"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

January 26, 2015

Baylisascaris schroederi

Larva of Baylisascaris procyonis, another
parasitic nematode in the same genus as
 Baylisascaris schroederi. Photo from here
To say that Giant Pandas are totally adorable is probably one of the least controversial statements you can make, I mean, just look them here and here. But if you are a fan of giant pandas, then today's parasite is public enemy number one. Baylisascaris schroederi is a species of parasitic roundworm that live as adults in the intestine of giant pandas, and in large numbers, they can form bowel obstruction and other more serious diseases.

The adult B. schroederi produce eggs which leave the host along with panda faeces. When the panda accidentally ingest the eggs, they hatch into larvae in the intestine and proceed to burrow through various bodily tissues, causing inflammation and scarring in the the intestinal wall, liver and lungs.

After a coming-of-age trip through the panda's various organs, the larvae return to the small intestine to grow up into an adult and get on with the business of being a grown-up parasite - laying lots of eggs. The eggs are really hardy and can stay viable in wet soil for many years, waiting for an unlucky panda to swallow them.

Because B. schroederi can stay viable, a panda can get repeated infected by parasite larvae, inflicting internal damage for years. Baylisacaris schroederi is actually one of the leading cause of death in giant pandas, and depending on the region, half or even all of the pandas in a given population might be infected.

To find out more about the ecology of these parasites, a team of scientists from China used a range of mitochondrial DNA markers to work out the population structure of B. schroederi. They collected adult B. schroederi found in giant pandas from ten geographical regions at three different mountain range in southern central China - Qinling, Minshan, and Qionglai.

They found that despite the geographic isolation of those mountain ranges, the gene pool of this parasite is fairly homogenise, indicating that somehow despite their isolation, cross-breeding is occurring between the parasite populations. Perhaps some pandas are visiting neighbouring mountain range and end up picking up worms and dropping off B. schroederi eggs while they were there. Or it might not be the pandas themselves that are moving around - seeing as the eggs can survive for years in wet soil, they may get transported through other means.

While the scientists found that the parasite population in Minshan has the highest level of genetic diversity, as a whole, B. schroederi has relatively low genetic diversity compared with other organism. But that is common feature with other roundworms in its family - the Ascarididae (of which the most well known species is Ascaris suum - the large pig roundworm) - regardless of their population size or geographical distribution.

Currently there are no vaccines available for B. schroederi and the main way through which this parasite is being controlled is with anthelminthic drugs. Despite their relative low genetic diversity, this has not stopped some populations from evolving drug resistance. That is why it is important for us to understand this parasite's population genetics - because if there is cross-breeding occurring, then the gene(s) for drug resistance can spread very quickly across the different population and pose a threat to the endangered giant panda.

Xie, Y. et al. (2014). Absence of genetic structure in Baylisascaris schroederi populations, a giant panda parasite, determined by mitochondrial sequencing. Parasites & Vectors 7: 1591.