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.

EtiologyAfrican 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

Host Range

HorsesMulesDonkeysZebras

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.

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.

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

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

Clinical SignsInfected 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.

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

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

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

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

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

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

Swelling of the supraorbital fossa

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