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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Date
2014
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Massey University
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Abstract
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.
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Keywords
Bovine tuberculosis, Tuberculosis in cattle, Tuberculosis in deer