"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

October 15, 2012

Marshallagia marshalli

Photo by Billy Lindblom
A host can be infected by many different species of parasites (see this post for example). While in some cases, co-infecting parasites can get along just fine, in others, co-infecting parasites end up competing with each other because they both use the same resources from the host. When it comes to such conflict of interest, the stronger competitors can often push other species aside, or even bar their entry altogether. So what can a parasite do in such a situation? Well, it can try and catch their competitors off guard by getting in during the off seasons.

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.

Marshallagia marshalli
egg from here
There are (non-parasitic) nematodes in Antarctica that can survive extreme cold, but it is not known if the free-living stages of some of their parasitic relatives can do the same. The two most abundant species of nematode worms in Svalbard reindeer are Marshallagia marshalli and Ostertagia gruehneri. For today's post, we will be focusing on a study which looked at the transmission dynamics of M. marshalli. Previous studies suggest that while other worms simply overwinter in the host and only lay eggs during summer, M. marshalli does not care for seasons; it just keeps laying eggs and infecting reindeer all year round, even through winter when their eggs and larvae will be resting on cold, snow-covered grounds.

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.

Reference:
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

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