Bovine TB: Research project summary
Project SE3013: Pathogenesis and diagnosis of tuberculosis in cattle – complementary field studies.
Project duration: 5 years
The study was designed to advance the understanding of cattle-to-cattle transmission of bovine tuberculosis (bTB) in Great Britain through detailed pathological and immunological investigation of cattle either naturally infected (“reactors”) or exposed to Mycobacterium bovis (“in-contacts”) and to assess certain diagnostic aspects of disease detection. The study provides reassurance that the slaughterhouse meat inspection is targeting areas of the body where visible lesions are most likely to be found.
The study also shows that there is good diagnostic agreement between the techniques of culture and histopathology for M. bovis for the confirmation of infection. Of the 200 reactor cattle selected, 55.5% had macroscopic visible lesions and, of the 200 in-contacts selected, 14% had macroscopic visible lesions. These in-contact animals were negative in the initial skin test. Some of these are at a very early disease stage and a portion of them would have been identified as reactors at a repeat skin test. However, the vast majority of these can be identified more quickly and accurately in the field by using blood based tests and these results provide solid support to the use of the BOVIGAM gamma interferon assay. These animals have been erroneously regarded as false positives by farmers and some vets in the past due to their skin test negative status, but holding the animals longer allowed them to develop visible lesions of tuberculosis.
Lesions were most commonly found in the thorax of both reactors and in-contact animals with secondary involvement of the head region. From experimental studies the lesion distribution is consistent with aerosol transmission between cattle. Extensive testing of nasal mucus in all animals failed to detect M. bovis being transmitted between cattle via this route and suggests that coughing is more important for transmission than general nosing of cattle.
Strikingly, statistical examination of the data found that lesion distribution varied between production class with dairy cows being less likely to have visible lesions and confirmed bTB than any other production class. This could not be explained and is a worthwhile area for further investigation. Additionally low selenium levels were associated with confirmed bTB status. The research shows that 7 weeks of supplementing cattle with a mineral lick (a block containing minerals for the cattle to lick) improved the selenium status of affected cattle back to their normal range. A cause and effect relationship is not established in this area.
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July 7, 2008