Epidemiological investigation of bovine tuberculosis outbreaks in Uruguay (2011-2013)

Abstract

Bovine tuberculosis (BTB) is a chronic disease of cattle caused by infection with the Mycobacterium bovis, and with an important zoonotic potential. Countries have widely recognized BTB as a major animal and public health problem. Uruguay is a South American country with a strong agriculture-based tradition and economy. Uruguay was one of the pioneers in the fight against BTB in the world in the late 1800’s, with continuously update of BTB-control measures, especially in dairy herds, which resulted in the design of the current national BTB control program. Under the current BTB control program, surveillance is based on test-and-slaughter strategies for dairy farms, and abattoir carcasses inspection looking for tubercles in all slaughtered cattle. When a farm is reported as positive, control measures include movement restrictions, testing of neighboring farms, and traceback of all animal movements performed up to two years previous to the outbreak. As a consequence of this surveillance program, BTB prevalence in Uruguay has been traditionally low (<11 outbreaks per year). However, between 2011 and 2013, the incidence of BTB increased, concerning farmers, industry, veterinarians, and the government authorities. The goal of this study was to assess the spatial dynamics of BTB in Uruguay in the 2011-2013 period, and the association between BTB and demographic and movement factors in the Uruguayan dairy industry. Data provided by the Uruguayan Ministry of Livestock, Agriculture, and Fisheries included information on geolocation, BTB status (positive/negative), and onset date. Also, individual information on movements from/to all farms during the 2008 to 2013 period in Uruguay was provided to obtain farm-level network characteristics. Fifteen, 26, and 16 BTB-outbreaks were reported in 42,759 (2011), 43,213 (2012), and 42,186 (2013) susceptible farms, respectively. The spatial distribution of incident outbreaks was assessed using the Cuzick-and-Edwards’ test, and the Bernoulli model of the spatial scan statistic. Significant (P=0.05) clustering at the second order of neighborhood was detected in 2012, and significant high-risk clusters were identified in southwestern (2011, 2012, 2013), northwestern (2012), and southeastern (2012) Uruguay. The presence of spatial aggregation in southwestern Uruguay (consistently in 2011 and 2012, and corresponding to a dairy production land-use) suggests an association between land management and BTB risk. Global clustering detected in 2012 suggests dissemination by close contact of BTB. The increase of BTB incidence in disparate regions of the country suggests that live animal movements may have played a role in spreading the disease. A subset of the dairy Uruguayan farms was assessed with a conditional multivariate logistic regression model to investigate the risk factors most likely associated with BTB, with three controls (n=171) per positive farm (n=57), accounting for spatial dependency. Ten demographic, and 51 movement epidemiological factors were evaluated as possible risk factors. The final model included large herds, high number of incoming cattle to the farm (>44 cattle), and purchasing steers as factors increasing the odds of BTB. The first two factors were in agreement with previous studies, highlighting the importance of control in terms of disease spread this practice in the cattle industry. The association between BTB incidence and purchasing steers may be associated with certain management practices for which this covariate may serve as a proxy. Results from this study were presented to and discussed with Uruguayan Animal Health Bureau members to address applicability of results. These results and discussions contributed to the design and implementation of disease management strategies intended to enhance the effectiveness of BTB control programs in Uruguay, with the ultimate objective of preventing or mitigating the impact of the disease in the human and animal populations of the country.