Johne's disease is on the increase. Research shows that one out of 10 animals moving through livestock auction facilities has Johne's disease, with infection rates at one out of about four dairy herds.
The single most significant hazard of maintenance and spread of infection are the subclinical animals—those that have the bacterium but have yet to exhibit clinical signs, say Bob Whitlock, a University of Pennsylvania veterinarian.
"The most prevalent transmission method within herds is fecal-oral," adds Scott Wells, a University of Minnesota veterinarian. "Two other common transmission methods are via colostrum and milk to calves and transplacental.
"If there's fecal material around anywhere, the opportunity for the organism's presence is there—and it's a risk," he says.
Researchers have found that it takes only a tiny bit of contaminated feces to infect herdmates and newborn and young calves. In fact, a calf can become infected from simply sucking on a contaminated teat.
Management practices are the preferred method of control for two key reasons: Treatment has not been shown to be effective, and vaccinating to protect animals against Johne's is not a viable option at this point.
To control Johne's:
- Remove clinically ill cows.
- Calving cows in a clean area.
- Raise replacement heifers separately from older cows and only add low-risk herd replacements—perhaps even limiting herd additions to animals from a test-negative herd.
- Feed calves either milk replacer or pasteurized milk.
Such practices can reduce prevalence of Johne's by as much as 50%, says Wells. He urges producers to learn more about Johne's by talking to their veterinarians and to become involved in the Voluntary Bovine Johne's Disease Control Program in their state.