Johnes disease is caused by a bacterium similar to TB called MAP which is resistant to most antibiotics making it impossible to treat in ruminants. It also has a thick tough outer shell and can survive in slurry, on pasture, and in water for many months.
Johnes disease is found worldwide and has become widespread among New Zealand dairy herds since it was first diagnosed in 1912. While the incidence of clinical disease in many herds is quite low, the subclinical phase results unseen production losses such as reduced yields, poorer fertility and susceptibility to other diseases.
The MAP bacterium is spread through dung and mostly infects new animals by ingestion of contaminated pasture, colostrum/milk, feed & water. Occasionally it can infect a foetus before birth in heavily infected cows. It affects the gut, gradually causing thickening and damage which eventually leads to the point where nutrients cannot be absorbed and protein from the body leaks into the faeces. It is usually several years after infection before this occurs and animals will shed the bacterium in their faeces months or years before they finally start showing the usual symptoms which include:
- progressive weight loss
- bubbly scours
- reduced milk yield
- swelling under the jaw (bottlejaw)
It has been shown that once MAP has been on a farm for several years, for every clinical case of Johnes there are about 10-15 other animals who are silently incubating the disease and spreading the bacteria in their faeces. The thin scouring cows just represent the tip of the iceberg!
Breaking the infection cycle
The risk of infection is greatest in calves during the first few days of life but remains high for the first 6 months and can still be a risk up to 18 months of age. After 18 months animals are generally resistant to new infections but if exposed to a very high level of infection they can still acquire the disease. Control measures are therefore focused around preventing new infections while identifying and culling high shedding cows.
Making use of LIC testing of milk samples at the herd test will help in the identification of those high shedding cows before they start showing symptoms, thereby reducing the infection of young stock through slurry & avoiding the unexpected forced culling of many of these cows in the following season as about 1/3rd of high positive cows will develop clinical johnes disease after calving in the next season. However because of the very long incubation period cows do not show up on a test until late in the disease process, therefore using a “test & cull” approach alone will never be successful and focusing on preventing new infections in young stock is just as important.
We can request milk testing, provide interpretation and help you in identifying the most effective ways to limit young stock exposure to MAP to produce a tailor made plan to suit your farms specific requirements.
We plan on hosting an evening seminar this winter which we will advertise nearer the time. In the meantime if you are interested in taking Johnes control further then come and talk to Teressa or Sam at the clinic.