African Horse Sickness Cases in South Africa

At least six African horse sickness (AHS) cases have been logged recently in the Eastern and Western Capes of South Africa, according to ProMED web site posts in early April. Last week, South African media outlets reported an outbreak of the deadly disease in the KwaZulu-Natal province, which borders the northeast of the Eastern Cape, but it is unclear how many horses are affected.

African horse sickness is spread by mosquito-like insects called midges. Affected animals can show clinical signs ranging from pulmonary distress to heart failure, and the disease is often fatal. Owners are encouraged to vaccinate their horses against the disease. Currently, AHS is contained to its namesake continent where nine serotypes of the disease circulate. (For more information on AHS, visit www.TheHorse.com/ViewArticle.aspx?id=5683.)

The virus is suspected to have been introduced to Plettenberg Bay in the Western Cape by horses transported from northern South Africa, where AHS serotypes 5 and 7 are circulating. Serotype 5 was isolated in the two cases in that area.

At least four equine deaths due to AHS serotype 2 occurred on a game farm in the Eastern Cape. Dr. G.H. Gerdes of the OIE (Office International des Epizooties) Reference Laboratory for AHS at the Onderstepoort Veterinary Institute, said in a ProMED post, "This particular serotype (serotype 2) has been present in the Eastern Cape since 2001 and was isolated in the 2003, 2004, and 2005 seasons."

The AHS outbreak in KwaZulu-Natal was reported on April 21 in the Cape Times. Horse owners reported at least eight deaths and widespread cases of the disease in the Nottingham Road area of the KwaZulu-Natal Midlands area. Other media reports have estimated anywhere from 50 to 1,000 deaths in the northern parts of the province, and horse owners quoted in the articles expressed concerns that the AHS vaccines were ineffective.

Dr. Philip S. Mellor is honorary professor in the School of Biological Sciences at the University of Aberdeen, head of the Department of Arbovirology, and head of the OIE reference lab for AHS at the Institute for Animal Health in Surrey, U.K. Mellor described the vaccines used against AHS as live attenuated preparations produced by Onderstepoort Biological products. "At the moment I understand they produce two polyvalent preparations--bottle 1 should contain vaccine against AHSV (virus) serotypes 1, 3, 4, and 5," he said, although he's been told that serotype 5 is not currently included. "Bottle 2 contains vaccine against AHS virus serotypes 2, 6, 7, and 8. Serotype 9 is not included because serotype 6 cross protects.

"No vaccine is perfect, and a proportion of vaccinated animals, hopefully very small, will always remain susceptible," Mellor added.

According to Gerdes, the disease is not endemic in South Africa except for the subtropical north. "The virus moves a variable distance down to the temperate South each season," said Gerdes. "The distance moved depends on climatic factors favoring the vector and the immune status of the population.

"AHS appears to have become entrenched in the Eastern Cape, possibly because of the large donkey population and the numerous zebras on the many game farms in which animals are (kept)," she added. The virus circulates in a subclinical form, which means the donkeys and zebras are carriers without showing clinical signs of the disease.

"At present, we have recovered isolates from six of the nine provinces in South Africa," Gerdes said.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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