Johne's disease Here is an informative text that is (with permission) from the veterinarian whose name is at the end. Johne's is a relatively new, or at least newly recognized, problem in sheep, but is on the increase.
Ron

The tests (for Johne's disease) we currently have available are very specific - meaning that, in most instances, a positive test result is reliable, but a negative doesn't necessarily mean that the animal is not infected.

The 'gold standard' diagnostic test for Johne's is the fecal culture - performed correctly, it can detect as few as 50 organisms per gram of feces; but, it takes 12-16 weeks before we can detect growth of the organism. Enhanced culture methods, with shorter turn-around times, are available, but the equipment needed to perform them is cost-prohibitive for many diagnostic laboratories in today's climate of budget crunches.

*Note* Mycobacterium paratuberculosis isolates from sheep and goats(and many from Brahman cattle, for some reason) are notoriously more difficult to culture under laboratory conditions than are most of those from dairy and British/Continental beef cattle.

DNA probe(PCR) tests are extremely specific, but sensitivity is lower than fecal culture - they will detect animals shedding as few as 1000 to 10,000 organisms per gram of feces. Test results will be available, usually, in about 3 days - but costs are higher, and you'll miss some animals which are shedding lower numbers of organisms. Not all diagnostic laboratories are equipped with the technology to perform PCR testing. *Note* - An animal classified as a 'high' shedder, one shedding high numbers of organisms, may be seeding the environment with as many as 60 million live M. paratuberculosis organisms per day, in its fecal output.

The ELISA test, performed on serum, has good sensitivity and specificity, but has some limitations. ELISA was not really developed to be a 'individual animal' test, but a whole-herd screening test. Yes, individual animals with clinical disease will be positive in most cases, but ELISA is not really good for just testing an individual animal here and there - or as a pre-purchase test on 'normal' animal, for that matter. In a whole herd test of a herd with a history of recent clinical cases, positive ELISA results are very reliable. However, sensitivity is such that as many as 15-50% of infected, but clinically normal, animals may still test negative.

Johne's Disease is very much an immunologic disease, and when I attempt to describe the frustrations of testing to producers and practicing veterinarians, I feel the need to give an abbreviated immunology lesson in an attempt to explain why serologic testing is not as clear-cut as it was for, say, bovine brucellosis.

A major point to consider is the phenomenon of "oral tolerance". Exposure to potential immunogens (in this case M. paratuberculosis) at a young age, via an oral route, often results in oral tolerance; that is, the animal's immune system does not recognize this organism or substance as 'foreign'.

The native Americans utilized this phenomenon (knowingly or unknowingly), by using poison ivy/oak leaves in cooking & wrapping foods - their young were exposed to PI/PO at an early age via an oral route and developed oral tolerance; hence, they weren't plagued by the skin rash so many of us develop when we are sensitized to PI/PO by way of skin exposure.

To me, Johne's Disease is a classical example of oral tolerance - calves/lambs/kids are exposed to the organism, either in utero, or via an oral route by way of fecal ingestion or colostrum/milk containing the organism. This slow-growing organism invades their intestinal tract and grows inside macrophages (white blood cells) in the submucosa of the intestine, where it is 'insulated' from the remainder of the immune system. Most disease-causing bacteria ingested by macrophages are killed, processed and presented to the lymphocytes, stimulating them to begin producing antibodies or expanding clones of 'killer' cells. M. paratuberculosis, however, is able to survive quite happily inside macrophages without being killed, so there is no stimulus for the animal to begin producing antibodies against it until the point in time at which the organisms have multiplied so greatly that they reach "critical mass", and surpass the tolerance threshold. It is only at this point in time - which usually corresponds pretty closely to the onset of fecal shedding &/or clinical disease, that the infected animal begins to produce antibodies detectible in serum.

So... most animals do not seroconvert (begin producing measurable antibodies) until just before they begin shedding organisms in detectible numbers in their feces or break with clinical disease, which may be as long as 2,4, 5, 8 or more years after they were initially infected.

Malnutrition & stress, as indicated by previous responders in this thread, have significant bearing on age at which infected animals 'break' , as well as when they 'break'. Certainly, the stress of calving and high lactation demand play a major role in initiating the onset of clinical Johne's Disease.

I doubt that nutritional management alone can eliminate Johne's Disease - actually, I KNOW it can't; but it certainly can go a long way in diminishing the economic impact of Johne's Disease in a herd. Actually, many of the management practices/changes that one could/should put in place in a well-developed Johne's Disease management plan will help with overall herd health and productivity.

Louis L. "Lucky" Pittman, Jr., DVM Veterinary Pathologist/Asst. Prof. Murray St. Univ.-Breathitt Veterinary Ctr. Hopkinsville, KY email at Lucky.Pittman@murraystate.edu

Toxoplasma abortion is caused by a microorganism, Toxoplasma gondii. It is spread by cats who pick it up from birds and mice. Kittens will get the disease soon after they begin to hunt. They will sicken quickly after being infected, have a fever and look poorly. They will have diarrhea for a week or ten days. The feces are loaded with infectious oocysts (eggs) and any sheep eating feed contaminated with the feces will get the disease unless they are immune. With pregnant ewes (and women) abortion is a frequent outcome. There is no vaccine or treatment for this disease.

Both cats and sheep gain lifelong immunity once they have had the disease. The only reasonable management practice is to eliminate kittens from the farm. The only practical way to do that is to have the female barn cats surgically sterilized. If one has 3-4 resident sterile female cats, they will drive away and stray females that show up, so kittens will cease to be part of the equation. See if you can cut a deal with a vet to do a batch of females all at the same time. I got a small discount for having five done at one time.

Note: This is an update to a listing in The Sheep Book, Ron Parker, 2002, Ohio University Press (Swallow Press) ISBN 0-8040-1032-3Note: This is an update to a listing in The Sheep Book, Ron Parker, 2002, Ohio University Press (Swallow Press) ISBN 0-8040-1032-3 see also hem.bredband.net/ronpar/tsb.html and www.ohioswallow.com/bookinfo.php?book_id=0804010323 I am not endorsing any of the listings - just listing what people send to me. Your comments welcome on any of the listings. Ron

Copyright © Ron Parker 2007-01-25 Permission to copy for free distribution is granted provided the information is quoted in its entirity - including these comments, but not including what follows below.

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