African horse sickness (AHS) is a severe, infectious, insect-borne, often fatal disease of horses caused by the AHS virus (AHSV). AHSV is a member of the genus Orbivirus and Reoviridae family. There are 9 different serotypes of AHSV. AHS is endemic to Africa, predominately found in the central tropical regions, however it spreads regularly to Southern Africa and occasionally Northern Africa. Other hosts that are affected by AHSV include donkeys, mules, and zebras. AHS is the only equine disease for which the World Organization for Animal Health (OIE) observes official recognition status, which requires that other countries immediately notify OIE of any outbreaks of AHS that occur in their country. A polyvalent, live-attenuated vaccine (LAV) against AHSV was developed in the 1930s. The vaccine is used widely in horses in endemic areas. The vaccine provides protection against all 9 AHSV types and is supplied in 2 separate vials, each containing different combinations of AHSV types.

AHS has both a seasonal and epizootic cyclical incidence, with increased incidences associated with drought followed by a heavy rain. Seasonally, most cases of AHS occur during the late summer and autumn. Major outbreaks of AHS are strongly linked with the warm (El Nino) phase of the El Nino/Southern Oscillation (ENSO). First cases of AHS outbreaks generally start in February, with the more serious outbreaks occurring in March and April. Immediately following the first frosts, which usually coincides with the end of April or the start of May, the disease abruptly disappears. However, with delayed frost or in areas such as parts of South Africa which doesn't usually get frost, usually continues to see cases of AHS well into May or June.

Four manifestations of AHS
AHS manifests as four different forms--subclinical form, subacute or cardiac form, acute respiratory or pulmonary form, or a mixed form (consisting of a combination of cardiac and pulmonary forms).

• Subclinical form: Also referred to as horse sickness fever, this form of AHS is the most mild and rarely causes death. Clinical signs include fever that slowly develops over a course of 4 to 5 days to where it peaks at (104 - 105°F (40 - 40.5°C) then drops back to normal.
• Subacute or cardiac form: Also known as the "dikkop" form of AHS, and is a milder form. Affected horses have a 50% chance at recovery. Clinical signs include acute onset of fever (102 - 106°F (39 - 41°C) that persists in horses for 3 to 4 days. As the fever starts to decline, various parts of the horse's body start to swell, most commonly the supraorbital fossa, eyelids, facial tissues, neck, thorax, brisket, and shoulder regions. A gradual decrease in swelling occurs in horses that recover over a course of 3 to 8 days.
• Acute respiratory or pulmonary form: Also referred to as the "dunkop" or simply "pulmonary" form, this is considered the peracute form of AHS. This form of AHS occurs when AHSV infects fully susceptible horses, such as foals that have lost their maternally derived passive immunity. Horses affected by this form of AHSV have an extremely poor prognosis, and have less than 5% chance of recovery.
• Mixed form: This form of AHS is considered to be the most common form observed in cases of AHS in horses. Horses affected by this form of AHS develop a combination of clinical signs related to the subacute or cardiac form and the acute respiratory or pulmonary form. Affected horses have a 20-30% chance of survival.

Transmission
AHSV is transmitted between hosts primarily by biting midges of the genus Culicoides, similar to bluetongue virus (BTV) in ruminants and equine encephalosis virus (EEV) in horses.

Incubation period
The incubation period for AHS is typically 7-14 days, however it can be as short as 2 days.