Evaluation of a number of ancillary tuberculin tests in cattle

Estimation of the relative sensitivity of the comparative tuberculin skin test in tuberculous cattle herds subjected to depopulation

Bovine tuberculosis (bTB) is one of the most serious economic animal health problems affecting the cattle industry in Great Britain (GB), with incidence in cattle herds increasing since the mid-1980s. The single intradermal comparative cervical tuberculin (SICCT) test is the primary screening test in the bTB surveillance and control programme in GB and Ireland. The sensitivity (ability to detect infected cattle) of this test is central to the efficacy of the current testing regime, but most previous studies that have estimated test sensitivity (relative to the number of slaughtered cattle with visible lesions [VL] and/or positive culture results) lacked post-mortem data for SICCT test-negative cattle. The slaughter of entire herds ("whole herd slaughters" or "depopulations") that are infected by bTB are occasionally conducted in GB as a last-resort control measure to resolve intractable bTB herd breakdowns. These provide additional post-mortem data for SICCT test-negative cattle, allowing a rare opportunity to calculate the animal-level sensitivity of the test relative to the total number of SICCT test-positive and negative VL animals identified post-mortem (rSe). In this study, data were analysed from 16 whole herd slaughters (748 SICCT test-positive and 1031 SICCT test-negative cattle) conducted in GB between 1988 and 2010, using a bayesian hierarchical model. The overall rSe estimate of the SICCT test at the severe interpretation was 85% (95% credible interval [CI]: 78-91%), and at standard interpretation was 81% (95% CI: 70-89%). These estimates are more robust than those previously reported in GB due to inclusion of post-mortem data from SICCT test-negative cattle.

Tuberculosis due to Mycobacterium bovis in humans in the south-west region of Ireland: is there a relationship with infection prevalence in cattle?

OBJECTIVE

To compare the incidence of tuberculosis due to Mycobacterium bovis in humans to the prevalence of M. bovis infection in cattle in south-west Ireland and discuss possible links between them.

SETTING

In the south-west region of Ireland, a mixed urban and rural community (pop. 536,000), there is a residuum of human tuberculosis caused by M. bovis.

METHODS

A retrospective analysis of the incidence of culture-positive M. bovis disease in humans in south-west Ireland from 1983 to 1994 and of the results of tuberculin testing in cattle from 1978 to 1994 for the same region.

RESULTS

One to five cases of human tuberculosis due to M. bovis were recorded per year while the overall prevalence of bovine infection fell gradually during the period of study from 467 tuberculin-positive animals per 100,000 cattle tested in 1983 to 158 per 100,000 in 1994.

CONCLUSION

The low incidence plateau of human tuberculosis due to M. bovis together with the decline in prevalence of animal infection in the overall period studied suggest a cut-off in the animal to human chain of infection at two points; the animal source and the ingestion of (now pasteurized) milk. This would suggest that disease in humans is now due to reactivation of previous foci of infection which were acquired when milk pasteurization was not compulsory. Based on this, we would anticipate a further reduction and possible elimination of human tuberculosis due to M. bovis in this region in the next 10-15 years.

The tuberculin test

Tuberculin tests in general use today rely on the response to intradermal injections of tuberculin with assessment of the injection site for swelling at 72 hours post injection. Estimates of the sensitivity of tuberculin tests range from 68-95% while specificity is estimated to be 96-99%. The sensitivity of the test is affected by the potency and dose of tuberculin administered, the interval post-infection, desensitisation, deliberate interference, post-partum immunosuppression and observer variation. Specificity is influenced by sensitisation as a result of exposure to M. avium, M. paratuberculosis and environmental mycobacteria and by skin tuberculosis.

In vitro immunodiagnostic assays for bovine tuberculosis

The immune response to mycobacterial infections in cattle is predominantly cellular in nature and current diagnostic tests for M. bovis are based on the measurement of T cell responses. The low sensitivity of serological assays for tuberculosis is therefore not surprising and serological tests will at best be used to complement rather than replace cellular assays. The recently developed bovine interferon gamma (IFN-gamma) assay is a rapid (24 hour) and simple whole blood in vitro assay, which in Australian field trials was found to be significantly more sensitive than the intradermal tuberculin test for the diagnosis of bovine tuberculosis. The problem of false-positive reactions, due to the cross-reactive nature of the antigen preparations used, can largely be overcome by using a comparative assay in which an animal's IFN-gamma response to bovine PPD and avian PPD are compared. Although reasonably M. bovis specific proteins have been identified and characterised, their use in either serological or cellular diagnostic assays is likely to be restricted due to the genetic diversity of the bovine immune response to M. bovis infection.

Suppression of skin reactivity to bovine tuberculin in repeat tests

In cattle sensitised with killed Mycobacterium bovis an intradermal injection of either 0.3mg or 0.1mg bovine purified protein derivative tuberculin in the caudal fold suppressed skin reactivity to both bovine and avian tuberculin in comparative cervical tuberculin tests carried out 4 and 7 days later. Complete return to original sensitivity did not occur in all cattle when re-tested 60 days later but this level of sensitivity was not significantly different from that measured in initial tests. There were large individual variations in skin reaction to the sensitising dose in all treatment groups as found by others working with naturally infected cattle.