Lurgan Vet

Iceberg Diseases in UK Sheep flocks

Understanding Iceberg Diseases

"Iceberg diseases" are chronic infectious conditions where only a small number of visibly affected sheep are seen — the "tip of the iceberg" — while many more animals silently carry infection underneath the surface.

These diseases quietly reduce flock health and productivity, often without obvious early signs.

The 5 iceberg diseases affecting UK flocks are:

Ovine Pulmonary Adenocarcinoma (OPA)
Ovine Johne’s Disease (OJD)
Maedi Visna (MV)
Caseous Lymphadenitis (CLA)
Border Disease (BD)

Because symptoms often develop late, iceberg diseases can spread extensively before being detected.
They impact flock performance by:

Shortening ewe lifespan and increasing culling rates.
Reducing fertility, causing more barren ewes and weaker lambs.
Slowing lamb growth and survival.
Leading to significant financial losses through poorer performance and higher replacement costs.

A study across 75 flocks in England and Wales (APHA Thin Ewe Project) found that 41% had at least one iceberg disease, with 32% of flocks affected by OJD and 11% by OPA.

OPA

What is Ovine Pulmonary Adenocarcinoma (OPA)?

OPA is a contagious lung cancer of sheep, caused by the Jaagsiekte Sheep Retrovirus (JSRV).
The virus infects the lung’s alveolar epithelial cells, causing them to grow uncontrollably and form tumours. After infection, sheep often remain healthy-looking for months or even years before developing signs. By then, lung damage is usually severe. OPA spreads mainly through respiratory secretions but there is also possible spread through colostrum and milk.

Symptoms of OPA

Early signs are subtle. Affected sheep often appear outwardly healthy while disease is quietly progressing.
When symptoms do appear, they include:

Breathing difficulties
Weight loss despite a good appetite
Poor exercise tolerance

In advanced cases, clear fluid may drain from the nostrils when the animal’s back end is lifted.
However, this only occurs in around 50–60% of positive cases.

Because symptoms are non-specific, OPA cannot reliably be diagnosed based on clinical signs alone.

Diagnosing OPA

Currently, there is no reliable blood test for OPA.
Post-mortem examination remains the gold standard for a definitive diagnosis.
Ultrasound scanning allows us to detect tumours in live sheep and are useful tool to screen a whole flock.

Controlling OPA: Early Detection is Key

At Lurgan Vet, we offer thoracic ultrasound scanning as a proactive tool to manage OPA within flocks.

Our recommended approach:

✔️ Act Early, Not Late

✔️ Regular Flock Screening

✔️ Immediate Culling of Positive Cases

✔️ Strict Biosecurity

✔️ Buy stock from scanned flocks

✔️ Don’t keep lambs from Positive Ewes

Ultrasound, when performed by an well trained vet, is currently the best available tool for detecting and managing OPA in live sheep. It’s not perfect but the benefits in reducing long-term losses are significant.

Why Take Action Against OPA?

Ignoring iceberg diseases like OPA can lead to devastating flock losses that take years to recover from.
Early diagnosis, rapid removal of infected animals, and strict biosecurity measures give you the best chance to protect your flock's health, welfare, and profitability.

At Lurgan Sheep Vet, you can be confident your flock is in experienced hands. Ann is an experienced OPA scanner who is involved in the Scottish Government’s OPA control project, a national initiative aiming to raise awareness and improve early detection of OPA across Scotland. She has the experience, knowledge and enthusiasm to help you tackle OPA and other Iceberg Diseases.

Scottish Government launches initiative to raise awareness | The Scottish Farmer

OPA Scanning