Tibet | Parasite Medical Program | Dr. Peter M. Schantz

Echinococcosis on the Tibetan Plateau:

Risk Factors for Infection in Tibetan Populations

The Tibetan Plateau is known as the third pole of the world with its high elevation, enormous land mass and unique nature and culture. Two species of the Echinococcus tapeworm parasite, E. granulosus and E. multilocularis, causes of the cystic (CE) and alveolar (AE) forms of echinococcosis, respectively, are known to afflict animals in this region. These infections can be transmitted to humans when they accidentally ingest the egg-stage of the tapeworm passed in the feces of infected dogs or foxes. Echinococcosis infections occur throughout the world; prevalence in humans is highest in populations in which interactions of dogs, domestic livestock and humans facilitate transmission between animals and result in high probability of exposure of humans to the infectious stage (eggs) passed in the feces of infected dogs.

Between June 1997 and June 1998, we carried out a systematic survey of echinococcosis in human and animal populations in 5 counties of this region.
Pastoralists by occupation, the people tend herds of domestic livestock, primarily yak and sheep, with smaller populations of cattle, horses and goats. Schools serve the populations in villages and towns; however, more than 80% of the herdsmen and farmer populations are "un-educated." We examined rural nomadic populations as well as persons living in towns, including students from 11 primary schools. Cases in humans were diagnosed by abdominal ultrasound, chest x-ray and blood tests. Abdominal ultrasound revealed suggestive space-occupying lesions in 435 persons (5.6%) of a total of 7702 who underwent the examinations. Upon critical review, 326 (4.2%) were diagnosed as CE and 109 (1.4%) as AE. We were able to evaluate the effectiveness of combining "imaging diagnosis" (ultrasound and x-ray) with blood tests for screening for this disease.
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The infection rates for CE varied from 0.9% to 8.1% in the 5 counties, and those for AE from 0.3% to 3.1%. For both forms of the infection, there was a tendency of increasing prevalence from agricultural areas to village areas to stock-raising areas. Female participants had consistently higher rates (6.8%, 258/3812) than male participants (4.6%, 177/3890); among the probable factors contributing to higher infection risk for women is their role in caring for dogs and livestock. For both sexes, prevalence of the two forms of infection increased with age from 0.5% (3/584) at 0-9 years of age to 16.0% (59/369) among persons older than 60 years. There were strong relationships between infection rates and occupation; the herdsman occupation was consistently the highest-risk occupational group for CE (prevalence 7.0%-13.5%). Most AE cases also occurred in herdsmen. There was also a significant inverse relationship between educational level and risk of infection, i.e., the higher the education level, the lower the infection rate. The infection rates in illiterate persons were 4.5%-17.0%, while the rate was 3.1% in those with primary school education level, 2.2% in middle school education level, and 1.1% in persons with college education. Children of families living in town/district areas had an average rate of 3.1% (39/1278).

Analysis of environmental risk factors disclosed a relationship between dog ownership/association and infection risk. Collection of data on infection in dogs and other animals revealed consistently high rates of infection in dogs of rural herdsmen, as well as stray dogs in the towns. It was apparent that dogs become infected with E. granulosus when their owners feed the cyst-laden viscera of home-slaughtered yaks and sheep to the dogs. Stray domestic dogs also had access to viscera of yaks and sheep slaughtered in the towns. E. multilocularis infection is maintained in wild animals in cycles involving foxes and their small mammalian prey. Stray dogs become infected when they roam the fields preying on the small mammals; this serves as an important source of infection for children and other persons in the towns.

In summary, on the Tibetan Plateau, as a result of a number of environmental and cultural characteristics, there exist very high infection rates for both the cystic and alveolar forms of echinococcosis.

These were the highest rates of infection by these two forms of infection ever documented, and transmission of the infection is closely related to the cultural and economic practices of the Tibetan populations. A large proportion of herdsmen (44.3% of 2650) admits to feeding viscera of livestock, even those with some parasitic lesions, to their dogs. More than half of persons living in towns (52.5% of 1124) feed their dogs on the offal of home-slaughtered livestock. Less than 1% treated viscera of livestock in a manner that would eliminate the infectivity of echinococcal cysts.

Neither the average local person nor students were knowledgeable about the diseases. Proposed efforts to control this zoonotic infection must be introduced with a strong educational program to inform people of the infection and how their behavior contributes to transmission and infection risk.

Peter M. Schantz, VMD, PhD
(now retired)
Division of Parasitic Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, Georgia, USA