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 result in minimal clinical signs, 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. Infection with EIAV appears to persist for life.
Transmission and Pathogenesis
EIA is a bloodborne infection; virus can be found free in the plasma or cell-associated, primarily in monocytes, macrophages, and endothelial cells. In nature, blood-feeding insects initiate most infections by mechanical transfer of infective blood between horses in close proximity. Horseflies, deer flies, and occasionally stable 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 insects are only mechanical vectors for the virus, infected equids appear to be the only reservoir of EIAV. Iatrogenic transmission has a high epizootic potential that can be avoided by standard precautions, eg, disposal or decontamination of needles and equipment between horses. A recent outbreak of EIA in Ireland raises serious concerns regarding the potential for direct or indirect horse-to-horse transmission of virus.
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 seropositive. These acute signs often go unrecognized in horses on pasture and may be accompanied by mild thrombocytopenia 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 thrombocytopenia, 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. In contrast, a recent outbreak in Ireland was associated with unexpectedly high morbidity and mortality.
In acute cases, the spleen and splenic lymph nodes are enlarged. In chronic cases, necropsy reveals emaciation, pale mucous membranes, petechial hemorrhages, 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.
Clinical diagnosis should be confirmed by serology. The agar gel immuno-diffusion (AGID, Coggins) test is internationally accepted; antigen sources include cell culture-propagated virus and recombinant proteins. A variety of 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 seropositive mares, which may possess maternal antibodies to EIAV after suckling colostrum. In the majority of cases, passive antibody against EIAV wanes to negative on AGID tests by 6–8 mo of age; however, detectable antibody may persist up to 12 mo, especially if ELISA testing is used.
The risk associated with maintaining 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. Inapparent carrier horses maintain low-level, variable degrees of viremia that may increase in stressful circumstances. As compared to seronegative healthy horses, inapparent carriers have increased serum globulin concentrations and lymphocyte subset changes that are consistent with immune activation or chronic inflammation. Because EIAV persists in each infected equid for life, most regulatory agencies assume all EIAV-positive equids pose the same high risk.
In the USA, seropositive horses must be placed under quarantine within 24 hr after the positive test results are known. The quarantine area must provide separation of at least 200 yd from all other equids. After a confirmatory test is performed, seropositive horses must be permanently identified using the National Uniform Tag code number assigned by the USDA to the state in which the reactor was tested, followed by the letter “A.” This identification may take the form of a hot brand, chemical brand, freezemark, or lip tattoo and must be applied by a USDA representative. Reactor horses must be removed from the herd by euthanasia, slaughter, or quarantine at the premises of origin. They may only move interstate under official permit to a federally inspected slaughter facility, a federally approved diagnostic or research facility, or to return to the premises of origin. After a reactor is detected in a herd, testing for EIA must be performed on all horses on the premises and repeated until all remaining equids on the premises test negative. These horses must be retested at 30- to 60-day intervals until no new cases are found. Quarantine on the premises is released when tests on the entire herd have been negative at least 60 days after the reactor equids have been removed.
All equids moved across state lines in the USA must be tested for EIA with a negative result within 12 mo prior to transport. All equids sold, traded, or donated within a state must have tested negative for EIA no more than 12 mo prior to change in ownership and, preferably, no more than 60–90 days. All equids entering horse auctions or sales markets are required to have a negative test prior to sale, or the horse must be held in quarantine within the state until the test results are known.
It is recommended that horse owners implement an EIA control plan for their premises. All horses should be tested at least every 12 mo as part of a routine health program. More frequent testing may be indicated in areas with high incidence of EIA. Owners of equids entering exhibitions or competitive events should present proof to event officials of a negative EIA test. All new equids introduced to a herd should have a negative EIA test prior to entry or be isolated while tests are pending. Vector control practices including application of insecticides and repellents and environmental insect control should be implemented. Good hygiene and disinfection principles should be maintained to prevent iatrogenic infection of horses with contaminated needles, syringes, or equipment.
Last full review/revision March 2012 by Debra C. Sellon, DVM, PhD, DACVIM