Infestation by Hypoderma tarandi in reindeer calves from northern Finland--prevalence and risk factors

Serum samples from 953 reindeer (Rangifer t. tarandus) calves-of-the-year from 21 reindeer herding co-operatives in Finland in slaughter season 2006/2007 were assayed for antibodies against Hypodermin C by an ELISA detecting IgG. Data on presence of Hypoderma tarandi larvae on 12,327 reindeer hides from 17 of the 21 herding co-operatives in slaughter season 2005/2006 were included for support. ELISA showed a seroprevalence of 60.9%, with no significant difference between females and males, and increase with latitude (southernmost and northernmost co-operatives examined, Pudasjärvi and Kaldoaivi, 11.8 and 100% of seropositives, respectively). The proportion of larva positive hides (range 0.5-60% per co-operative) was low compared to the proportion of seropositives. Also the proportion of larva positive hides increased with latitude. Our findings indicated that high latitude combined with open landscape, presence of low vegetation and high reindeer density provided more favorable conditions for sustaining of high degree of warble fly infestation, and furthermore, that any possible effect of ivermectin treatment on infestation rate was ruled out by the higher effect by the above factors.