African horse sickness

Alert

African horse sickness is prohibited matter under the Biosecurity Act 2014.

African horse sickness is an infectious, non-contagious insect-borne viral disease of equids, with horses, donkeys, mules and zebras as the primary hosts. This virus is also known to affect dogs.

African horse sickness has not been reported in Australia.

African horse sickness does not affect humans.

If you suspect the presence of this disease in any species of animal, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Disease Watch Hotline on 1800 675 888.

African horse sickness is a serious disease in horses where the mortality rate can range from 50% to 95%. The threat of African horse sickness entering Australia has increased due to notification of recent outbreaks in Thailand. It is possible for African horse sickness to spread southward to Australia if adequate biosecurity measures are not in place. It is important for horse owners, those that work with horses and the general community to have an awareness of African horse sickness and know what to do if they suspect the presence of this disease.

Cause

African horse sickness is caused by infection with the African horse sickness virus, a member of the genus Orbivirus in the family Reoviridae. There are 9 serotypes of the virus.

Other names

  • African horse plague
  • Equine plague

Description

Disease caused by infection with the African horse sickness virus has four main manifestations (see Symptoms below). While features vary between the various forms, fever, oedematous swellings, respiratory distress and high mortality are notable presentations. However, one of these manifestations, horse sickness fever, is an important subclinical form of the disease; this variant form is characterised by undulating fever but other signs are mild and it rarely results in death.

Distribution

African horse sickness is endemic in tropical and subtropical areas of Africa. Outbreaks outside of Africa have included: the Near and Middle East (1959–63), Spain (1966, 1987–90), Portugal (1989), Yemen (1997) and the Cape Verde Islands (1999). Occurrence of the disease is seasonal and based on environmental conditions that support the breeding of its insect vectors. Culicoides imicola is regarded as the main African vector and its expansion into the Mediterranean Basin of Europe and Asia now threatens those areas and beyond.

In the Asia and Pacific region there was a notification in February 2020 of unresolved outbreaks of African horse sickness in Thailand. The proximity of these outbreaks and the potential for southward movement of the virus are of heightened importance to Australia.

With respect to Australia, the capacity of endemic Culicoides spp (especially C brevitarsis) and mosquitoes to transmit the African horse sickness virus is unknown.

Hosts

  • The usual hosts are all species of Equidae, especially horses, mules, donkeys and zebra.
  • Dogs are susceptible but are dead end hosts
  • Life cycle

    Equids:

    African horse sickness is transmitted by Culicoides spp midges and is not contagious between horses. In Africa, zebras are the natural reservoir hosts of the virus and the main vector species is Culicoides imicola. The efficiency of potential vectors within Australia to transmit the virus is yet to be definitively determined.

    Moist mild conditions and warm temperatures tend to favour the breeding of insect vectors. Wind has been implicated in the dispersal of infected vectors in some epidemics.

    Transmission depends on the development of sufficient viraemia (the presence of virus in the blood) to infect potential insect vectors. Zebras are considered the natural reservoir hosts of the African horse sickness virus, where viraemia has been shown to last up to 40 days. Viraemia in horses can extend for 21 days but 4 to 8 days is more usual.

    Transmission through parenteral injection of blood containing the virus is possible.

    Animals recovering from the infection do not remain carriers of the virus.

    Dogs:

    Viscera, meat and blood from infected horses can be a source of the virus. Dogs eating infected horse material can succumb to a fatal peracute pulmonary form of the disease. Because dogs are not preferred hosts for the Culicoides spp in endemic areas they are not considered to be significant in the epidemiology of the disease.

    Affected animals

    • Horses, donkeys, mules and zebras are the primary hosts for African horse sickness.
    • Dogs eating viscera and blood from infected horses are also known to be affected. While antibodies have been reported in several other species, they are not considered to be significant in the epidemiology of the disease.

    Symptoms

    Incubation period:

    The incubation period varies with the form of the disease and can range from 2 days up to 14 days.

    Clinical signs:

    Mortality rates vary with the species affected, previous contact with the disease and the form of the disease: horses (50% to 95%); mules (50%); and donkeys (European and Asian types: 10%).

    Lifelong immunity following recovery is uncommon as different serotypes are not necessarily cross-protective.

    The clinical signs associated with the four main forms of the disease are shown below:

    • Subclinical form (Horse sickness fever)
      • fever (40–40.5°C)
      • general malaise for 1–2 days
      • very rarely results in death.
    • Subacute or cardiac form
      • fever (39–41°C)
      • swelling around the eyes, face, neck, thorax, brisket and shoulders
      • mortality rate usually 50% or higher
      • death usually within 1 week.
    • Acute respiratory form
      • fever (40–41°C)
      • difficulty breathing, spasmodic coughing, dilated nostrils with frothy discharge
      • redness of conjunctivae
      • nearly always fatal
      • death usually within 1 week.
    • Mixed form (cardiac and pulmonary)
      • occurs frequently
      • respiratory signs of a mild nature that do not progress
      • oedematous swellings and effusions
      • mortality rate 70–80% or higher

    Impacts

    • High mortality rate in horses. The main routes for introducing African horse sickness virus include: movement of infected equids and incursion of infected Culicoides midges by wind dispersal.
    • Restricted animal movements for sporting, recreational and farming purposes
    • Restricted international trade in equids and derived products.

    Risk period

    The number of competent insect vectors is favoured by moist mild conditions and warm temperatures. In some epidemics wind has been implicated in dispersal of infected Culicoides spp.

    Monitoring and action

    Surveillance

    Monitoring for competent vectors:

    Culicoides imicola is regarded as the main vector for African horse sickness virus in Africa and its northward movement into the Mediterranean basin of Europe and Asia poses new risks for movement of the disease. Culicoides imicola is known to be present in Thailand but it has not been detected in Australia.

    Insect traps are deployed throughout Australia as part of the National Arbovirus Monitoring Program (NAMP) for bluetongue virus. These traps are used to monitor for incursions of exotic Culicoides spp that are capable of transmitting the bluetongue virus. The viruses causing bluetongue and African horse sickness share Culicoides imicola as a competent transmission vector.

    The capacity of endemic Culicoides spp and other insect species to transmit African horse sickness has not been definitively determined.

    Awareness of endemic differential diagnoses:

    • anthrax
    • equine infectious anaemia
    • Hendra virus infection
    • purpura haemorrhagica.

    Reporting:

    African horse sickness has not been reported in Australia. Horse owners and those that work with horses should be aware of the clinical signs and if you suspect the presence of this disease in any animal, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Disease Watch Hotline on 1800 675 888.

    Control

    Australia has strict import conditions on equids to prevent the entry of African horse sickness (and other equine diseases).

    Comprehensive plans to respond to and control an outbreak of African horse fever exist at a national level. The nationally agreed strategy is outlines under Australian Veterinary Emergency Plan (AUSVETPLAN).

    Control measures during an incursion are aimed at reducing horse contact with vectors including:

    • using insect repellents on horses
    • housing horses under midge-proof netting
    • preventing insects feeding off infected horses
    • insecticides and other measures to reduce insect populations in the environment.

    A vaccine is available in Africa and is being used as part of the response to the Thailand outbreak. The vaccine is not available for use in Australia.

    Further information