· What is it?
Johne’s disease is a bacterial disease caused by Mycobacterium paratuberculosis. This bug is closely related to the one that causes TB, and the two diseases share many features. There may also be a link between Johne’s disease and Crohn’s disease in humans. Johne’s causes a progressive untreatable wasting disease in adult sheep, cattle and deer. Diarrhoea is often present in these animals when the disease becomes advanced.
There are two strains of Johne's disease, one largely affecting cows and the other sheep. These two strains are generally not interchangeable but occasionally cross infection may occur. Goats and deer can be affected by either strain.
· How is it spread?
M. paratuberculosis is shed in the faeces of infected animals. It can survive in the environment for up to a year. Infection normally occurs in young animals, when it is picked up orally through suckling contaminated udders, or drinking milk from a Johne’s infected cow, or through ingesting contaminated soil. There is a small chance of transmission across the uterus during pregnancy in diseased cows. It appears that animals become immune to picking up the infection by the time they are six months of age.
· What does it do?
Whilst a large proportion of stock are probably exposed to the organism, relatively few will go on to develop clinical disease (disease you will notice). The organism will not affect some at all, while others will become subclinical (silent) carriers of the disease. These subclinical carriers (while appearing normal) will be shedding the bacteria in their faeces and acting as a source of infection to young stock. In time some will develop clinical Johne’s disease. This often occurs up to five years after they pick up the infection.
Clinical Johne’s disease typically appears in older animals; the onset is normally between three and five years of age. It first shows as a watery scour, which is unresponsive to treatment. The scour may be intermittent. Cows with Johne’s tend to waste away. There may be a degree of “bottle jaw” present due to excessive protein loss. Subclinical cows, while not showing these signs, may have decreased milk production and fertility.
· Control of Johne’s disease.
There is no treatment for Johne’s disease.
Diagnosis of subclinical animals is made difficult by the absence of a reliable screening test, although research work is currently being done on this at the University of Otago.
Control is based on two principles:
Because most infections take place before six months of age this is where the focus for reducing the problem is placed.
In some areas a vaccine is available for prevention of Johne’s disease. Due to the Tb status of this area the vaccine is unable to be used in Otago.
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