Johne’s disease is a chronic wasting disease caused by the microbacterium avium paratuberculosis, (MAP), resulting in gut inflammation, which means the gut struggles to absorb nutrients, leading to wasting and death.
Veterinary surgeon Dr Jenny Hull, of Black Sheep Farm Health, Northumberland, says: “There are two strains of MAP; one which affects cattle and one that affects sheep.
“However, there appears to be cross-transmission of these strains between cattle and sheep. This is important if sheep share grazing with cattle, especially if the herd is a high health status herd in a Cattle Health Certification Standards accreditation scheme for Johne’s.
“A recent study showed 64 per cent of UK sheep flocks tested positive for the disease.”
The main source of infection for uninfected flocks is bought-in stock. Disease is spread through faeces, as well as across the placenta and through colostrum and milk to lambs. Bacteria can survive in the environment for up to 18 months.
Johne’s in sheep is an iceberg disease, which means for every affected case there are often another 10 15 shedding but without signs.
Other iceberg diseases include maedi-visna (MV) and ovine pulmonary adenocarcinoma (OPA).
Infection is picked up mainly in young animals less than six months old. However, usually clinical signs do not become visible until two to three years of age.
Signs are non-specific and include:
■ Thin to emaciated, but bright, leading to being too weak to stand in the very late stages.
■ Increased death rate, especially among ewes.
■ Reduced lambing and rearing percentages.
■ Non-responsiveness to worm and fluke treatments.
■ Scouring (not a common feature of the disease, unlike cattle).
Dr Hull says: “It is important to know a flock’s status, especially if co-grazing with cattle. Testing can include submitting thin sheep for post-mortem or submitting 10 pooled faecal samples from thin, cull ewes.
“Also consider any other causes of lean ewes, such as fluke, MV, OPA, broken mouth, trace element deficiencies or simply undernutrition. Many of these tests can be combined in a cull or barren ewe investigation at weaning.”
There is no treatment for Johne’s. Control measures should focus on breaking the cycle of transmission from ewe to lamb and, thereby, reducing significant economic losses.
This includes culling thin ewes early and the use of vaccination. There is only one vaccine available for sheep which is a one-shot, lifetime injection.
Replacement sheep are best vaccinated at four weeks to six months of age. The younger they are vaccinated, the more effective the vaccine.
The vaccine reduces the amount of MAP shed by sheep by 90 per cent, thus reducing the cycling of infection. It is not an overnight cure, but a long-term reduction of disease within a flock. Vaccination has also been shown to reduce deaths from Johne’s by 90 per cent.
FOR FLOCKS OF UNKNOWN STATUS
■ Determine flock disease status by investigating barren or cull ewes before they are sold, or fallen lean ewes; this may involve a combination of sampling blood, dung and post-mortems
FOR FLOCKS WITH CONFIRMED JOHNE’S
■ Vaccinate all replacements each year for lifetime cover; this will not eliminate Johne’s from a flock, but significantly reduces shedding in faeces and keeps levels low enough to minimise the impact on productivity
■ Keep extra replacements and cull hard for any sign of wasting or leanness; shedding increases in the later stages of the disease
■ Buy vaccinated stock, such as rams, or vaccinate incoming stock
FOR FLOCKS CONFIRMED OR SUSPECTED AS FREE FROM JOHNE’S
■ Keep it out by breeding your own replacements
■ Buy vaccinated animals or vaccinate bought-in animals on arrival