April 14, 2022
Johne’s is a sneaky disease that isn’t detectable until an animal has been infected a long time—and then loses weight and has diarrhea.
Dr. David Smith, an epidemiologist at the University of Nebraska, says bringing in new cattle is how it comes to your ranch.
First described by Heinrich Albert Johne (pronounced yo-knee) in 1895, the pathogen causing this disease wasn’t identified until 1911. Mycobacterium paratuberculosis is a bacterium similar to the one that causes tuberculosis in humans. It affects ruminants (cattle, sheep, goats, deer, elk, etc.) but multiplies very slowly after the animal becomes infected; it may take 2 to 10 years for animals to develop signs, though they are shedding bacteria in manure (to infect other animals) long before they appear to be sick.
Smith says the incubation period averages 3 to 4 years--sometimes as long as 10 or more years. Healthy-looking cattle can be shedding bacteria, infecting other animals, though the ones with diarrhea are the worst shedders, passing millions of bacteria in their manure.
There are 3 stages of the disease. Stage 1 is subclinical and not detectable with any tests. The infection progresses slowly and may take months or years to reach stage 2, a subclinical shedder. Stage 2 cattle are usually more than 2 years old and appear healthy, but shed enough bacteria to be detectable by fecal culture, though blood tests may not find it. Animals in stage 2 are a hidden threat unless the animals are being checked for Johne's with fecal cultures or PCR tests—which are faster and now more commonly used than cultures. Stage 3 is clinical disease, with diarrhea and weight loss.
How it's spread
Usual transmission is by consuming contaminated feed or water or nursing a dirty udder. Young calves are at high risk of picking up bacteria from manure, and can also be infected before birth if the dam has Johne's. The calf picks up the bacteria before or soon after birth, but seems healthy until symptoms appear several years later--yet part of that time is shedding bacteria into the environment to infect more calves.
Calves can also be infected by colostrum from an infected cow. Calves given colostrum from dairy cows may be at risk. Several cases of Johne's have been traced to embryo transplant calves with Holstein mothers, says Smith.
Effects of the disease
The infection causes gradual thickening of the intestinal lining, making it difficult to absorb fluid and nutrients—resulting in weight loss and diarrhea. Temperature and appetite remain normal, but the animal gradually loses weight and its hair color may fade. Eventually the bacteria spread to other parts of the body, and in later stages are circulating through the blood and lymph system. There is no effective treatment--the disease is not responsive to antibiotics, and is always fatal. As the animal enters stage 4, it becomes emaciated, then dies. Once an animal is diagnosed with Johne's disease it should be sent to slaughter before the final stages. The meat is safe for human consumption. In stage 4, diarrhea is watery, the animal becomes weak and often develops edema between the lower jawbones. A fluid-filled lump under the jaw can also be a sign of parasitism, liver damage, or brisket disease, according to Smith.
Sometimes stage 3 (appearance of symptoms) is brought on by stress, like calving. A few animals that break with diarrhea may seem to recover temporarily, relapsing again the next time they are stressed. Many stockmen are not aware they have Johne's in their herd, since they cull subclinical animals before they show actual signs of disease. Cows during the long incubation period (before obvious signs of diarrhea and weight loss) often have higher rates of mastitis, lower milk production, more infertility and longer calving intervals.
Smith recommends culling any older cow that becomes thin. “If you are rigorous in culling animals with poor body condition, this can reduce the number of animals that may be shedding bacteria. If you can minimize the risk of calves exposed to manure from these individuals, this can help slow down transmission,” he says.
Johne’s is impossible to diagnose in early stages, and it’s also difficult to clean up an infected herd. “A group of researchers was given money from USDA to study the organism and methods of control, and scientists are working on better diagnostic tests. At this point several tests are used. There are better culture methods and PCR tests today that take less time than they did 15 years ago but are no more accurate. Scientists have also worked on tests to measure cellular immune response, to try to pick up the infection at an earlier age, but these tests have not been successful,” says Smith.
“It would be a huge breakthrough if we could tell which animals are infected, at an early age, before they start shedding bacteria. Currently we can’t find them until they are spreading the disease. Test results are most reliable when the animals have become thin and have diarrhea, and by that time the damage is already done,” Smith says.
There are vaccines against this disease but they are not widely used. “Most states don’t allow them because they confound the TB tests. It’s also a dangerous vaccine because if people accidentally inject themselves they could get large granulomatous nodules at the site of injection. In states that use it, the vaccine must be administered by a veterinarian and with approval of the state veterinarian,” says Smith.
“Early on, the states had funds from USDA to test herds and certify the herds that were free of the disease. But people lost interest in this because they had to bear the expense of the testing, for no added value to their cattle if their herds were certified free.”
Recommendations for beef herds
“About 18 years ago we did a study in Nebraska beef cow herds, using risk assessment developed by the Johne’s Working Group. The main risks are related to young calves exposed to manure from infected cattle. If a thin cow, or a cow with diarrhea, was present at calving time, this was the greatest risk. Other risk factors included having that thin cow with the calves prior to weaning, or calves having access to manure-contaminated sources of water. Prior to calving you should remove any cows that are thin or have diarrhea or signs suggestive of Johne’s, and control the water that calves can access,” says Smith.
Don’t put a suspicious cow upstream from your calving cows. Even if you take her out of the main herd, it won’t help if she’s still defecating in a water source.
“Herds with Johne’s often had calves drinking from water sources that cattle could defecate in,” says Smith. Fencing off ponds, ditches, and having a piped-in source of clean water can be a way to help prevent this disease. Changing from late winter/early spring to summer when cows can be spread out on pasture could make a difference.
“In a big pasture a calf is less likely to get contaminated manure in its mouth,” he explains. Butif weather is wet and cattle are more congregated than normal or calves drink from puddles, the contamination factor becomes a bigger issue.
“You might segregate your herd, keeping older cows and their calves separate from younger cows and their calves. Most people who find Johne’s disease in their herd decide to live with it and just try to minimize spread—and not spend money on testing because the tests are so inconclusive. Unless you are a seedstock producer, you may not make the effort required to completely get rid of it.
“If you don’t have it currently, you can keep your herd free of it by not bringing in new cattle, or always buying bulls and replacement heifers from a safe source. Use of AI could minimize the number of bulls to bring in,” says Smith.
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