African Horse Sickness: a vaccine in progress

African horse sickness (AHS) is the most lethal disease known to horses. Fortunately for American horses, AHS is currently enzootic, meaning only a limited number of cases occur each year, in sub-Saharan Africa. However incursions into southern Europe, the Middle East, and even the Indian subcontinent occurred in the last century.

"While modified live virus vaccines are available in enzootic countries of sub-Saharan Africa to immunize horses against AHS virus, these vaccines are not licensed in North America and are not problem-free," reported Alan Guthrie, BVSc, MedVet, PhD, from the University of Pretoria's Equine Research Centre in South Africa.

Some of the problems associated with modified live virus AHS vaccines include:

•Occurrence of vaccine reactions, including death;
•Variable response to vaccination;
•Difficulty immunizing young horses due to interference from maternal immunity; and;
•The potential for reversion to virulence (infectivity) of vaccine virus.
In light of these limitations, Guthrie and a group of international collaborators, including scientists from Canada, France, and California, developed and preliminarily tested a new vaccine against serotype 4 of AHS. This new vaccine is a recombinant canarypox-vectored vaccine that circumvents many of the above-described problems.

"In addition, we are able to distinguish between horses that have been vaccinated against AHS and those that were naturally infected," Guthrie said.

Despite the current limited geographic localization of AHS, Guthrie said the vaccine comes at a pivotal time since.

"The global spread of diseases spread by insects, such as AHS, has highlighted the need for a new generation of AHSV vaccines that are both safe and effective to prevent AHSV infection," he said. "This vaccine will be particularly valuable should AHSV spread into regions of the world that were previously free of AHS."

The study, "Protective immunization of horses with recombinant canarypox virus vectored vaccine co-expressing genes encoding the outer capsid proteins of African horse sickness virus," is scheduled to be published in an upcoming edition of the journal Vaccine.