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EQUINE INFECTIOUS ANAEMIA IN INDIA :  PRESENT STATUS AND CONTROL POLICY
M.P.Yadav, S.K. Dwivedi and S.K. Srivastava

Courtesy : Festschrift - Dr. S. Ramachandran

Equine infectious anaemia (EIA) was recognized as an infectious disease in the late nineteenth century and was one of the first diseases to be assigned a viral etiology (1). EIA virus is classified as a lentivirus within the family Retroviridae. Infection with the EIA virus can result in three defined clinical syndromes-acute disease, chronic disease or an inapparent carrier state. Not all infected horses progress through all syndromes. Some animals progress through acute disease and recurrent episodes to become inapparent carriers of the virus for life. Within 7 to 30 days after infection with the virus, horses may develop acute disease with anorexia, fever (40°C to 41°C) and petechiae on the mucus membranes. The incubation period is normally 1-3 weeks but can be as long as 3 months. 

Bio-data

Anaemia is not a common feature in an acute infection, but severely affected animals may develop epistaxis, anaemia and ventral pitting edema. Horses may die during this phase. Levels of virus are high in the blood during peak febrile response (2) and viral antigen is found in most tissues (3). Mildly affected horses will only show slight depression, decreased appetite and mild fever. Once infected with EIA virus, a horse remains infected for life. Animals that survive the initial acute episode, may progress to chronic disease. The acute phase usually subsides within a few days. Then, the horses exhibit recurring cycles of illness, which appear initially at about biweekly intervals (2). The frequency and severity of recurring episodes decreases with time and 90% of the episodes occur within one year of infection (4). Such horses are almost always sero-positive. 

Clinical manifestations alone cannot provide an unequivocal diagnosis of EIA. Horse inoculation or sero-diagnosis provides confirmatory diagnosis. However, since for diagnosis horse inoculation is not practically feasible, sero- diagnosis is the commonly used procedure for diagnosis of EIA. For the sero- diagnosis, the immunodiffusion (ID) test has been accepted as an official test. The technique was first reported by Coggins and Norcross (5) and was found to be a convenient and reliable procedure. In addition to ID, complement fixation (CF), complement fixation inhibition (CFI) and virus neutralization (VN) tests are available for the sero-diagnosis of EIA, but their applicability is considerably limited because CF antibody is usually demonstrated only for a short period of time after the first febrile reaction (6) and VN antibody is active only against serologically homologous strains of EIA virus (7,8).

Detection of EIA virus in a horse with an equivocal ID test reaction (9) led to the search for more sensitive tests. ELISA has been successfully used for sero-diagnosis of EIA (10,11,12). Rattan et al. (13) used an indirect ELISA to detect EIA at an early stage of infection with a greater sensitivity than the Coggins ID test. Rattan et al. (14) also developed a rapid biotin-avidin ELISA (BA-ELISA). Anti- idiotypes to EIA viral antigen have been developed to differentiate between EIA positive and negative horses (15) using ELISA. 
The diagnostic tests accepted by the USDA as valid and reliable test for the diagnosis of EIA include horse inoculation test, the Coggins ID test, and the competitive ELISA (16).

Present status 
Occurrence of EIA in India was first recorded at Bangalore by National Research Centre on Equines (NRCE) in 1987 (17) on the basis of clinical signs and Coggins ID test. Since then, NRCE is engaged in the surveillance and monitoring of this disease. An outbreak of EIA was diagnosed at Royal Calcutta Turf Club (RCTC), Calcutta, during April 1988 and cases of EIA were detected at Royal Western India Turf Club, Mumbai in 1987, and 1990 and 1996 (19). Surveillance and monitoring of the disease by Coggins test coupled with clinical history, elimination of the infected animals, restriction on the movement of horses, mandatory Coggins negative status of all the incoming and outgoing horses and hygienic measures to reduce the insect vector population were successful in the control and elimination of the infection. From 1987 till March 1999, a total of 60,823 samples were screened at NRCE, out of which 228 (0.374%) were found to be positive (20) as against 3.29% recorded during 1987. None of the samples, out of the 9973 samples screened during April 1999 to March 2001, was found positive. 

Control
As the EIA virus is transmitted by blood-sucking insects and iatrogenic route, measures to control the insect population and the principle of “One horse-one needle” are helpful in controlling the disease. From the past experience, it can be said that continued surveillance and monitoring of the disease by Coggins test, mandatory elimination of positive reactors, restrictions on the to-and-fro movements of the equines from an infected premises till all the population of the said premises had proved negative in all the three testings carried out at monthly intervals within a period of 90 days. Mandatory Coggins negative status for the moving equines and other zoo-sanitary measures can help in the successful control of the disease. Furthermore, strict quarantine measures for the imported equines are mandatory. EIA has been managed in most countries by the imposition of strict quarantine regulations (21).

To eradicate EIA from the country, testing of the representative equine population at one point of time, elimination of positive reactors and three subsequent testings within an year and strict quarantine measures can pay rich dividends. It is incumbent upon the scientific community to develop effective immunogens and / or chemotherapeutic agents to inhibit EIA viral replication in vivo. Till the advent of these approaches, the policy of test and slaughter remains the most appropriate tool to control this dreaded malady of equines.

References

  1. Vallee, H. and Carre, H. (1904). Cr. Acad. Sci., 139: 331.
  2. Issel, C.J. and Coggins, L. (1979). J. Am. vet. med. Ass., 174: 727.
  3. McGuire, T.C. et al. (1971). Am. J. Path., 62: 283.
  4. Kono, Y. (1973). Recurrences of Equine Infectious Anaemia. Proc. 3rd lnt. Conf. Equine Infectious Diseases. p.175. Karger, New York.
  5. Coggins, L. and Norcross, N.L. (1970). Cornell Vet., 60: 330.
  6. Koho, Y. and Kobayashi, K. (1966). Natn. lnst. Anim. Hlth. Qt., 6: 204.
  7. Henson, J.B. et al. (1969). J. Am. vet. med. Ass., 155: 336.
  8. Kono, Y. (1969). Natn. lnst. Anim. Hlth. Qt., 9: 1.
  9. Issel, C.J. and Adams, W.V. Jr. (1981). J. Am. vet. med. Ass., 180: 276.
  10. Suzuki, T. et al. (1982). Vet. Microbiol., 7: 307.
  11. Shen, D.T. et al. (1984). Am. J. vet. Res., 45: 1542.
  12. Archambault, D. et al. (1989). J. Clin. Microbiol., 27: 1167.
  13. Rattan, B. et al. (1993). Indian J. Anim. Sci., 63: 910.
  14. Rattan, B. et al. (1998). Indian J. Comp. Microbiol. lmmunol. Infect. Dis., 19: 79.
  15. Rattan, B. et al. (1996). Anti-idiotypes to equine infectious anaemia viral antigen. Souvenir III Annual Conf. IAAVR. p. 51.
  16. Calabough, D.L. (1990). Vet. Med., 9:1007.
  17. Uppal, P.K. and Yadav, M.P. (1989). Vet. Rec., 124: 514.
  18. Yadav, M.P. and Uppal, P.K. (1995). Int. J. Anim. Sci., 10: 311.
  19. Yadav, M.P. et al. (1998). Indian J. Comp. Microbiol. lmmunol. Infect. Dis., 19: 50.
  20. Anonymous (1998-99). Annual Report. National Research Centre on Equines, Hissar, India.
  21. Issel, C.J. et al. (1989). Devl. Biol. Standard, 72: 49.

Authors Corresponding address: 

Dr. M.P. Yadav
 Director, Indian Veterinary Research Institute, Izatnagar - 243 122, India

Dr. S.K. Dwivedi
Project Director, National Research Centre on Equines, Sirsa Road, Hissar - 125 001, India

Dr. S.K. Srivastava
Scientist, National Research Centre on Equines, Sirsa Road, Hisar - 125 001, India

The views expressed in this article are of the author(s), and any clarifications can be obtained from the author(s).