Echinococcosis on the Tibetan Plateau

The Tibetan plateau of western China has been shown to have a very high prevalence
of human cystic echinococcosis (CE) caused by Echinococcus granulosus and human
alveolar echinococcosis (AE) caused by Echinococcus multilocularis, with the
domestic dog suspected of being the primary definitive host for the transmission of
both parasites to humans in this locality. A purgation study of 371 dogs in Shiqu
County, Sichuan Province during 2002 – 2003 resulted in an E. multilocularis
prevalence of 12% and an E. granulosus prevalence of 8%. These crude prevalences
were then adjusted, based on the known sensitivity of arecoline purgation for the
detection of E. granulosus and a suggested sensitivity for the detection of E.
multilocularis. In addition, it was assumed that some immature parasites of either
species could be misidentified morphologically and wrongly assigned. This resulted in
credible true prevalence intervals of between 13 – 33% for E. multilocularis and 8 –
19% for E. granulosus. Risk factors associated with the acquisition of canine
echinococcosis were evaluated based on responses to a questionnaire administered to
dog owners. Male dogs were more likely to be infected with Echinococcus spp. than
female dogs (P < 0.05) and dogs allowed to roam were more likely to be infected with
E. multilocularis (P < 0.05). E. granulosus and E. multilocularis abundance and
prevalence were then fit to mathematical models to evaluate transmission parameters.
Abundance based models, assuming the presence and absence of immunity, were fit
for both parasites using Bayesian priors, maximum likelihood techniques, and Monte
Carlo resampling techniques. When the models were compared, using the likelihood
ratio test for nested models, the model assuming the presence of immunity was the
best fit for E. granulosus infection, with a mean abundance of 80 parasites per dog
and an average infection pressure of 560 parasites per year. In contrast, the model
assuming the absence of immunity was the best fit for E. multilocularis infection, with
a mean abundance of 131 parasites per dog, and an average infection pressure of 334
or 533 parasites per year assuming a 5 or 3 month parsite lifespan respectively. The
prevalence data for both parasites was then fit to a set of differential equations
modeling the transition between infection states in order to determine number of
infectious insults per year. Infection pressure was 0.21, with a 95% credibility interval
of 0.12 – 0.41, infections per year for E. granulosus and 0.52, with a 95% credibility
interval of 0.29 – 0.77, infections per year for E. multilocularis, assuming a 5 month parasite lifespan or 0.85, with a 95% credibility interval of 0.47 – 1.25, infections per
year, assuming a 3 month E. multilocularis lifespan in dogs.
Since Shiqu County has an extremely high prevalence of both human AE and CE, the
SF-12 v2 quality of life survey was utilized to evaluate the extent to which morbidity
associated with echinococcosis should be accounted, and verified a significant
reduction in mean health scores for individuals diagnosed with abdominal
echinococcosis compared to an age and gender cross-matched population. Results of
an ultrasound survey, which screened 3135 subjects, indicated a prevalence of
approximately 5% for both AE and CE and an adjusted overall combined prevalence
of 9.5%. The burden of disease associated with echinococcosis, utilizing disability
adjusted life years (DALYs), was calculated using Monte-Carlo techniques to model
uncertainty in the prevalence estimates and disability weights. Total numbers of
DALYs lost due to echinococcosis, for the current population of 63,000, was
estimated to be 50,933 (95% CI 41,995 – 61,026) and suggests an average of
approximately 0.81 DALY lost per person. Human losses, associated with treatment
costs and loss of income due to morbidity and mortality, in addition to production
losses in sheep, goats, and yaks due to E. granulosus infection were also evaluated. A
control program based on the biannual deworming of dogs with praziquantel and the
vaccination of sheep and goats was then suggested based on the infection pressure of
E. granulosus and E. multilocularis for the region. The median estimated cost of the
program would be approximately U.S.$56,000 per annum, which is a fraction of the
estimated combined livestock and human financial losses due to the disease. Overall
cost for the proposed control program is within the World Health Organization’s
second most cost-effective band of less than U.S.$150 per DALY averted, however,
cost per DALY averted would be less than U.S.$25 dollars for the human health
sector if cost-sharing was implemented between the public health and agricultural
sectors based on proportional benefit from control.