Paratuberculosis

Also known as

Johne’s disease

Description

This is a contagious, chronic, and usually fatal infection that affects primarily the small intestine of ruminants. Johne’s disease is the common name for clinical signs, but these signs don’t appear until the disease is already well established in a herd of cattle. Due to the long incubation period, diarrhea and weight loss don’t show up until an animal has been infected for a long time.

For many years, beef producers were unaware of this disease, or thought it only affected dairy cows. A few beef herds are infected, creating more risk for introduction of this diseases into other herds. Ten years ago it was estimated that about10% of beef herds had at least one positive cow, and 68% of all dairy herds had one positive cow.

There’s less incidence in beef herds partly because animals are spread out more, with less exposure to manure, since the bacteria that cause Johne’s are shed in feces. If beef herds are confined at calving they may be more at risk.

Incubation period—between the time of infection and when signs appear--is relatively long. Cattle generally do not show signs until they are at least 2 to 5 years old. Someone buying bulls, heifers or young cows may inadvertently bring home Johne’s disease because animals that young do not yet show signs.

There is always risk when bringing in new animals, because there is still no good way to detect this disease in young animals. The animals that break with Johne’s disease as adults probably became infected as young calves.  

The number of cows in a herd that are skinny and have diarrhea is not representative of the problem. A producer might have only one or two cows each year that become thin and get culled, but for every one of those thin cows, there are other cattle in some stage of the disease, that may be already shedding bacteria.

For every obvious case of Johne’s disease among dairy cattle on a farm, 15 to 25 other animals are likely infected. Johne’s has been called an iceberg disease because you only see a small tip. Most of the problem is underneath the surface. If a producer is just selling the cows with diarrhea, this is not addressing the real problem.

Affected cows lose weight but still have good appetite and are not sick. They have a thickened intestinal wall and can’t absorb nutrients; feed just goes on through. Any adult animal that eats well but has watery diarrhea and is losing weight might have Johne’s.

There are 4 stages of the disease.  Stage 1 is subclinical (no signs) and generally not detectable with current tests. Typically, this stage occurs in calves, heifers, and young stock under 2 years of age or animals exposed at an older age.

Current tests (including fecal culture and serological tests) cannot detect infection in these animals. Research to develop new tests to detect the disease in such animals is ongoing. This stage progresses slowly over many months or years.

It is possible that some animals recover from this early phase of infection. In most cases the infection progresses slowly until the animal reaches stage 2, in which it becomes a subclinical shedder.

Stage 2 cattle are usually more than 2 years old and appear healthy, but are shedding enough bacteria to be detectable by fecal culture, though blood tests may not pick it up. Animals in stage 2 pose a hidden threat unless a producer is checking for Johne’s with fecal cultures. 

These animals pose a major but often hidden threat of infection to other animals through contamination of the environment, and may or may not progress to Stage 3.

Stage 3 is clinical disease, with diarrhea and weight loss (due to gradual thickening of the intestinal lining). These signs usually don’t show up until after 2 years of age, but in some instances as early as 12 to 14 months.

Usually the younger an animal shows signs, the more likelihood of high infection rate within the herd—and greater level of contamination.  Animals in this stage have advanced infection, and clinical signs are often brought on by stress, such as calving.

Signs at this stage include acute or intermittent diarrhea, weight loss despite normal appetite, and decreased milk production. The infected animal might respond favorably to parasite control if it is infested with parasites along with Johne's disease. The “recovery”, however, is brief and it soon goes downhill again.

Some animals appear to recover but relapse in the next stressful period. Most of them are shedding billions of bacteria, and fecal tests give positive results. Many animals are positive on blood tests as well. Clinical signs may last days to weeks before the animal progresses to Stage 4.

As the animal enters stage 4, it becomes more emaciated. The diarrhea is watery, the animal becomes weak and often develops “bottle jaw” (edema and swelling between the lower jawbones). Animals at this stage of the disease will not live very long—perhaps a few weeks at most.

The fluid-filled lump under the jaw can also be a sign of parasitism or liver damage, or brisket disease, however, and not just Johne’s. Animals in Stage 4 that are culled and sent to slaughter may not pass inspection for human consumption.

Signs

• Diarrhea
• Weight loss in spite of good appetite
• Swelling under the jaw

Causes

Johne’s disease is caused by Mycobacterium paratuberculosis, a bacterium similar to ones that cause leprosy and tuberculosis in humans. It affects all ruminant animals such as cattle, sheep, goats, deer, elk, antelope, bison, etc.

Bacteria are shed in the feces of an infected animal, and picked up by susceptible animals ingesting contaminated feed or water, or a calf nursing a dirty udder. Calves can be infected before birth if the dam has Johne’s.

Holsteins were often used as embryo transfer recipients in the past, but not so much anymore, since Johne’s can be passed to the fetus, or to the calf through the colostrum and milk.  If you purchase an infected pregnant female, there is a risk for the offspring to be infected. There is a risk if you use colostrum from an infected dairy or buy a dairy calf, or an older dairy cow to use as a nurse cow for orphan calves.

Johne’s disease usually enters a herd when healthy but infected animals (Stage 1 or 2) are introduced. Cattle are most susceptible to infection in the first year of life. Calves most often become infected by swallowing small amounts of infected manure from the calving environment or udder of the cow.

In addition, calves can become infected in the uterus or by swallowing bacteria passed in milk and colostrum. Studies have shown that up to 25% of calves are infected in utero if the cow is in Stage 3 of the disease. Calves may become infected by exposure to contaminated manure any time in the first year of life.  

Cattle of any age can become infected. Even older animal’s resistance can be overcome by high doses of bacteria over time manure-contaminated feed bunks or water sources.

Intensive grazing (such as mob grazing or any rotational grazing where cattle are closely confined on pasture and moved frequently) can be a risk unless you have a closed herd with no chance for Johne’s to be introduced.

If a large number of cattle are jammed into small areas of pasture, they have more risk of ingesting fecal material. It’s an efficient way to use grass, increase stocking rates and pasture health, but you must practice good biosecurity and make sure you don’t bring in an animal that could infect your cattle.

Prevention

The most important thing is to not let your herd become infected, taking biosecurity precautions to avoid introducing animals that might pose a potential risk, such as cattle bought at an auction. Since most cattle coming into beef herds are purebred bulls, it is important that purebred breeder control it within their herds.

Testing is important for them, partly because the animals are more valuable. They are not as worried about losing 60 pounds at weaning (due to a cow with Johne’s not milking well); they are worried about losing a valuable animal too soon or selling an infected one to somebody.

A seedstock producer doesn’t want to send this problem to customers’ herds.

Testing is an important tool, but current tests still have limitations; we have to wait until the animal has either an immune response, or starts shedding detectable levels of the organism in feces, and by that time the animal is an adult.

The challenge of identifying infected young animals is still a roadblock. It would be a huge breakthrough if we could tell which animals are infected, at an early age, before they start shedding bacteria. We really can’t find them until they are already spreading the disease to other cattle.

The closer that animal gets to being clinical (showing signs of disease), the more likely we will get a positive test result. Part of the problem is that the test results are most reliable when the animals have become thin and have diarrhea, and by that time the damage is already done.

When tests for pre-purchase screening—testing healthy animals to bring into the herd—it doesn’t matter whether the test is a fecal culture or PCR, or serology, most of the infected animals are missed because they are not tested at all. A negative test result is almost meaningless. A positive culture result means the animal is positive, but a positive serology result (unless it’s in a herd that has many animals being screened) is inconclusive because most of the animals that test positive don’t have the infection.

Most veterinarians recommend blood test every year to check for antibodies, particularly for purebred breeders. If they get a positive on the blood test (usually an ELISA) they need to do a fecal culture. There can be some false positives with the ELISA test, but if a cow is positive on the blood test and positive on the fecal culture, that animal should be culled.    

If it’s valuable cow, the owner may opt to do surgery before culling her, taking a biopsy of the intestine to get a more definitive diagnosis. You can get the same answer from a necropsy, if you euthanize the cow. If the blood test and fecal culture are both positive the body can be sent to a diagnostic lab to confirm the disease.  

Without control efforts, incidence of Johne’s gradually mushrooms in a herd; within 10 years of the disease entering a herd, young cows (3 to 4 years of age) could be dying.

One of the best way to minimize spread of this disease is to make sure no thin cows or cows with diarrhea are present in the calving herd. Body condition scoring is a good practice, and if some animals have low body condition when the rest of the herd looks good, this should raise a warning flag. Those animals deserve a closer look and should not be present during calving, to protect the rest of the herd.

The key is minimizing exposure of newborn calves to contaminated conditions—especially manure from older cows. There are things that can be done in a dairy, such as isolating the calf as soon as it’s born, that are more difficult in beef herds. The best recommendation is to minimize confinement and crowding.

Two important factors are animals that look like they might have the disease, and water sources that might be contaminated with manure. Don’t put a suspicious cow upstream from your calving cows.  Even if you take her out of the main herd, this won’t help if she’s still defecating in a water source. Fencing off ponds, ditches and sloughs, and having a piped-in source of clean water can help prevent this disease.

Consult with your herd health veterinarian to figure out the best options for your own herd, and try to reduce risks for introducing Johne’s. Some herds have established levels of confidence (through testing) that Johne’s is not an issue. Information about the herd you are considering purchasing from can be very valuable.  Even knowledge there have been no animals that have had clinical signs of Johne’s is better than nothing when evaluating risk. Do some checking before buying.

For herds that are not infected, take precautions against introduction of Johne’s disease. Keep a closed herd or require that replacement animals come from test-negative herds. Some States offer Johne’s certification to test-negative herds. The new Uniform Program Standards for the Voluntary Bovine Johne’s Disease Control Program (APHIS 91–45–014) outline a new, voluntary national Johne’s classification program that helps identify risk of infection in participating herds.

The key to preventing Johne’s infection is to know that herds get infected only when infected animals are added to the premises. Testing the herd of origin is much more reliable than testing only the purchased animals.

If you have an established herd and test feces from some of the cattle, you only have to find one positive culture to know that the disease exists and is circulating in that herd. Some people do environmental samplings, particularly in dairies—running a swab through the alleyway. All they need to do is find the bacteria in the culture and they know that at least one cow is shedding it.

In beef herds it’s more likely you’d use the serology (blood) test. Any animal that tested positive could be followed up with a culture from feces. Again, you only need to grow it from one of them to know the herd is infected. Then you are trying to find the individual.

Treatment

There is no effective treatment--the disease is not responsive to antibiotics, and it is always fatal. This disease can be eradicated from a herd, but takes a good plan and diligent effort to accomplish it.