Paratuberculosis (Johne’s disease) in Cattle
What is Johne’s disease?
Johne’s disease is an infectious disease in cattle and other ruminants. It is caused by the bacterium Mycobacterium avium subspecies paratuberculosis, commonly known as MAP. It is closely related to the organism that causes tuberculosis and can survive on pasture for many months. The infection is very contagious and young animals are more susceptible to the disease than adults. This disease damages the intestines and results in persistent diarrhoea, weight loss and protein-losing enteropathy, followed by death.
What causes it?
The bacterium attaches to the wall of the lower small intestine, known as the ileum. As an immune response, infected tissues then attempt to regenerate new healthy tissue, which leads to visible thickening in the intestines. This, subsequently prevents nutrient absorption, resulting in weight loss.
In a dairy herd, Johne’s disease reduces milk yield well before other signs of the disease occur. Persistence of MAP in the environment and lack of effective vaccines lead to complicated on-farm control. This in turn causes significant economic loss in an affected herd. Evidence suggests a link between MAP and human disease, and some countries have now begun to approach Johne’s disease control as a food safety issue. In many infected herds, the annual mortality rate from Johne’s disease is estimated to be as high as 5 to 10 per cent.
How does it spread?
It is generally spread through the faecal-oral route. This is when animals eat bacteria from infected faeces along with contaminated grass or feed. Johne’s disease is also transmitted across the placenta, saliva and through milk or colostrum, but this is less common. When the microbe is excreted, it can then contaminate the soil or water.
What are the clinical signs of Johne’s disease?
Cattle usually become infected with M. paratuberculosis as calves but often don’t develop clinical signs until they are 2 to 5 years old. The most consistent clinical sign in sheep and cattle is weight loss, despite a good appetite. It is important to differentiate this disease from internal parasites which can cause similar symptoms. The infected animal takes months to years to show clinical signs of the disease. Because of this, an infected animal could be shedding the organism in its faeces and spreading the disease to other animals in the herd silently. There is currently no cure for Johne’s disease.
How do we diagnose it?
It is important to look out for signs of the disease and take steps to screen the herd for presence of MAP. There is a blood test that detects the antibody to MAP produced by infected cattle. However, cattle tend to only produce this antibody quite late in the infection. As a result, sometimes diagnosis does not happen until necropsy.
Detection of MAP in faecal samples has an advantage over blood testing, as it confirms the presence of the bacterium. It is a non-invasive test, which prevents additional stress to the animals. PCR tests can detect shedding animals. These tests can detect the pathogen itself in faeces and milk with very high sensitivity. However, if an infected animal is not shedding the bacterium, it will not show up in the faeces or milk. As a result, false negative results are possible. Usually, in a herd with Johne’s disease, up to 60% of infected cows would not be shedding at any particular time.
How do I prevent Johne’s disease?
While it is difficult to completely prevent a wide-spread disease like Johne’s disease, there are measures you can take to reduce the likelihood of transmission.
- Maintain a closed herd
- Only buy animals if you know their Johne’s status
- Closely evaluate the body condition of all adult animals and test if any show signs
- Improve or maintain strict biosecurity to control many diseases
- Avoid using equipment potentially contaminated with manure or soil from properties with the disease, or forming herds with an unknown disease status
- Do not feed colostrum from cows of unknown MAP test status to calves
- Use colostrum from MAP test‐negative cows can to build a colostrum bank
- Clean teats thoroughly to ensure clean colostrum collection
How can I manage Johne’s disease in my herd?
If testing shows a positive result for Johne’s disease in a herd, it can then be managed. There are three basic strategies for management, and a combination of these will likely be the best option.
- Prevent new infections
Maintaining very strict biosecurity measures will help to prevent new infections in a herd. Keep MAP-positive cows away from healthy cows and ensure they do not pass the infection on. Calves are highly susceptible to the bacterium, so it is vital to make sure they do not catch it. Ensuring good hygiene and careful management will help to reduce risks.
- Manage infected animals: test‐and‐manage/cull programs
Test all animals so that you know their status. Once tested, groups can be divided into positive and negative animals, and managed accordingly. In some cases, culling of infected animals might be necessary.
- Improve resistance to paratuberculosis
While vaccination is currently not that common, there is a vaccine available in the USA. In the rest of the world, selective breeding is a more important method of disease control. Some animals might be especially susceptible to the disease, and these should not be allowed to breed. Conversely, if an animal is resistant to the disease, it is a good candidate for breeding.
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Sweeney, et al. (2012). Journal of Veterinary Internal Medicine, 26(6), pp.1239–1250.