"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 10, 2013

Paragordius varius

Photo of adult worm by Matthew Bolek
The nematomorphs, or horsehair worms, are well-known for their ability to persuade their insect host to jump into a pool of water, thus allowing the adult worm to escape and reproduce. After mating, the adult worm lays eggs which comes out in these long, white, spaghetti-like strings (see photo on the right). The eggs hatch into free-swimming larvae that then infect aquatic invertebrates such as freshwater snails, mosquito larvae, or other small aquatic critters. Once it infects this host, the larva takes between 5-14 days to develop into a cyst stage which is ready to infect a cricket where it can mature into an adult.

The trouble with studying a parasite like the horsehair worm is that because they have multiple hosts in their life cycle, in order to keep them in a laboratory you would have to also maintain colonies of all its host animals on stand-by to act as sacrificial hosts for the hairworm larvae to infect. Additionally, those little invertebrates are not always "in season" and they may not be available in sufficient number when the infective stages of the parasite are available for experimentation.

If scientists can somehow put the life cycle of these parasites on hold at each stage until suitable hosts become available for the parasites to infect, not only would it become less logistically challenging to maintain them in the laboratory, it would also allow scientists to carry out more detailed studies on their life cycles. Fortunately, there is an aspect of their biology that may allow scientists to do just that - the parasite we are featuring today - Paragordius varius - along with other hairworms that live in temperate regions are capable of surviving through winter either as a dormant larva or a cyst inside an aquatic invertebrate that waits until spring comes when there are cricket hosts around. During the winter months the larval or cyst stage of the parasite simply stay in a state of suspended animation as their surroundings freezes over.

This is also good news for scientists who wish to study them - these worms' ability to survive freezing means that the larval stages can be "put on hold" until suitable hosts become available. To explore the tolerance limit of these parasites, a team of scientists put some P. varius larvae and snails infected with P. varius cysts under a series of different conditions including freezing at -30°C or -70°C for 15-30 days or dried out at room temperature or -70°C for the same period of time.

Photo of P. varius larva from Nematomorpha.net
They found that larvae that has been frozen in water at both -30°C or -70°C still managed to infect snails once they have been thawed and they did it just as well as those that were not previously frozen. The only group that did not fare as well were the larvae that have been dried out at room temperature for a month. The cyst stage of the parasite were not as hardy as the larvae and experience a slight decrease in the number of cysts in snails that have been frozen compared to those that were not, and being dried out dramatically decrease the survival of P. varius cysts. Nevertheless those that did survive the freezing process were still able to infect crickets once they were thawed. And while P. varius seems to cope better with getting frozen rather than being dried out, the team who conducted this study also found that the cysts in the snail were better at surviving desiccation at -70°C than at higher temperatures.

So not only did this study reveal an interesting adaptation that allow these hairworms to complete their life cycle in temperate regions, it also discovered a way of making it easier for scientist to study them in the future. What had originally evolved in these parasites as a way for them to put their life on hold during those freezing winter may now also be the key for researchers to find out more about them.

Bolek, M. G., Rogers, E., Szmygiel, C., Shannon, R. P., Doerfert-Schrader, W. E., Schmidt-Rhaesa, A., & Hanelt, B. (2013). Survival of larval and cyst stages of gordiids (Nematomorpha) after exposure to freezing. Journal of Parasitology 99: 397-402.

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