|Photo by Billy Lindblom|
During their life-cycle, many parasites go through a free-living stage where they spend some time in the outside environment; either as an egg or a spore, or as a larva that has just hatched or while they are moving from one host to the next. Outside the cozy interior of they host, they can be exposed to some pretty harsh conditions. The parasite we are looking at today is found in the gut of Svalbard Reindeer (Rangifer tarandus platyrhynchus), which live on the Svalbard archipelago in the high arctic. During winter, Svalbard reindeer do not migrate, but instead move around the local area in search of any forage that is still accessible, which is not easy as the ground becomes completely covered by snow during winter. So not exactly the most cozy environment, especially not for the microscopic larval worms which infect these reindeer.
egg from here
At Spitsbergen, Norway, a group of researchers conducted an experiment to find out if reindeer did indeed pick up any additional worms during winter. To do so, they first fed some reindeer with anti-parasite drugs just before winter to purge them of any worms they already had. The drug wears off after a month, so the deer can start picking up worms again during winter if there are any infectious parasites around. What they found was that in the treated reindeer, after the purge there was no increase in O. gruehneri throughout winter, but the number of adult M. marshalli steadily increased, indicating the reindeer were picking up M. marshalli larvae throughout this period.
Marshallagia marshalli is a generalist parasite which also infects a wide range of hoofed animals ranging from sheep in Saudi Arabia, to saiga antelopes in Kazakhstan, to bighorn sheep in Montana, and reindeers in the Arctic - unlike O. gruehneri which is a reindeer specialist. While you'd expect that the reindeer specialist would have evolved such cold-resistant larvae, instead it simply refrains from laying eggs during winter so that transmission only occurs during summer. Because M. marshalli is a parasite of ruminants in dry deserts, their ability to survive such cold conditions might simply come with being able to infect hosts in generally arid and inhospitable environments. The caveat here is that M. marshalli might be a species complex (a group of closely-related lineages which have been classified as a single species due to their similarities), and the species/sub-species that infects Svalbard reindeer might have evolved to withstand the cold as a specialised adaptation for the conditions found in the high arctic.
So why has M. marshalli evolved such cold-resistant larvae instead of doing what O. gruehneri does and simply lay their eggs during summer when the larvae will be exposed to more favourable conditions? As mentioned above, Ostertagia gruehneri is a reindeer specialist, so perhaps in order for M. marshalli to have a fighting chance while sharing a host a well-adapted specialist like O. gruehneri, it needs to come in from the cold. Given how the infection dynamics of these two parasites are so seasonally-dependent, it is unknown how future climate change will affect their respective abundance in their hosts, and what consequences this will have on the reindeer population.
Carlsson, A.M., Justin Irvine, R., Wilson, K., Piertney, S.B., Halvorsen, O., Coulson, S.J., Stien, A., Albon, S.D. (2012) Disease transmission in an extreme environment: nematode parasites infect reindeer during the Arctic winter. International Journal for Parasitology 42:789-795