Echinococcosis on the Tibetan Plateau:
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.
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
Division of Parasitic Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, Georgia, USA