"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

July 28, 2011

Acanthocephalus galaxii

The brown trout (Salmo trutta), a popular angling species, was introduced to the waters of New Zealand in 1867 and has become very well established in the local freshwater system. The trout have made New Zealand their own all-you-can-eat buffet, feeding on many of New Zealand's native freshwater fishes. But other native fauna have also been getting intimate with the trout in a different way. It turns out that during its time in Aotearoa, the brown trout has also picking up a new parasite - Acanthocephalus galaxii, which normally infects a little native fish call the roundhead galaxias (Galaxias anomalus).

Furthermore, the parasitic worm has actually become more abundant in the introduced trout than in the native galaxids - presumably because when compared with the tiny native fish, the much larger trout gobbles up more amphipods (the crustacean which carries the larval stage of A. galaxii). But this isn't necessarily good news for the parasite. Once they get into the trout, because of physiological incompatibility with the introduced host, the parasites are unable to reach maturity. So the trout actually acts as a kind of dead-end sink for the worm, which in turn reduces parasite burden on the native fishes.

So even while the trout might be chomping up native galaxids by the mouthful, they also are inadvertently reducing their parasite burden - though I doubt that would give much comfort to the little galaxids fleeing from a hungry trout!

Paterson, R.A., Townsend, C.R., Poulin, R. and Tompkins, D.M. (2011) Introduced brown trout alter native acanthocephalan infections in native fish. Journal of Animal Ecology 88: 990-998.

July 18, 2011

Isospora lesouefi

Isospora lesouefi is a coccidian parasite which infects the Regent Honeyeater (Xanthomyza phrygia), an endangered species of bird found in Australia. This parasite was found and described during a parasitological survey conducted on a group of honeyeaters at Taronga Zoo as a part of their captive breeding programme.

Before the birds can be released into the wild, their health needs to be assessed and a part of that procedure involves determining their parasite load. For animals that you want to keep alive, this usually involves counting the number of parasite eggs or spores found in their faeces. But here's the tricky bit - it turns out that I. lesouefi keeps to a daily timetable. The researchers in this study found that bird faeces collected in the afternoon contained about 200 times more oocysts (the parasite's infective stage) than those collected in the morning. Other species of Isospora also keep similar shedding schedules, and it is likely to be an adaptive trait which minimise the oocysts' exposure to desiccation and ultraviolet radiation.

This study illustrates the importance of taking multiple samples, as well as understanding the life history of the parasites when you want to obtain an accurate picture of parasite burden, and its actual impact on the health of an animal.

Morin-Adeline, V., Vogelnest, L. Dhand, N.K., Shiels, M., Angus, W. and Šlapeta, J. (2011) Afternoon shedding of a new species of Isospora (Apicomplexa) in the endangered Regent Honeyeater (Xanthomyza phrygia). Parasitology 138: 713-724

July 4, 2011

Myxidium sp.

When species of plants and animals are introduced to a new environment, this can often lead to some unexpected consequences. The parasite for today is Myxidium sp. - a myxosporean that lives in the liver and brain of native frogs in Australia. But in addition to the native amphibians, this parasite is also found in the invasive cane toad. The cane toad was introduced into Australia to control cane beetles, but has since become one of the most famous posterchildren of invasive species. While Myxidium was originally thought to have been a "present" brought to Australia by the cane toad, recent research indicates that it might actually be native to Australia.

The infamous cane toad does play a role in the story of Myxidium, but in a different manner to what was originally suspected. A collaborative group of researchers from Australia and the Czech Republic found that instead of bringing Myxidium to Australia, the toad has become embroiled in an ecological phenomenon known as "spillback". This is when a native parasite adopts a newly introduced host, this new species turns out to be a better host for the parasite than the native species it was originally infecting, and the parasite propogates more successfully in the new host species.

This can have dire consequences for the original host because the introduced species acts as an ampilifier for the parasite. As a result, the original host become exposed to more of the parasite than ever before. Because many parasites often have dose-dependent effects, this can mean a parasite, which would otherwise be tolerated, can become debilitating or even deadly to its original host.

Reference (and photo):
Hartigan A, Fiala I, Dyková I, Jirků M, Okimoto B, et al. (2011) A Suspected Parasite Spill-Back of Two Novel Myxidium spp. (Myxosporea) Causing Disease in Australian Endemic Frogs Found in the Invasive Cane Toad. PLoS ONE 6(4): e18871. doi:10.1371/journal.pone.0018871