Johne's (YO-neez) disease is a chronic wasting disease of ruminants caused by the bacterium Mycobacterium avium subspecies paratuberculosis. This bacterium is closely related to the organism 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 or MAP.
Johne's disease has historically been considered a dairy disease, but many beef and goat herds and sheep flocks have Johne's disease; it can infect any ruminant. Johne's is a slowly progressive and invariably fatal disease. It affects livestock producers through excessive culling, death losses, increased feed costs, 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 due to its "iceberg" effect. For every clinical case of Johne's in a herd, there can be 15 to 25 animals subclinically infected. The "iceberg" of Johne's steals profits through reduced production, increased secondary diseases, culled animals, and increased feed costs.
Johne's disease is usually transmitted when the animal is very young, and 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, contaminated udders, or 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 future 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 can shed the organism in their manure, milk and semen; the number of organisms increases over time. Infection through contaminated semen is not believed to be an issue due to the age of the animal at breeding and route of infection (genital vs. intestinal).
Johne’s is considered a disease of confinement. Housing and management systems that increase the likelihood of animals being in contact with manure also increase their likelihood of contracting Johne’s disease. Beef cattle on rangeland have extensive areas available to them for grazing, therefore are at low risk of contacting manure and ingesting the Johne’s organism. Dairy cattle in confined freestall housing generally have loose manure that can splatter and contaminate water troughs and feedbunks. Cattle, sheep and goats on drylots are also at increased risk. The Johne's organism can live for up to 12 months on the ground.
As mentioned above, Johne’s bacteria attack the lining of the intestine which eventually results in thickening of the intestinal wall. Consequently, nutrients can't be absorbed through the intestinal wall and the animal loses weight. Because nutrients and fluids can't be absorbed, the animal develops diarrhea, which also contributes to weight loss.
Animals affected with Johne's disease usually exhibit clinical signs between two and five years of age, but they can be older (10 to 15 years of age) or younger (as young as 10 months). Males and females can be affected. Affected animals appear unthrifty, thin and weak. They usually have a good appetite and no fever. Profuse diarrhea is a hallmark of the disease in cattle, but much less pronounced in sheep and goats. Despite months to years of infection, signs often appear suddenly after a major stress or change such as giving birth or being transported.
The disease does not respond to antibiotics. No medication or dietary change is effective.
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 their herd is infected until many years have gone by; by this time, the disease has spread to many more animals. 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.
Many infected animals are negative to all existing tests early in the course of the disease. After the animal begins mounting a specific immune response to the bacteria or shedding bacteria in feces, a diagnosis via laboratory testing is possible.
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 determination can be made. 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 diagnose Johne's disease. It is inexpensive and results are available in a few days. Care must be taken in interpreting ELISA results. There can be false positives. Thus, 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. Additionally, information on an individual animal is less valuable than information on the herd of origin. How many other animals were tested and what were their results?
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 and certify herd Johne's status. Ask your prospective seller if he or she participates in such a program and what the herd’s 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 herd. It is better to test the entire herd (or a significant proportion of the herd) and buy animals only from herds in which all animals test negative. Involve your veterinarian in any discussion of laboratory results.
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 available in most states are designed to assist producers in controlling Johne's and have a wealth of expertise to help.
A Johne’s disease vaccine is available in the U.S. It is approved for use in calves less than 30 days old. Through proper use it can decrease the number of cows that develop Johne's-related diarrhea and weight loss, but it will not alter infection rates. The control recommendations listed below are generally more effective and widely used than the vaccine. Vaccination is a rare recommendation except in some instances of very high rates of infection on individual farms.
Johne's control programs will vary from herd to herd. Involve a Johne's certified veterinarian in assessing your situation and designing 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:
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 animals already infected, control programs must identify infected animals early so they can be removed.
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 about 50% of cases, if a cow exhibits clinical Johne's disease, her last calf became infected while in the uterus. To help stop the perpetuation of Johne’s disease, these calves should be culled along with their clinically-ill mothers.
In the beef industry, Johne's control relies primarily on testing to identify infected cows. Cull 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.
On sheep and goat farms, emphasis should be placed on not inadvertently importing the disease through infected dairy cow colostrum or milk or grazing on contaminated pastures. For producers who cannot maintain a closed herd, herd additions should only come from test-negative farms.
For a Johne's control program to be successful, all farm personnel, veterinarians and others intimately involved in an operation must fully cooperate.
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