African Horse Sickness
African horse sickness (AHS, virus AHSV) is an infectious noncontagious virus in the family Reoviridae and genus Orbivirus that is transmitted to domestic and wild equids (horses, mules, donkeys) by biting midges (no-see-ums) in the genus Culicoides (6). The virus is nonenveloped and the icosohedral 85-nm-diameter capsid contains the 10 segments of linear double-stranded RNA genome of approximately 25 kilobases (4, 5). The disease is found in Africa and is endemic in the central tropical regions of the continent. Epizootic, the disease has occured in the Middle East and Europe, Spain and Portugal. Pictures shown here were taken during an outbreak in Spain. Due to the seasonal increase in the vector, disease is most prevalent from mid summer to early fall. The disease was first recognized in South Africa in 1780 (2).
Horses and mules are most susceptible to AHSV; donkeys suffer milder disease, and although zebras get infected they rarely show signs of disease. There are four recognized forms of the disease: (i) pulmonary, (ii) cardiac, (iii) combined pulmonary and cardiac, and (iv) mild or asymptomatic. Approximately 7 to 14 days postexposure horses will develop fever (104 to 106oC) for a few days followed by respiratory signs including increased respiratory rate, standing with head extended and nostrils flared (Fig. 1 and 5), abdominal heaving, sweating, and frothy nasal discharge due to pulmonary edema (Fig. 1–4 show the progression of the discharge). Red swollen eyelids (Fig. 4 and 5) and swelling of the supraorbital fossae (the indentation that is usually above the eyes) (Fig. 1, 2, 4, 5, and 6) are common. Mortality usually occurs within 4 to 8 days of clinical onset. In horses, mortality ranges between 70 and 95%; in mules it is usually around 50%, and in donkeys only 5 to 10%. Upon necropsy examination, a frothy fluid is found in the lungs (Fig. 7) and myocarditis (inflammation of the heart) with petechial hemorrhages can be seen with the accumulation of fluid in the pericardial sac and lungs (Fig. 8). A frothy discharge is often observed in the trachea (Fig. 9) and muscle tissue (1, 2, 3).
Infected horses remain viremic for approximately 18 days, and donkeys and zebras for 28 days. It should be noted that although zebras become infected, normally they do not show signs of clinical disease. There is no treatment for this disease, but several vaccines are available for prevention (2, 3). Diagnosis is performed using an enzyme-linked immunosorbent assay or virus neutralization assay (using paired acute and convalescent serums, 2 weeks apart, and assayed for a four-fold increase in serum neutralizing antibody titers). This assay detects antibodies in the serum; if animals have had an immune response, antibodies will neutralize the ability of the virus to produce plaques in a plaque assay (this titer should go up at least four-fold if the infection is current).
1. Committee on Foreign Animal Diseases of the United States Animal Health Association. 1998, revision date. The Gray book of foreign animal diseases, 6th ed. United States Animal Health Association, Richmond, Va. [Online.] http://www.vet.uga.edu/VPP/gray_book/FAD/index.htm.
2. Fenner, F., P. A. Bachmann, E. P. J. Gibbs, F. A. Murphy, M. J. Studdert, and D. O. White. 1987. Veterinary virology, p. 587–590. Academic Press, Inc., Orlando, Fla.
3. Roberts, W. A., and G. A. Carter. 1976. Essentials of veterinary virology, p. 106. Michigan State University Press, East Lansing, Mich.
4. Roy, P. 2001. Orbiviruses, p. 1679–1728. In D. M. Knipe and P. M. Howley (ed.), Fields virology. Lippincott Williams and Wilkins, Philadelphia, Pa.
5. Tyler , K. L. 2001. Mammalian reoviruses, p. 1679–1728. In D. M. Knipe and P. M. Howley (ed.), Fields virology, Lippincott Williams and Wilkins, Philadelphia, Pa.
6. Walton T. E., and B. I. Osburn. 1991. Bluetongue, African horse sickness, and related Orbiviruses. Proceedings of the Second International Symposium. CRC Press, Boca Raton, Fla.