Johne’s disease is something that many dairy farmers haven’t had to deal with, but that doesn’t mean it isn’t present in their herd.
It is caused by the bacterium Mycobacterium avium subspecies paratuberculosis (MAP), which affects cattle and other ruminants.
MAP bacteria come from animals that are already infected and infectious on a farm. These are typically older animals.
These animals shed the bacteria in:
- Dung;
- Colostrum;
- Milk;
- The womb: Calves can be infected in the womb if the dam is infected. This becomes increasingly common as the disease progresses in the dam.
Johne’s disease causes loss of body condition, diarrhoea and death; there is currently no cure for the disease.
Because there is no cure, the best defence against Johne’s is to remove infected animals and maintain high levels of biosecurity on farms.
Some herds complete regular screenings to detect cows that may be, or are positive for the MAP bacteria or for cows that have antibodies present.
Johne’s disease
Ahead of the spring-calving season on farms, when a large number of calves will be born, farmers should be taking the opportunity to screen their herds for the disease.
There are two main tests that can be used to identify animals with Johne’s disease.
These are:
- Tests that find MAP bacteria directly;
- Tests that find an antibody response against MAP bacteria.
Tests looking for bacteria are performed on dung or milk samples while tests looking for antibodies are performed on individual animals’ blood or milk samples.
MAP bacteria can spread from infectious animals to susceptible animals for years before the first animal in the herd develops signs of Johne’s disease.
Unseen spread means that by the time the first animal develops clinical signs on a farm, there could be a number animals infected within the herd.
If cattle are culled for low performance then unseen spread may continue for much longer without an animal being seen with clinical signs.
Controlling spread
According to Animal Health Ireland (AHI), some common management practices can dramatically increase the rate of the spread of Johne’s disease on a farm.
Spread occurs whenever an infectious cow feeds or is in direct or indirect contact with several calves/young animals that are not her own.
This can occur when pens are inadequately cleaned between calvings; by feeding pooled colostrum or milk; having group calving accommodation; having adult cows share accommodation with several calves or young animals; and spreading slurry from infected cows onto land grazed by calves and young animals.
AHI said that these activities present an increased risk of rapid spread of Johne’s disease on a farm.
Purchasing stock poses a risk too, so when purchasing stock from farms it may be worthwhile finding out about the herd’s health status and whether Johne’s disease has been screened for.
Colostrum management is also vital and any cows that are suspected of having Johne’s should not be used to feed calves.
Milk from these cows poses a risk to calves and could result in them becoming infected with the disease.