Iceberg Diseases in Sheep

There are several chronic wasting diseases of sheep that are often insidious in onset, but can be economically costly. They are often referred to as “iceberg diseases”, as the noticeably thin ewes are the visible tip of the iceberg, with the vast majority of their negative impact hidden below the surface.

What are these diseases?

Johne’s disease

Johne’s disease is caused by the bacteria Mycobacterium avium paratuberculosis, and affects sheep, cattle, goats, deer and other ruminants. While animals are infected early in life, the disease usually only becomes apparent above 2.5 years of age. Symptoms in sheep and goats are often confined to weight loss.

Infection can be detected by post-mortem examination of thin ewes, blood sampling thin sheep or, the most sensitive method, submitting faecal samples from thin sheep for pooled PCR testing. The latter is available from SAC for £75.40 for 10 animals.

If Johne’s disease is having an impact on your flock, then consider either attempting to eliminate it, though this will be a slow process, or using the Gudair vaccine to reduce the impact it has. There is an accreditation scheme:


Caseous lymphadenitis (CLA) is caused by the bacteria Corynebacterium pseudotuberculosis. It causes abscesses in lymph nodes. These are mainly superficial, often round the head and neck. A rarer form is when the affected lymph nodes are mainly internal, often in the lungs.

It is spread through coughed-up bacteria (from sheep with the internal form) or discharging pus (from the superficial form) getting into cuts on the skin, or being ingested or inhaled.

Diagnosis may be made by culturing the bacteria from abscesses (either superficial ones, or internal ones found on post-mortem) or by blood sampling animals.

If you think your flock may be affected by CLA, contact us for more advice. Make sure you isolate any animals with discharging abscesses from the rest of the flock until healed up (ideally outside, as the bacteria survives less time in the cold and damp than in the dry).


Maedi-Visna (MV) is caused by the Maedi-Visna Virus. The virus causes the accumulation of scar tissue within the lungs and udder, leading to breathlessness, exercise intolerance and a hard udder with poor milk production. More rarely, infected animals develop neurological signs. Once infected, animals never clear the infection. Flocks with widespread infection often have significant reductions in productivity (20-40%).

Diagnosis is usually made by blood sampling. A screening test of up to 12 animals is available from the SAC for £37. If suspicious signs are seen on post-mortem this can be tested by sending off the affected organs.

Control depends on the level of disease in the flock. We can advise on the options available to you and what might be the best option for your situation. An accreditation scheme exists:


Ovine pulmonary adenocarcinoma (OPA) is caused by the Jaagsiekte Retrovirus (JSRV). This virus transforms the fluid secreting cells of the lung into tumour cells. Affected sheep show breathlessness, exercise intolerance and repeated moist coughing.

Diagnosis may be made on post-mortem with lab testing to confirm OPA. Ultrasound scanning of the lungs can identify affected sheep (once the tumours have reached a certain size) but not infected sheep. There is no blood test.