Skin test fails – good examples of anergic cows Print this pagePrint this page

Back in April 2008 a Friesian cow with chronic mastitis was reported as a possible source of TB infection to three calves which were positive to the skin test. Whilst the dairy herd had confirmed disease, this cow had passed 11 TB skin tests since 2003. In February 2008 the three calves sucking milk from her were positive to the TB skin test, but they did not have visible lesions at post-mortem examination. The cow passed the skin test. In April 2008, the next three calves (three months old) which were sucking from her also reacted positively to the TB test, but this time one of them had visible lesions at post-mortem. The cow again passed a skin test. The housing conditions of these calves made contact with wildlife extremely difficult, and the three calves only sucked milk from the suspect cow so badgers were not implicated.

Permission was sought from the cow’s owner to carry out a gIFN blood test, in order to find out if the cow was the source of infection of TB to the calves (and maybe to the rest of the dairy herd) as it was suspected that the cow might be an anergic animal. Both gIFN test and a Rapid Antibody test were positive.

In view of the results, the animal was slaughtered. The carcass was condemned because of the amount of TB lesions found (generalised TB). Visible lesions were found in the head nodes, broncho-mediastinal nodes, mesenteric nodes, supramammary glands, udder tissue and liver. All lesions found were typical of TB. Samples of lesioned material were submitted for mycobacterial culture to the VLA in June, and were returned as positive for M. bovis spoligotype 17 (SB0263 from “http://www.Mbovis.org” www.Mbovis.org). This spoligotype was within its geographical home range, and therefore not unusual for the area of this breakdown.

Also of interest, with similar cases is this paper, ‘Outbreak of bovine tuberculosis featuring anergy to the skin test, udder lesions and milkborne disease in young calves’ by M. G. Houlihan, MVB, MSc, MRCVS1, F. W. Dixon, BVM&S, MRCVS2 and N. A. Page, BVSc, MRCVS2 (http://veterinaryrecord.bmj.com/content/163/12/357.abstract):

Abstract.

A severe outbreak of bovine tuberculosis in a 1300-head, multisite dairy herd in Great Britain had several unusual features, including anergy to the tuberculin skin test, milkborne disease in calves and a farm cat, and a risk of human infection. The outbreak was controlled by culling 221 cattle over 15 months, by using the γ-interferon (γ-IFN) test and by the examination of milk samples. The γ-IFN test detected infected animals that were not detected by the skin test.

Information extracted from Animal Health’s ‘Official Veterinarian Newsletter”, Issue 2, November 2008 - see http://animalhealth.defra.gov.uk/about/publications/ov/ov-newsletter/OV_newsletter_2.pdf


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