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When Johne’s hits home

Dr. Meaghan Crawford’s empathy for the family that discovered Johne’s disease in their young beef herd was evident as she spoke about her involvement with the case during her time as a veterinary student at Calgary.

A cow and two heifers showing severe weight loss and diarrhea were brought into the rural clinic where she was doing her fourth-year rotation in early 2015. Their body condition scores were three to four on a nine-point scale, and they had soft, watery feces without blood or mucus. Otherwise they were bright and responsive with normal temperatures, respiration and heart rates. The cow and one heifer were found to be about six months pregnant and the other was open.

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a cow with Johne's disease
a cow with Johne's disease

The family had purchased 300 females from three sources during the five years leading up to the case. The producer said he had noticed some thin cows and there had been some mortalities here and there over the years, but he hadn’t been overly concerned until these three animals rapidly went downhill. He thought five others could potentially be affected in the same way.

One by one the veterinary team went through the possible-problem list. Stress and dietary issues were ruled out because even though multiple animals were observed to be ill, it was a small percentage of the herd. None of the potential common problems, such as salmonella, bovine viral diarrhea, hardware disease or parasites, fit the case.

Crawford says Johne’s disease was a big suspect because it causes chronic diarrhea and loss of body condition without fever, going off feed or other signs of illness. Their research turned up one source herd that may have had a history of thin, poor-doing cows, but lingering questions remained because the three suspect animals were so young and so sick. Signs of Johne’s disease don’t usually start showing until animals are four to seven years of age and even then usually only pop up in one animal at a time… unless the infection is heavy. They feared this could be what the producer was facing.

Johne’s is caused by Mycobacterium avian subspecies paratuberculosis (MAP) bacteria spread in the manure, colostrum and milk of infected animals. Cattle almost always pick up the infection as calves, usually before six months of age, by consuming MAP bacteria in manure on the cow’s udder, bedding, feed, pasture and in water. Some calves and most mature animals are able to build immunity to MAP over time and don’t become permanently infected. If an animal can’t fight off the bacteria, it hides in the immune cells of the ileum (between the small and large intestine) from where it slowly does its damage by thickening the wall of the large intestine. Affected animals often have good appetites, but eventually start wasting as the intestine thickens to the point where it is unable to absorb protein.

An infected animal’s fate is sealed because there is no cure. The animal will either succumb to the disease or get culled for being a poor-doer or raising poor calves without a producer ever suspecting Johne’s disease. The greater concern is that infected animals shed MAP bacteria off and on throughout their lifetimes.

The nature of the disease with intermittent shedding and late onset of illness makes it very difficult to catch in the early stages, Crawford explains. Blood tests to detect antibodies to Johne’s disease, fecal samples to culture MAP bacteria, and fecal samples for PCR (polymerase chain reaction) to detect MAP DNA are accepted diagnostic tools. But producers aren’t likely to test seemingly healthy animals and fecal samples collected when animals aren’t shedding will give false negatives.

Blood tests were used to confirm Johne’s disease in the three suspect animals because the team needed a quick answer, whereas it can take as long as eight weeks to see positive results from fecal cultures.

“The blood test results came back with astounding numbers,” Crawford says. An antibody ratio above 0.7 relative to a positive control serum is considered positive and all three animals tested well above 2.0.

The gold standard is necropsy followed by microscopic evaluation, so the producer agreed to sacrifice the cow and open heifer. Seeing the characteristic signs of Johne’s disease confirmed their worst fears. The large intestine was thick walled and ropey and the associated lymph nodes were enlarged. Tissue samples from the inflamed lining of the intestine and ileum from each animal came back positive for MAP with the pathology report classifying the infection as severe in all samples.

Fecal samples from 90 young cows were collected that summer for PCR analysis to get an idea of the infection rate at the herd level. Manure from four or five cows was pooled in each of 19 samples and 30 per cent of the samples came back positive. Individual fecal samples then had to be collected from each animal represented in each of the positive pooled samples to be tested to identify the positive individuals. Testing costs quickly start adding up.

Approximately 30 positive individuals were identified that summer. The producer chose to manage the positive animals separately, calve them out this spring and ship the cows directly to slaughter this fall. None of the female calves will be kept as replacements because of their potential to perpetuate the disease cycle.

The latest round of testing was this spring when seven of the 16 pooled samples came back positive. Subsequently, 17 of the individuals represented in positive pooled samples were identified as positive or suspect.

The producer is committed to doing everything possible to eliminate Johne’s disease from the herd by continuing with the test-and-cull strategy and carefully managing positive animals to contain and reduce the spread of the bacteria.

“Unfortunately, the damage has already been done and the producer might be looking at eventually culling half the herd,” Crawford says. “Aside from how heartbreaking the case is, the discussion currently is about how remarkable the infection rate is in these young cows, which is unheard of to those familiar with this case.”

Her take-home message is that Johne’s disease, once considered a dairy disease, is most definitely a beef cattle disease, too. Alberta statistics show that approximately 14 per cent of the province’s beef herd and 50 per cent of the dairy herds have tested positive for Johne’s, but she suspects the prevalence is higher than most people think.

“Johne’s disease is known as an ‘iceberg’ disease because for every sick animal there will be several infected animals that don’t show signs. It’s this subclinical population that causes the most loss for producers because those animals will spread the bacteria and may birth smaller calves that gain weight more slowly,” Crawford explains.

The first step for producers is to learn about Johne’s disease because prevention doesn’t cost a thing and is far easier than management after the fact.

If you are going to buy cows, pairs or heifers, ask questions, be selective in your choices and buy from reputable sources, Crawford advises. Purchase colostrum only from herds that are negative for Johne’s disease and manufacturers of commercial colostrum that use a method to kill MAP bacteria.

Never sell cattle suspected or confirmed positive for Johne’s disease through any channel that could end with them being breeding animals in another herd.

Remember, infection starts with the calf. Vigilant management at calving time will not only reduce the risk of calves picking up MAP bacteria, but will help control many other calfhood diseases.

It’s important to realize that eradicating this disease from a herd is at minimum a five-year project that requires a close working relationship with your veterinarian and a good understanding of the disease. Some progress can be made to minimize losses by early detection and shipping thin cows and those with diarrhea, but much more rapid progress is made by full-herd fecal testing.

This can easily be done at pregnancy-testing time by the veterinarian, a registered veterinary technician, or by the producer. The fecal test is the most useful test for surveillance because it is the most accurate for detecting early infections, while blood tests are useful to confirm the disease in animals already showing signs.

In the event of a positive test result, your veterinarian will be able to help with a risk assessment and deciding on a management strategy that’s suitable for your operation.

For more information, visit the Beef Cattle Research Council web page, where there are also links to fact sheets on recent research, and the Canadian Johne’s Disease Initiative website.

A normal small bowel lining is smooth.

A normal small bowel lining is smooth.
photo: UCVM

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