Testing has showed that the mysterious respiratory disease that had began to broaden across Iceland early in April, was not influenza.

Iceland is known to be one of three countries in the world, free of the disease and the ash from a recent major volcanic eruption was also ruled out.

The contagious infection could have been spread due to equine herpesvirus, the investigation showed. With the weakening of the animal's immune system, a streptococcus infection was able to take hold and cause the nastiest of the symptoms.

The research had been undertaken at Keldur, the Institute for Experimental Pathology, a website in Iceland reported.

Halldor Runolfsson, Chief Veterinary Officer at Keldur, held a discussion with two veterinarians, who had also been conducting study into the epidemic.

Runolfsson said, "There are indications that viral infections cause the beginning of the horse fever and streptococci infections the more difficult part".

Testing of samples from infected horses had discovered the existence of the herpesvirus, but some more testing is essential.

The presence of bacterium Streptococcus equi Zooepidemicus, had been seen in many horses with longer-lasting cases of the disease, MAST, Iceland's food and veterinary authority had already confirmed.

The contagion mainly implicated the upper respiratory tract, at first with grave nasal discharge, a dry cough and mild conjunctivitis. While in some cases the nasal discharge turned mucopurulent. Temperature remained standard in most cases, but tolerance to exercise was reduced.

The reach among stabled horses proved that horses in Iceland had slight or no immunity to the infectious agent.