Equine Infectious Anemia: Introduction - The Merck Veterinary Manual

Equine Infectious Anemia: Introduction

Equine infectious anemia (EIA) affects Equidae and is caused by an equid-specific lentivirus in the retrovirus family, equine infectious anemia virus (EIAV). Although the majority of persistent infections appear to have minimal clinical consequences, EIA may be seen in epizootic form with high morbidity and mortality. Infection can be accurately diagnosed with laboratory tests. Because there are no effective and safe vaccines, many countries have established control programs based on serologic testing.

Transmission and Pathogenesis:

EIA is a bloodborne infection; virus can be found free in the plasma or cell-associated. Infection with EIAV appears to persist for life and, in nature, blood-feeding insects initiate most infections by mechanical transfer of infective blood between horses in close proximity. Tabanids, horseflies, and deer flies appear to be the most efficient vectors because the pain of their bite initiates host defensive behavior that interrupts feeding and results in additional host-seeking behavior. As EIAV has not been shown to multiply in insects, infected equids appear to be the only reservoir of the virus. Iatrogenic transmission has a high epizootic potential that can be avoided by standard precautions, eg, disposal or decontamination of needles and equipment between horses.

Clinical Findings:

Exposed equids generally support viral replication for days to weeks before antibodies to EIAV can be detected. The incubation period ranges from 10 to ≥45 days, usually lasting 21-42 days after natural transmission. Peak viremia often occurs during a febrile episode before the horse becomes test-positive. These acute signs often go unrecognized in horses on pasture and may be accompanied by a mild reduction in platelet counts and transient inappetence. Often, infection is noted only after routine surveillance testing for EIA or when the horse develops recurring clinical bouts of fever accompanied by marked platelet reductions, petechial hemorrhages, anemia, depression, weight loss, cachexia, and dependent edema (hallmarks of the chronic form of EIA). EIAV infection can therefore present as an inapparent infection or as an acute or chronic disease. The clinical manifestations are determined in part by the viral strain and dose, and the genetic makeup and status of the immune system of the equid. For example, strains of EIAV adapted by rapid serial passage in horses can kill horses within 14 days of infection but may have no clinical effect on donkeys. Likewise, strains that produce no or mild clinical disease in adults have killed immunologically immature fetuses or immunodeficent foals. Frequently, EIAV enters a herd and is transmitted silently until the chronic form of the disease is noted. By that time, a high percentage of the herd can be infected.

Lesions:

Equine infectious anemia, mucous membranes

In acute cases, the spleen and splenic lymph nodes are enlarged. In chronic cases, necropsy reveals emaciation, pale mucous membranes, subcutaneous dependent edema, splenomegaly, and enlarged abdominal lymph nodes.

Microscopically, there is proliferation of reticuloendothelial cells in many organs, and periportal and perisinusoidal collections of round cells in the liver with accumulations of hemosiderin in Kupffer’s cells. Perivascular lymphoid accumulations may be seen in other organs also. In some horses, there is proliferative glomerulitis with glomerular deposition of immunoglobulins (IgG) and complement.

Diagnosis:

Clinical diagnosis should be confirmed by serology. The agar gel immunodiffusion (AGID, Coggins) test is internationally accepted; antigen sources include cell culture-propagated virus and recombinant proteins. ELISA tests for detection of antibody against EIAV antigens are accepted in many countries and aid in the practical diagnosis of EIAV infection. The ELISA tests can be done in minutes (compared with 1-2 days for AGID test results) and used under field conditions. In all cases, positive ELISA tests should be confirmed by AGID before regulatory actions are taken because of the higher rate of false positive results. When combined, ELISA and AGID testing affords the highest level of sensitivity and specificity.

Treatment and Control:

No specific treatment or vaccine is available. As EIAV-infected equids present the only known source of infection, antibody-positive animals should be kept at a safe distance (~200 m) from other equids. The only recognized exception to this rule is the progeny of test-positive mares, which may possess maternal antibodies to EIAV. In the majority of cases, passive antibody against EIAV wanes to negative on AGID tests by 6-8 mo of age.

The risk associated with maintaining EIAV-infected breeding stock varies. Field studies have indicated excellent success in raising test-negative foals from inapparent carriers of EIAV. The risks of infection in utero increase dramatically if clinical signs of EIA are seen in the mare prior to parturition. Unfortunately, it is not possible to accurately determine the risk posed by any EIAV-infected equid. As EIAV persists in each infected equid for life, most regulatory agencies assume all EIAV-positive equids pose the same high risk.