Johne’s is a contagious, chronic and sometimes fatal infection caused by the organism Mycobacterium avium subspecies paratuberculosis.
The term ‘Johne's disease' is used only to describe the clinical disease in ruminants, however there is a human ailment called ‘Crohn's disease' that resembles Johne's in many ways. The risk to humans from contracting Johne's is still unclear, but Johne's disease is widely recognised as a potential risk to our dairy and meat export markets.
Up to 47% of dairy herds and 76% of sheep flocks in New Zealand contain variable levels of infected animals. The associated financial losses, and practicalities of controlling the disease are important considerations.
Infection with Johne’s occurs in young animals up to 18 months of age through ingestion of infected milk or faeces. The disease can survive months in the environment with the correct conditions. Despite the early age of infection, clinical disease doesn’t usually occur until years later with the average age of disease being five to six years old – unless there has been a high level of exposure in which case earlier onset of disease may be seen.
Financial implications arise not only from the loss of clinically affected animals but also subclinically affected animals. Subclinically affected animals, despite appearing healthy have a decreased milk yield, resulting in significant losses in a herd with a high prevalence of disease. Clinically affected animals present with severe and rapid weight loss, submandibular oedema or ‘bottle jaw’, and scouring. Onset of clinical disease is often associated with stressors such as calving or poor nutrition. There is no treatment for Johne’s therefore infected animals are condemned to slaughter.
Stringent management of the disease is difficult due to its route of transmission and the inaccuracy of tests for Johne’s. To control the disease, infected animals must be identified and culled, and young stock must be prevented from becoming infected. Tests on blood samples, faecal samples, or milk samples can be used to detect Johne’s disease. These tests are useful in establishing a test and cull protocol, however animals must be shedding at sufficient levels to detect the disease. Eradication purely through test and cull is difficult but this approach helps to reduce high-risk animal numbers hence the level of exposure for other animals.
Preventing infection in calves and young stock isn’t practical due the routes of transmission for the disease and long environmental survival time. Reducing the level of exposure however can be done through several management strategies:
- Effluent management and avoiding using effluent for irrigation of paddocks.
- Removal of high-shedding cows from the herd contributing to the calf milk.
- Grazing heifers away from the dairy platform will reduce the exposure providing the grazing location isn’t highly contaminated.
- Vaccines are available for sheep but not cattle due to the interference of the vaccine with TB testing.
In conclusion, Johne’s is a disease that has the potential to have considerable implications for a herd. Careful management protocols are important due to reducing the exposure to young stock and removing high shedding animals from the herd.
If you have any concerns about Johne’s, please do not hesitate to contact your vet for further advice.