What is African Horse Sickness?

African Horse Sickness is a variant of the Bluetongue virus of sheep which is principally carried and transmitted to horses by Culicoides species midges.

It is endemic in the African continent and is characterised by clinical signs and lesions associated with respiratory and circulatory impairment. Up to 90% of infected horses die. There is no efficient treatment. Prophylactic vaccines are available in Africa but none is approved for use in the EU.

Insecticides, repellents and screens to repel the vector midges are a crucial element in preventative horse management.

There are nine distinct types of the Reoviridae family which cause AHS.

Although AHS is not directly contagious (ie it is not transmitted directly between horses), it can remain in infected horses for up to 18 days, enabling it to be transferred via the biting midges.

Control of the disease in the UK would be achieved by slaughter of infected animals, destruction of the carcasses, and the establishment of a protection zone of at least 100 kilometres radius around infected premises. This, together with a surveillance zone of at least a further 50 km remains in force for at least 12 months. The scale of the zone is because virus movement over long distances via windborne infected vectors is possible.

Its appearance in the UK could spell the death knell for horse racing and all other forms of equestrian sport.

Dogs which have eaten infected horse meat may also become carriers of the disease.

Horse owners should look out for any signs of illness in their horses, such as:

fever
sweating
lack of energy
breathing difficulties
coughing
discharge from the nose
excessive salivation
restlessness
swelling of the eyes and/or head.

If any of these symptoms occur, put the horse in a separate closed stable and consult your veterinarian as soon as possible.

The World Organisation for Animal Health (OIE) says that theIncubation period of AHS is usually 7-14 days, but may be as short as 2 days. It also gives the following information:

Clinical diagnosis

Subclinical form: fever (40-40.5°C) and general malaise for 1-2 days
Subacute or cardiac form: fever (39-41°C), swelling of the supraorbital fossa, eyelids, facial tissues, neck, thorax, brisket and shoulders. Death usually within 1 week
Acute respiratory form: fever (40-41°C), dyspnoea (difficulty breathing), spasmodic coughing, dilated nostrils with frothy fluid oozing out, redness of conjunctivae, death from anoxia (respiratory failure) within 1 week
A mixed form (cardiac and pulmonary) occurs frequently: pulmonary signs of a mild nature that do not progress, oedematous swellings and effusions, death from cardiac failure, usually within 1 week
In the majority of cases, the subclinical cardiac form is suddenly followed by marked dyspnoea and other signs typical of the pulmonary form
A nervous form may occur, though it is rare

Lesions

Interlobular respiratory form: oedema (fluid) of the lungs, hydropericardium (fluid around the heart), pleural effusion (fluid in the chest cavity), oedema of thoracic lymph nodes (swelling), petechial haemorrhages in pericardium (pin-point haemorrhages in the membrane surrounding the heart)
Cardiac form: subcutaneous and intramuscular gelatinous oedema, epicardial and endocardial ecchymoses (bleeding into the tissues of the heart), myocarditis (inflammation of the heart muscle), haemorrhagic gastritis

Differential diagnosis

Anthrax
Equine infectious anaemia
Equine viral arteritis
Trypanosomosis
Equine encephalosis
Piroplasmosis
Purpura haemorrhagica

In the 1950s an epidemic of AHS spread from India to the near East, leading to the death of an estimated 800,000 equines. In the mid-1960s it broke out in Spain, where it was controlled by a rigorous slaughter policy, only to appear again in both Spain and Portugal in 1988-90 and 1989 respectively.

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