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. While bTB prevalence in Uruguay has been low (<11 outbreaks/year) for the past 50 years as a consequence of a national control program, annual incidence increased in 2011 through 2013-15, 26 and 16 infected herds each year, raising concerns from livestock stakeholders and the government. The goal of this study was to assess the spatial dynamics of bTB in Uruguay from 2011 to 2013 and the association between bTB and potential demographic and movement risk factors at the herd level using data provided by the Uruguayan Ministry of Livestock, Agriculture, and Fisheries. Clustering of incident outbreaks was assessed using the Cuzick-Edwards' test and the Bernoulli model of the spatial scan statistic, and a conditional multivariable logistic regression model was used to assess risk factors associated with bTB in a subset of Uruguayan dairy farms. Significant (P<0.05) global clustering was detected in 2012, while high-risk local clusters were detected in southwestern (2011, 2012, 2013), northwestern (2012), and southeastern (2012) Uruguay. Increased risk of bTB in different regions of Uruguay suggests a potential role of animal movements in disease dissemination. Larger herds, higher numbers of animals purchased, and incoming steers to the farm were associated with increased odds of breaking with bTB, in agreement with previous studies but also suggesting other additional sources of risk. These results will contribute to enhanced 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.

Evaluating empirical contact networks as potential transmission pathways for infectious diseases

“…Uruguay experiences a very low farmlevel incidence of bTB, with typically fewer than 30 new infected farms detected annually. Dairy farms in Uruguay are tested annually for bTB using a tuberculin skin test, which has limited sensitivity to detect infected animals [42]. Movement of animals between farms is a commonly cited source of between-farm spread of livestock pathogens [18,42,43], though local factors such as wildlife reservoirs and fence-line transmission have also contributed to bTB transmission in other countries [44].…”

Optimal surveillance strategies for bovine tuberculosis in a low-prevalence country

“…We evaluated surveillance strategies that eliminated active surveillance in low-risk dairies, while maintaining existing surveillance measures on high-risk dairies (Table 2) 11 . Specifically, lower risk farms that failed to meet cut-off values for herd size and/or number of animals received did not undergo annual skin testing (no active surveillance), whereas high-risk farms that exceeded cut-off values underwent annual skin testing.…”

“…However, to optimize the implementation of surveillance and control measures, additional research is needed to clarify which herds and locations are associated with higher risk for disease introduction and transmission, and to develop methods to identify high-risk herds at an early stage of infection. Animal traceability systems, which have been implemented in many countries, provide an ideal opportunity to empirically assess movement-related bTB risk and to simulate the potential between-farm spread of bTB through the cattle industry 11–14 .…”

“…Despite considerable investment in a test-and-cull program for the control of bTB, the incidence of the disease has increased since 2008 (~4 farms per year in the early 2000s to ~22 per year in 2012–2014), raising concern among stakeholders and animal health agencies 11 . No wildlife reservoir has been identified within Uruguay, and all detected cases have been in dairy farms.…”

“…No wildlife reservoir has been identified within Uruguay, and all detected cases have been in dairy farms. The current surveillance program includes a combination of active surveillance in dairy farms (annual intradermal tuberculin testing; caudal fold test -CFT- with confirmation via the comparative cervical test-CCT) and passive surveillance in all farms (slaughter inspection of carcasses for bTB-like lesions) 11 . Risk-based surveillance has the potential to optimize ongoing control programs in low prevalence countries such as Uruguay, balancing the efficacy of control efforts while minimizing investment of human and financial resources.…”