Introduction

Johne's Disease is a chronic wasting disease of ruminants caused by the bacterium Mycobacterium avium subspecies paratuberculosis. This bacterium is closely related to the bacterium that causes tuberculosis in cattle and humans. The disease was named for Dr. H.A. Johne, a German veterinarian, who first described this disease in 1895. It is also known as paratuberculosis.

Johne's Disease has historically been thought a "dairy" disease, but many beef herds have Johne's disease. Johne's is a slowly progressive disease and impacts dairy and beef producers through excessive culling, death losses, sub-optimal milk production, and decreased fertility. It is a possible cause for litigation if knowingly infected breeding stock are sold as non-infected animals.

The real danger of Johne's Disease is called the "iceberg" effect. For every clinical case of Johne's in a herd, there can be 15 to 25 animals subclincally infected. The "iceberg" of Johne's steals profits through reduced production, increased secondary diseases, culled animals, and increased feed costs.

Clinical Signs

The bacteria attack the lining of the intestine and causes thickening of the intestinal wall. This causes two things to occur: (1) nutrients can't be absorbed through the intestinal wall, so the animal starts to lose weight and get thin, and (2) because nutrients and fluids can't be absorbed, the animal develops diarrhea.

Cattle affected with Johne's usually exhibit clinical signs between 2 and 5 years of age, but animals can be older (10 to 15 years of age) or younger (as young as 10 months). Cows and bulls can be affected. Affected animals appear unthrifty, thin and weak. They usually have a good appetite and no fever.

The disease does not respond to antibiotics. No medication or dietary change is effective.

Transmission

Johne's Disease is usually transmitted when the animal is very young, probably within the first 24 hours of life. Johne's can be transmitted from the mother to the calf before the calf is born. Young calves can ingest the organism from contaminated colostrum, contaminated milk, sucking on contaminated udders, or sucking on any other contaminated surface. Specialized cells in the wall of the intestine take up the bacteria. Normally, an invading bacterium would be killed, priming the immune system to strengthen itself against futere invasion. However, some of the organisms that cause Johne's Disease are able to survive this process. As time passes, more and more specialized cells (white blood cells) are recruited to try to kill the bacteria, causing the intestinal wall to thicken with infected cells and disease-fighting cells.

Animals affected with Johne's Disease shed the organism in the manure; the number of organisms increases over time. The bacteria are in the semen and feces of infected bulls. In general, artificial insemination is not believed to pose a large risk for the spread of Johne's. Additionally, since most large AI centers routinely test bulls for Johne's Disease, use of semen from these centers should not present a risk for introduction of the disease.

The Johne's organism can live for up to 12 months on the ground.

Clinical Course

Animals infected early in their lives initially show no sign of Johne's Disease. The time from initial infection to onset of clinical signs (diarrhea and weight loss) is generally two to five years. After this incubation period, infected animals may begin shedding the Johne's bacteria in manure at very low levels. In most cases, the number of bacteria shed increases with increasing age of the animal.

At some time, the animal may develop clinical Johne's Disease. Because Johne's Disease can take so long to manifest itself, producers may not realize that the herd is infected until years down the road. Before animals reach the point of chronic diarrhea and weight loss, they may have already been culled for other reasons, including mastitis, poor production or reproductive failure. Hence the "iceberg" effect.

Diagnosis

The most definitive way to diagnose Johne's is by culturing feces for the bacteria. The Johne's organism is very slow-growing, taking up to 16 weeks of incubation in the laboratory before a final result can be given. A negative test (no growth of the organism) does not necessarily mean the animal is not infected. Even if infected with M. avium paratuberculosis, young animals often do not shed the organism. Often, animals will not shed in detectable numbers until just a few months before they develop diarrhea.

A blood test called the Enzyme Linked ImmunoSorbant Assay (ELISA) can be used to diagnose Johne's Disease. It is inexpensive and results are available in a few days. Care must be taken in interpreting ELISA results, however. If an ELISA test is positive, the animal must be evaluated in light of other information, such as presence or absence of clinical signs and a follow-up fecal culture. Interpretation of any laboratory result should be discussed with the herd's veterinarian.

Prevention

The best way to avoid introducing this disease into a herd is to be as certain as possible that animals brought into the herd are not infected. Many states have voluntary Johne's programs that establish the Johne's status of the herds. Ask your prospective seller if he or she participates in such a program and what the herds status is.

Pre-purchase screening of animals may be of some benefit. However, it is important to understand the limitation of the tests. An ELISA test on one animal in a herd may not indicate the presence or absence of Johne's Disease in the hert. It is better to test the entire herd and buy animals only from herds in which all animals test negative. Involve your veterinarian in any discussion of laboratory results.

Control

If Johne's is present within a herd, control requires a long-term commitment. The most effective method is a combination of identifying infected animals through systematic testing and management changes. This two-pronged attack centers on removing infected animals to decrease the amount of shedding in the herd and protecting the youngest, most susceptible animals. The voluntary Johne's programs in most states are designed to assist producers in controlling Johne's and have a wealth of expertise to help.

Johne's control programs will vary from herd to herd. Involve your veterinarian from the ground floor in your control program. Herd size, geographic location, type of enterprise, number of infected animals and herd management need to be taken into account. No one program will work for all operations, but there are some control measures that will be universal:

• Cull clinical Johne's cows immediately

• Follow the Johne's control program outlined by your state or veterinarian

• Maintain clean calving areas

• Spread manure on non-pasture land

• Limit access to low-lying wet areas

• Do not contaminate feed for young stock with manure.

Conclusion

Control of Johne's Disease can be attained only by halting the transmission of Mycobacterium avium paratuberculosis within the herd. Since there is no effective treatment for cows already infected, control programs must identify infected cows early so they can be removed from the herd.

In the dairy industry, removal of the newborn calf to a Johne's free environment and feeding non-infected colostrum and milk are key parts of Johne's control programs. The goal is to produce a Johne's free calf. This assumes that no intrauterine transmission has occurred. In general, in a cow that exhibits clinical Johne's disease, about 50% of the time, the last calf she had was infected while in the uterus. These calves should be culled, not sold to propagate the disease on other premises.

In the beef industry, Johne's control relies primarily on testing to identify infected cows. Cull culture-positive cows and maintain clean calving/nurse pasture areas. Prompt removal of calves from dams is not a viable option for most producers, but in the case of an extremely valuable individual it may be a wise endeavor. Again, this assumes that no intrauterine transmission has occurred.

For a Johne's control program to be successful, all farm personnel, veterinarians and others intimately involved in an operation must fully cooperate.

Links

Johne's Information Center, University of Wisconsin

Johne's Disease, USDA-APHIS

Johne's Disease, Iowa State University

Johne's Pictures, University of Wisconsin

Approved Johne's Disease culture laboratories, APHIS

Approved Johne's Disease serology laboratories, APHIS

Johne's Disease Veterinary Outreach, Pennsylvanian State University

International Association for Paratuberculosis