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23rd February, 2019

Johnes Disease – Explained

Johnes Disease is caused by the bacterium Mycobacterium Avium Paratuberculosis (MAPs), a distant relative to the bacteria that causes tuberculosis. The disease is a contagious, progressive bacterial infection that restricts the absorption of nutrients.

An example of a cow with Johnes Disease

Clinical signs are

1. long lasting diarrhoea and
2. extreme weight loss despite maintaining appetite.

Long Incubation Period

Animals usually become infected with Johne’s disease as calves. No sign of the disease is seen for years with the average incubation period of 5 years but it can range from 2-10 years. Animals who are infected may appear normal and spread the disease to other animals in the herd before showing signs themselves.

How is it spread?

Johne’s Disease usually enters a herd through the purchase of an infected animal that sheds the bacteria in their manure. This in turn can infect feed and/or water and spread the disease within the herd. Calves are particularly susceptible to picking up the disease. Calves can also pick up the disease from their mother’s milk.

How can it affect my herd?

Johne’s Disease can have a significant financial impact on the herd through
• Reduced Milk Production
• Increased Involuntary Culling
• Loss of Heifer Sales
• Reduced Beef Production

How to detect Johne’s Disease

1. Milk Samples.
Colostrum is the primary source of Johne’s disease. It can be identified by milk samples taken from individual cows at the time of milk recording.
2. Dung Samples
Further testing will be required using dung samples to get a definitive assessment of the disease in the herd.

Stress will exacerbate the expression of Johne’s
Any form of stress in the herd will exacerbate the expression of Johne’s disease. Therefore if your herd is losing body condition because of poor silage quality and the failure to feed a good quality supplementary concentrate, the immune system of your cows will be challenged. This will increase the risk of clinical cases of Johne’s disease.

What is the most important thing I can do?

The single most important thing any farmer can do is to manage calf rearing particularly for potential breeding animals. When multiple infected cows are identified, the risk of baby calves becoming infected is high and changes need to be made to the management of the Johne’s cows, especially around calving.

The Dept. of Agriculture offer a list of guidelines on Johne’s disease.

Animal Health Ireland plan to implement an eradication programme for Johne’s disease, which is welcome. Biosecurity is an integral element in control. Many continental beef cattle were imported as foundation females over the past 10 years. These have been a source of the disease transmission in Ireland.

In conclusion, Body Condition Scores (BCS) need to be managed properly to avoid loss in the dry cow period and no more than 0.5 BCS loss in the first six weeks post calving. Colostrum management has to entail the avoidance of bulk feeding of same as the risk of transmission of Johne’s disease is greatest in colostrums.
Dr. Dan Ryan is a Reproductive Physiologist and can be contacted at