This disease is chronic, debilitating and irreversible and is caused by the organism Mycobacterium avium subspecies paratuberculosis (Map). It progressively damages the intestines, a bit like the human condition Crohn’s, and ultimately results in the animal’s death. It’s a disease that primarily affects ruminants but has been found in rabbits.

Cattle can become infected at any age although infection is most common during the first few weeks of life. Animals are generally infected by ingesting Map from infected faeces in contaminated bedding, on udders, teats, buckets or by drinking infected colostrum or milk. The infectious agent is shed in large numbers and can even cross the placenta meaning calves can become infected before they are born.

Multiplication of bacteria is slow so it takes several years before signs of disease are evident, usually when the animal is 3-6 years of age. It is, however, possible for a beast as young as 18 months to show signs of clinical disease.

Johne’s Disease is one of the most difficult diseases to control because there is such a long time between infection and clinical signs; it is difficult to diagnose infection during this ‘quiet’ phase.

Long before the disease becomes apparent in a herd, many animals will already have been exposed to infection. For every diseased animal seen there will be a group of cattle where the disease is already affecting their milk output or fertility.

The most obvious clinical signs are scouring and weight loss although the animal itself continues to appear bright and maintains a good appetite. Some may even appear to rally and improve in condition but this is a period of remission as the disease is irreversible.

As the disease progresses, fertility of both bulls and cows will be reduced and the animal becomes emaciated. Eventually, ‘bottle-jaw’ or oedema is visible and death (or culling) is the inevitable outcome.

The exceptionally long incubation period and the fact that animals are shedding the causal agent before they show clinical signs of disease means that the control of Johne’s Disease is very difficult. To eradicate it requires substantial commitment by the farmer, vet and laboratory. It is largely based on screening animals and removing/culling those that are infected and by introducing and maintaining strict biosecurity measures.

A live vaccine can be administered to the brisket area of calves within their first month of life. Some argue it is best only used in heavily infected herds while biosecurity precautions are put in place to reduce Map infection. It is not, however, a long-term solution to prevent infection and could also interfere with the skin test for TB. Live vaccines do not tend to be favoured by many countries because of their potential risk of causing disease in humans.

Practical control measures that can be adopted to limit losses in a diseased herd include:

• Testing animals with clinical signs and culling those with Johne’s Disease as well as those where disease can’t be ruled out
• Trace calves from infected dams and don’t use them as breeding replacements
• Minimise the risk of faecal contamination of feed and water sources; keep troughs clean.
• Keep calving pens/yards as clean as possible, as well as the udders/teats of freshly calved cows, to minimise the risk of exposure of newly born calves to infection.
• Avoid feeding pooled colostrum and waste milk
• Aim to restock only from accredited herds, especially bulls.
• Avoid co-grazing with other livestock (limiting contact with sheep is advisable) and controlling rabbit populations are sensible precautions