SYMPTOMS and SIGNS
African horse sickness (AHS) is not contagious. In susceptible horses, AHS usually begins with an acute fever, followed by the sudden onset of severe respiratory distress. Infected animals often stand with fore¬legs spread, head extended, and nostrils fully dilated. Other clinical signs may include rapid breathing, forced expiration, profuse sweating, spasmodic coughing and a frothy nasal drip of serum and fibrin. Breathing difficulty usually progresses rapidly, and the animal often dies within a few hours after the respiratory signs appear. In less acute cases the horse may develop extensive swelling of the head neck and front legs.
CAUSES and RELEVANCE to FLORIDA
African horse sickness is caused by an arthropod-borne virus. There are nine serotypes of the virus. It is spread by midges (Culicoides species). Mortality can be as high as 95 percent in some forms of this disease. Asymptomatic or mild infections may occur in zebras, African donkeys, and horses previously infected by another serotype of the virus. Dogs are susceptible to infection if they eat infected meat. African elephants have been found to carry antibodies to the virus, but there is no evidence of virus replication in this species.
African horse sickness is endemic in sub-Saharan central and east Africa. This disease often spreads to southern Africa and occasionally to northern Africa. Outbreaks have been seen in Egypt and other parts of the Middle East, as well as in Spain. The disease has never been recorded in North America.
Similar to the situation with West Nile virus, insect vectors already exist in the U.S. that are considered capable of transmitting the AHS virus. The related bluetongue virus is transmitted in Florida by several species of Culicoides. Bluetongue is a hemorrhagic disease that affects sheep and deer and was probably introduced to the U.S. in the 1930’s. The widespread distribution of bluetongue virus in Florida livestock indicates that the introduction of AHS virus to the state could rapidly lead to an extensive epidemic that would be a disaster for the equine industry. The greatest danger to Florida probably exists through the inadvertent importation from Africa of a reservoir species of AHS virus. Currently the zebra is considered the reservoir species, but the epidemiology of AHS is not fully understood and it is possible that other wild animal reservoir species are unidentified.