Pembrokeshire's livestock farmers are warned to be vigilant when they buy stock to avoid infecting their herd with a respiratory disease.
Infectious Bovine Rhinotracheitis (IBR) is the most common of the three viruses which cause respiratory disease in cattle.
Virtually all farms with an IBR problem have bought it in, according to NADIS vet Mike Howe.
"The main source of the virus is not animals with disease, but animals that have recovered from disease, as these cattle still have the virus in their body," said Mr. Howe, who is based in Pembrokeshire.
IBR is caused by the bovine herpes virus and symptoms include a nasal discharge, red eyes, a loss of appetite, coughing and difficulty with breathing. The first sign is often a dramatic drop in milk yield.
Blood testing for antibodies can identify infected cattle. Diagnosis on a herd basis can be made using a combination of blood and milk tests.
A report by Richard Laven, manager of the Scottish Agricultural College's Surveillance Centre, suggests that vaccination in the face of an outbreak can help stop the virus spreading.
Antibiotics are also useful in preventing and treating infection by bacteria, which often develops after IBR starts.
With several effective vaccines on the market, farmers should discuss with their vet which of these is best for them, said Mike Howe.
"The main problem with most vaccines is that they produce antibodies which cannot be distinguished from those caused by natural infection," he said.
"Thus, vaccinated cattle cannot be separated from latently infected cattle. A new vaccine is now available which does not produce the same antibodies as natural infection. This is helpful if a farmer is trying to eliminate rather than control IBR."