Johne’s Disease, or Paratuberculosis, is a chronic, contagious disease of ruminants and certain types of game which is caused by the bacterium Mycobacterium avium subspecies paratuberculosis. This bacterium is closely related to the organism that causes Tuberculosis or TB. The disease occurs worldwide and is regarded as one of the biggest threats to the livestock industry in countries such as the UK, Australia, New Zealand, and Spain.
In South Africa, sporadic cases of paratuberculosis in cattle were reported since the first case was confirmed in 1923. In sheep, the disease was limited to two flocks in the previous Transvaal. In 1993, the first case in sheep was however confirmed in the Western Cape.
Affected or carrier animals can excrete large quantities of the bacterium in their dung. The paratuberculosis bacterium is very resistant and can survive under favourable conditions (a moist, cool environment), for more than a year in dung or water. Susceptible animals ingest the bacterium orally through contaminated grazing or, in younger animals that are more susceptible than older animals, by suckling on contaminated teats. The bacteria enters the intestinal tract where it multiplies. Some infected animals become carriers and can excrete the bacterium through their dung for their entire lives. Most infected animals remain subclinical carriers; in other words, they remain apparently healthy and show no symptoms of the disease. Others may, however, after an incubation period that can vary from six months to as long as several years, develop clinical symptoms. The percentage of a herd that develops clinical symptoms is usually 1-2% per year but can be as high as 10% per year. Factors such as malnutrition, trace elements deficiency and internal parasites may increase the number of clinical cases.
In sheep that show clinical signs, chronic emaciation is observed, but the droppings are usually normal. Diarrhoea is typically not one of the disease’s symptoms in sheep. Due to the disease’s long incubation period, clinical cases are usually only observed in sheep from 2-tooth and older sheep.
Affected animals die after a few weeks or months due to starvation because the bacterium causes the intestinal wall to thicken, with the result that nutrients cannot be absorbed. There is no treatment for the disease, but a vaccine is available on prescription from a State Veterinarian.
There are many other causes for emaciation in sheep, such as malnutrition, a deficiency in trace elements, internal parasites, abscesses, etc. and it is very important that Johne’s Disease is confirmed if it is at all suspected. It is a Controlled Disease and any suspicious or confirmed cases must be reported to the closest State Veterinarian.
In view of the above, the only advice for a producer with an infected flock is the following:
Acknowledgement to Dr S Davey , State Veterinarian Malmesbury for this article.