A stallion in Gloucestershire is the first to contract an exotic virus in two years.
It is fighting the Equine Viral Arteritis (EVA) - the first time the disease has been seen in the UK since December 2010.
The Department for Environment, Food and Rural Affairs has confirmed the case.
The horse will remain under breeding restrictions until shown to be free of the virus, and it is understood that it has not been used for breeding since arriving in the UK in April.
EVA is a low impact exotic disease found in horses, transmitted through mating, semen, or contact with aborted foetuses. Coughing and snorting can also spread the disease, but it cannot spread to humans.
There is no treatment available for the virus, which can cause abortions, fever, depression, lethargy, stiff movement, runny nose, conjunctivitis and swelling of the lower leg, around the eye and the reproductive organs.
Vets in the area are asked to remain vigilant for signs of infection, and anyone suspecting the disease must immediately notify the duty vet at the Animal Health and Veterinary Laboratories Agency.