Disentangling Temporal and Environmental Effects on Hematological Values in Prairie Rattlesnakes (Crotalus viridis)

Hematologic assessment is a common clinical tool used to characterize both individual and population health. In ectothermic organisms, the hematologic response is influenced by external factors such as temperature and season, especially in temperate species. Yet, specific environmental effects have received little direct attention. Therefore, to improve the diagnostic utility of hematologic assessments in reptiles, it is imperative to characterize physiological variation in diagnostic parameters. Our objective was to determine the impacts of temperature and season on commonly assessed components of the complete blood count, including packed cell volume (PCV), total solids (TS) and white blood cell count (WBC). We used a group of 17 long-term captive Prairie Rattlesnakes (Crotalus viridis). Animals were randomly assigned to either a control group kept in an environmental chamber at a constant temperature of 25°C or an experimental group housed in an environmental chamber which reflected average ambient temperature throughout the year (5°C-32°C). We collected blood samples twice monthly for 13 months. We found WBC decreased as the mean previous 14-day temperature increased, with no effect of season. Changes in total WBC were driven by increased numbers of lymphocytes and decreased number of heterophils and azurophils at higher temperatures. We found a difference in the effect of time since the start of the experiment between control and experimental groups in PCV, with no change in our control group, but a decrease in our experimental group through time. Total solids were higher in the control group, but there was no direct effect of temperature or season. Therefore, we conclude environmental conditions, rather than season, drive variation in hematologic parameters of otherwise healthy snakes and should be included in the interpretation of hematologic assessments in wild reptiles.