EIA Transmission Through The Air

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A report from the Troytown Veterinary Hospital in County Kildaire, Ireland, is looking like the first known incident of spread of the equine infectious anemia (EIA) virus by aerosol (through the air).

Equine infectious anemia the disease that a Coggins’ test checks for is a viral disease causing anemia and potentially death with acute infections. If the horse survives the acute phase of the infection, a chronic carrier state can develop where most infected horses are showing no symptoms, but the virus is still present in their circulating blood cells.

Acutely infected and ill horses also have free virus circulating in their blood. At this stage, the infection can be spread from horse to horse via biting flies that have serum containing free virus on their mouth parts. This was the only known way for the disease to naturally spread from horse to horse.

However, at the Troytown Hospital, all horses that had been housed in the same barn as a horse that turned out to have EIA became infected without any biting flies being involved. The infected horse was bleeding from the nose and snorting, dispersing her blood and the virus into the air.

The EIA virus is a lentivirus, in the same family as the human HIV/AIDS virus. Although this is the first-known instance where biting flies or shared needles were not involved in the spread, it really shouldn’t be too surprising that this can happen by direct contact with whole blood of an EIA-infected horse.

In 1982, a research group (Issel et al) showed that one teaspoon or less of whole blood from asymptomatic Coggins-positive horses injected into negative horses was enough to cause infection in 85% of the horses that were injected.

Many people are upset by the current laws that require all Coggins-positive horses be either euthanized or isolated from negatives not infected with this virus, but the experience in Ireland drives home the point that contact with whole blood from infected horses is dangerous. While this one was actively infected and likely had very high levels of virus, the earlier work has already shown that even small amounts of whole blood from apparently healthy carriers contains enough virus to cause infection.

Like the AIDS virus, the EIA virus can cause infection when blood comes in contact with mucous membranes or breaks in the skin. It’s also been documented that injections of corticosteroids, which mimics a stress response and is a widely used medication, can cause enough immune system suppression that the virus again begins to multiply in carrier horses, making them capable of spreading infection via flies. For these reasons, we support the current recommendations regarding Coggins-positive horses.