Anobservational case–control study was conducted to investigate risk factors for confirmed bovine tuberculosis
(bTB) infection in cattle reacting positively to the single intradermal comparative cervical test
(SICCT) in Northern Ireland in the years 1998, 2002 and 2006.
Macroscopic lesions were detected at slaughter (positive visible lesion (VL) status) in 43.0% of reactor
cattle, whilst 45.3% of those sampled were confirmed as bTB positive due to the presence of lesions or
positive histopathology/mycobacterial culture (positive bTB status). In 97.5% of the reactors, the VL status
and bTB status were either both negative or both positive. Generalized linear mixed model analyses were
conducted on data of 24,923 reactor cattle with the variables herd identifier, local veterinary office (DVO)
and abattoir being used asrandomeffects within all the models generated at univariable and multivariable
level. The other variables within the dataset were used as fixed effects. Significant risk factors associated
with VL status and bTB status at multivariable level (p < 0.05) included age at death, breed, sex, test year,
net increase in skin thickness at bovine tuberculin injection site, epidemiological status of skin test, total
number of reactors at the disclosure test, mean herd size and prior response to the skin test.
These risk factors are likely related to the time since infection, the strength of the challenge of infection
and the susceptibility of the animal. These findings are important as the detection of visible lesions and
the confirmation of bTB are an integral part of the overall bTB control programme in Northern Ireland
and the veterinary meat inspection and hygiene programme. The visible lesion status and bTB status of
an animal can affect the way in which bTB breakdowns are managed, since failure to detect visible lesions
and recovery of Mycobacterium bovis can lead to a less stringent follow-up after other risk factors have
been taken into account.