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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.
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
Host Range
Horses
Mules
Donkeys
Zebras
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) >
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.
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