Johne’s is a bacterial infection that results in chronic diarrhea that leads to weight loss, wasting and eventual death.
The disease is primarily seen in mature cows.
The bacterium that causes the disease is referred to as MAP (Mycobacterium avium paratuberculosis) and the disease is sometimes also known as paratuberculosis.
The MAP bacterium that causes Johne’s disease has a thick, waxy cell wall that makes the bacteria very resistant in the environment. It can survive in soil or feces for more than a year and transmission occurs primarily when animals ingest infected manure or milk.
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The bacteria are also secreted in milk and colostrum from infected cows. Transmission is very age dependent and older animals require a much higher exposure to the bacteria to become infected.
Most animals are infected as young calves before they are six months old, but it is a very slow progressive disease and clinical symptoms may not appear until animals are four to five years old.
The infection causes a gradual thickening of the intestines of the animals, making them less able to absorb nutrients. The primary clinical sign is weight loss and chronic watery diarrhea. The affected animals often remain bright and alert and continue to eat, despite gradually losing weight. Clinical symptoms often initially occur shortly after calving.
A recent paper was published in the journal Frontiers in Veterinary Science by a research team at the University of Saskatchewan’s Western College of Veterinary Medicine led by Dr. Cheryl Waldner and her graduate student, Paisley Johnson.
The study was part of the Canadian Cow-Calf Surveillance Network project funded by the Beef Cattle Research Council and Saskatchewan Agriculture.
Blood and fecal samples were collected at pregnancy checking time from 159 cow-calf herds from across Canada. There were 3,171 cows (older than two years) sampled from these cow-calf herds. In Western Canada, 110 herds were sampled, while 49 herds were in Eastern Canada. About 15 percent of the herds sampled were mostly purebred cattle, 81 percent were mostly commercial cattle and four percent of the herds had a 50-50 mix of purebred and commercial cattle.
The prevalence of cows that tested positive for MAP antibodies in the blood (serology test) was still low at 1.3 percent and only 0.9 percent tested positive on the fecal PCR test (pooled testing confirmed by individual testing).
There was a significantly higher prevalence of test positive animals in Eastern Canada (2.1 percent) when compared to Western Canada (0.5 percent) based on the pooled fecal PCR testing.
The higher prevalence of Johne’s disease in Eastern Canada was hypothesized to be because of the higher prevalence of MAP in dairy cows and the greater concentration of the dairy industry in eastern provinces.
Only 20 samples were tested per herd and so the researchers couldn’t determine if herds were positive for Johne’s disease or not based on this sample. However, using the pooled fecal PCR test, it was estimated that about 5.6 percent of the cow-calf herds tested had at least one cow testing positive for MAP. The prevalence of positive herds was eight percent in Eastern Canada and five percent in Western Canada.
The other important finding was to evaluate the performance of the two diagnostic tests that are commonly used to test for MAP infections in cows.
The researchers showed the ELISA blood test had the poorest performance, with a sensitivity of 36 percent. In other words, this test would only correctly identify 36 percent of the mature cows infected with the MAP bacteria as test positive.
The pooled fecal PCR (pooling five cows together into one fecal sample and testing that) was the next most sensitive test with a substantial improvement to 54 percent sensitivity.
Finally, the individual fecal PCR had the highest level of sensitivity and would detect the highest proportion of cows infected with MAP. It should be noted that there are cost considerations in these test choices as the ELISA blood test and the pooled fecal PCR are sometimes used because they are less expensive compared to individual fecal PCR testing.
The study showed that Johne’s disease continues to be prevalent in our cow-calf herds, although at a lower level than we have seen in Canadian dairy herds.
It also demonstrated that fecal PCR testing is more accurate for identifying cases of Johne’s disease in beef cattle within infected herds.
The data from this study has been incorporated into an online Johne’s testing decision tool, which is available through the Beef Cattle Research Council website at www.beefresearch.ca/tools/johnes-disease-calculator/.
John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.
