EIAV Project Development Background
Equine Infectious Anemia (EIA), or "swamp fever," is a viral disease affecting horses, donkeys, and mules caused by Equine Infectious Anemia Virus (EIAV), a lentivirus closely related to HIV-1. Transmission typically occurs through blood-borne vectors such as horseflies and deer flies, or mechanically via contaminated needles or instruments. Direct horse-to-horse transmission without a vector is rare, as the virus is not highly contagious. Once infected, horses become lifelong carriers, with the virus targeting macrophages and persisting through ongoing immune responses. EIA presents in three clinical forms: acute, chronic, or inapparent (asymptomatic) infection.
The clinical signs of EIA vary widely. In acute infections, horses may develop high fever, anemia, swelling, and depression, with mortality possible in severe cases. Chronic infections involve recurrent episodes of fever, weight loss, and anemia, alternating with periods of remission. Inapparent carriers show no symptoms but harbor the virus, posing a potential risk for transmission under favorable conditions. Diagnosis is primarily through the Coggins test, which detects antibodies against EIAV with high specificity. ELISA tests are also used for rapid screening but are less definitive than the Coggins test.
There is no vaccine or cure for EIA, making prevention and control critical. Measures include routine testing, quarantining or euthanizing infected horses, and controlling transmission vectors like flies. Preventative steps, such as sterilizing equipment and testing horses during transport or events, are essential to limit outbreaks. EIA is a notifiable disease in many countries due to its economic impact and the lifelong infectious nature of affected animals. EIAV's ability to evade the immune system through antigenic variation and cause periodic relapses underscores the disease's unique challenges to equine health.
The EIAV extension layer was originally developed in association with a study of EIAV origins, focussing on an isolated population of wild American horses. The wild horse population on Shackleford Banks, North Carolina, believed to descend from Spanish horses abandoned in the 16th century, has a unique genetic heritage. In the 1990s, a significant portion of this population was found to be infected with Equine Infectious Anemia Virus (EIAV), a lentivirus related to HIV-1. Infected horses were relocated to a quarantine facility to preserve their genetic lineage.
The wild horses of Shackleford Banks are a unique population of feral horses that roam the Shackleford Banks, a barrier island in the Outer Banks of North Carolina, USA. It is believed that the Shackleford Banks horses are descendants of Spanish mustangs brought to the Americas during the colonial period, possibly as early as the 16th century. Shipwrecks, abandoned settlements, or intentional release by early settlers may have led to the establishment of the herd on the island.
In 2008--2009, three quarantined horses (SB1, SB2, and SB3) developed severe clinical signs of Equine Infectious Anemia (EIA) after over a decade of appearing healthy, demonstrating that long-term inapparent carriers can experience severe disease recrudescence. Two of these horses were euthanized due to the severity of their symptoms.
Genetic analysis including virus isolated from these horses revealed strains unique to the Shackleford horses are distinct from other global EIAV strains. These findings suggest a long-standing, geographically constrained EIAV lineage in this population.