Johne’s disease or paratuberculosis, is a chronic wasting disease primarily in ruminants. The term Johne’s disease refers to the clinical disease condition of chronic weight loss and diarrhea. This disease is caused by an acid-fast, gram positive, intracellular bacterium, Mycobacterium avium subspecies paratuberculosis (MAP). It is estimated that 22% of U.S. dairy herds and 8% of U.S. beef cow-calf herds are infected with MAP. Young animals seem to be more susceptible to this disease than older animals, but a high dose of organisms could also infect older animals.

Pathophysiology

Calves that are born from infected dams are more likely to become infected especially if the neonate suckles on the dam for colostrum. The infected dam will excrete organisms through the milk to pass onto the offspring, or teats that the calf will be nursing off of will be covered with manure with organisms present. If calves are born in a contaminated birthing facility, those neonate calves can also consume feces-contaminated soil. Infections can also occur in utero when the pregnant animals are in a latter stage of the disease where the bacteria can disseminate to infect internal organs. Following ingestion of Mycobacterium avium subspecies paratuberculosis during the perinatal period, the bacilli will penetrate the gastrointestinal mucosa and be taken up by M cells of the small intestine and macrophages in the lymphoid tissue (Peyer’s patches in the terminal ileum). The bacteria will multiply intracellularly, which will cause a diffuse granulomatous tissue reaction. The bacteria will then spread gradually through the lymphatics to the regional lymph nodes. This will eventually turn into a systemic infection spreading to the liver, mammary gland, uterus, pulmonary lymph nodes, and peripheral lymph nodes. The clinical disease does not usually occur in animals younger than 19 months due to long incubation period.

Pathology

There are specific lesions for Johne’s disease. This disease produces a granulomatous enteritis. The gross lesions include chronic, segmental thickening of the ileum, cecum, and proximal colon. Affected areas will have variable thickened, rough, rugose mucosa. There can also be multiple foci of mucosal ulceration and mesenteric lymphadenopathy. On histopathology, there will be an accumulation of foamy macrophages with a large number of acid-fast organisms within the lamina propria of the intestines, especially the ileum. The ileum will look thickened on histopathology, due to the infiltration of macrophages and lymphocytes.

Clinical Signs

There are four different stages of clinical disease, which all have different clinical signs. Infected animals can advance to different stages slowly or progressively, but it depends on the age when the animal was infected, dose of organisms, and the immune response of the host.

Diagnostic Testing

Johne’s disease if very difficult to diagnose since there is a very long incubation period. Signalment with clinical signs can help in making a diagnosis. There are numerous tests that are used in attempt to diagnose Johne’s disease, but certain tests are considered the “gold standard” when diagnosing.

Fecal culture is considered the “gold standard” test for identifying individual infected animals with M. avium subspecies paratuberculosis. The disadvantages of this test are the higher cost than ELISA test and a long incubation period of 12-16 weeks. A liquid culture has been developed to help reduce the incubation period to 35 days or less. This test can detect more infected animals than using the ELISA test, and can detect mycobacterial organisms 1 year or more before clinical signs develop. Farms that have a heavy burden of mycobacterial organisms might have more false positive fecal cultures. This is due to mycobacterial organisms that are passed in the manure can contaminate the feed cattle are eating. When non-infected cattle eat the contaminated feed, the organisms will be passed within their manure, which will give a positive fecal culture. Farms can perform a pooled fecal sample since there will be a higher percentage of animals represented and will equal out the cost for the test.

The Enzyme-Linked Immunosorbent Assay (ELISA) detects and quantitates the animal’s serum antibody response to an antigen derived from Mycobacterium avium subspecies paratuberculosis. Animals that show a high antibody titer are more likely to be infected with MAP. ELISA is a good screening test for those animals that are more at risk of becoming infected. This test has a 2-4 day rapid turnaround time, is low cost, and a large number of samples can be taken and processed. Cattle should not be culled if positive on ELISA test since the specificity is not 100%. There can be cows that have a false positive ELISA, so this test should not be used for a definitive diagnosis. A negative test result does not indicate the cow being tested does not have MAP. ELISA can be used as a herd screening tool to determine if MAP is within the herd, even though it is less likely to detect the lightly infected cattle. If one cow has a positive ELISA and confirmed positive on fecal culture, that certain cow being tested is infected with MAP. Those kinds of results highly suggest there are other infected cattle on the farm.

There is a milk ELISA test that diagnoses antibodies to MAP, which the specificity and sensitivity is similar to the serum ELISA. Cows in early and late lactation have a higher amount of antibodies compared to cows in midlactation. Producers can perform “target testing” on cows at dry off period to determine if that certain cow is more likely to be infected with Johne’s disease since more antibodies likely to be present. This test can help save labor costs since cows on Dairy Herd Improvement Association (DHIA) testing each month will be taking milk samples in the parlor, and milk samples can also be taken for Johne’s detection. Cows will not have to be restrained at a different time for blood collection to be performed. Dairy producers can also use bulk tank milk to quantify the levels of MAP within the herd. This could also help identify positive herds for Johne’s disease, decrease in testing costs, and market Johne’s free milk.

The Agar Gel Immunodiffusion (AGID) or rapid Johne’s test (RJT) can be beneficial only if the individual animal being tested is showing diarrhea and weight loss. This test will detect antibodies to Mycobacterium avium subspecies paratuberculosis. AGID has a high specificity, but low sensitivity. The difficulty about this test is the failure to detect animals that are not showing clinical signs of the disease, but those animals will be positive on fecal culture. A negative test result does not indicate the cow being tested does not have MAP.

Polymerase chain reaction (PCR) test will identify specific DNA segments (IS900 or HSx protein) in M. avium subspecies paratuberculosis. Real-time PCR (qRT-PCR) will detect a low concentration of organisms in fecal samples. This kind of PCR will quantify the mycobacterial concentration in the fecal sample. This test is very beneficial due to the 100% specificity and very high sensitivity. A composite of environmental manure (up to 6 samples from different areas) from the whole herd can be tested with PCR to detect herd infection. A negative test result with a negative history of the disease increases the confidence that the animals on the farm are at a low risk of becoming infected with MAP.

Histopathology of the gastrointestinal tract and adjacent lymph nodes should only be performed on cattle showing clinical signs. Examining tissues from cattle that have subclinical Johne’s disease may not reveal any acid-fast organisms within the tissue. If organisms are not visualized, this does not indicate the cow is not positive for the disease. Other tissues will need to be examined if organisms are not identified. Visualizing acid-fast organisms within macrophages on an intestinal biopsy sample is positive for Johne’s disease. PCR test on a fecal sample is a better diagnostic test (faster and more sensitive) than performing a histopathologic examination on a tissue sample.

Direct DNA probe identifies organisms in feces with IS900 genes. The advantage to this test is the short turn-around time. It only takes three days for the results. The disadvantages include low sensitivity, high cost, and an experienced technician needs to perform the test on special equipment.

There are diagnostic tests that might become beneficial in the future. One example is the cellular immunity test, which is an ELISA that measures IFN-gamma. IFN-gamma is released by T-lymphocytes when specific mitogens are present. Recent studies suggest this test might be more sensitive than other currently available tests.

Treatment

Sadly there are no treatments that are effective for Johne’s disease. Numerous pharmaceuticals (eg. Clofazimine and isoniazid) have been used, but clinical signs only improved and did not treat the disease. Vaccines have been approved in several states in the United States, but the vaccine is limited to infected herd. The vaccine has to be approved by the state veterinarian and a protocol for identification of the animals has to be performed.

Prevention

This disease can have severe devastating economic effects on a farm due to this disease can spread throughout the herd, cause debilitation of animals, decrease milk production, decrease fat and protein yield, increase susceptibility to other diseases, loss of genetic potential, loss of export market, increase medical costs, reduce weight at slaughter, premature culling, poor feed conversion, and increased calving interval. This disease produces a risk of infecting non-infected herds through buying infected but apparently normal calves or cattle at a sale. The best way is to prevent this disease from occurring by having a strict neonatal calf protocol, keeping equipment separate, annual herd testing, isolating infected cattle, and culling infected cattle. Baby calves should be born in a disinfected, clean maternity stall and must be removed from the dam immediately. Calves should be fed colostrum from an ELISA-negative or fecal culture negative cow. Equipment used for manure removal should be kept separate from the feeding equipment. If a farm is infected with MAP, the farm should test all animals older than 24 months of age for many years to help eradicate the disease from the farm. If the farm is at a low risk for the disease or is negative, but has replacement heifers coming into the herd, then the farm should perform a pool fecal culture. If that pooled fecal culture returns positive, then individual testing will need to be performed to find the heavy shedders. Dairy herd producers should concentrate on culling heavy and super-shedders from the farm to reduce the environmental contamination with M. avium subspecies paratuberculosis.

This disease can be devastating to a farm and dairy producer. By having strict protocols with neonatal calves and buying replacement stock, this disease should never step onto a farm. It is a good idea to know the ‘411’ about Johne’s disease when dealing with dairy cattle.