"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

August 24, 2020

Hexametra angusticaecoides

In 2016, a group of crested geckoes in captivity suddenly got sick soon after they were transferred to a terrarium that previously held some Madagascan mossy geckoes. Within a short period of time, the geckoes started dying of a mysterious illness. Of the ten that were held in that terrarium, only a single gecko survived.

Photos of Hexametra emerging from moribund geckoes from Fig 1, 3, and 4 of the paper

When the dead geckoes were dissected, it was found that they had massive worms that were tearing their ways through the their innards. Some of the worms had even started emerging through the skin.
One gecko was lucky enough to receive treatment in time to save it. It was initially given fenbendazole and pyrantel - two commonly used medications for treating parasitic worm infections - but they had no effects. So a surgical procedure was performed on the lizard to remove the deadly nematodes from under its skin, followed by a dose of levamisole. About two weeks after the surgery, the surviving gecko managed to recover to full health.

Between the nine dead gecko and the sole survivor, over 50 worms were retrieved. The worm in question was Hexametra angusticaecoides - a parasitic nematode which commonly infects reptiles. Some of you who have  been following this blog for a while might recognise the genus Hexametra from a post back in 2016 where another species of that parasite was found in the body of a captive false coral snake.

So how did a bunch of crested geckos ended up with all these worms? Tracking down the original source of infection was a bit tricky, given some were sourced from a breeder in Canada, while other were sourced from a pet shop in Germany. Furthermore, they had been kept separately in different terrariums until they were combined into a single enclosure, soon after which the worms began appearing. However, it could be that particular terrarium which was responsible for the worms. Prior to housing the crested geckoes, that enclosure had been occupied by some wild-caught Madagascan mossy geckoes, Uroplatus sikorae.  

The harm caused by Hexametra to its host is likely due to its relatively large size, and its tendency to move around within the host's body instead of staying in place. Additionally, since the crested gecko is an exotic host for the parasite, this pairing would not have happened naturally. Hosts which have had a history of coevolution with their parasites would have also evolved mechanisms for tolerating or offsetting some of the more harmful effects of the parasite. But hosts which have never encountered those parasites would have no such adaptations and are thus exposed to the full effect of the parasite's presence. On top of that, the stress of captivity might have made the geckoes less able to tolerate any kind of parasitic infection.

The exotic pet trade, in addition to driving the poaching, smuggling and distribution of wildlife worldwide, also bring together animals which would otherwise never come into contact with each other, along with their many parasites. Like the majority of parasites, there is simply insufficient basic information about the ecology, life cycle, and life history for most reptile parasites, let alone what effects they might have if they end up in hosts which they have never encountered before.

Barton, D. P., Martelli, P., Luk, W., Zhu, X., & Shamsi, S. (2020). Infection of Hexametra angusticaecoides Chabaud & Brygoo, 1960 (Nematoda: Ascarididae) in a population of captive crested geckoes, Correlophus ciliatus Guichenot (Reptilia: Diplodactylidae). Parasitology, 147: 673-680.