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| Histo slide of lungs from horse with EIA - some macrophages have eaten RBC's (hemosiderosis) |
Equine Infectious Anemia (EIA)
Equine infectious anemia (EIA) is typically a wasting disease of horses, but can result in dependent and pulmonary edema. On this particular farm, EIA is unlikely, as all horses on the farm had a negative Coggins test in May. Equine infectious anemia is a reportable disease.
Infectious agent: An enveloped ssRNA virus in the Family Retroviridae, Genus Lentivirus.
The hallmark of this family of viruses is the presence of reverse transcriptase enzyme. What is unusual about this enzyme?
| Reverse transcriptase converts the ssRNA genome of the virus to dsDNA molecule that can integrate into the host genome. It can therefore persist in the host and cause chronic disease. In addition, lentiviruses cause immunodeficiency. |
History: Horseflies present; horses sick with recurrent bouts of anemia and fever.
Clinical Signs: Recurrent cycles of fever and anemia with chronic wasting; dependent and pulmonary edema can occur; death.
How is EIA transmitted?
| It is transferred horizontally in body fluids by fomites and biting insects (i.e. needles and horseflies). The virus does not require the horsefly for replication—the horsefly just carries it from one animal to the next like a contaminated needle. Transplacental transmission is possible; venereal transmission is rare. |
Pathogenesis: Virus replicates in monocytes and macrophages. Viral antigen “sticks” to the host RBCs leading to their immune destruction and thus anemia. Cycles of fever and anemia occur. The immune system eventually produces enough circulating antibody to control the viral replication and disease symptoms. However, anitgenic variants of the virus are produced within the infected horse that are resistant to the neutralizing antibody. These mutant viruses cause another round of fever and anemia to start up in the animal. How does this possibly relate to this case? Well, decreased plasma proteins (fewer RBCs and platelets) lead to a decrease in plasma oncotic pressure which can cause a dependent and pulmonary edema.
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| Histo slide of lungs from horse with EIA - some macrophages have eaten RBC's (hemosiderosis) |
Diagnosis: Presence of antibody is diagnostic, because this this virus is integrated into the horse’s genome and is persistent. An agar-gel immunodiffusion assay (AGID) called the Coggins Test is used to identify antibody to the EIA virus, and therefore identify infected animals.
Treatment and Prevention: Supportive care. However, due to its persistent nature and the likelihood of transmission to uninfected horses, antibody positive horses are usually destroyed. There is no vaccine.
| Why is it counterproductive to vaccinate? |
