The discovery of three more cases of equine infectious anaemia has raised fears among Ireland's multi-million euro breeding and racing industry.
The Agriculture Department revealed tests on three animals had proved positive for Equine Infectious Anaemia (EIA), also known as "swamp fever".
The animals had already been identified as high-risk due to their contact with infected horses. After the cases were confirmed, the Department restricted 13 individual premises and a further 280 individual horses.
EIA has been detected in a small number of horses in counties Meath and Kildare after it is believed some foals were injected with infected serum.
The Department said 12 cases proved positive out of 2,300 tests carried out and examined by the Irish Equine Centre.
Officials said they were continuing to identify and trace all horses who had been in contact with those infected.
There is no known cure for the virus.
Any animals which test positive for the virus, which spreads between horses, donkeys and mules, are generally culled to prevent it spreading further. The virus is usually transmitted through blood-sucking insects or through the use of blood contaminated instruments or needles.