Temporal association of ambient temperature and relative humidity with Spirocerca lupi infection of Onthophagus sellatus: a 14-year longitudinal study

The parasitic nematode Spirocerca lupi causes major morbidity and mortality in dogs. The scarab beetle Onthophagus sellatus is its major intermediate host in Israel. We investigated the prevalence of beetle infection by S. lupi in different years between 1994 and 2008. The average monthly maximum and minimum relative humidity (RH) and ambient temperature (AT) throughout the study period were calculated based on daily meteorological data. The infection prevalence decreased over the study period, possibly due to a chronological change resulting from increased preventive treatment of dogs against S. lupi, or climate change. Multivariate analysis was performed for these two hypotheses. Under the first hypothesis, chronological change was forced into the model, and environmental variables were inserted stepwise. The final model included beetle-collection date, minimum RH (RH min) during the month preceding beetle collection, its interaction with maximal AT (AT max) during that same month, and the interaction of maximal RH (RH max) and AT max, during the month of beetle collection. Under the second hypothesis, chronological change was not forced. The final model included RH max during the month of beetle collection, average RH (RHave) during the month preceding beetle collection, and its interaction with AT max during the latter month. The results suggest that under both hypotheses, RH and AT during the month preceding beetle collection influence S. lupi's ability to develop and survive in O. sellatus, and may be used to predict the risk to dogs of S. lupi infection.