Epidemiology of Brucellosis in Yaks in the Tibet autonomous region of China

Brucellosis, caused by members of the genus Brucella, is a highly contagious production-limiting disease and one of the most important zoonosis in many countries of the world, including China. Prior to the study outlined in this thesis, few studies on the epidemiology of brucellosis in Tibet had been undertaken. Consequently, the aims of this study were to determine the epidemiological characteristics and economic impact of brucellosis in yaks in Tibet. In a study examining historical data, significant differences were found in the spatial and temporal distribution of brucellosis in both livestock and humans (p<0.05). In the period from 2011 to 2013 there was a positive correlation between the seroprevalence in livestock and humans (r=0.93). Brucellosis was shown to be more common in the spring/summer seasons when parturition occurred. A cross-sectional serological study of 1,523 randomly selected yaks belonging to 181 herders was conducted in Damxung, Maizhokunggar and Yadong counties. Sera were tested using a Rose Bengal Test (RBT) and a Competitive immune-enzymatic assay (C-ELISA) and the results interpreted in parallel. The individual yak seroprevalence was 2.8% (95%CI: 2.0, 3.7) with a herd level seroprevalence of 18.2% (95%CI: 12.9, 24.6). At the individual animal level, age and production system were significantly associated with seropositivity in a multivariable logistic regression model. The odds of Brucella infection were significantly higher in older yaks (3-5 years old, OR=4.51; 95%CI: 1.53, 19.29; >5 years old, OR=3.89; 95%CI: 1.23, 17.21) compared to younger yaks (<3 years old). The odds of seropositivity for yaks managed under an agro-pastoral production system was 2.9 (95%CI: 1.48, 5.86) times higher compared to those managed under a pastoral production system. At the herd level an association between seropositivity and a history of herd-abortions was observed (OR=4.98, 95%CI: 1.48, 16.62). Surprisingly vaccination of calves in Pali township of Yadong county was not associated with a lower level of infection (p=0.49 and p=0.99 for individual and herd level data, respectively). A total of 317 yak pastoralists were interviewed using a structured questionnaire to determine their knowledge, attitudes and practices (KAP) relating to brucellosis. Although 60.6% of the respondents had heard of the disease, there was an overall low level of knowledge about the disease. Pastoralists did, however, adopt management/husbandry practices which would reduce transmission of the disease to humans and other animals. Multivariable logistic modelling showed that better knowledge was predicted by age (≥50 years old, OR=1.98; CI: 1.15, 3.46), production system practiced (pastoral, OR=10.57; CI: 5.47, 21.54), education level (primary/secondary school, OR=2.19; CI: 1.22, 3.96) and number of persons in a household (≥6 persons, OR=2.77; CI: 1.59, 4.91). Difference in attitudes and practices were predicted by education level (primary/secondary school, OR=1.72; CI: 1.03, 2.88), number of persons in a household (≥6 persons, OR=2.80; CI: 1.68, 4.76) and production system practiced (pastoral, OR=2.43; CI: 1.38, 4.33). An economic evaluation of brucellosis found that the disease could result in a loss of US$ 3,126,256.68 (95%CI: US$2,006,644.50, US$4,559,176.80) in the total population of yaks over a six-year period in Damxung and Maizhokunggar counties and Pali township of Yadong county in Tibet, with an average loss per yak estimated at US$ 1.42 (95%CI: 0.91, 2.07) annually. Through benefit-cost analysis, vaccination was found to be the most economically sound control method with a benefit-cost ratio (BCR) of 3.19 (95% CI: 2.17, 4.66) and a net present value (NPV) of US$313,354.87 (95%CI: US$157,678.46, US$541,061.80). In a sensitivity analysis the NPV of the vaccination control program was shown to be most sensitive to the loss from an abortion. In contrast the price of yaks that were slaughtered had the largest influence on the NPV for the test-and-slaughter control program and the combination control program (vaccination and test-and-slaughter programs). It is concluded that both public health education and implementation of a routine vaccination program are needed to effectively control brucellosis in yaks on the Tibetan plateau.