The present study used a battery of diagnostic tests to confirm that the herd was infected with M. a. paratuberculosis
. The sensitivity and specificity of the diagnostic tests depend among other factors on the prevalence of the infection with M. a. paratuberculosis
within the herd, and will thus give different results from herd to herd. However, in a herd that has been infected for many years, it is usual that at least 1 animal will show clinical signs. The infection in such an animal is usually quite easily confirmed by faecal culture and serology. About 25% of the remaining clinically healthy animals in the herd will be infected, but only 1/4 of these will be detected by faecal culture [35
]. In the present herd, no animals showed clinical signs of paratuberculosis, but 1 animal was found to shed bacteria in the faeces. Therefore, in this herd of 45 animals, the prediction would be that about 11 (25%) of the animals were infected. 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
, although clinical signs would have been expected particularly in the 9 animals that were 5 years or older.
In general, the diagnostic results of the immunological tests showed a weak trend towards younger animals having raised IFN-γ tests and older animals having raised serological tests. There were however exceptions, and this limited the ability to state categorically that one test should be used in young animals and another in older animals. A raise in the cell mediated immunity (CMI) response in young animals and in the antibodies in older animals has been a common finding in many paratuberculosis studies. Experimental trials carried out in cattle showed that the CMI response can be detected shortly after the infection [12
], and the high proportion of CMI reactors observed during the first 2 years of life indicated that the majority of individuals become infected during this period. Investigations in sheep and goats have shown a relationship between pathological findings and the CMI response [25
], and it has been suggested that the CMI response gives protection against the development of diffuse lesions. Our results indicate that a CMI response persisted in the animals for several years following infection, which possibly explains the limited clinical problems in the herd. Production of antibodies is often correlated with progression of the infection [3
], and in our study, 3 of the 4 cows with histopathological lesions had high levels of antibodies.
In the present study, pathological and bacteriological examinations detected the infection in 4 animals. A few other animals had small granulomatous inflammatory lesions in the intestine devoid of demonstrable acid-fast bacilli or foreign material and could therefore have been due to M. a. paratuberculosis
infection. This type of lesion was however no more frequent in seropositive than in seronegative animals, and many seropositive animals had no histopathological lesion indicative of paratuberculosis. More exhaustive tissue sampling for both histopathology and bacteriology may have confirmed infection in additional animals, since discrete subclinical lesions can be widely distributed throughout the intestinal tract and mesenteric lymph nodes [35
The 4 confirmed positive animals were all older than 4–5 years. In animals up to 4 years of age the IFN-γ immunoassay would appear to be the relevant screening test, while a test measuring antibodies would be preferable in animals from 3 years and older. In cattle, however, the age of the animals can have an impact on the IFN-γ results. False positive reactions have been observed when the IFN-γ test has been applied to calves less than 15 months of age [15
]. Furthermore, cross-reactions with other mycobacteria are common [11
], reducing the specificity of both serological and IFN-γ assays. These cross-reacting mycobacteria are common in the environment [32
], and could well have caused some of the immunoreactions in the animals in the present study. Results from the Norwegian surveillance and control programme for paratuberculosis [4
], have shown that about 8% of Norwegian cattle are seroreactors. A follow-up study of these seropositive cattle has shown that the reactions were false positive, and were probably caused by environmental mycobacteria [8
The clinical problems with paratuberculosis in cattle in Norway have been insignificant compared with those in goats during the second half of the last century, and there has been uncertainty whether the M. a. paratuberculosis
strains in goats in Norway are pathogenic in cattle [28
]. However, several observations indicate that strains isolated from one animal species can infect other species [9
], and that strains isolated from one animal species and orally administered to another species have led to infection [1
]. Paratuberculosis had been a clinical problem in goats on the present farm several years before the present study was conducted, and the infection might well have existed in cattle in a subclinical form. The same RFLP patterns were found in the M. a. paratuberculosis
strain isolated from cattle in our study as in a strain isolated from a goat on the farm several years previously. This RFLP pattern is the predominant type in Norwegian goats (Djønne
, unpublished observations) and in cattle from Europe and the United States [23
]. These findings indicate that the same M. a. paratuberculosis
strain has infected goats and cattle.
Our observations do not exclude that the present strain shows different pathogenicity for cattle and goats, but there are factors other than animal species that should be considered when evaluating the pathogenicity of M. a. paratuberculosis
for cattle and goats. These factors include management conditions and breed resistance. The management conditions are quite different for cattle and goats in Norway. The cattle units are small, the young calves are usually separated from their dams shortly after birth, animals older than 1 year are usually housed in separate stalls, and the average age of the cows is low (3.9 years of age). All of these management factors have been shown to reduce the spread of infection in a herd [13
]. The goat kids, however, are often born in pens where several goats are housed. Thus, one single offspring might suckle several dams, and the risk of infection with faecal material from a bacterial shedder should therefore be higher in goats than in cattle. In the present herd, in contrast to the general management condition described above, 9 cows were older than 4 years, and 3 of these were 9 years old. In addition, the young calves were allowed to move freely among adult cows, which might have exposed several individuals to contact with one single shedder.
Paratuberculosis was considered to be a clinical problem in the Norwegian cattle population during the first part of the 20th century. At that time, different local cattle breeds made up the cattle population in Norway. After 1970, the majority of the population was drawn from the Norwegian red cattle breed, which is a hybrid of many different breeds. In recent decades, paratuberculosis has been considered a minor problem in the cattle population in Norway, and clinical cases were not reported between 1979 and 2001. Thus one can speculate that the Nor-wegian red cattle breed is more resistant to clinical infection with M. a. paratuberculosis than the local cattle breeds.
The present study shows that the infection might be subclinical in cattle herds, and may be overlooked if immunological, pathological and bacteriological investigations are not performed.