Winter can be tough on invertebrates. Even endotherms can find winters a bit grim if it’s extremely cold. Harsh freezing conditions are unforgiving, especially to those who cannot regulate their own body temperature. Parasites that have life stages involving intermediate hosts fare somewhat better than those free-living in the environment. But there is still risk to their survival, especially if the intermediate host is an invertebrate. In this way, the cold winter temperatures regulate the populations of parasites by increasing their mortality.
The effects of warmer winters on parasite abundance, and as a consequence, host health, have been known for some time. A neat example of this comes from the Canadian arctic, in the relationship between the nematode Umingmakstrongylus pallikuukensis and its definitive host is the muskox (Ovibos moschatus). The nematode has an intermediate host, usually a slug, where larvae develop to infective stages. When the infective stage nematodes are consumed by a muskox, they take up residence in its lungs. Heavy infections can accumulate as the host gets older, causing serious pathology and can even kill muskox (Kutz et al. 2009). The accumulation was key, because under ‘normal’ (i.e., cold) winter conditions, it takes two years for the nematodes to reach maturity in their intermediate hosts. But if it warms up, they can be ready to infect muskox in only one year, without having their development arrested during a winter (Kutz, et al. 2002). This is bad news for the muskox, because it means that far more nematodes will be around at any one time.
Another effect of warmer winters is increased survival of ticks. While not yet present in the Arctic, the winter tick, Dermacentor albipictus, feeds on moose (Alces alces) in parts of southern Canada and the northern United States. Cold winters play a large part in regulating the populations of the ticks, as many larvae perish during winter as they wait for a moose to walk past. The time it takes for females to reach maturity is also thought to be regulated by a combination of time and temperature (Hueffer et al. 2011), meaning that warmer weather will go some way to speeding up the development of the ticks. The ticks stay on their moose hosts for their whole life, feeding and moulting. This means that moose can be subject to thousands of ticks feeding on them continuously from winter to the start of summer. Moose that suffer heavy infestations are called ‘ghost moose’. With up to 400,000 individual ticks feeding on a single individual (Kutz, et al. 2009), the ghost moose suffer hair loss, anemia, emaciation and secondary infections, and can die. Increased mortality of moose populations in northern US have been linked to increased abundance of ticks (ref Washington Post), which will affect trophic cascades in the boreal forests the moose live.
These are just two examples of how parasites interact with their hosts. By adding environmental perturbation (be it climate, urbanisation, habitat loss etc.), the dynamics of these interactions will change. Distribution changes, host switches, alteration to phenology and emerging pathogens are all on the cards as we hurtle through the 21st century.
Hueffer et al. (2011) Acta Veterinaria Scandinavica 53, doi: 10.1186/1751-0147-53-17
Kutz et al. (2002) Canadian Journal of Zoology 80, 1977-1985.
Kutz et al. (2009) Veterinary Parasitology 163, 217-228.