Evidence of TB during post-mortem

Is an animal infected with bovine TB if bovine TB infection was not found in a post-mortem examination?

The following extracts explain why post-mortem examination does not reveal all incidences of infection.

About 6% of confirmed reactor cattle in GB show no grossly visible lesions at post-mortem examination. Given the high throughput of animals in slaughterhouses and the fact that many confirmed reactor cattle have only one or two small grossly visible lesions, or none, it is perhaps unrealistic to expect slaughterhouse inspection to provide anything other than cursory surveillance. This is not to underestimate its important role in protecting public health, but its role in surveillance is secondary to herd testing. ¹

Q7. Negative culture results from the lab must mean no infection is present?
A7. No. Detection of M. bovis by culture is affected by many factors including the sampling process, with visibly lesioned animals giving a greater chance of detecting infection. Animals at early stages of disease and latently infected animals do not present with visible lesions at post-mortem and will result in some animals escaping detection. ²

5-10% of latently infected humans develop clinical tuberculosis during their lifetime through re-activation of the latent infection (re-activation tuberculosis). The argument that latently infected individuals (culturenegative NVL, skin test reactors for example) constitute a continuous and unpredictable source of reinfection, is equally valid for cattle as it is for human TB. At the early stages of infection, or in latently infected cattle, a period will occur when M. bovis appears to be absent because the bacillary load is not large enough to be detected by culture. In addition, the pathological changes caused by the bacilli are not yet profound enough to be detected during routine abattoir inspection. However, cellular immune responses will be detectable in these animals at an earlier stage of infection than the pathological changes caused by the disease (e.g.visible lesions), or before the bacterial loads exceed the numbers necessary to be able to culture M. bovis from tissue samples. ³

Q1. Is it true that skin test positive animals that show no signs of diseased tissue at post-mortem examination (known as NVL or no visible lesions) have never had the infection?
A1. The false positive rate for the skin test is very low (1 in 1000) and so it is very likely that any animal that tests positive is infected, regardless of whether this is confirmed at post -mortem. ²

References

1. Final Report of the Independent Scientific Group on Cattle TB
2. Bovine TB- the facts
3. Government Veterinary Journal

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