Distribution of Mycobacterium avium subspecies paratuberculosis in tissues of experimentally infected steers
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Tissue culture was carried out on 30 experimentally MAP inoculated calves in 5 different age groups (2w, 3m, 6m, 9m and 12m) with two different doses and on 3 uninfected calves (control), after euthanasia at 17 months of age. In total 21 tissues per animal were taken from different locations: duodenum, jejunum (mid and distal part), ileum (proximal, mid and distal part), ileocaecal valve, caecum, colon (spiral colon and transverse colon), rectum, lymph nodes corresponding with the previously mentioned gastro-intestinal tract samples (except for the spiral colon, transverse colon and rectum), hepatic lymphnode, tonsil, retropharyngeal lymph node and the superficial inguinal lymph node. After decontamination and 48 days of incubation at 37°C a confirmatory identification of MAP with F57 qPCR was performed. In total 19 of the 31 infected calves (61,3%) were tissue culture positive. The tissue with the highest probability for detection of an infected animal was the ileocaecal valve with 0.19, considered very low. In 45,2% of the infected animals MAP was found in the GI tract tissue, in 25,8% of the animals had positive tissues for the associated lymph nodes of the GI tract and for the other lymph nodes (hepatic, retropharyngeal and superficial inguinal) MAP was found in 12.9% of the animals. Out of the 19 tissue culture positive animals, MAP was detected in 73,7% of the animals with GI tract tissues. Considering that all MAP inoculated animals are successfully infected, which is perhaps not true, the results suggest that tissue culture, considered to be the gold standard, detects a lower percentage of infected animals than expected, this leads to underestimation of test agreements. Tissue samples from multiple sites are necessary to detect 100% of the infected animals. The probability of detection of an infected animal per location is too low to use as single detection method of an infected animal. Dissemination of MAP takes already place in the subclinical stage of JD instead of the previous assumed clinical stage of JD.