Asymptomatic Plague: Qinghai, China, 2005

Now that we know the Tibet-Qinghai plateau region is where Yersinia pestis originated and the region where subsequent pandemics arose, I think its time to look more closely at regional outbreaks and case studies.

In this region, the marmot (Marmota himalayana) is the primary reservoir for Yersinia pestis. This large communal burrowing rodent is hunted by local Tibetan tribesmen for both meat and pelts. Butchering marmots has long been considered a risk factor for contracting plague via their fleas, aerosols or skin abrasions. To investigate the exposure of marmot hunters to plague, Chinese epidemiologists collected serum from 120 Qinghai villagers, 68 male hunters and 52 female family members, along with 120 negative controls from the non-endemic area of Beijing. None of the villagers or controls reported having a fever within the last two years.

The results are eye-opening and illustrates the importance of occupational exposure. Over a third of the male villagers had an antibody response to Yersinia pestis. Only 2% of their female family members produced an antibody response. Wether two fever-free years are enough time to determine if they had symptomatic plague in the past is an open question. Their letter to Emerging Infectious Diseases does not provide much information on the test subject’s histories or oral reports.

Table 1: Plague antibody assays (Li et al, 2005)

The epidemiologists explained this high level of immune protection to the use of prophylactic antibiotics by marmot hunters. They suggest the presence of tetracycline or sulfamethaoxazole, common prophylactic antibiotics in Tibet, in  their system at the time of exposure would be enough to prevent a symptomatic infection while still giving them an immunizing dose of bacteria.

The use of prophylactic antibiotics is, of course, a double-edged sword. It is clearly preventing symptomatic infections and probably outbreaks. The Chinese epidemiologists credit  most outbreaks in Qinghai to marmot hunters who either a lack of prophylactic antibiotics or have ineffective antibiotics.  On the other hand, the use of antibiotics is possibly encouraging them to harvest the easier to catch, sicker marmots. The use of prophylactic antibiotics also promotes antibiotic resistance in Yersinia pestis.

Li, M., Song, Y., Li, B., Wang, Z., Yang, R, Jaing, L., and Yang, R. Asymptomatic Yersinia pestis, China. Emerging Infectious Disease, 2005, 11 (9): 1494-1496.