High winter loads of Oestrid larvae and Elaphostrongylus rangiferi are associated with emaciation in wild reindeer calves

The Oestrid flies Cephemyia trompe and Hypoderma tarandi and the nematode Elaphostrongylus rangiferi are important parasites of Rangifer spp. The larvae of Oestrid flies develop in the throat (C. trompe) and skin (H. tarandi) of their host during winter while E. rangiferi develop in the CNS. Oestrid pupation, and development of E. rangiferi larvae from first- (L1) to infective third- stage in the environment during summer are highly temperature dependent. We investigated the possible negative effects of these parasites on the winter body-condition of wild reindeer calves. Two year-classes (generations) of calf, born in a warm (2014) and cold (2015) summer respectively, were examined for changes in body condition between autumn and spring, in relation to the parasite load determined in the spring. The body condition in the autumn was assessed as carcass weight, while the body condition in the spring was assessed as carcass weight, supplemented by an evaluation of fat reserves in various bodily locations. Oestrids were counted directly whereas the E. rangiferi quantification was based on faecal counts of L1 larvae. The abundance of infections for Oestrids and E. rangiferi were significantly greater in the 2014 generation than in the 2015 generation. The mean carcass weight decreased between autumn and spring for the 2014 generation but increased in the 2015 generation. Emaciation in the spring was documented (fat reserve evaluation) in 42% and 7% of calves in the 2014 and 2015 cohorts, respectively. There was a significant correlation between high parasite load and the probability of emaciation. The mean summer temperature in 2014 was 2.6 °C higher than the mean for 2015, and 1.0 °C higher than the mean for the last 30-years. Our findings suggest that following a warm summer, high loads of Oestrids and E. rangiferi may cause emaciation and potentially deaths among the calves.