The term “Iceberg disease” is used to describe a disease where only a small proportion of the infected population are noticed, ie the tip of the iceberg. The remaining infected population are the iceberg that is hidden underwater. The small proportion of the population that reach the tip of the iceberg are those clinically, and often chronically, affected.

In sheep, the classic iceberg diseases include: MV (Maedi Visna), Johnes disease, CLA (Caseous Lymphadenitis), ovine pulmonary adenocarcinoma (OPA), and Border disease. The extent of a problem within a flock may be underestimated due to the large percentage of sheep affected that only show vague or no clinical signs, but these cause inefficiency and are production-limiting.
These diseases should be included in investigations into poor flock production, especially where poor fertility and thin ewes are a problem. Investigations can have large costs associated with them due to vet time and testing, as well as cost of replacements further on, but long-term production and farming efficiency should increase and costs recuperated.


A highly infectious viral disease, characterised by a long incubation period of several months to years which leads to a progressive loss of condition. It is predominantly spread via milk and nasal discharge, but infection can also occur via the placenta to unborn lambs. Clinical disease can be respiratory or neurological, but these are rare to see.

An area of MV within an infected lung. Note the swollen tissue where the lung has been cut and the pinkish-grey discolouration Photo source: Farm Post Mortems Ltd

Johnes Disease

A bacterial disease of the small intestine. Resulting in chronic inflammation and poor absorption of nutrients, leading to reduced fertility and chronic weight loss. The equivalent disease in cattle receives a lot of attention and has had control schemes in place for many years. In sheep, there are no obvious or specific signs – unlike in cattle where profuse diarrhoea and progressive weight loss are seen. The disease is spread in the same way, by faecal contamination of water and feed, and off teats when lambs are sucking, with the highest risk period for becoming infected being in early life.

Enlarged lymph nodes (arrows) and oedema may also occur in cases of Johne’s and may accompany intestinal thickening and pigmentation


Sheep become infected with CLA bacteria through abrasions on the skin. The bacteria then travel to the nearest lymph node where abscesses develop. Most commonly we see lesions around the head and neck. Infection is spread between sheep by direct contact with discharging lesions or airborne transmission; thus grouping sheep for management purposes increases risk of spread. The bacteria can also survive in faeces, hay and straw for up to 55 days, even longer in low temperatures.

A large CLA lesion in the mediastinal lymph node within the chest cavity. Note the concentric rings of an onion-like lesion, however, this should not be used to definitively diagnose CLA. For confirmation, bacterial culture is required to isolate C. pseudotuberculosis


Contagious lung tumour caused by a virus related to the MV virus. It is a major production impairment and the animal can eventually reach a stage of disease where they suffer respiratory distress. Transmission is via oral discharges and milk, with some spread likely through the virus’s survival in the environment.

A classic lesion of OPA, although the tumour may be found throughout the lungs Photo source: Hal Thompson, Richard Irvine and Noelia Yusta, School of Veterinary Medicine, University of Glasgow

Border disease

This is a viral disease that causes ‘hairy shaker’ lambs. It is closely related to bovine viral diarrhoea virus (BVD) in cattle. Border disease can be spread in several ways including nose-to-nose contact, via the placenta to the unborn lamb, and in colostrum and milk. Birth of a persistently infected lamb results in a hairy shaker, and unfortunately very few persistently infected lambs survive. If a non-pregnant ewe becomes infected with the virus, poor fertility may follow. Due to the nature of these iceberg diseases, they are nearly all associated with older animals. It is time to stop attributing weight loss and reduced production to ageing and to start investigating the possible reasons.

Lambs affected by BD may present in different ways, including clinically normal, weak and small with failure to thrive, or as classic ‘hairy shaker’ lambs with nervous signs and fleece changes (increased crimp or increased pigmentation of the fleece).

Contact the vet team for help on where to start.

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Photo source: Farm Post Mortems Ltd & Hal Thompson, Richard Irvine and Noelia Yusta, School of Veterinary Medicine, University of Glasgow