Equine lentiviruses

Equine Infectious Anemia (EIA), also known as "swamp fever," is a viral disease that affects horses, donkeys, and mules. It is caused by an equid-specific lentivirus called Equine Infectious Anemia Virus (EIAV). Important aspects of the disease are as follows:

Transmission:

• Blood-borne transmission is the most common mode of spread, typically via biting insects such as horseflies and deer flies. Mechanical transmission through contaminated needles, surgical instruments, or blood transfusions can also occur.
• The virus is not highly contagious; direct transmission from one horse to another without a vector is rare.

Pathogenesis:

• After infection, the virus establishes a lifelong infection in the host, similar to other lentiviruses.
• EIAV primarily infects macrophages, leading to persistent viral replication and an ongoing immune response.
• Infected horses can have three clinical outcomes: acute, chronic, or inapparent (asymptomatic) infections.

Clinical Signs:

1. Acute infection: Infected horses can develop severe symptoms such as high fever, anemia, swelling of the legs, and depression. Mortality is possible, though some horses recover and become chronic carriers.
2. Chronic infection: Horses may exhibit recurring fevers, weight loss, anemia, and lethargy. Infected animals alternate between remission and periods of illness.
3. Inapparent carriers: These horses show no outward signs of the disease, but they carry the virus and can potentially transmit it.

Diagnosis:

• The Coggins test (agar gel immunodiffusion test) is the traditional diagnostic method for detecting antibodies against EIAV.
• Other tests, such as ELISA, are also used for rapid screening, but Coggins remains the gold standard due to its specificity.

Control and Prevention:

• There is no vaccine or effective treatment for EIA, so controlling the spread relies on testing and quarantining infected animals.
• Infected horses must either be euthanized or permanently isolated, as they remain lifelong carriers and potential sources of infection.
• Preventing transmission through fly control, sterilization of equipment, and routine testing, especially for horses at events or those being transported across state or country borders, is essential.

Disease Impact:

• Outbreaks can be economically devastating, as infected animals often have to be euthanized or isolated for life.
• The disease is notifiable in many countries, meaning authorities must be alerted when cases are confirmed.

Immune Response and Viral Persistence:

• EIAV exhibits antigenic variation, allowing it to evade the immune system by changing the proteins that antibodies recognize. This leads to intermittent clinical relapses.
• Horses that recover from clinical disease are not cured; they continue to harbor the virus and can transmit it under favorable conditions (e.g., through vectors or direct contact with blood).

EIAV’s ability to persist in infected animals while periodically causing disease makes it a serious concern for equine health, similar in some ways to how HIV-1 persists in humans. However, its restricted host range and lower contagion level make it a unique veterinary challenge.