Horses at four New Jersey farms remain under quarantine as animal health authorities investigate a possible outbreak of the neurologic form of equine herpesvirus.

Equine herpesvirus can cause respiratory problems, especially in young horses, and spontaneous abortions in pregnant mares. The neurologic form of the virus is highly contagious among horses and is often fatal.

The investigation began last week after two horses at a Monmouth County farm were euthanized after receiving positive results in presumptive testing for the neurologic form of equine herpesvirus type 1 (EHV-1). Confirmatory testing continues. Another horse was also euthanized after exhibiting clinical signs of the disease.

Two other Monmouth County farms and one in Gloucester County are also under quarantine. All four had either received a horse from the farm where the horses were euthanized, or have horses that had contact with animals from that farm