Johne’s disease (JD) is a relatively unknown disease which affects sheep, goats, cattle and even camelids throughout the world and is endemic in most European countries. It is widespread throughout the UK and becoming a key issue affecting the UK sheep industry today.
Prevalence of JD is difficult to estimate as no single test can reliably detect all stages of infection and disease. After a prolonged incubation period of up to 2 – 4 years from initial infection, symptoms can include chronic weight loss, scouring, reduced productivity and a reduction in fleece quality in sheep. The disease is caused by a bacterium called Mycobacterium avium subspecies paratuberculosis. It affects the animal by slowly proliferating in the intestine, which stimulates the immune response and causes inflammation. The intestine wall then becomes thickened which leads to absorption of nutrients being impaired, so the animal is effectively being starved of nutrition eventually leading to weight loss, loss of body condition and death.
The main route of infection is via ingestion of contaminated food and drink and young animals in particularly are susceptible to infection via this route. The disease is particularly troublesome as ‘subclinically’ infected animals, i.e. animals who show no visible symptoms, cannot be reliably detected using the current diagnostic tests, meaning that infection can be easily spread by apparently healthy animals. Several studies have been conducted investigating the zoonotic potential of paratuberculosis and at present no connection has been shown between contact with animals and Johne’s disease. However some researchers have suggested a link between Johne’s disease in animals and Crohn’s disease in humans, although this link has yet to be proved.
Treatment for JD requires prolonged use of antimicrobials and is extremely expensive, making it inappropriate for a flock of sheep and is only ever considered for particularly rare or valuable animals, for example in zoos or wildlife parks. Culling out suspected cases is therefore the simplest option. Vaccination, however, is considered an affordable option for both sheep and goats and is being widely used in Spain. The vaccine reduces clinical disease and shedding but does not prevent infection, so the disease can still be transferred between animals, and it is not possible to differentiate between vaccinated and naturally infected animals using current diagnostics.