Tibetan sheep carry the plague

On the highlands of China and Tibet, people slaughtering and eating Tibetan sheep are at risk of contracting plague.

One third of China’s sheep population comprises this hardy highland breed, which are reservoirs for the infamous human plague agent Yersinia pestis, and local researchers are concerned.

Indeed, new research on the ecology of plague outbreaks on the Qinghai-Tibet Plateau, published in the journal PLOS Neglected Tropical Diseases, warns that “the hazards of Tibetan sheep plague should not be underestimated”.

The plague bacterium is usually transmitted to humans via the parasitic fleas that feed upon infected rats, as seen in the Black Plague outbreaks of the past. The same route applies today in areas that still suffer from outbreaks, including Madagascar and India.

On the Qinghai-Tibet Plateau, deaths from plague are not occurring from infected fleas – instead, they are all associated with the skinning and butchering of Tibetan sheep. Some individuals have also been infected from eating under-cooked sheep meat, resulting in a nasty version of the disease, known as intestinal plague.

Y.pestis can occur in a wide variety of mammal species, but Wei Li of the Chinese Centre for Disease Control and Prevention (CDC) and colleagues decided it was timely take a closer look at the Tibetan sheep-related plague events occurring on the plateau.

The region has an abundant rodent fauna, including hamsters, pika, jerboas and voles – but the primary plague host in this region is the marmot (Marmota himalayana), a large beaver-like rodent.

To confirm the vector transmission process, and track down how the marmots were passing the bacterium to the sheep, the researchers extracted DNA from 38 strains of Y. pestis isolated from humans, Tibetan sheep, and marmots. They also noted the location and timing of all plague events in both sheep and humans since 1956.

Through this analysis, Li’s team confirmed that the Tibetan sheep plague originated in marmots, and the human plague strains, in turn, were derived from the sheep.

Previous studies have revealed that Y.pestis does not transmit from sheep to sheep, begging the question of how the animals catch it from marmots. Like other outbreaks around the world, plague events on the plateau are related to the fleas of the region and their ecology.

Plague outbreaks in marmots occur in June and July, but in sheep some months later, with an October and November peak. Life is harsh on the plain, which lies at an altitude of 4000 metres. To help absorb nutrients in such a harsh landscape, Tibetan sheep have a gut bacteria flora quite unlike that of their low altitude cousins.

During the study, the researchers observed Tibetan sheep sniffing and licking dead marmots, perhaps as a nutritional boost. This behaviour would enable marmot fleas to switch hosts, thus transmitting the bacterium.

Despite the risk of sheep-to-human transmission, local education and public health campaigns appear to be working.

“From 1997 to 2016, no human plague cases were caused by Tibetan sheep, due to active prevention and intervention measures in Qinghai, even though Y. pestis was still isolated from local Tibetan sheep and Tibetan goats on the Qinghai-Tibet Plateau,” the researchers note.