Animal Industry Division,
Alberta Agriculture Food & Rural Development,
#307B, 6909 - 116 Street, Edmonton,
AB, T6H 4P2 Canada
E-mail: ollis@agric.gov.ab.ca
Take Home Message
Johne's disease is a progressive, debilitating disease of all ruminant animals for which there is no treatment. It is caused by Mycobacterium paratuberculosis, which has been implicated as a cause of Crohn's disease in humans. Like all infectious microorganisms, the rate of spread of M. paratuberculosis is directly related to the number of infected animals in a herd. Johne's disease is a disease of adult animals that was contracted in the first few months of life. This disease can be controlled, but it requires commitment and persistence over a period of time.
Why Discuss Johne's Disease?
Johne's disease is insidious. Only one of 20 to 25 infected animals will show clinical signs of the disease. Therefore, infected animals may exist in a herd without the owner being aware of it. A recent study in the United States found that 44% of US milk producers had never heard of this disease. This is in spite of the fact that an estimated 30% of US dairy operations contain infected animals (S.J. Wells, personal communication). Unless producers are aware of the existence of this condition, they are unlikely to implement measures to reduce the spread of the infection.
This is a disease of adult cattle in which the infection was acquired during the first few months of life (1). The time between an animal becoming infected and when it shows signs of disease is very long, 6 months to 15 years (2). These facts have important implications on the choice of control measures employed in a dairy herd.
There is growing evidence that the financial losses associated with this disease can be significant, even in animals that are not showing clinical signs of disease. The losses associated with clinically affected animals are substantial, but these animals are only indicators of a potentially much larger problem.
Many in the livestock industry believe that Johne's disease cannot be controlled and, therefore, are not prepared to implement measures to prevent it's spread. This attitude is likely responsible in part for the increasing prevalence of the disease. There is an acute awareness of Johne's disease in other parts of the world and the potential impact on trade of Canada's dairy, meat, and other animal products cannot be ignored.
Finally, there is concern regarding the potential association between Mycobacterium paratuberculosis, the cause of Johne's disease, and Crohn's disease in humans. The cause of Crohn's disease is still unknown, but if a link between this human condition and M. paratuberculosis is proven, the consequences on ruminant livestock operations could be dramatic.
What is Johne's Disease?
Johne's disease, or paratuberculosis, is a chronic, progressive, and debilitating disease of the intestinal tract of domestic, exotic, and wild ruminant animals (2,3). Most cattle developing clinical disease are at least two to three years of age. Clinically affected cattle have a chronic, non-responsive diarrhea which leads to muscle wastage and severe loss of body condition. There is no cure for Johne's disease and all clinically affected animals eventually die.
Johne's disease is recognized world wide. It was first recognized in North America in 1908. In 1927, there was only one known infected herd in Ontario (4). In recent years, the prevalence of Johne's disease is increasing. This could reflect an actual increase in the disease or the improved recognition of the condition (5). Likely both factors are involved.
About the Organism
Johne's disease is caused by Mycobacterium paratuberculosis, which is related to the bacteria that cause tuberculosis and leprosy. In fact, Johne's disease and avian tuberculosis are now thought to be caused by variants of the same bacterium (4, 7). Mycobacterium paratuberculosis is not easy to grow in the laboratory because it has special culture requirements (1). It is very slow growing (8) and cultures must be held up to 16 weeks before being declared negative (2, 9, 10).
Because of it's composition, Mycobacterium paratuberculosis is resistant to many disinfectants, as well as environmental degradation. It may survive at least a year in stagnant water, manure, and damp soil (1,11,12). This organism can also survive freezing at minus 14C for up to a year (2). However, it will only survive up to seven days in urine (2) and it's viability is reduced by the ensiling process (13).
The organism is susceptible to 10 minutes exposure to 5% formalin, 1:32 cresylic disinfectants and 1:40 phenol (2).
Is M. paratuberculosis killed by routine pasteurization? A study in 1993 found that high temperature-short time (HTST) pasteurization killed 95% of bovine isolates, but only 70-80% of human isolates of M. paratuberculosis (14). Interestingly, as the rate of cooling was increased, so was the survival rate of the bacilli. Recent work in Ireland is inconclusive and further studies are ongoing (I.R. Grant, personal communication).
How is Mycobacterium paratuberculosis Spread?
Transmission of M. paratuberculosis to susceptible animals is primarily by oral ingestion of feed and water contaminated by infected feces (1, 2, 5, 11). Fecal shedding of M. paratuberculosis from an animal with clinical disease can be as high as 5 trillion organisms/day. These animals are the primary source of environmental contamination on the farm (15).
Infected runoff can contaminate a property and continually grazed pasture will remain infected indefinitely (1). Acidic conditions in the soil are associated with an increased prevalence of the disease, while alkaline soil caused the disease to be more self-limiting (5, 16).
Mycobacterium paratuberculosis has also been detected in the colostrum and milk of infected cattle (17). Its presence in milk appears to be related to the level of fecal shedding, up to nearly 35% of animals with advanced clinical disease have M. paratuberculosis in their milk (18). In spite of this, some researchers suggest that infected milk is not an important means of infecting young calves (13, 14), presumeably because the number of organisms in milk is relatively low.
Transplacental transmission does occur; 8.6% of cows shedding large numbers of M. paratuberculosis in their feces gave birth to an infected calf (17).
Mycobacterium paratuberculosis has also been isolated from semen, but artificial insemination does not appear to be an important mode of transmission (13).
Cattle of all ages can become infected with M. paratuberculosis, but calves less than four to six months of age are the most susceptible (1). The experimental infective dose for calves is much lower than that of adult cattle (15). The reasons for this may be the higher acidity in the pre-ruminant calf intestine or the normal microflora has not yet become established in the calf's intestine (19).
It is not known why only about a third of exposed calves become chronically infected with M. paratuberculosis. Calves raised in contact with infected adults or kept in unsanitary conditions have a higher risk of becoming infected (20).
Development of the Disease
Mycobacterium paratuberculosis usually multiplies in the lining of the intestine and/or the associated lymphoid tissue. However, it can also multiply within the host's macrophages (2, 3). The infected macrophage can carry the organism to other tissues in the animal's body. Although spreading throughout the body was always considered to be rare (20), recent evidence contradicts this. Mycobacterium paratuberculosis has been isolated from many tissues of slaughtered animals, including the testes and epididymus in bulls (R.H. Whitlock, personal communication).
The animal's initial immune response is cell mediated. Later in the course of the infection, antibodies appear, likely in response to the release of M. paratuberculosis bacilli by dying macrophages. Some researchers suggest that the clinical signs of diarrhea in affected animals may be the result of the animal's own immune response (2). Inflammatory cells responding to the presence of M. paratuberculosis may cause damage to the lining or wall of the intestine, impairing it's function and causing malabsorption and diarrhea. It is interesting that clinical disease is often reduced or disappears during pregnancy when the cow's immune response is suppressed. Clinical disease then appears or recurs after the stress of parturition.
Only about 5% of infected animals will develop clinical signs of disease (11). In other words, those animals that happen to develop Johne's disease represent the 'tip of the iceberg' on a farm. For every animal showing clinical signs of Johne's disease, there may be up to 20 to 25 animals with subclinical infections of M. paratuberculosis (5, 11). These subclinically infected animals represent a major challenge to controlling or eradicating M. paratuberculosis.
Economic Significance of Johne's Disease
The true prevalence and economic losses associated with Johne's disease are unknown. This is because there are no practical diagnostic tests available that reliably detect subclinical infections of M. paratuberculosis in the living animal (1, 2, 3, 21, 22, 23).
In slaughterhouse studies, estimates of the prevalence of infected animals are 1.6 to 18% in the USA (24,25), 4% in New Zealand (26) and 5.5% in Ontario (27). Most of the slaughter animals examined were dairy cattle. In one of the studies, 2.9% of dairy culls and 0.8% of beef culls were infected (24).
Although clinical Johne's disease is not an infrequent diagnosis in Alberta, we have no idea what the prevalence of infected animals might be. Because of this, it is difficult to put an economic value on losses in Alberta associated with this disease.
A study in the Netherlands found that infected animals not showing clinical signs of disease produced 16% less milk in their last lactation compared to two years earlier (28). This translates to approximately $800/year per infected animal in the herd, valuing milk at $55 per hectoliter.
Infected slaughter animals were reported to weigh an average of 81 kg less than uninfected culls (21). Clinically affected animals often weigh substantially less when culled.
At the Fifth International Colloquium on Paratuberculosis in October, 1996, a preliminary study found that days to conception in subclinically culture-positive and ELISA-positive animals was 37 days longer than in those testing negatively (Y.J. Johnson-Ifearulundu, personal communication). Another researcher reported that, although the Linear Score was significantly lower, the higher culling rate and lower milk production in subclinically infected dairy cows resulted in a net financial loss. The decreased milk production was detected in the second lactation (D.J. Wilson, personal communication).
Other countries are beginning to take steps to control Johne's disease. Sweden slaughters all animals that test positively for M. paratuberculosis. A voluntary, industry driven control program is now operating in Australia in an effort to control the spread of M. paratuberculosis in cattle and sheep. This activity will intensify if a link between M. paratuberculosis and Crohn's disease is established.
Detection and Control of Mycobacterium paratuberculosis
The diagnosis of animals with clinical Johne's disease is relatively easy. Adult animals with chronic, non-responsive diarrhea and progressive emaciation, coupled with a normal appetite, are not much of a challenge for an experienced veterinarian or owners of infected herds.
Although Johne's disease has been observed for over 160 years, there is still no unconditional diagnostic test to detect subclinically infected animals (1, 2, 3, 4, 19, 22, 23, 29). Until a definitive test is developed to detect subclinically infected animals, adequate evaluation of any past or proposed control program is difficult (7). A reduction in the number of clinically diseased animals is a crude and inadequate measure of success because the clinical cases represent only about 5% of infected animals.
Factors to consider before embarking on a control program should include the following.
In an infected herd of cattle, at any one time there are potentially four groups of animals. This is important in understanding the limitations of the various diagnostic tests currently available.
The following diagnostic tests have been used in various jurisdictions with variable success.
Rectal or Fecal Smears
Staining fecal smears with an acid-fast stain is fast and cheap and of value when used to confirm animals with clinical signs (M.T. Collins, personal communication). Rectal biopsy or fecal smears only detect 25 to 56% of subclinical infected animals (2, 31).
Fecal Culture
Although a herd fecal culture will only detect 30 to 40% of the infected (and shedding) cattle at any one point in time (9), this is the only test available which has the lowest risk of false positive results. The risk of culling innocent animals is minimal. A major problem with fecal culture is that replacements under 18 months of age seldom shed significant numbers of M. paratuberculosis in their feces. Therefore, this test is restricted to animals over two years of age.
Depending on the prevalence of infection in a herd, up to three consecutive whole herd cultures would detect most of the infected animals (32). Culling of culture-positive animals can be very successful in eliminating clinical cases of Johne's disease as well as fecal shedding (33).
The main disadvantage of fecal culture is the slow growth of M. paratuberculosis. Cultures must be maintained for at least 12 to 16 weeks before being discarded as negative (1,9). Obviously, reporting will be delayed and this test is unsuitable for confirming clinical cases of Johne's disease.
Serology
Serology is useful on a herd basis and has the advantage over fecal culture of much faster turnaround for the results. It cannot be used on individual animals with any confidence except to confirm a clinical case of Johne's disease. Both the ELISA and AGID have a high specificity so there are few false negative results.
ELISA tests are now available and have a sensitivity that varies from about 15 to 87%, depending on the stage of infection the animals are at (M.T. Collins, personal communication). The ELISA is proving extremely useful as a herd screening test because it can be performed in several hours and is relatively cheap.
The problem with serology is that the test is trying to detect antibodies in animals that may not have any. I mentioned earlier that the initial immune response was cellular. Antibodies cannot be detected in experimentally infected calves until they are at least 18 month of age. This is similar to fecal shedding which does not occur consistently until at least 18 months of age.
Gamma Interferon
This is a new laboratory test that has been developed as a measure of the animal's cellular immune response to M. paratuberculosis. It shows promise in being able to detect infected animals before they have developed antibodies or started to shed the organism in the feces (J.R. Stabel, personal communication). It may have particular value in replacements under the age of 18 months of age. One of the technical difficulties with this new test is that the chilled blood samples must be delivered to the laboratory within hours of being taken.
Histology
At postmortem, histological lesions and acid fast organisms can be detected in the ileocecal valve and associated lymphoid tissue (34). Unfortunately, obtaining this type of specimen from live animals requires surgery and can only be justified in very valuable animals. Recently a study was conducted on slaughter specimens and culture was found to be superior to histological examination for detecting infected animals (R.T. Whitlock, personal communication).
DNA Probe
Several reports at the Fifth International Colloquium on Paratuberculosis in October, 1996 indicated that the sensitivity of these probes is still less than fecal culture. However, work is ongoing to combine the use of DNA probes with culture techniques in the hope of reducing the amount of time required to detect a positive culture.
General Control Recommendations
An epidemiological model of Johne's disease indicated that minimizing contact between infected cows and young calves was the most important factor in reducing the rate of spread of M. paratuberculosis (35). As a result, one of the goals of a control program for Johne's disease should be to protect young calves from exposure to the organism. It is in this light that the following points will be discussed.
Culling Infected Animals
Cattle affected with clinical disease, diarrhea, and wasting, should be culled as quickly as possible. These animals can shed as many as 5 trillion organisms into the environment every day (36) and are a major source of infection to other animals. To put this into perspective, a single gram of feces from these animals contains enough M. paratuberculosis bacteria to potentially infect up to 10,000 neonates (37).
It is controversial as to whether the offspring of infected animals should be culled as well from the herd. For years, this recommendation has been based on the perceived higher risk of these calves being infected, transplacentally, via the dam's colostrum and/or milk or exposure to fecal contamination (1). However, recent work indicates that only calves from 'heavy fecal shedders' should be culled as these are the calves most likely to be at risk from the above (38).
Environmental Cleanup
Because M. paratuberculosis is relatively resistent to environmental degradation, a buildup of infection pressure is quite likely to occur. Corrals and sheds should be cleaned frequently and well drained. Manure should be spread on crop land, avoiding hay fields and pastures to expose the organism to dry conditions and sunlight (13). Raising the pH of the environment by the use of lime has been suggested and may be practical in barns and sheds (16).
Drainage from corrals and sheds should not contaminate pasture, hay fields, or sources of drinking water. All sloughs should be either drained and filled in or fenced off to prevent access by cattle. Permanent pastures should be avoided.
Drinking water should be piped to well-maintained waterers which prevent fecal contamination. Dry cows and replacements should not be housed together.
Segregate Calves
The maternity area should be completely separate from the lactating and dry herds, as well as the calf raising area. It should be clean, dry, and well-bedded and used for one cow at a time. A thorough cleaning between cows with the liberal use of hydrated lime is advisable.
Remove the calf from it's dam as soon as possible and put it into a clean, dry, individual pen. If possible, prevent suckling of the dam. Provide colostrum from animals that are culture-negative at least twice for M. paratuberculosis. If herd testing hasn't been done, consider pasteurizing the colostrum, keeping in mind that denaturation of up to 20% of the immunoglobulins is likely (M. Meylan, personal communication). Remember to wash and dry the udder.
All replacements should be raised completely separate from adult animals until 24 months of age. Calf hutches are particularly good for this purpose. Calf chores should be done before those with older cattle or clean boots and coveralls worn when working in the calf area. If possible, avoid feeding milk and provide a high quality milk replacer.
Herd Additions
As the number of purchased replacements increases, the risk of adding infected animals to the herd increases (35). Maintaining a closed herd policy is the best way to avoid purchasing infected animals. If purchasing of outside animals is necessary, investigate the Johne's status of the source herd. Recommendations from the University of Wisconsin now include buying only from herds with more than one negative herd test, either fecal culture or ELISA serology (M.T. Collins, personal communication). The second best criteria is a herd which has had one negative herd test, either fecal culture or ELISA serology.
Summary
Johne's disease has been recognized as a disease entity for over 160 years. It is only relatively recently that the consequences and potential ramifications of this condition are beginning to be appreciated. Research has produced and improved new methods of detecting Mycobacterium paratuberculosis and identifying infected animals. These advances have facilitated the development of successful control programs. However, eradication of Johne's disease may still be an elusive goal. The commitment of the livestock industry, including individual producers and veterinarians, will be necessary to minimize the further spread of Mycobacterium paratuberculosis.
References
Pemberton, D.H. 1979. Diagnosis of Johne's disease in cattle using mesenteric lymph node biopsy: accuracy in clinical suspects. Aust Vet J 55: 217-219.