Equine Infectious Anemia

Equine infectious anemia (EIA) is a persistent infection of horses and other equids caused by a lentivirus (family Retroviridae).  Common names for EIA include swamp fever, mountain fever, and malarial fever.  EIA is clinically characterized by recurring episodes of fever, hemolytic anemia, icterus, depression, edema, and weight loss. 

Unlike other lentiviral infections, most horses with EIA eventually progress from a clinical stage of viremia and fever to an asymptomatic stage.  Infected animals mount strong humoral and cell mediated immune responses but are unable to completely clear the virus, resulting in persistent infection.  Currently, no effective treatments or vaccines for EIA are available.

EIA is a bloodborne disease.  Transmission to horses most often occurs during interrupted feeding of large hematophagous insects of the family Tabanidae (deerflies and horseflies).  Flies serve as mechanical vectors, and the likelihood of transmission is highest when flies feed on febrile, viremic horses.  Iatrogenic transmission may result from use of contaminated blood transfusions, hypodermic needles, or surgical instruments.  Transplacental transmission from dam to foal has also been described.  EIAV actively replicates in many tissues with particular tropism for cells of the monocyte-macrophage system.  Acute disease is associated with extensive viral replication within tissue macrophages.

   The three clinical stages of EIA are acute, chronic, and asymptomatic.  The clinical stage may be influenced by virus strain, infective viral dose, and individual host factors.  Thrombocytopenia is the earliest and most consistent hematologic abnormality associated with acute EIA.  Other signs of acute infection include fever, lethargy, petechial hemorrhages, and anemia.  Chronic EIA is characterized by recurring episodes of fever, depression, anemia, icterus, lymphadenopathy, petechial hemorrhages, dependent edema, and weight loss.  In most horses, the severity and frequency of the clinical signs decrease over time, and surviving animals eventually enter the asymptomatic stage of infection.  Asymptomatic animals remain infective for life.  The asymptomatic stage is reversible.  Recrudescence of clinical disease is associated with immunosuppression, including admini-stration of exogenous corticosteroids.

   Gross lesions of EIA infection vary depending on the clinical stage of the disease.  Lesions associated with active infection include generalized lymphadenopathy, hepato-spleno-megaly, anemia, widespread hemorrhages, and edema.  Common microscopic lesions include hepatic lipidosis, hepatocellular necrosis, hemosiderosis, and perivascular lymphocytic infiltration of most tissues.  Necropsy of horses during the asymptomatic stage of infection is often unremarkable.

   Two serologic diagnostic tests, the Coggins agar gel immunodiffusion test (AGID) and a competitive enzyme-linked immunosorbent assay (cELISA) are recognized by the United States Department of Agriculture (USDA) as valid and reliable for the diagnosis of EIA.  However, the Coggins AGID test is considered the international standard for diagnosis of EIA, with accuracy at least 95%.  Both tests detect antibodies to the viral p26 core protein, which stimulates a strong humoral response in most infected horses.  The cELISA test will detect antibodies somewhat earlier and at lower concentrations than the AGID test, but false positive tests have occurred, and positive cELISA test results are confirmed using the AGID test.

   Control of EIA focuses on identification and elimination of carrier animals.  Most US states require horses to have a negative EIAV test result within six months of entry, and all EIAV-positive horses must be reported to the Indiana Board of Animal Health.  The options for EIAV-positive animals include euthanasia, permanent identification and lifelong quarantine, slaughter, or shipment to a research facility.  Interstate travel of EIAV-positive animals is prohibited by federal law except for three conditions: 1) the return of the animal to its origin, 2) the transport of the animal to slaughter, and 3) movement of the animal to a research facility or diagnostic laboratory.  Fly control and prevention of iatrogenic infections are also essential for minimizing the spread of EIA.

- by Casey Shake, Class of 2006
-edited by Dr. Kim Maratea, ADDL Graduate  Student

References:

1. Issel CJ and Cook FR: 1993.  A Review of Techniques for the Serologic Diagnosis of Equine Infectious Anemia.  J Vet Diagn Invest 5:137-41.

2. Leroux C, Cardore J-L, Montelaro RC: 2004.  Equine infectious anemia virus (EIAV): what has HIV's country cousin got to tell us?  Vet Res 35: 485-512.

3. Oaks JL, Long MT, Gaszler TV: 2004.  Leukoencephalitis Associated with Selective Viral Replication in the Brain of a Pony with Experimental Chronic Equine Infectious Anemia Virus Infection.  Vet Pathol 41(5): 527-32.

4. Reed SM, Bayly, WM and Sellon DC: 2004.  Equine Internal Medicine, 2^nd ed. Elsevier USA.  Pp 733-35.

5. Sellon DC: 1993.  Equine Infectious Anemia.  Vet Clin North Am Eq Prac 9(2): 321-36.

6. Smith BP: 2002.  Large Animal Internal Medicine, 3^rd ed. Elsevier USA: 1056-58.