Only a small percentage of cattle test positive for the bacteria that causes Johne’s disease, but almost half of herds surveyed in Canada and the United States include infected animals.
Most of those carrying the disease will never develop the clinical symptoms, but could pass the disease to offspring and others in the herd, said Robert Tremblay, in infectious cattle disease specialist with the drug company Boehringer-Ingelheim Vetmedica.
Johne’s disease is a chronic infectious wasting disease most commonly seen in dairy cattle and associated with diarrhea, and weight and productivity loss.
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It was first identified more than a century ago and has seen steady growth.
A 1999 survey of 53 Alberta dairy farms found that the number of cows likely to be infected with the mycobacterium avium subspecies paratuberculosis, or MAP, a bacteria that causes Johne’s, was between 7.3 and 14.2 percent. The range was caused by the sensitivity of the test and depended on how far evolved the disease was in the animal.
Fifty-eight percent of the tested herds were likely to contain infected cows.
In 1999 and 2000, Alberta researchers found 2.1 percent of 3,632 beef cows tested positive.
Johne’s infection rates are similar in the United States, where surveys have been conducted since the early 1980s. A 1996 survey there found 3.4 percent of dairy cows with Johne’s, with slightly higher rates among cows in larger herds. The survey estimated 21.6 percent of herds have an infection rate of 10 percent or more.
MAP can remain in soil, manure or water for almost a year because of resistance to heat, cold and drying.
The infection can spread through manure-contaminated feed and water. Prenatal infection is also possible and milk and colostrum can transmit the disease from dam to calf.
An animal can carry the bacteria for 10 years before showing clinical symptoms.
“So it means we could leave animals in herds that could spread it,” Tremblay said.
He said better diagnostic tools and more reliable tests are needed to help veterinarians detect it earlier.
The consolidation of the dairy industry will mean a rise in infection rates, said Michael Collins of the University of Wisconsin. He estimated that half of the world’s dairy herds are infected with MAP.
“It is an epidemic,” he said.
There is growing concern about the role played by MAP in Crohn’s disease in humans and the risk that food produced from MAP-infected animals poses to people susceptible to developing Crohn’s. Crohn’s, like Johne’s, is an inflammatory bowel disease.
The incidence of Crohn’s is steadily rising, said Collins, citing a number of possible environmental risk factors.
Milk is one possible source. It could also include well water contamination. Collins cited a high incidence of Crohn’s in Minnesota where children were known to swim regularly in rivers with high coliform levels.
Collins said moving cattle between herds and countries can increase the risk, but feeding waste milk to calves also spreads the disease, with the young stock most susceptible.
“Farmers have to rescue themselves from this disease,” he said.
More research is needed to prove to farmers “the productivity-limiting aspects of this disease.” Affordable lab tests are also a must.
Collins said education is the first hurdle in eradicating the disease.
Biosecurity is the next step.
He said farmers should test the herds from which they are buying cattle, rather than just the purchased animals.
More practical control measures can include removing calves from cows within 30 minutes of birth, keeping calving pens clean, feeding colostrum only from one cow to one calf and not pooling colostrum.
Feed only milk replacer, never feed waste or transition milk, and isolate heifers from the adult herd and its manure for at least six months.
A more aggressive measure might be to stop raising heifers and use contract suppliers with a well-designed program for Johne’s control.
Still more aggressive is the implementation of annual Johne’s ELISA tests on the entire adult herd and culling infected animals.
His accelerated plan includes more ELISA testing, fecal cultures and gamma interferon, and culling all positive cows and the last daughter born to positive cows.
The most aggressive plan would include doing those three tests twice a year on all animals older than six months, followed by culling. He talked about a herd he is familiar with that eliminated Johne’s in one year using this last plan.
He said test-and-cull is generally the only option for beef cow-calf herds.
Collins said there is no cure for Johne’s and effective vaccinations are years away.
Regular testing could lead to a herd certification program that might result in premiums and marketing benefits for those producers involved, he said.
