Johne’s disease is a contagious, bacterial disease that is responsible for considerable financial losses in infected livestock.
It isn’t just the financial costs that need to be considered, Johne’s disease also imposes welfare issues on infected animals and there are growing concerns over implications to public health.
Johne’s disease is caused by bacteria that are shed in to the environment by infected animals including sheep, cattle or other ruminants.
About the Bacterium
The causative bacterium of Johne’s disease, also known as paratuberculosis, is Mycobacterium avium subspecies paratuberculosis, which is commonly abbreviated to MAP.
MAP is quite resistant and can withstand freezing, high temperatures and drying. These factors make it difficult to effectively control the bacterium in the environment making spread from environment to livestock more likely.
Fortunately, MAP can only replicate within an animal host, but the bacterium is easily shed in the faeces of an infected animal and can contaminate the environment. Food, water, bedding and all other aspects of an animal’s surrounding environment can easily become a source of infection for uninfected ruminants.
Signs & Symptoms
Wasting and diarrhoea are typically the only clinical signs that indicate an animal is suffering from Johne’s disease. This makes it difficult to diagnose without further investigation as diseases such as liver fluke, coccidiosis and parasitic gastroenteritis share the same symptoms.
Rapid weight loss is typical of Johne’s disease and will continue to be a problem even when the infected animal is being fed an appropriate diet.
MAP causes internal inflammation of the intestine that reduces its ability to function correctly. It is this reduced function that contributes to the rapid weight loss and diarrhoea associated with Johne’s disease.
Not all animals infected with MAP will show clinical signs of the disease. In fact, for every animal in a flock that shows signs of infection, there will be numerous other carriers of the bacterium not displaying symptoms, these are known as asymptomatic carriers.
It is asymptomatic carriers that make controlling the disease difficult, as it is almost impossible to control what cannot be seen. This means the infection can be prevalent in a herd without clinical signs of the disease being apparent.
Animals can become infected at any stage of their life and remain without symptoms for the duration of infection, but there is always the risk of the disease developing at a later date in carriers. The exact cause of the transition from a carrier state to developing Johne’s disease is not yet known.
Controlling Johne’s Disease
To effectively control Johne’s disease, the spread of MAP must be stopped and its introduction in to farms, flocks and herds prevented. This requires that infected animals are removed from the herd when diagnosed with Johne’s disease, but the initial diagnosis of an entire herd can prove quite costly.
To prevent MAP being introduced on to a farm, careful considerations must be made when purchasing in new stock. Ensuring that new stock is Johne’s free will prevents asymptomatic carriers arriving on your farm and shedding MAP unknowingly, posing a potential infection risk to the rest of your flock or herd.
One of the primary routes of infection for newborn lambs and calves is via their mother’s colostrum. Young animals consuming MAP-infected colostrum will likely go on to be asymptomatic carriers of MAP or may even go on to develop Johne’s disease.
To prevent the transmission of Johne’s disease to newborn lambs and calves, we should therefore provide them with Johne’s disease-free colostrum.