Johne’s Disease (Paratuberculosis)
Summary written by Karen Stevenson BSc, PhD and George Caldow BVM&S, MSc, CertCHP, DipECBHM, MRCVS
Taken from Moredun Foundation Newsheet Volume 4 No. 13 (2006)
- Johne’s disease (paratuberculosis) is a chronic intestinal disease that predominantly affects ruminants. It is characterised by severe weight loss, loss of body condition and diarrhoea (in cattle) culminating in death.
- The disease is caused by infection with a bacterium called Mycobacterium avium subspecies paratuberculosis (referred to as Map), which is a very slow growing organism that can survive for long periods in the environment.
- The disease is spread mainly by the ingestion of faeces from an infected animal. Infected animals can also pass on the infection in colostrum or milk and across the placenta to unborn animals. The organism can be found in the semen of infected bulls, but it is thought that this is of negligible importance in the spread of the disease.
- Young animals are more susceptible to infection than adults.
- The disease has a long incubation period of 2 to 4 years during which time the animal may show no clinical signs of disease and shed Map intermittently. Animals in this stage of the disease are said to be ‘subclinically infected’ and act as ‘carriers’ of the disease.
- The disease is usually introduced to a farm through the purchase of subclinically infected stock that show no sign of the disease.
- Different strains of Map can infect a broad range of hosts including cattle, deer, sheep, goats and some wildlife species.
- The diagnosis of Johne’s disease is problematic and there is no single diagnostic test that can detect all stages of the disease. Subclinically infected animals are particularly difficult to diagnose and test results may be negative.
- Diagnostic tests are available to detect Map in faeces and milk. These are faecal smears, bacteriological culture and polymerase chain reaction (PCR). There are also diagnostic tests that detect circulating antibodies to Map in infected animals. The most common test is the enzyme-linked immunosorbent assay (ELISA).
- There are two main objectives in the control of Johne’s disease; 1) Prevent the introduction of the infection by purchasing uninfected stock and 2) Reduce the impact of the disease in infected herds and move towards freedom from the disease.
The fine detail of control varies with the species and the production system, but biosecurity and reducing the amount of faecal contamination that young stock are exposed to are central to any control programme.
- Implementing a Johne’s disease control programme after consultation with your veterinary surgeon will help to protect your animals against Johne’s disease.
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