AFRICAN HORSE SICKNESS
Aetiology Epidemiology Diagnosis Prevention and control References
AETIOLOGY
Classification of the causative agent
Viscerotropic virus, family Reoviridae, genus Orbivirus
Resistance to physical and chemical action
Temperature: Inactivated by 50°C/3 hours; 60°C/15 min pH: Survives between pH 6.0 and 12.0 Chemicals: Inactivated by ether and ß-propiolactone 0.4% Disinfectants: Inactivated by formalin 0.1%/48 hours. Also phenol and iodophores Survival: Survives at 37°C/37 days EPIDEMIOLOGY
- Mortality rate in horses is 70-95%, in mules it is around 50%, and in donkeys it is around 10%
Hosts
- Reservoir host still unknown
- Usual hosts: horses, mules, donkeys, zebra
- Occasional hosts: elephants, onager, camels, and dogs (after eating infected blood or horsemeat)
Transmission
- Not directly contagious
- Usual mode of transmission: Culicoides spp., i.e. biological vector
- Occasional mode of transmission: mosquitoes - Culex, Anopheles and Aedes spp.; ticks - Hyalomma, Rhipicephalus
- Moist mild conditions and warm temperatures favour the presence of insect vectors
- Virus movement over long distances via windborne infected vectors has been suggested
Sources of virus
- Viscera and blood of infected horses
- Semen, urine and all shed and secreted products
- Viraemia in horses may extend for as long as 18 days, but usually lasts for fewer days - about 4-8 days. In zebras and donkeys viraemia may last up to 28 days
Occurrence
As its name indicates, AHS is a truly African disease that is endemic in the central tropical regions of the continent, from where it spreads regularly to Southern Africa and occasionally to Northern Africa. A few outbreaks have occurred outside Africa, such as in the Near and Middle East (1959-63), in Spain (1966, 1987-90) and in Portugal (1989)
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Bulletin
DIAGNOSIS
Incubation period is usually 7-14 days, but may be as short as 2 days
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, spasmodic coughing, dilated nostrils with frothy fluid oozing out, redness of conjunctivae, death from anoxia 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 of the lungs, hydropericardium, pleural effusion, oedema of thoracic lymph nodes, petechial haemorrhages in pericardium
- Cardiac form: subcutaneous and intramuscular gelatinous oedema, epicardial and endocardial ecchymoses, myocarditis, haemorrhagic gastritis
Differential diagnosis
- Anthrax
- Equine infectious anaemia
- Equine viral arteritis
- Trypanosomosis
- Equine encephalosis
- Piroplasmosis
- Purpura haemorrhagica
Laboratory diagnosis
Procedures
Virus isolation
- Suckling mice or cell culture (BHK, MS, VERO)
Virus identification
- ELISA
- Virus neutralisation (Serotyping)
- Polymerase chain reaction (PCR)
Serological diagnosis
- ELISA
- Complement fixation
(prescribed tests in the Manual)
- Immunoblotting
Samples
Virus isolation
- Blood specimens obtained at the peak of the fever are preserved in OPG (50% glycerol, 0.5% potassium oxalate, 0.5% phenol) or heparin 10 IU/ml and transported at 4°C to the laboratory
- Spleen, lung and lymph node samples collected from freshly dead animals are preserved in 10% buffered glycerine and transported at 4°C to the laboratory
Serology
- Serum: preferably paired samples should be taken 21-days apart and kept frozen at -20°C
PREVENTION AND CONTROL
- No efficient treatment
Sanitary prophylaxis
- Virus identification (group and type)
- Slaughtering of affected horses and destruction of cadavers
- Vector control (insecticides, repellents, screens)
- Identification of vaccinated horses
Medical prophylaxis
- Vaccination of non-infected horses:
- polyvalent vaccine
- monovalent vaccine (better when virus has been typed)
- monovalent inactivated vaccine (only for sero-type 4)
REFERENCES AND OTHER INFORMATION
- Reference experts and laboratories
- Classified as an OIE List A disease (A110)
- Chapter 2.1.11. in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.
- Terrestrial Animal Health Code
- Other references - see the Index
- World Animal Health .
- Current Animal Health Status (Disease Information)
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Contact : scientific.dept@oie.int