Although commonly associated with cattle, Johne’s disease can also affect sheep and often goes unnoticed within flocks as it is more difficult to diagnose. Wendy Short speaks to expert Dr Karen Stevenson to find out more.
There is no doubt Johne’s disease costs the sheep industry thousands of pounds in lost productivity each year, but it is impossible to provide accurate figures on financial losses says Dr Karen Stevenson of the Moredun Research Institute, Edinburgh.
“In cattle, one obvious indication of Johne’s disease is persistent and severe scouring. Sheep do not always scour, but they will lose body condition and become emaciated,” explains Dr Stevenson.
“However, these symptoms can also be caused by a heavy worm burden or several other possible conditions. Most thin ewes are culled from the flock without being tested or sent for post-mortem examination.
“It is thought cross-infection can take place between sheep and cattle, because identical strains of the causative bacterium have been isolated from the two species on the same farm. However, at present it is not clear how easily or frequently this occurs.”
There are two major strain types of Johne’s, she explains. Both infect sheep, although one type is thought to be more virulent for sheep than the other.
Johne’s grows very slowly under laboratory conditions and this factor hampers the progress of research.
The debilitating effect of Johne’s is caused by inflammation of the intestines, says Dr Stevenson. An affected sheep will continue to eat but its digestion is impaired and it cannot absorb sufficient nutrients.
The animal will gradually waste away, with the disease eventually proving fatal.
Infected animals shed Johne’s in their faeces and milk and the disease has a long incubation period - usually two to four years.
Sheep with Johne’s may harbour the disease for this length of time without showing any outward signs, although they may infect others, including any cattle on the farm.
While Johne’s cannot multiply outside host animals, it can survive for many months in the environment and is resistant to both hot and freezing temperatures. Some, but not all, infected animals can be detected using the Johne’s serum test and there are tests to detect
Johne’s in faeces. However, no single test can positively identify the disease at all its stages. The most reliable method of confirming Johne’s can only be carried out post-mortem.
“There is a lifetime Johne’s vaccine for sheep, but rather than safeguarding the animal from contracting the disease, it will only act to limit the clinical symptoms and reduce bacterial shedding,” says Dr Stevenson.
“Vaccination is used widely in Spain, where significant increases in productivity of milking flocks have been reported as a result. A further complication is that vaccinated animals produce antibodies, so they cannot be distinguished from those suffering from Johne’s when tested.”
Young animals are much more vulnerable to infection than adults. In most cases, the route of infection is the ingestion of contaminated faecal material, commonly through contaminated pasture or bedding.
Lambs can contract Johne’s from their dams while still in the womb and it can also be passed on via colostrum and milk. Another route of transmission is through infected semen.
While Johne’s is notoriously difficult to eradicate, there are several measures producers can take to reduce the risk of their flocks becoming infected says Dr Stevenson.
“The greatest risk is buying in diseased animals. Ideally, replacements should be bought from farms which have had no positive diagnoses for at least three years.
“It is also a good idea to try and minimise faecal contamination; lambing pens and water troughs should be kept as clean as possible, for example.
“Some producers remove lambs before they suckle and rear them on pasteurised colostrum and milk powder but this is not a practical option on most farms. A more suitable alternative would be to vaccinate the lambs.”
Farmers also need to keep in mind it is a risk to co-graze sheep and cattle if either species is infected with Johne’s. It may be worth considering the vaccination of sheep in these circumstances. Vaccination of cattle is not advisable as it interferes with tuberculin testing she adds.
Such are the complexities of Johne’s disease management Dr Stevenson strongly advises individual producers develop control programmes on their farms, in consultation with their vet.
Her research work at Moredun relies on farmers sending in sheep with suspected Johne’s so the disease and its effect can be studied in more detail. She would welcome enquiries from anyone who believes the disease may be present in their flock.
“We have voluntary control programmes for Johne’s in cattle, but due to the difficulties of diagnosis and the high costs of control, plus the low value of sheep, no such arrangement exists for ovines.
“At the moment, there is no funding available for a survey to determine the prevalence of Johne’s in our national sheep flock and tackling the disease in sheep remains a challenge. The problem is exacerbated by the nature of Johne’s, as well as the issues surrounding its diagnosis and treatment,” she says.