Abstract
The Norwegian surveillance and control programme for paratuberculosis revealed 8 seroreactors in a single dairy cattle herd that had no clinical signs of Mycobacterium avium subsp. paratuberculosis (M. a. paratuberculosis) infection. Paratuberculosis had been a clinical problem in goats several years previously in this herd. All 45 cattle were culled and a thorough investigation of the infection status was conducted by the use of interferon-γ (IFN-γ) immunoassay, measurement of antibodies, and pathological and bacteriological examination.In the IFN-γ immunoassay, 9 animals gave positive results, and 13 were weakly positive, while 19 animals were negative. In the serological test,10 animals showed positive reactions, and 5 were doubtful, while 30 animals gave negative reactions. There appeared to be a weak trend toward younger animals having raised IFN-γ and older animals having raised serological tests. Histopathological lesions compatible with paratuberculosis were diagnosed in 4 animals aged between 4 and 9 years. Three of these animals had positive serological reaction and one animal gave also positive results in the IFN-γ immunoassay. Infection was confirmed by isolation of M. a. paratuberculosis from 2 of these 4 animals. One single bacterial isolate examined by restriction fragment length polymorphism (RFLP) had the same profile, B-C1, as a strain that had been isolated from a goat at the same farm several years previously.Despite many animals being positive in one or both of the immunological tests, indicative of a heavily infected herd, none of the animals showed clinical signs and only one cow was shown to be shedding bacteria. A cross-reaction with other mycobacteria might have caused some of the immunoreactions in these animals. It is also possible that the Norwegian red cattle breed is resistant to clinical infection with M. a. paratuberculosis.
Highlights
Paratuberculosis is a chronic infectious enteritis in ruminants caused by M. a. paratuberculosis
Serology and the IFN-γ assay detected 17 positive and 11 weakly positive/ doubtful animals in either one or both of the tests, indicating that more than half of the herd was infected. This finding is consistent with a cattle herd heavily infected with M. a. paratuberculosis, clinical signs would have been expected in the 9 animals that were 5 years or older
1999), reducing the specificity of both serological and IFN-γ assays. These cross-reacting mycobacteria are common in the environment (Tell et al 2001), and could well have caused some of the immunoreactions in the animals in the present study
Summary
Paratuberculosis is a chronic infectious enteritis in ruminants caused by M. a. paratuberculosis. Serological examinations were used to screen the herds, and on average about 8% of the animals tested were found to be seroreactors (Djønne et al 2001a). These findings might indicate that the infection is more widespread in the Norwegian cattle population than has been assumed during the last 20 years. In one cattle herd included in the Norwegian surveillance and control programme for paratuberculosis, 8 of 18 dairy cows were found to have positive seroreactions. Four of these animals had high levels of antibodies. The aim of the present study was to investigate thoroughly the infection status in this herd at the time of slaughter, by the use of IFN-γ immunoassay, and serological, pathological, and bacteriological examination
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