The state's agriculture secretary, Douglas Fisher, said four horses purchased from a Texas herd subsequently found to be infected with the disease were brought into the state.
"At this time, two of four New Jersey horses purchased from an infected herd in Texas in 2008 have tested positive for the disease," Fisher said.
The fact the horses were bought in 2008 suggests some of the 101 horses so far identified in Kleberg County, Texas, with the disease may have been harbouring it for some time.
"Additional testing on the imported horses and contact horses is under way," Fisher said.
"Quarantines have been placed on the affected premises and precautions implemented to prevent the spread of this disease to other horses."
Fisher urged veterinarians and horse owners to watch their horses, donkeys and mules for signs of the notifiable disease, caused by protozoa which can be transmitted by ticks or by contaminated needles.
Once infected, an equine can take seven to 22 days to show signs of the illness. Symptoms include fever, anaemia, jaundiced mucous membranes, swollen abdomens and laboured breathing.
Infected horses also may have roughened hair coats, constipation, and colic. In milder form, the disease can cause equine to appear weak and show lack of appetite.