Introduction:Predicting the timing and locations of future mosquito-borne disease outbreaks has the potential to improve the targeting of mosquito control and disease prevention efforts. Here, we present and evaluate prospective forecasts made prior to and during the 2016 West Nile virus (WNV) season in South Dakota, a hotspot for human WNV transmission in the United States.Methods:We used a county-level logistic regression model to predict the weekly probability of human WNV case occurrence as a function of temperature, precipitation, and an index of mosquito infection status. The model was specified and fitted using historical data from 2004-2015 and was applied in 2016 to make short-term forecasts of human WNV cases in the upcoming week as well as whole-year forecasts of WNV cases throughout the entire transmission season. These predictions were evaluated at the end of the 2016 WNV season by comparing them with spatial and temporal patterns of the human cases that occurred.Results:There was an outbreak of WNV in 2016, with a total of 167 human cases compared to only 40 in 2015. Model results were generally accurate, with an AUC of 0.856 for short-term predictions. Early-season temperature data were sufficient to predict an earlier-than-normal start to the WNV season and an above-average number of cases, but underestimated the overall case burden. Model predictions improved throughout the season as more mosquito infection data were obtained, and by the end of July the model provided a close estimate of the overall magnitude of the outbreak.Conclusions:An integrated model that included meteorological variables as well as a mosquito infection index as predictor variables accurately predicted the resurgence of WNV in South Dakota in 2016. Key areas for future research include refining the model to improve predictive skill and developing strategies to link forecasts with specific mosquito control and disease prevention activities.
Mosquito surveillance has been conducted across South Dakota (SD) to record and track potential West Nile virus (WNV) vectors since 2004. During this time, communities from 29 counties collected nearly 5.5 million mosquitoes, providing data from over 60,000 unique trapping nights. The nuisance mosquito, Aedes vexans (Meigen) was the most abundant species in the state (39.9%), and most abundant in most regions. The WNV vector, Culex tarsalis Coquillett (Diptera: Culicidae), was the second most abundant species (20.5%), and 26 times more abundant than the other Culex species that also transmit WNV. However, geographic variation did exist between WNV vector species, as well as relative abundance of vector and nuisance mosquitoes. The abundance of Ae. vexans decreased from east to west in South Dakota, resulting in an increase in the relative abundance of Cx. tarsalis. Other species are reported in this study, with various relative abundances throughout the different regions of South Dakota. WNV infection rates of mosquitoes showed that Cx. tarsalis had the most positive sampling pools and the highest vector index of all the species tested. This study addressed the need for an updated summary of the predominant mosquito species present in the United States Northern Great Plain and provides infection rate data for WNV among these predominant species.
Greater Sage-Grouse (Centrocercus urophasianus) is a species of conservation concern and is highly susceptible to mortality from West Nile virus (WNV). Culex tarsalis, a mosquito species, is the suspected primary vector for transmitting WNV to sage-grouse. We captured, radio-tagged, and monitored female sage-grouse to estimate breeding season (April 15 to September 15) survival, 2016–2017. Deceased sage-grouse were tested for active WNV; live-captured and hunter-harvested sage-grouse were tested for WNV antibody titers. Additionally, we trapped mosquitoes with CO2-baited traps 4 nights per week (542 trap nights) to estimate WNV minimum infection rate (MIR). Eight sage-grouse mortalities occurred during the WNV seasons of 2016 and 2017, 5 had recoverable tissue, and 1 of 5 tested positive for WNV infection. Survival varied temporally with sage-grouse biological seasons, not WNV seasonality. Survival was 0.68 (95% CI: 0.56–0.78; n = 74) during the reproductive season (April 1 to September 15). Mammalian predators were the leading suspected cause of mortality (40%), followed by unknown cause (25%), avian predation (15%), unknown predation (15%), and WNV (5%). These results indicate WNV was not a significant driver of adult sage-grouse survival during this study. Three sage-grouse (1.9%; 95% CI: 0.5–5.9%) contained WNV antibodies. We captured 12,472 mosquitoes of which 3,933 (32%) were C. tarsalis. The estimated WNV MIR of C. tarsalis during 2016 and 2017 was 3.3 and 1.6, respectively. Our results suggest sage-grouse in South Dakota have limited exposure to WNV, and WNV was not a significant source of sage-grouse mortality in South Dakota during 2016 and 2017. Based on our finding that a majority of sage-grouse in South Dakota are susceptible to WNV infection, WNV could potentially have an impact on the population during an epizootic event; however, when WNV is at or near-endemic levels, it appears to have little impact on sage-grouse survival.
In 2016, we compared susceptibility to the insecticide, permethrin, between the West Nile virus vector, Culex tarsalis Coquillett, and a major nuisance mosquito, Aedes vexans (Meigen), using baseline diagnostic dose and time values determined using the CDC bottle bioassay protocol. Mosquitoes were collected in the wild in Brookings County, South Dakota, situated in the Northern Great Plains of the USA. The determined diagnostic dose and time were then used in 2017 to validate these measurements for the same 2 mosquito species, collected at a second location within Brookings County. The diagnostic dose was determined for multiple time periods and ranged from 27.0 µg/ml at 60 min to 38.4 µg/ml at 30 min. There was no significant difference detected in mortality rates between Cx. tarsalis and Ae. vexans for any diagnostic time and dose. For practical purposes, mosquitoes in 2017 were tested at 38 µg/ml for 30 min; expected mortality rates were 93.38% for Cx. tarsalis and 94.93% for Ae. vexans. Actual 2017 mortality rates were 92.68% for Cx. tarsalis and 96.12% for Ae. vexans, validating the usefulness of this baseline at an additional location and year.
Parasites can influence host population health and dynamics and are often an overlooked aspect of the ecology of ecosystems. Understanding the impacts of ecological interactions between parasites and small mammals can provide insights into ecosystem dynamics. We live trapped small mammals within the Apostle Islands archipelago (2017-2020) and assessed factors influencing the presence of mites from the Trombiculidae family. Archipelagos provide unique research opportunities because in small mammal-parasitic trombiculid systems, they are essentially closed systems. We detected trombiculids on 47% of Myodes gapperi (Vigors, 1830; red-backed vole) individuals but rarely detected trombiculids on other species. We developed and ranked a set of a priori logistic regression models of trombiculid presence relative to habitat quality, host abundance, body condition, sex, and sexual maturity to identify factors significant in predicting trombiculid infection for M. gapperi. Parasitic trombiculids were more likely when M. gapperi abundance was high and body condition was poor, however it is unknown whether trombiculids affect condition or if trombiculids are more likely to parasitize hosts in poor condition. The significance of host abundance may indicate density-dependent transmission. Our work suggests that host density and body condition are important factors influencing parasitism by trombiculids in M. gapperi populations.
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