Johne’s Disease
By Allen Roussel, DVM MS, professor and associate department head,
Large Animal Clinical Sciences Texas Veterinary Medicine Center

Johne’s disease is a chronic wasting disease of cattle that causes weight loss, diarrhea and eventually death.

It is caused by a bacterium, Mycobacterium avium subspecies paratuberculosis, which is a close relative to the organisms that cause tuberculosis in people, birds and cattle.
While a 1997 survey showed 70 percent of beef cattle producers had never heard of Johne’s disease, it is not new Ñ it was first described in 1895.

There are several reasons it is relatively unknown. First, until recently, it was relatively difficult to diagnose in a live animal. Second, it closely resembles gastrointestinal parasitism.

Prior to the development of the excellent dewormers available today, many cases of Johne’s disease were written off as “a bad case of worms”.
Third, the prevalence of the disease in beef cattle nationwide is low compared to its prevalence in dairy cattle.

It’s estimated 50 percent of all dairies and 10 percent of all beef operations in the United States have one or more infected cows. However, it seems the prevalence in beef cattle is not uniform throughout all geographic regions or segments of the beef cattle industry. Prevalence seems to be higher in purebred cattle than in commercial cattle.

In a survey in 2000 in Texas, we found three percent of purebred cattle had a positive blood test and about 45 percent of herds had at least one blood test-positive cow.
However, the test is not perfect and not all blood test-positive cows are infected.

The disease
Johne’s disease is unlike any other cattle disease. While it is caused by a bacterium and is contagious, it seldom spreads from a mature animal to another mature animal. It spreads from a mature animal to a calf.

Infected cows pass large numbers of Johne’s bacteria in their manure. Transmission most commonly occurs when calves ingest manure-contaminated feed or water.

The younger the calf, the more susceptible it is to infection.

The organism is also shed in colostrum and milk. It can even infect a fetus in the uterus.
This transmission from an infected cow to its own calf by colostrum, milk or in the uterus is probably an important means of disease spread in beef cattle.

Although calves are typically infected at an early age, signs of the disease seldom show up until the animal is at least two years old.

During the latent period of three to five or more years, the cattle look perfectly normal. Then they begin to lose weight and have diarrhea. Even after signs occur, affected cattle maintain a good appetite.

Attack method
The bacteria do not produce toxins or kill cells. Instead, they stimulate the cow’s own immune system to attack. White blood cells crowd into the gut to kill the bacteria. They fail in this task and squeeze out the gut’s cells, decreasing digestion and the absorption of nutrients. Diseased cattle literally starve to death while eating all they can.

There is no treatment. There is a vaccine available in some states, but its use is regulated because it interferes with the tuberculosis test.

Testing for Johne’s
Another feature of Johne’s disease is the bacterium takes two to four months to grow in the laboratory.

While culturing the manure is the best way to positively identify an infected cow, the test only detects about half of all infected cattle.

Several blood tests are available which are relatively accurate when used on cattle with clinical signs. But when used on cattle without clinical signs, blood tests detect less than half of infected cattle.

The older the animal the more likely the blood test is to find the infection. It is very difficult to detect an infected animal, by any test, before it is two years of age. This is why the national effort is aimed at identifying low-risk herds from which producers may purchase cattle without a test.

The economic impact
The economic impact of Johne’s disease in beef cattle is undetermined. In heavily infected dairy herds, the cost is estimated to be as high as $227 per year per cow inventory (not per infected cow). Most of that loss is due to lost milk production.

We assume infected beef cattle also produce less milk and wean lighter calves. Salvage valve is reduced because of weight loss. Replacement costs are increased due to premature culling.

For seedstock producers, losses mount due to premature death, decreased productivity of genetically superior cattle, lost export sales, and potentially, from litigation.

The link between M. avium paratuberculosis and Crohn’s disease, a similar disease of people, is being investigated. So far, there is no proof of a link, but there is enough evidence to cause concern. If a firm link is established, Johne’s disease will take on a new significance.

On the horizon
For five years, USDA has appropriated several million dollars annually specifically for Johne’s disease programs.

More than half of these funds have been directed to producers for risk assessments, testing and other control measures.

Participation is voluntary and testing results are confidential. The level of federal funding for each state is predicated on the level of Johne’s disease control activity within the state. Most states have Johne’s Disease Committees consisting of producers, veterinarians and state animal health officials. The committees oversee the Johne’s Disease Control Program within the state.

For more information about Johne’s disease, consult your veterinarian, Texas Cooperative Extension Service, your state Johne’s Disease coordinator - Dr. Mike Pruitt, mike.pruitt@oda.state.ok.us, 405/522-8396 and Dr. Andy Schwart, andys@tahc.state.tx.us, 800/687-4603 - or visit www.johnes.org.