This disease is a chronic, progressive, wasting condition that affects ruminants and is caused by the organism Mycobacterium avium subspecies paratuberculosis (Map).
The infectious agent is shed in large numbers in faeces, can cross the placenta and can be found in colostrum and milk.
Animals are generally infected by ingesting the agent and young animals are considered to be the most susceptible to infection.
Farmers should look for diarrhoea, poor milk yield and weight loss in cattle three to five years-old with onset often following calving or other stressful event (sale, transportation etc). There is no fever and the animal maintains a good appetite until the terminal stages. Clinical signs may continue for several months with the cow/bull becoming emaciated, and then being culled for economic/welfare reasons.
Vaccination has been used as an aid in the control of Johnes in many countries and can be imported into the UK under license. Inoculation is given into the brisket area of calves less than one month old (preferably less than one week) producing considerable local reaction. Vaccinated herds/flocks have much reduced clinical cases and losses but Johne’s disease will not be eradicated. Vaccination may be the most cost-effective option for commercial beef herds breeding their own replacements and experiencing considerable loses from Johne’s disease.
A survey which started in 2006 estimated the prevalence of Map infected herds in the UK was 34.7%
It is estimated that there were 1,000 cases of Johne’s disease in the UK during 2000, increasing to 2,400 in 2004 affecting between 20 to 50 per cent of UK herds. In infected herds the annual culling/mortality rate may be 1 to 5%. However, losses due to subclinical disease (weight loss, reduced milk yield; poor fertility) are also substantial. The financial losses are estimated to be £2600 in a 100 cow dairy herd with clinical cases but this is likely to be a gross underestimate.
Control is difficult because of the long incubation period, shedding of infection by animals before they show clinical signs, and diagnostic techniques with poor detection rates in the early stages of disease.
Eradication requires a substantial commitment by the farmer, veterinarian and local laboratory and is based upon the identification and removal of infected animals. Blood testing and/or faecal examination may be done every 6-12 months with slaughter of positive cases. Two consecutive herd negatives may indicate eradication.
Practical control measures that can readily be adopted to limit losses in a diseased herd include:
- Rapid culling of diseased animals.
- Minimise faecal contamination of food, water and pasture e.g. by raising feed and water troughs, strip grazing, use of mains/piped water rather than surface/pond water, avoiding spreading yard manure on pasture, and maintaining good hygiene in buildings/yards and calving boxes in particular
- Separate newborn calves from dams at birth and rear by bucket with artificial colostrum/milk (only possible for dairy calves).
- Do not feed waste milk to calves
- Do not raise calves from known infected dams as breeding replacements.
- Re-stock only from accredited herds especially bulls.