito-virus interactions in ways that impact pathogen transmission. We investigated the effect of two daily fluctuating temperature profiles on Aedes aegypti vector competence for dengue virus (DENV) serotype-1. A large diurnal temperature range of 18.6 C around a 26 C mean, corresponding with the low DENV transmission season in northwestern Thailand, reduced midgut infection rates and tended to extend the virus extrinsic incubation period. Dissemination was first observed at day 7 under small fluctuations (7.6 C; corresponding with high DENV transmission) and constant control temperature, but not until Day 11 for the large diurnal temperature range. Results indicate that female Ae. aegypti in northwest Thailand are less likely to transmit DENV during the low than high transmission season because of reduced DENV susceptibility and extended virus extrinsic incubation period. Better understanding of DENV transmission dynamics will come with improved knowledge of temperature effects on mosquito-virus interactions.

We collected a total of 15,329 mosquitoes during weekly sampling in Davis, CA, from April through mid-October 2006 at 21 trap sites uniformly spaced 1.5 km apart over an area of approximately 26 km(2). Of these mosquitoes, 1,355 pools of Culex spp. were tested by multiplex reverse transcriptase-polymerase chain reaction, of which 16 pools (1.2%) were positive for West Nile virus (WNV). A degree-day model with a developmental threshold of 14.3 degrees C accurately predicted episodic WNV transmission after three extrinsic incubation periods after initial detection. Kriging interpolation delineated that Culex tarsalis were most abundant at traps near surrounding agriculture, whereas Cx. pipiens clustered within residential areas and greenbelt systems in the old portion of Davis. Spatial-temporal analyses were performed to test for clustering of locations of WNV-infected dead birds and traps with WNV-positive Cx. tarsalis and Cx. pipiens; human case incidence was mapped by census blocks. Significant multivariate spatial-temporal clustering was detected among WNV-infected dead birds and WNV-positive Cx. tarsalis, and a WNV-positive Cx. pipiens cluster overlapped areas with high incidences of confirmed human cases. Spatial analyses of WNV surveillance data may be an effective method to identify areas with an increased risk for human infection and to target control efforts to reduce the incidence of human disease.

Although horse cases frequently are reported during West Nile virus (WNV) outbreaks, few investigations have focused on the epidemiology of this transmission. From April to October 2003 to 2005, mosquito abundance and infection were monitored 3 days per week at an equine research facility at the University of California, Davis. Thirty-two nonvaccinated horses enrolled as controls in a vaccine study were bled monthly, and their serum was tested for evidence of WNV infection by plaque reduction neutralization test (PRNT). In 2004, one positive Culex pipiens pool was associated with a single horse that presented with confirmed WNV disease in late September. The annual incidence of clinical and subclinical WNV infection in the nonvaccinated horses was 16%, with an apparent to inapparent ratio of 1:4 among infected horses. In 2005, two Culex tarsalis and two Cx. pipiens WNV-positive pools were associated with an equine infection incidence of 62%, with an apparent to inapparent ratio of 1:17. The majority (79%) of 70 blood-engorged Cx. pipiens fed on birds and the remaining on equines (21%). Conversely, Cx. tarsalis fed primarily on equines (n = 23, 74%), followed by birds (n = 7, 23%) and 1 (3%) fed on a lagomorph. These data indicated that nonvaccinated horses were a sensitive indicator of WNV activity and that their risk of infection was associated with the presence of infection in Cx. pipiens and Cx. tarsalis, which served as both enzootic and bridge vectors amplifying WNV among birds and transmitting WNV to horses.

Abstract. In a previous study, a new flavivirus genome sequence was identified in Culex tarsalis mosquitoes obtained in Alberta, Canada and was shown to be genetically related to but distinct from members of the insect-specific flaviviruses. Nonstructural protein 5-encoding sequences amplified from Cx. tarsalis pools from western Canada have shown a high similarity to genome sequences of novel flaviviruses isolated from mosquitoes in California and Colorado. Despite wide distribution of this virus, designated Calbertado virus, strains demonstrate a high degree of nonstructural protein 5 nucleotide (> 90%) and amino acid (> 97%) identity. The ecology and geographic range of Calbertado virus warrants further study because it may potentially influence transmission of mosquito-borne flaviviruses, including important human pathogens such as West Nile and Saint Louis encephalitis viruses.

Although herons and egrets in the family Ardeidae frequently have been associated with viruses in the Japanese encephalitis virus serocomplex, communal nesting colonies do not appear to be a focus of early season and rapid amplification of West Nile virus (WNV) in California. Evidence for repeated WNV infection was found by testing living and dead nestlings collected under trees with mixed species ardeid colonies nesting above in an oak grove near the University of California arboretum in Davis and in a Eucalyptus grove at a rural farmstead. However, mosquito infection rates at both nesting sites were low and positive pools did not occur earlier than at comparison sites within the City of Davis or at the Yolo Bypass wetlands managed for rice production and waterfowl habitat. Black-crowned night herons (Nycticorax nycticorax) were the most abundant and frequently infected ardeid species, indicating that WNV may be an important cause of mortality among nestlings of this species.