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Data from: Greater sage-grouse survival varies with breeding season events in West Nile virus non-outbreak years

Citation

Parsons, Lindsey (2021), Data from: Greater sage-grouse survival varies with breeding season events in West Nile virus non-outbreak years, Dryad, Dataset, https://doi.org/10.5061/dryad.4xgxd2581

Abstract

Greater Sage-Grouse (Centrocercus urophasianus) are a species of conservation concern and are 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‒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 one 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−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 Culex tarsalis. Estimated WNV MIR of Culex 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.      

Methods

We annually captured breeding-age male and female sage-grouse near active leks March−May, as well as at high sage-grouse use areas in August and September using nocturnal spotlighting and a long-handled net (Giesen et al. 1982, Wakkinen et al. 1992). We aged and sexed captured sage-grouse based on plumage and morphological characteristics (Crunden 1963, Beck et al. 1975, Bihrle 1993). We fit female sage-grouse with a 21.6 g necklace-type Very High Frequency (VHF) radio-tag with an 8-hour mortality switch (model A4060, Advanced Telemetry Systems, Isanti, MN, USA). We weighed all birds at the time of capture to ensure that radio-tags were less than 3% of body weight (Kenward 2001). Both male and female sage-grouse were fit with a uniquely numbered aluminum butt-end leg band (National Band & Tag Company). All animal handling procedures were approved by the Institutional Animal Care and Use Committee at South Dakota State University (IACUC approval # 15-074A).

We monitored VHF signals of all radio-tagged females twice weekly from April 15– June 14 and daily from June 15- September 15. Daily monitoring allowed us to reduce bias associated with lag-time in recovery of corpses in determining true cause-specific mortality (Bumann and Stauffer 2002, Stevens et al. 2011) as well as increase our probability of finding fresh tissue for WNV testing. If a sage-grouse radio-tag signal was detected on mortality mode, the sage-grouse and VHF radio-tag were recovered immediately, and cause of death was investigated.

To test for previous WNV infection in sage-grouse, we used a 22-gauge needle to collect 1 ml of blood from the brachial vein (Gregg et al. 2006) of captured sage-grouse. In addition to captured sage-grouse, we opportunistically collected blood from the hearts of harvested sage-grouse during a limited hunting season in 2016. Whole blood was placed in a 2 ml blood tube with a clot activator, then centrifuged within 12 hours of collection. Serum was decanted, frozen, and maintained at -20°C until shipped for testing (Walker et al. 2007). Serum samples were tested for WNV antibodies using a Plaque-reduction neutralization assay at Cornell University’s Animal Health Diagnostic Center (Naugle et al. 2004, 2005, Walker et al. 2007). Samples were considered negative for WNV antibodies if titers were not detected at the minimum serum dilution (1:10). Radio-tagged sage-grouse were recaptured at the conclusion of the study. Blood samples were collected and tested for WNV antibodies again; radio-tags were removed. This allowed us to document any radio-tagged sage-grouse that had become infected with WNV, developed antibodies, and survived the duration of the study.

To test for active WNV infection in dead sage-grouse, we recovered any available tissues (intestines, proventriculus, liver, kidney, lung, heart and brain) from mortalities that occurred from June 1 ‒ September 15. Recovered tissues were frozen until testing for presence of WNV using a Reverse Transcriptase-PCR Assay at Cornell University’s Animal Health Diagnostic Center (Porter et al. 1993, Shi 2001).

Adult mosquitoes were trapped June 1‒September 15 using standard miniature light traps with photocell-controlled CO₂ release (John W. Hock Company; Model 1012.CO2) 4 nights per week. CO₂ traps were set to deliver 0.5L CO₂/min (Vincent et al. 2020). Captured mosquitoes were frozen and sorted to species. Female mosquitoes contribute to the transmission of WNV due to their requirement to consume blood meals prior to oviposition (Hubert et al. 1954) and although there are examples of male mosquitoes infected with WNV (Anderson et al. 2006, Unlu et al. 2010), they are typically not of interest when researching WNV (US EPA 2017). Therefore, we identified female mosquitoes and sorted each to species based on morphological characteristics using a dichotomous identification key (Darsie and Ward 2005). Female Culex tarsalis were placed in vials of up to 50 individuals, homogenized, and tested for WNV using a TaqMan Reverse Transcriptase-PCR Assay, as described in Lanciotti et al. (2000) with slight modifications.

Usage Notes

Readme files are included for each sub dataset.