A new case of Equine Infectious Anaemia (EIA), commonly known as swamp fever, has been confirmed by the Department of Agriculture and Food.
This, the 22nd case of the disease to have been found here, occurred in a "high risk" group of horses that had been exposed to the disease.
Scientists who have been attempting to discover how the disease entered the country for the first time have identified a group of animals that had contact with the first case of the disease in late May.
They are also continuing to trace all horses that were in direct contact with any of the confirmed cases and have put in place an intensive testing regime to ensure that all direct contacts are tested for any evidence of the disease.
The department has also placed more than 20 farms under restriction and the movement of horses from these premises has been forbidden or placed under tight control.
Last month the department announced it would give financial assistance to owners to have their horses tested for the disease.
Meanwhile, the department's Special Investigation Unit is continuing its investigation into the possibility that the disease was brought into the country in contaminated blood serum from abroad.
Blood-based serum injections are used to build up the immune system in thoroughbred foals and the team is looking at the possibility that contaminated serum was imported into the State illegally.
The disease is thought to have broken out in a Co Meath farm when a foal died. When the mare was being treated at a veterinary centre, the disease spread to other animals.
It has now been found in counties Meath, Kildare, Dublin, Limerick and the first case of the disease in Northern Ireland was reported on September 1st in Magherafelt, Co Derry.