1. African Horse Sickness  African horse sickness (AHS) is an infectious but noncontagious, arthropod-borne, peracute to subacute, often fatal disease of Equidae characterized by respiratory distress or cardiovascular failure.  It is transmitted in the field by at least two species of Culicoides.

2. Etiology African Horse Sickness is caused by an Orbivirus of the family Reoviridae Family: Reoviridae Genus: Orbivirus Nine different serotypes of the virus have been described A viscerotropic virus

3. Host Range Horses Mules Donkeys Zebras

4. Approximately 70-95 % of all horses developing the disease will die the mortality percentage for mules is only about 50 percent the mortality percentage for donkeys only 10 percent.

5. Dogs infected with this virus may have the pulmonary form of the disease. In a survey in Egypt, antibodies to AHS virus were detected in sheep, goats, camels, buffalo and dogs.

6. Pathogenesis:  Zebras are the natural host and reservoir; transmission is via biting arthropods and midges (Culicoides sp.) are the most important vector  Infection results in damage to the circulatory and respiratory systems resulting in serous effusion and hemorrhage in various organs and tissues

7. Pathogenesis:  Horse is bitten by an infected midge > initial viral replication in regional lymph nodes > primary viremia > infection of target organs (endothelial cells and mononuclear cells of the lung, spleen, and lymphoid tissue) > secondary viremia > virally induced endothelial cell damage and activation of infected macrophages with subsequent cytokine production (IL-1, TNFa) > increased vascular permeability > edema

8. Clinical Signs Infected horses remain viremic for approximately 18 days, although the fever may be present for another 4 to 8 days, if the animal should live that long. Despite their reduced mortality, the viremic stage in donkeys may last for up to 28 days. Zebras appear to be very similar to donkeys in this regard.

9. Clinical Findings Pulmonary (peracute) form: Dyspnea, tachypnea, fever, coughing Frothy nasal exudate, widely spread forelegs and extended neck Three to five day incubation; death within minutes to hours

10. Clinical Findings Cardiac (subacute) form: Fever, depression, signs of colic Supraorbital and palpebral edema, subcutaneous edema of the neck and chest, absence of edema in the lower limbs Seven to fourteen day incubation; death within four to eight days

11. Clinical Findings Cardiopulmonary (mixed) form: Mixture of the cardiac and pulmonary form Commonly a subclinical cardiac form is followed by marked dyspnea Five to seven day incubation; death within three to six days Horse sickness fever (mild form): Fever; minimal clinical signs

12. Gross Findings Pulmonary form: Copious amounts of fluid and froth in the airways Peritracheal, periaortic, and pulmonary edema Hydrothorax, edematous lymph nodes Hyperemia and petechia of intestinal serosa and mucosa

13. Gross Findings Cardiac form: Supraorbital and palpebral edema Prominent gelatinous, yellow edema of the neck (nuchal ligament), head, and chest Hydropericardium Epicardial and endocardial petechia and ecchymoses Edema of the gastrointestinal tract

14. Microscopic Findings Lung: Alveolar exudate composed of fibrin, protein rich fluid, and few mixed inflammatory cells Interstitial, alveolar, subpleural, and perivascular edema Heart: Epicardial and endocardial hemorrhage Multifocal myocardial necrosis, hemorrhage of great vessels Subcutaneous and intramuscular tissue: Edema Lymph nodes; gastrointestinal tract: Edema

15. Swelling of the supraorbital fossa

16. AHS: The animal is depressed and showing edema of supraorbital fossa, neck and chest