Risk factors for detection of recurrent bovine tuberculosis in New Zealand cattle and deer herds 2005-2011 : a dissertation presented in partial fulfillment of the requirements for the degree of Master of Veterinary Studies (Epidemiology) at Massey University, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand
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In New Zealand, under the national bTB eradication strategy, bovine tuberculosis (bTB) has had a sustained decrease in prevalence since its peak in 1994 at 1700 infected herds. With the success of control measures, recurrence of infection in cattle and deer herds that have previously tested to a clear status is a problem that has recently become more apparent. Uncontrolled movement of cattle and deer from these herds pose a risk to the bTB eradication strategy. A retrospective cohort study was conducted to identify herd-level risk factors for bTB recurrence. Data were collected from 356 New Zealand cattle and deer herds that met the following criteria: (1) a culture positive case of bTB had been identified on or after 1 June 2006 (the index bTB episode), (2) the herd status had been cleared by testing that was completed by 1 November 2010, and (3) the clear status was not achieved by destocking. The outcome of interest for this study was defined as detection of a subsequent bTB episode before the end date of the study, 5 May 2011, by routine tuberculin testing or by slaughter surveillance. Herds were censored if no further bTB episode occurred by 5 May 2011. A Cox proportional hazards model was developed to quantify the magnitude of a series of herd-level risk factors on the daily hazard of bTB recurrence. Disease control area was included in the model as a fixed effect to account for confounding. There was a positive relationship between the daily hazard of recurrence and: (a) the number of bTB episodes in a herd prior to the first episode (HR [hazard ratio] 3.2 for two prior episodes, 95 % CI 1.2-8.5; HR 86.7 for five prior episodes, 95 % CI 13.8-580), (b) the presence of more than one bTB positive animal at the index bTB episode (HR 2.3: 95 % CI 1.2-4.3) and (c) the presence of one or more cleared test-positives at the final clearance test at the index episode. The proportional hazards assumption was violated for the latter variable so a time dependent covariate was introduced into the model to account for the variable effect of the presence of cleared test-positives at the final clearance test over time. The monthly hazard of recurrence during the first two years after clearance was significantly increased in herds with one or more test-positive animals at the final test (HR 2.8: 95 % CI 1.2-6.4), but this association was no longer significant more than two years after clearance (HR 1.5: 95 % CI 0.6-3.6). We conclude that the presence of unresolved infection in a herd is a contributor to further bTB episodes in the first two years after clearance. TBfree New Zealand is reviewing policies to increase the sensitivity of detecting residual infection before clearance and to intensify post clearance testing and movement tracking in herds with risk factors.
Bovine tuberculosis, Tuberculosis in cattle, Tuberculosis in deer