Production-limiting ‘iceberg diseases’ in sheep impact the health and welfare of affected animals and their lambs and reduces the profitability, sustainability and resilience of the industry.
Awareness of iceberg diseases is low, with a recent survey suggesting fewer than 5 per cent of farmers screen for them routinely.
Lis King, AHDB sheep scientist, says: “The extent of the problem within a flock can be underestimated as visibly diseased sheep are usually just the tip of the iceberg, which is why these diseases are sometimes referred to in the industry as ‘iceberg’ diseases.
“The impact of flock infection with these diseases is not immediately obvious to the owner or their vet, which means there may be significant unnecessary and preventable production losses before the disease is realised.
“None of these diseases are treatable with medication or antibiotics and the complex network of sheep movements between breeders presents significant challenges for control of these diseases and biosecurity.
“It is essential that farmers begin to develop an appreciation of the existence, significance and management of these iceberg diseases in order to stem the spread. “
ICEBERG DISEASES OF EWES
A NEW AHDB technical manual for vets, consultants and farmers provides practical industry guidance and describes how to prevent and manage these diseases.
It can be downloaded from the AHDB website at ahdb.org.uk, or hard copies ordered by email at firstname.lastname@example.org, or by phone on 024 7647 8834.
Farmers are advised to:
Be vigilant: Think about thin ewes; why are they thin? Consider screening for disease or send cull ewes for post-mortem examination.
■ Post-mortem: For unexplained deaths, consider the value of a post-mortem examination.
■ Testing: Where abortion occurs, consider sending samples for testing.
■ When buying replacement sheep (rams and ewes), keep diseases you do not have out of your flock:
■ Cross-infection between cattle and sheep: Cattle farmers who are in BVD or Johne’s control programmes that have sheep in close contact with cattle should consider screening sheep for border disease and ovine Johne’s to prevent hampering efforts to control BVD and Johne’s in their cattle.
■ Ask for advice: Before screening for disease, discuss with your vet what the tests involve and the implications of a positive result.
■ Border disease: Also known as hairy shaker disease, it is a less commonly diagnosed cause of abortion and is closely related to BVD.
Both diseases are caused by members of the pestivirus family of viruses. Border disease virus (BDv) is present throughout the UK, with an estimated prevalence of between 30.4 and 37.4 per cent. Incidence has significantly increased in the last 40 years.
The impact of BDv infection is widespread as it affects both reproductive performance and lamb performance.
■ Caseous lympadenitis: This is caused by the bacteria corynebacterium pseudotuberculosis and was first diagnosed in the UK in goats in 1990, then in sheep in 1991.
It is now classed as endemic. Initially thought to be found primarily in pedigree flocks producing terminal sires, it is now increasingly seen in commercial UK flocks, usually introduced by infected rams.
It has been suggested that feeding via troughs facilitates the spread of CLA within UK sheep systems. Experience from other countries shows that, if left uncontrolled, up to 60 per cent of adults within a flock may become infected
■ Maedi visna: A highly infectious disease, caused by the maedi visna virus. It is characterised by a long incubation period of anywhere from several months to years, which leads to a progressive loss of condition, reduced flock production and poor economic performance.
It may only be noticed when prevalence within a flock reaches 50 per cent. Infected ewes have a 9 per cent reduction in conception rate compared to uninfected ewes of a similar age within the same flock.
■ Ovine Johne’s disease: This is also known as ovine paratuberculosis. It is a chronic bacterial disease of the small intestine caused by mycobacterium avium subsp. Paratuberculosis (MAP). MAP was first reported in cattle in Europe in 1895 and has spread throughout the developed world and parts of the developing world.
It is a multispecies pathogen, well-recognised as a major production-limiting disease in cattle and goats in Europe. In the UK, there is no reliable estimate of prevalence, but a recent survey of 51 larger commercial sheep enterprises found 64 per cent tested positive for the disease.
■ Ovine pulmonary adenomatosis (OPA): Also known as jaagsiekte or ovine pulmonary adenocarcinoma, it is found globally, with the exception of Australia and New Zealand, where it has never been reported, and Iceland, which is the only country to have eradicated the disease.
The prevalence of OPA in the UK is not known, although abattoir cull ewe and fallen stock surveys have reported a prevalence of between 0.9 and 5.6 per cent. With an incubation period of six months to three years, the disease is rarely seen in lambs, with most clinically affected sheep between three and four years old. Subclinical sheep may be culled from the flock without diagnosis.