Survival through the Arctic winter: How gastrointestinal nematodes influence fitness in reindeer.

AIM: The student will specifically test the hypothesis that, M. marshalli, a gastro-intestinal nematode that infects reindeer over winter when are nutritionally stressed, depresses host performance and ultimately reduces survival. RATIONALE: Although theoretical models to explore the potential role of parasites in regulating host population dynamics were developed thirty years ago (Anderson & May 1978, May & Anderson 1978) only two studies, one of red grouse (Hudson et al 1992) and another of reindeer (Albon et al 2002), provide empirical support that macroparasites (e.g: nematodes (helminths) rather than viruses) can regulate vertebrate populations. Both 'model' systems had the benefit of no competing hosts and were amenable to experimental manipulation (treatment with anti-parasite drugs). In Svalbard reindeer, experimental treatments were repeated over many years to effectively control for stochastic variation associated with the prevailing weather, and were integrated with detailed observation of individuals (Albon et al 2002), thereby avoiding the limitations of cross-sectional studies (Stien et al 2002). However, unlike the grouse-caecal worm system there are two species of nematode co-dominant in reindeer hosts. While one, Ostertagia gruehneri, depresses fecundity when infection increases, which, in turn is related to the host population density two years earlier (Albon et al. 2002), the influence of the second, Marshallagia marshalli, is unknown. Interestingly these two species appear to have evolved contrasting seasonal transmission cycles, thereby potentially avoiding competition in the host (Irvine et al 2000). While O. gruehneri has a free-living stage in the summer, typical of gut nematodes, remarkably M. marshalli is transmitted in winter and appears to accumulate in reindeer between October and March (Irvine et al 2001). These contrasting life histories mean that it is very likely that the two species have differential impacts on the host. APPROACH: In April 2009 and 2010, 40-50 reindeer will be caught and treated with a rumen bolus delayed drug delivery system giving a placebo for 180 days and then an anti-parasite drug for 30 days to remove the burden of gut parasites. The efficacy of the delivery system was demonstrated in a pilot study in 2006/07 where the median number of adult gut parasites in the abomasa of treated reindeer in autumn was 1,130 (N=3), an order of magnitude lower than in the untreated controls (N=6; median 11,610). 39 reindeer were treated in April 2007 and a further 40 -50 will be treated in April 2008, ensuring that the student has around 200 treated subjects across a total of five years. SPECIFIC PREDICTIONS: 1) After 4 months (in February), reindeer treated with the delayed-release bolus will have a lower abundance of parasites than untreated controls, and these parasites will be largely M. marshalli. 2) By April, the proportional decline in body weight and body fat will be lower in the treatment groups than in the untreated controls. 3) Differences in relative change in body condition between treated animals and untreated controls will increase with winter severity (total precipitation Oct-Apr) and host density. 4) The probability of survival to the following winter will be highest in treated animals. TRAINING: The student will receive an induction into theoretical population ecology, and specifically host-parasite interactions, through the University of Lancaster. The CASE partner (Macaulay Institute) will provide practical training in animal capture-mark-recapture, field experimentation and monitoring, as well as advanced statistics facilitated by Biomathematics and Statistics Scotland (BioSS). The student will learn basic parasitological laboratory methods, as well as molecular techniques at the University of Aberdeen, where PCR methods will be used to distinguish both larval and adult stages of the nematode species (Dallas et al. 2000; Irvine & Dallas., 2002).