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Newsletter of Penn Dutch Cow Care October 2005

Hi Folks,

I was visiting with a former professor the other day who is an expert in Johnes disease research. Dr. Robert Whitlock and his team at New Bolton Center at the University of Pennsylvania School of Veterinary Medicine have been studying Johnes disease and its causative organism, Mycobacterium avium paratuberculosis (MAP), for more than two decades. There have been some interesting developments in Johnes information recently and I'd like to pass it along to you. Rather than re-hashing the usual management changes that need to be made to control Johnes (in a nutshell - keep infected cows away from calves), an interesting discovery has been made within the last year.

First, rather than testing each individual in a whole herd, especially larger herds, it is possible to get an idea of the level of Johnes, if present, by strategic environmental sampling of manure. This would mean collecting a small amount of manure from areas that cows congregate in, such as the barn yard, box stalls, calving areas, laneways to pasture, walking areas in free stall areas, milking parlor holding areas and slurry drop-offs into a manure pits (i.e. gutters). Composite manure samples would have 4-6 samples of equal volumes from each site in that area so that the mix would fit in a small disposable dish. Each pooled (mixed) sample would represent that particular area. Include dry cow areas. Know which cows are contributing to each area's sample. A farm diagram should be drawn so that the same locations within the area could be re-tested at later dates. Each area's composite sample of manure would then be sent in to the lab and cultured. This takes 3-4 months for results to be completed.

Environmental samples can: determine if the herd is infected with MAP, determine the extent of MAP contamination in the environment, monitor management changes, estimates herd Johnes prevalence and is more economical than individual fecal sampling. Instead of sending in 50 samples from a 50 cow herd, perhaps only 4-5 pooled samples would be needed.

Now for the breaking news - there are cows which are "super-shedders" of Johnes. These animals can severely contaminate a farm area (or areas over time). While conventional wisdom says that only the youngest calves get infected with Johnes, adult animals sharing the area with a super-shedder cow can become "pass-through" shedders. This means that non-infected animals can become low-level shedders of Johnes exposed to the manure of a super-shedder. They can become infected within 12-24 hours of exposure to manure from a super-shedder. Interestingly, when a super-shedder is identified and removed, the newly infected animals revert to non-shedding status, although upon biopsy they may indeed harbor the bacteria in the ileo-cecal lymph nodes and other abdominal areas. So, while a super-shedder is obviously of great concern, other animals that become pass-through low level shedders should cause concern because they also could contaminate calves with the organism in their manure.

A similar contagious picture is observed with persistently infected BVD animals. While BVD is a virus and Johnes is a bacterium, it seems clear now that one animal, either a BVD PI or a Johnes super-shedder, can ruin a whole herd's health. The two diseases are very different, but until a Johnes super-shedder or BVD PI is identified, that animal will severely contaminate the environment until it is culled. Screening a herd for BVD PI identification can entail screening the bulk tank milk but it is often more revealing to take ear notch samples from each individual animal so a pathologist can identify the viral particles embedded within the ear tissue. In a similar manner, screening a herd for Johnes prevalence can now be done confidently with environmental samples rather than drawing manure or drawing a blood sample from each cow. Identifying areas with abnormally high culture results can help to narrow down which animals may have caused a high result. At that point testing individual animals should be done.

For rapid screening of a clinical suspect, the AGID test is still used. This test shows true positives. But if the animal is not diseased enough, it will come back negative. A more sensitive test is the ELISA, which is more likely to pick up lightly infected animals. Using a PCR test can be rapid and very specific, but the animal needs to be expressing enough of the organism for the test to pick it up. The gold standard for Johnes identification is the fecal/manure test, but it takes 3-4 months for results. By strategically sampling each area where cows congregate, the slower fecal test can identify areas of cows which are potential cause for concern in a statistically valid and economical fashion.

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