A second case of the deadly equine infectious anaemia (EIA or swamp fever) has been confirmed in a horse in Devon.
The disease came to light after the owner called in a private vet to examine the sick horse.
A Defra spokesman said the case in Devon and the case in Northumberland are not being linked at present.
Defra has launched an investigation into how the horse came to contract swamp fever (EIA), and the premises on which the horse was kept is under restrictions.
Another two horses on the premises are being tested for evidence of equine infectious anaemia.
But the whereabouts of the premises has not been disclosed.
Defra chief vet Nigel Gibbens said: “The risk of notifiable disease is ever present.
“This case demonstrates the importance of owners being vigilant and identifying illness in their animals and consulting their vet who should then report any signs of exotic disease to the Animal Health Agency.”
In Northumberland, the horse infected with EIA has been humanely destroyed, and the premises in Ashington is under restrictions.
It arrived in the country in a group of six horses, five of which have tested negative, and a further 11 horses on the premises have also tested negative.
Horse owners in Northumberland have contacted H&H with concerns that they have received little information.
But a Defra spokesman said: “The local authority has put in place notices at relevant points advising horse owners not to exercise their horses in this area.
“We do not disclose the precise details of an infected premises.”