The impact of gastrointestinal nematodes on wild reindeer: experimental and cross‐sectional studies

Summary It is well known that gastrointestinal nematodes can have a significant impact on the growth of farmed ruminants. The clinical signs of these parasites are often subtle, with production losses mainly due to reductions in host appetite and gut function. However, little is known about the impact of this widespread group of parasites on wild ruminants. We use experiments and cross‐sectional data to investigate the effects of gastrointestinal nematodes on a wild host, the Svalbard reindeer. Individually marked reindeer were treated for parasites. Their body mass, back fat depth and fecundity were compared with the control group one year later. The effect of treatment on worm burdens was investigated in a subsample of animals that were culled 3 and 6 months after treatment. Also, the relationship between the intensity of infection and body mass, back fat depth and host pregnancy rates was investigated in cross‐sectional data from culled reindeer. The anthelmintic treatment caused an increase in the body mass, back fat depth and fecundity of the reindeer. Treatment depressed the abundance of adult parasites of Ostertagia gruehneri for at least 6 months, but had no significant effect on the abundance of adults of the other dominant parasite species, Marshallagi marshalli , or the abundance of larval stages of either species. In the cross‐sectional study, the probability of a reindeer being pregnant in late winter was negatively related to the abundance of adult O. gruehneri when controlling for host body mass. However, no clear evidence were found for an effect of parasitism on host condition in the cross‐sectional study. Our experimental results show for the first time in a natural ruminant host population that gastrointestinal nematodes can have a significant effect on host condition and fecundity. However, effects of infection on host condition was not detectable in the cross‐sectional study. Also, we show that the experimental effects on host condition and fecundity is most likely to be due to a negative effect of O. gruehneri , while the experimental design did not allow detection of potential negative effects of M. marshalli .