African Horse Sickness
Introduction | The Disease | Vaccines | Research at the AHT | Links
African horse sickness is a highly fatal disease, which affects horses, mules and donkeys. It is caused by African Horse sickness virus (AHSV), which is a member of the Orbivirus group. The disease is not directly contagious between horses, but is transmitted by the bite of infected biting Culicoides midges.
The virus is present in sub-Saharan Africa and outbreaks have also occurred in Morocco, the Middle East, Spain (1966, 1987-90) and Portugal (1989) in the past. AHSV has never been present in the UK.
The spread of disease is influenced by climatic conditions which favour the spread of Culicoides midges (vectors), including wind dispersal, warm, moist weather and high rainfall. Current changes in the global climate therefore mean there is an increased risk of AHSV reaching the UK.
This has been highlighted by the first UK outbreak of Bluetongue virus (BTV) in 2007. BTV is closely related to AHSV and also belongs to Orbivirus group. It causes disease in ruminants (such as cattle, goats, deer and sheep), with symptoms generally being most severe in sheep.
The Disease
Zebras may be infected without showing signs of disease. However, in susceptible horse populations the mortality rate may be as high as 90%.
There are four different forms of the disease. The clinical signs differ depending on the form:
Acute
Short incubation period of 3-5 days. High fever, redness of conjunctivae, severely laboured breathing, coughing, profuse discharge from the nostrils. 90% horses dying within a week.
Cardiac
Incubation period 7-14 days. Swelling of eyelids, facial tissues, neck, and shoulders (subcutaneous oedema), 60% mortality as a result of heart failure.
Mixed
Incubation period 5-7 days. Combination of the above two types, the disease shows itself initially by mild respiratory signs followed by the typical swellings of the cardiac form. Death usually within 1 week.
Horse Sickness fever
Mildest form, characterised by a fever with low temperatures in the morning rising to a high peak in the afternoon.
Currently, there is no specific treatment for AHSV infection in horses. Supportive veterinary care is recommended.
Vaccines
Live attenuated AHSV vaccines for the horse are available in Africa, but are not licensed for use in the UK.
Research at AHT
Vaccination plays an essential role in the control and prevention of the disease and vaccine development is the main focus of AHS research at the AHT.
Although AHSV vaccines are commercially available in South Africa, concerns still exist over their use in countries where the disease is not endemic because they contain live AHSV.
Current work at the AHT is concentrated on the development of vaccines which make use of the recombinant viral vector - modified vaccinia Ankara (MVA). Using this strategy an AHSV gene is incorporated into the MVA genome so that the recombinant MVA expresses the AHSV protein encoded by that gene. This virus is unable to complete replication in horses (so cannot be transmitted between horses), but the AHSV protein is produced before replication stops. The expressed AHSV protein is therefore expressed by infected horse cells and may be detected by the horse’s immune system.
AHSV research at the AHT is carried out in collaboration with Cambridge Infectious Diseases Consortium and the Institute for Animal Health, and is supported by the Biotechnology and Biological Sciences Research Council (BBSRC) and the Department for the Environment and Rural Affairs (Defra).
AHSV Links
http://www.defra.gov.uk/animalh/diseases/notifiable/africanhorse/index.htm
