"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 15, 2015

Taenia serialis

Many parasite can cause health problems for their hosts, but aside from those that infect humans and domestic animals, it is not entirely clear just how much impact most parasites are having on the host population. Of course, the problems caused by parasites for a host goes beyond direct pathology; for social animals, parasitism can also affect how individuals interact within a group.

(A) Frodo the gelada and (B) the T. serialis larvae that spilled from her back
Photo from Fig 1 of the paper
In this post, we will be discussing a study which investigated the impact of a tapeworm on a population of gelada baboons (Theropithecus gelada) in Ethiopia. The tapeworm in question is Taenia serialis, which is related to some more well-known species of tapeworms include the beef tapeworm and the pork tapeworm. Despite being commonly used in first year biology textbook as a "typical" example of a tapeworm, Taenia is anything but typical in terms of its life cycle compared with most other tapeworms.

Like other parasites that have a complex life cycle, the larval stage dwells in an animal known as the intermediate host - this is where the parasite grows to a certain size before being eaten by a predatory animal which serves as the final host (definitive host), where it will mature into a sexually reproducing adult worm. Taenia does something different in its intermediate host - an adaptation found in the evolutionary play book of the digenean flukes and some other parasites. Instead of merely growing larger and await consumption by the final host, they asexually multiply inside the intermediate host - making many genetically identical copies of themselves and forming cysts which contain hundreds or even thousands of larval clones.

As you can imagine, having a slowly growing bag of worms lodge inside your body is not good for your health (it actually served as a plot device in an episode of House), but just how much does it impact a population of wild animals? The paper featured today is the result of a long term study stretching from January 2007 to June 2013 monitoring the health and demographic data of 16 gelada bands on the Guassa Plateau located on the western edge of the Great Rift Valley on the Ethiopian Highlands. The research group kept track of 348 individual geladas over the course of the six and a half year study, noting their health, reproductive status, and any birth or death. These monkeys are also commonly infected by a species of Taenia which uses the geladas as an intermediate host.

The final host for this parasite is most likely the Ethiopian Wolf (Canis simensis) - which shares the same habitat with the geladas. Even though this carnivore usually only hunt small mammals such as rodents, they are known to scavenge on gelada corpses - which is probably how they become infected with T. serialis. When geladas accidentally ingest tapeworm eggs which had come from the wolf's faece, the parasite proliferate in the monkey, forming cysts or bladdders which can become visible as protrusions on the skin. While the cysts are grotesque, this allows researchers to monitor infections in the monkeys without coming into direct contact them (which might affect their natural behaviour). But while the cysts are clearly recognisable on the gelada's skin, one cannot simply identify a parasite via skin cysts alone - a closer examination is necessary.

Fortuitously (for science anyway), during the course of their study they were able to obtain some parasite material for identification due to a serendipitous event. Some members of the research group noticed an adult female gelada they named Frodo had a large parasite cyst on her back. At some point, the cyst ruptured and spilled out a bunch of parasite larvae, enabling the researchers who were following Frodo at the time to collect some of the parasites for examination, and subsequently identify them as T. serialis. While this tapeworm is usually known to infect rabbits as an intermediate host, on the Ethiopian Highlands, they infect geladas.

Overall, the researchers found that one in six of the monkeys they monitored had at least one T. serialis cysts, and most of those afflicted were adults with one-third of the adult population showing signs of infection by the tapeworm at some point. Those infected monkeys are more than twice as likely to die than their uninfected comrades, and this tapeworm's impact extends beyond the individual directly infected with it. Infants born to tapeworm-infected mothers are twice as likely to die before their first birthday compared with infants that have mothers with no signs of infection, and infected female monkeys also experience a longer lag period between the birth of each offspring.

Male monkeys also lose out due to T. serialis infection - geladas are polygamous species that organise themselves into so-called one-male units (OMU), each consisting of a single male with a harem of females. The researchers observed that tapeworms infection compromises the male monkeys' ability to hang on to their harems and infected geladas are more likely to lose in a dispute with any new (uninfected) challenger(s) that appear on the scene.

The impact of parasites on most wildlife is not well-understood, and often their effects are not immediately visible without a sustained long-term ecological and demographic study. Even natural levels of infection can have profound impact on host population, as seen with the effects of T. serialis on geladas. Therefore when it comes to wildlife conservation, it important to be mindful of parasites and the hidden role they play on the stage of nature.

Reference:
Nguyen, N. et al. (2015). Fitness impacts of tapeworm parasitism on wild gelada monkeys at Guassa, Ethiopia. American Journal of Primatology 77: 579-594.

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