Iceberg diseases: Viral ‘lung cancer’ found in Irish sheep flocks

An iceberg disease is a term used by the medical profession to describe a disease which has a large number of undiagnosed cases so that what is seen clinically is a small representation of the total.

In the sheep industry, it’s a phrase that is generally used to describe diseases that are insidious, production-limiting, slow in onset and diagnostically challenging.

At last week’s Teagasc National Sheep Conference, Fiona Lovatt – a UK-based vet – gave over 200 farmers a run down of a number of iceberg diseases, including: OPA (jaagsiekete); ovine Johne’s disease; CLA (caseous lymphadenitis); and maedi visna (MV). All of these diseases are notifiable to the Department of Agriculture in Ireland.

She said: “The detection and control of iceberg diseases is quite a challenge and they often involve hidden costs that you won’t necessarily see on the surface. The tests are sometimes problematic and it’s difficult to tell whether they are in a flock.”

When it comes to dealing with iceberg diseases, she said, putting your head in the sand is not the answer.

“If you either buy sheep in or you sell sheep – even if it’s only the odd ram – it’s really important to you to know whether you have these diseases in your flock; it has huge implications for your reputation if people buy a disease that has stemmed from your flock.”

OPA (jaagsiekete)

One of the iceberg diseases which Lovatt spoke at length about was ovine pulmonary adenocarcinoma (OPA), which is caused by the Jaagsiekte sheep retrovirus (JSRV).

“OPA is a lung cancer and it’s actually terrifying because it’s spread in the same way as the common cold. The sheep produce the virus in their snot and they cough it up.

“They get a frothy white discharge from their noses and it has a very long incubation period. From the period of time from when the sheep pick it up, they don’t actually develop it for quite a few years. We see it mainly in sheep that are above two years old.”

Sheep lungs with large OPA tumours. Image source: Teagasc National Sheep Conference proceedings

Symptoms and spread

Sheep present with increased respiratory rate and a soft cough. They remain bright with a good appetite unless a secondary bacterial pneumonia develops (this becomes more common in affected flocks). Later in the disease, lots of clear, frothy fluid flows from the nostrils when the head is down.

The virus is spread by the respiratory aerosol route and via contaminated troughs and water. It can also be transmitted via milk and colostrum.

When it comes to controlling OPA, Lovatt said: “Avoid putting your head in the sand about it. If you have any inclination about OPA in your flock, you definitely want to get it investigated.”

She continued: “If you find you have it in your flock, you need to identify and get rid of those infected animals as quickly as possible. The better your records are, the better you can get on top of it quickly.

“We can also try to stratify the flock into age groups; it’s more of a problem in older sheep and we don’t want it to spread from older animals to younger sheep.

It’s not simple to deal with, but you have to deal with it as soon as you find it in your flock, as it will only get worse and spread.

When it comes to diagnosing OPA, she said that no blood serology testing is possible, as no immune response is generated. Ultrasound scanning can be used to identify infected animals, but only when the cancerous growths are greater than 2cm in size. Unfortunately, the best method of confirming the presence of OPA on your farm is through the use of post-mortem.

“It is really difficult to prove negative and the best way, sadly, to diagnose it is by post mortem.”

Presence in Ireland

“I am afraid to say OPA has been found in sheep all over Ireland and it’s usually found in clustered flocks. It is probably the biggest iceberg disease that is facing our industry and there’s a lot that we don’t know about it,” she said.

The prevalence of OPA in Ireland appears to be similar to the UK with JSRV-positive sheep identified in the counties of Donegal, Kerry, Kilkenny, Offaly, Tipperary, Waterford and Wicklow.

A study published in the Journal of Veterinary Science, which investigated nearly 2,000 sheep, found that 1.6% were found to be infected with JSRV and 0.5% with OPA.