EIA can be fatal for horses but has no human health implications
Two horses imported into Britain from Romania have tested positive for swamp fever, in the first case of the virus entering the UK for more than 30 years.
The horses, in Wiltshire, are to be put down, the Department for Environment, Food and Rural Affairs has announced.
Seven other horses from Romania tested negative for the disease. A horse from Belgium transported at the same time is still to be tested.
The British Horseracing Authority urged vigilance for signs of the disease.
Equine infectious anaemia - EIA or swamp fever - is a virus affecting horses, mules and donkeys that can cause fever, anaemia, emaciation and death. It has no human health implications.
It is transmitted by exchange of blood by biting insects, and infected horses carry the disease for life and pose a permanent risk to other animals.
Defra chief veterinary officer Nigel Gibbens said: "This is the first case of equine infectious anaemia-infected animals being imported into Great Britain since 1976 and shows the success of our post-import testing regime.
"These were apparently healthy horses carrying a notifiable disease that we are keen to keep out of Great Britain.
"After considering the risk I have decided to take appropriate action and humanely destroy these two horses that tested positive."
The British Horseracing Authority (BHA) said the infected horses were not racehorses or animals used for breeding.
Defra advice was that horse racing was unlikely to be affected, and that the risk of the virus spreading was not high because "it is spread by biting flies and such spread is unlikely at this time of year and with the current weather", a BHA statement said.
"Nevertheless, racing should not be complacent," it added.
The authority said it had "advised all its veterinary officers and relevant racing stakeholders to be aware of the signs of this disease".
"This emphasises the need for continued vigilance for all horse diseases in the UK."