Johne’s disease, also known as paratuberculosis, is a bacterial disease of cattle and other ruminants for which there is no cure.
It is caused by the bacterium mycobacterium avium subspecies paratuberculosis (MAP).
Cattle usually become infected as calves early in life by drinking or eating milk or food contaminated with the bacteria, which are shed in the dung or milk of infected adult cattle.
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.
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.
Johne’s disease
Being that the majority of Irish cows calf during the spring, the spring is a high risk period for possible spread of infections.
Regular herd screening is the best method of control and prevention within herds, it is likely that some herds that have never screen could have infected cows that have not yet shown clinical signs.
There are two main tests that can be used to identify animals with Johne’s disease.
They 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.
Prevention
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 have 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.
Animal health
Some farms already screen their herds for MAP bacteria, but many don’t. Screening is highly effective and should be performed on farms.
Positive cows or inconclusive cows should be culled from the herd and their off-spring should also be looked at, as they are likely to have come into contact with the bacteria.
Johne’s disease is currently not declared to be zoonotic, but there are conflicting data on the involvement of the causative organism in Crohn disease – a chronic granulomatous enteritis of unknown cause in people.
However, MAP is consistently detected by PCR in people with Crohn disease.
So, it is advised when dealing with positive or potentially positive animals that you are cautious at the very least.