"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

May 13, 2013

Cyrtosomum penneri

The Atractidae is a family of nematodes (roundworms) that are found in the intestines or lungs of various vertebrate animals. Instead of producing eggs, the adults produce larvae that are ready to infect as soon as they leave their mother's womb. While it is well-known that a host already parasitised by an atractid nematode can infect themselves again thanks to the infective larvae, it was not entirely clear how this parasite get passed between different hosts.

Photo taken by Charles R. Bursery
Used with permission from Gerrut Norval
For the paper we are looking at today, a team of scientists studied a species of atractid nematode that infects lizards - Cyrtosomum penneri - and conducted a series of experiments to figure out how this parasite is transmitted in Brown Anoles. They did this by administering larval worms in a number of different ways to lizards that they had previously de-wormed with anti-parasite drugs. To ensure that they could tell afterwards if the worms they found in the anoles were the ones they administered, the scientists labelled them with a fluorescent dye (the same type as the ones I used in experimental infections to track larval flukes in bivalves: see here and here)

The larvae of many parasitic nematodes infect their hosts through being accidentally swallowed, usually while their hosts are feeding (this is commonly how mammalian herbivores like sheep, cattle, and horses become infected). However, when these scientists tried to do the same with C. penneri larvae by pippetting larval worms down the lizards' throats, none of the larvae were successful in establishing in the host.

Instead, they found the feces of anoles that had been fed parasite larvae were full of dead worms - presumably they were killed by gastric acid. Indeed, the environment in which C. penneri is usually found, down in the lower intestine, is pretty benign comparing with the acidic milieu of the stomach. But when they pipetted larval C. penneri into the cloaca of the lizard, it worked every time. Instead of having a separate opening, lizards have a cloaca - a common opening for their intestinal, urinary, and reproductive tracts. When lizards mate, they bring their cloacae together - and this is when C. penneri gets transmitted.

Yes, that's right - C. penneri is a sexually transmitted infection.

In the mating trials run by those scientists, male lizards infected with the nematode passed it on to the female they mated with every single time, whereas the female lizard only passed the STI to the male in seven out of the ten trials they ran. In addition to anoles, C. penneri is also found in a few other lizard species such as the Mediterranean House Gecko and Eastern Fence Lizard. But while it infects those other lizards in addition to the Brown Anole, for some reason it does not appear to parasitise the Carolina Anole... which means that it would be interesting to consider what happens to the parasite when something like what you see at this link happens...

Langford, G. J., Willobee, B. A., & Isidoro, L. F. (2013). Transmission, host specificity, and seasonal occurrence of Cyrtosomum penneri (Nematoda: Atractidae) in lizards from Florida. Journal of Parasitology 99: 241-246

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