Meet the parasitic worm that kills giant pandas

I’ve got worms… nathaninsandiego, CC BY-NC-ND

Giant pandas aren’t dying like they used to. In the early 1980s, starvation accounted for more than nine out of ten deaths. However, over the past three decades a parasitic gut worm has replaced that as the dominant killer.

New research from Sichuan Agricultural University, published in the journal Parasites and Vectors, provides a better understanding of the ecology of this parasite, and may help preserve the few pandas that remain in the wild.

Baylisascaris schroederi is a species of parasitic roundworm that live as adults in the intestine of giant pandas. In large numbers, they can form a bowel obstruction and lead to more serious diseases. The adult B. schroederi produces eggs which leave the host along with panda faeces. The eggs hatch into larvae inside the intestine and proceed to burrow through various bodily tissues, causing inflammation and scarring in 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, which mainly involves 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 of the tendency of the eggs to stay in the surrounding environment for so long, a panda gets repeatedly infected by B. schroederi larvae, inflicting internal damage for years. B. schroederi is one of the leading causes 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 track the geographical location and movement of B. schroederi. This type of DNA is found inside the powerhouses of a living cell called mitochondria.

Thank you for nothing, parasite. Momotarou2012/WikiMedia, CC BY-SA

They collected adult B. schroederi found in giant pandas from ten geographical regions, at three different mountain ranges in southern central China – Qinling, Minshan, and Qionglai. They found that, despite the geographic isolation of those mountain ranges, the gene pool of these parasites is fairly homogeneous, indicating that somehow despite their isolation, cross-breeding is occurring between the parasite populations.

Perhaps some pandas are visiting the neighbouring mountain range and end up picking up worms and dropping off B. schroederi eggs. 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 by 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 a relatively low genetic diversity compared with other organisms. But that is a 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.

No vaccines for B. schroederi exist. The parasite is currently controlled using anthelminthic drugs, which stun or kill the worm. Despite the relatively low genetic diversity, this has not prevented some populations from evolving drug resistance.

Improved understanding of the parasite’s population genetics will help us to make decisions about conservation. If cross-breeding occurs frequently, then the genes for drug resistance could spread quickly amongst the population. Given that there are fewer than 3,000 giant pandas alive in the wild, there is very little room for error.