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JOHNE’S DISEASE SUMMARY
This is a chronic, progressive, wasting, ultimately fatal disease that affects all ruminant species (including cattle, sheep and goats) and is caused by the bacteria Mycobacterium Avium subspecies Paratuberculosis. The strains tend to be fairly species specific and if cross-infection does occur it is likely to be rarely. The organism invades the cells of the intestine wall resulting initially in thickening and then in malfunction. Affected animals usually start to loose condition and then start to scour. Death or culling usually follows within a short period. There is no treatment. HI Health will concentrate on preventing infection of “free” herds and, where possible, eradicate Johne’s from infected herds.
The agent is shed in large numbers in the faeces of affected animals but can also be passed on to calves via colostrums. Animals are infected by ingestion of the agent and calves are probably infected in utero as well. Young animals are considered most at risk from infection. Although the signs of disease can start as early as six months of age, usually they occur in cattle older than 18 months. In heavily infected herds most cases occur in animals aged 2 to 4 years. Infection is almost always introduced into the herd through the purchase of infected breeding stock. The progeny of confirmed cases are almost certain to be infected and so should be culled from the herd when the positive status of their dam is confirmed. Strains of the organism appear to be specific to each different ruminant species and if cross infection occurs it is rare.
Johne’s testing may be carried out on blood and/or faecal samples, but the majority of Johne’s testing done through HI Health is the ELISA test for antibody in the blood. A major drawback of these tests is that they cannot detect affected animals until shortly before they start to show clinical signs. This slows down the process of eradication.
Exclusion of Johne’s disease from the herd is therefore a sensible option given the potential losses associated with herd infection. Since concern about potential for human disease is frequently raised those who are currently free should make every effort to retain this status. It is currently believed that eradication is worth attempting although the process is difficult especially in heavily infected herds.
Johne’s Disease Screening & Eradication Programme
Herd Test
For Johne’s disease all animals of 2 years of age and over are blood
sampled. In herds where there has been no evidence of the disease we would
follow up any positive animals by testing by PCR. Otherwise all reactors
are removed. The offspring of any cows that react should also be removed
from the breeding herd and their current calf not retained for breeding.
The herd test is repeated each year.
Johne’s Disease Monitored Free Programme
To be monitored free from disease, two clear tests of all animals of 2 years
and over are required at an interval of not less than 12 months and not
longer than 24 months apart. If a test and cull programme has been carried
out before this, three clear annual tests are required. As the incubation
period for Johne’s disease is measured in years it is not possible
to give an absolute assurance of freedom from infection until many years
have passed. That is why the programme is called “Johne’s Disease
Monitored Free”.
The herd test is carried out every second year and all cull cows are tested in the intervening year. However any non-home bred animals from herds that are not Monitored Free of Johne’s disease must be tested annually. The cull screen involves collecting blood and faecal samples from all animals two years of age or older prior to removal from the herd. In every case the blood test will be carried out but the faeces samples will only be cultured where the animal tests positive by the blood test. Where it proves impractical to test cull animals a full herd test must be carried out annually.
Johne’s Disease Bio-security Measures
Added animals from herds that are not Monitored Free of Johne’s disease
always constitute a risk of infection and therefore should not be added to
the herd if possible. Where this is not possible it is preferable to have
them blood sampled for Johne’s disease on the farm of origin. If positive
the animal cannot enter the herd and there will considerable savings in time
and expense.
On entry to the herd the animal/s must be placed in isolation and tested
in accordance with CHeCS rules. The animal must be tested with blood and
faecal samples. Only when the results are negative can the animals enter
the herd. Where a group of animals has been purchased from one source and
one or more tests positive, none of that group can enter the herd.