African horse sickness

African horse sickness virus, a member of the genus Orbivirus in the family Reoviridae. There are 9 serotypes of the virus.

African horse sickness (AHS) is endemic in tropical and subtropical areas of Africa. Distribution is influenced by environmental conditions and the presence of appropriate insect vectors.

In 2020, African horse sickness was detected in Thailand and Malaysia. These were the first occurrences of AHS outside the African region in 30 years.

Recent expansion of the distribution of the main African vector, Culicoides imicola into the Mediterranean Basin of Europe and Asia has increased the risk of spread.

The recent outbreak in Thailand has increased the risk of spread to Australia.

African horse sickness is a non-contagious disease transmitted by Culicoides biting midges. The virus replicates in insect and equine hosts.

Equids:

Natural infection between equids is transmitted by biting midges. African horse sickness is not transmitted directly between infected equids.

It is not known if the biting midges and mosquitoes of Australia, including C. brevitarsis, are able to transmit African horse sickness virus.

Natural transmission requires sufficient amounts of virus to be in the equine's blood to infect insect vectors. The infectious period varies with the species:

• Zebra – up to 40 days
• Horses – usually 4–8 days, up to 21 days.

Infection can also be spread by injection of infected blood.

The incubation period ranges from 2 to 14 days.

Recovered animals are immune, clear the virus and do not remain carriers. However, they remain susceptible to disease due to other serotypes.

Dogs:

African horse sickness can be transmitted to dogs that eat the internal organs, meat, or blood of infected horses.

However, as dogs are not preferred hosts for the Culicoides spp, they are not significant in the epidemiology of African horse sickness.

Humans that eat products from infected horses are not affected.

Horses:

Clinical signs associated with the 4 main forms of the disease are:

• 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
  • case fatality 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
  • mild respiratory signs that do not progress
  • oedematous swellings and effusions
  • case fatality 70–100% or higher.

Case fatality varies with the species, previous exposure to disease and the form of the disease:

• horses (50% to 100%)
• mules (50%)
• donkeys (European and Asian types: 10%)

Dogs:

• peracute pulmonary disease
• death.

• High morbidity and mortality rate in horses
• Restricted animal movements for sporting, recreational and farming purposes
• Restricted international trade in equids and derived products

• movement of infected equids
• wind dispersal of Culicoides midges.

African horse sickness is not spread by animal carcasses or products.

In endemic areas disease is seasonal, occurring when mild, warm, moist conditions favour the breeding of Culicoides biting midges. These vectors are most numerous and active after rain.

Horses:

Owners should monitor their horses for signs consistent with African horse sickness (see symptoms). Clinically similar diseases of horses that are present in Australia include: anthrax, equine infectious anaemia, Hendra virus infection, and purpura haemorrhagica.

If you believe African horse sickness is present, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Animal Disease Hotline on 1800 675 888.

Vectors:

Insect traps are deployed throughout Australia as part of the National Arbovirus Monitoring Program (NAMP). This program monitors for incursions of exotic Culicoides spp that can transmit animal diseases.

While Culicoides imicola is known to be present in Thailand, it has not been detected in Australia.

A nationally agreed strategy for responding to an outbreak of African horse fever is included under the 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.

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