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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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