Johne's disease

What is it?

• A chronic infection involving the small and large intestines
• Characterised by persistent diarrhoea, gradual emaciation and great weakness

Signs

• Very slow onset with infected cattle unlikely to show symptoms for some time
• Pointers to look for include an unexplained drop in milk yields, diarrhoea in an individual adult animal and loss of condition

Disease spread

• Calves up to six months of age are the only animals in which infection can become established, but because the incubation period is two years or more, the disease may not be seen until after first or second calving at least
• The calf becomes infected usually from contact with faeces from infected cows in the calving yard or from being fed contaminated colostrum or milk from infected cows
• There is evidence it can persist in a subclinical form for eight to 10 years or more, and may be a cause of chronic poor growth and disappointing production. These cows will be intermittent excretors of infection, spreading the disease within a herd
• A stressful calving may trigger the disease

Testing

• Once the bacteria starts to be shed in the dung, which typically occurs between two and seven years of age, the animal will produce antibodies that can be picked up in an individual cow milk test

Treatment

• Once the disease is well established, it is invariably fatal

Prevention

• Attention should be paid to stop the spread of infection, especially to calves
• Identify all test-positive animals and keep them separate, or cull them
• Clean individual calving pens between calvings to reduce exposure of newborns to manure from infected adults
• Avoid manure contamination of feed and water sources
• Remove a calf from an infected mother immediately after birth to stop disease spreading through colostrum and dung
• Only use colostrum from Johne’s-negative animals
• Only buy stock from herds with a known health status

Vaccination

• Vaccination is an option for some herds but is generally viewed as a ‘last resort’ for high incidence herds where all other options have been explored and found impractical
• It should be given to calves in the first month of life. While it will reduce the number of animals developing the later signs of the disease, it does not remove the infection from the herd. The vaccine modifies the animal’s response to the pathogen but does not eliminate infection entirely
• Once cattle have been vaccinated there is no test to distinguish between true infection and vaccinated cattle.
• There is no simple exit strategy from vaccination; the herd remains infected as vaccinated animals still shed infectious bacteria

Johne’s and bTB

• Vaccination complicates the interpretation of diagnostic tests and can also interfere with the TB skin test leading to false positive reactions