African Horse Sickness

Thu, 21 Mar 2024

FEEVA animation on African Horse Sickness (AHS)

What is AHS?

AHS is a frequently fatal disease of horses caused by infection with an Orbivirus with 9 serotypes (types 1-9), there is limited cross protection between most serotypes
AHS virus is related to another Orbivirus called Bluetongue virus (BTV)
The horse is the most susceptible equine species, followed by donkeys. Zebra are the least susceptible with the infection being largely subclinical
AHS is considered endemic in sub-Saharan Africa but occasionally breaks out in other countries, most commonly following movement of infected zebra

Transmission AHS virus

AHS is not directly contagious between horses and viral transmission is by biting Culicoides midges
AHS virus is maintained in a cycle of infection between equine hosts (involving intrinsic viral incubation) and midge vectors (extrinsic viral incubation)
Zebra are critical to the endemic maintenance of AHS in Africa
Maintenance of AHS in non-endemic areas would probably require mechanisms for over-wintering of AHS virus

Clinical syndromes of AHS

AHS virus is transferred to blood from infected biting midges, with viral replication in the regional draining lymph nodes
Dissemination to the lungs, spleen and other lymph nodes is through a marked secondary viraemia leading to severe vascular permeability with disseminated haemorrhage and oedema
There are four clinical syndromes associated with AHS
- Pulmonary form (‘Dunkop’), which presents as acute disease, fever, dyspnoea, sweating, pulmonary oedema, terminal frothy discharge (90% fatal)
- Cardiac form (‘Dikkop’), which presents as subacute disease, fever, oedema – head, neck, chest, supra-orbital fossa, conjunctivitis, +/- colic (50% fatal)
- Mixed form, which is as the name suggests a mixture of signs of Dunkop & Dikkop (80% fatal)
- Horse sickness fever, which is milder and non-fatal and occurs in partially immune animals, either by vaccine or following survival after natural infection

Diagnosis

Virus detection in blood or tissues (spleen, lung or lymph nodes), traditionally using virus isolation but now based on detecting viral RNA using reverse-transcription (RT) PCR
Serological diagnoses can be made using virus neutralization (VN) test or ELISA detecting antibodies against viral protein 7 (VP7)
World Organisation for Animal Health (WOAH) recommended tests are RT-PCR based on segment 8 (encoding VP7) and VP7 ELISA

Aims of AHS investigation and control in non-endemic areas

Identify any suspect cases & determine if AHS positive as quickly as possible
Eliminate AHS virus infection as quickly as possible to avoid spread to endemic midge vectors
Determine the route of introduction of AHS virus (e.g. recent live animal import, transfer of infected vector)
Regain AHS free status as soon as possible based on use of disease control zoning principles

Use of zones for AHS control

Zones are declared on confirmation of disease, together comprising the Restricted Zone (RZ)
- Control Zone (CZ): at least 20km from infected premises (IP)
  - Vaccination of all equidae permitted
- All suspect holdings under surveillance within the zone
- All movements prohibited
Protection Zone (PZ): at least 100km from IP
- Vaccination of all Equidae may be permitted
Surveillance Zone (SZ): at least another 50km
- No vaccination permitted

AHS vaccination

The AHS vaccine currently used in South Africa is based on 7 live attenuated vaccine strains (polyvalent) presented as two separate injections (bottles 1 and 2) given at least 3 weeks apart before highest risk periods
- Bottle 1: serotypes 1, 3 and 4
- Bottle 2: serotypes 2, 6, 7 and 8
- Cross protection is conveyed between serotypes 6 and 9 and 8 and 5
Viral surveillance has demonstrated recombinations between field and live-attenuated vaccine strains in South Africa
So use of polyvalent live attenuated vaccine to control AHS in non-endemic areas may present issues in restoring AHS free status
Safer, monovalent AHS vaccines with DIVA (differentiate infected from vaccinated animals) capability are required to assist effective control and eradication of AHS in non-endemic areas

More information

FEEVA