United States Department of Agriculture 

Animal and Plant Health Inspection Service

What is African horse sickness?

African horse sickness (AHS) is an insect-borne 

infectious disease of equids. The disease occurs in 

four clinical forms and, depending on the disease 

form and species of equid, it can have a very high 

mortality rate. The AHS virus (AHSV) is not 

contagious through contact between animals, it is 

carried by an insect or mechanical vector (e.g.,

needles). Although equids are the primary hosts, 

natural disease is rarely reported in other species. 

AHS is not a threat to human health.

Where has AHS been detected?

• The disease is normally found in tropical 

equatorial regions of sub-Saharan Africa. In 

subtropical regions of Africa, AHS exhibits a 

seasonal occurrence from late summer to 

autumn.

• Historical outbreaks previously occurred in the 

Near and Middle East, the Iberian Peninsula, 

Morocco, and as far east as Pakistan and India.

In 2020, it spread for the first time to Thailand 

and Malaysia in Southeast Asia. 

• AHS is a World Organisation of Animal Health 

(OIE) notifiable equid disease. Consequently, 

any illness in equids that may be AHS must 

be reported immediately to State animal 

health officials and USDA.

How is AHS transmitted?

AHS is a non-contagious, infectious vector-borne 

disease, which means that it is not transmitted by 

direct contact among horses. Instead, it is usually 

spread by infected insects.

o Midges serve as the primary vector.

o Mosquitoes, some biting flies, and other 

arthropods such as ticks, might also transmit 

the virus from one equid to another. 

o Because climatic factors influence the 

breeding of midges, AHS occurs seasonally. 

A period of drought followed by heavy rains

might lead to an outbreak in an endemic 

area or in an area with an introduction of an 

infected equid. 

o Although AHS has not been detected 

domestically, the United States does have 

species of insects that could transmit AHSV 

among equids. 

Clinical signs of AHS

The signs and severity of disease depend upon 

the clinical form of disease as well as the species

of equid infected. The disease can present in the 

following forms:

o The pulmonary form of AHS begins with fever 

and progresses within a few days to 

respiratory failure and death in about 95 

percent of affected horses, the most sensitive 

equid species. Clinical signs include

depression, profuse sweating, labored 

breathing, pulmonary edema, severe 

coughing, and frothy nasal discharge. 

o The cardiac form of the disease has clinical 

signs that include a mild fever; swelling of the 

face, head, neck, and chest; and depression. 

The equids may also exhibit signs of colic. 

With this form of AHS, death can occur in 

about half of cases or more within 4 to 8 days. 

Information Sheet

May 2021

African Horse Sickness

Photo by Canva.com 
United States Department of Agriculture 

Animal and Plant Health Inspection Service

o The mixed form of AHS includes clinical signs 

of both the pulmonary and cardiac forms,

beginning with mild respiratory signs, followed 

by edema, cardiac failure, and death within 3-6 

days. The case fatality rate is around 70 

percent. 

o The fourth form of the disease is known as

African horse sickness fever. Donkeys and

zebras, and vaccinated horses with partial 

immunity to AHSV, may develop the milder 

form of disease, characterized by low-grade 

undulating fevers, lethargy, and shortness of 

breath. This form is rarely fatal.

o Horses are more susceptible to the pulmonary 

and mixed forms of disease with the highest 

mortality rates (50% to 95%). Mortality rates in 

mules average 50% with European and Asian 

donkeys’ mortality rates averaging around 5%

to 10%. African donkeys and zebras are the 

least susceptible equids and thought to be the 

reservoirs of the virus. Zebras are believed to

have been responsible for movement of the 

virus to unaffected countries, including the 

recent 2020 outbreak in Thailand.

What are control measures if AHS is found 

in the United States? 

If AHS is found here, it is critical for horse owners 

to follow biosecurity precautions such as:

o Implementing insect-control measures (as 

well as destroying possible midge breeding 

sites)

o Stabling equids from dusk to dawn, preferably 

in insect-proof housing

o Not re-using needles among multiple equids

o Properly inactivating AHSV in potentially 

contaminated areas using heat above 140oF,

pH less than 6 or greater than 11, or acidic 

disinfectants.

Animal health officials may take some of the 

following actions to limit the spread of AHSV:

o Delineate an area of infection and establish a 

quarantine zone.

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o Restrict equid movements within, into, and 

out of the infected area.

o Consider euthanasia for infected horses with 

severe clinical signs that are unlikely to 

recover.

o Implement quarantine measures to separate

infected horses from noninfected horses to 

prevent spread of the virus.

o Release quarantines, as appropriate, related 

to the disease situation.

o Require all equids in affected areas to be 

housed from dusk to dawn.

o Implement insect-control measures.

o Monitor all equids daily for signs of the 

disease.

o On approval from USDA APHIS, use an 

authorized, appropriate vaccine to protect

susceptible horses within a quarantined area.

How could AHSV enter the United States?

AHSV could enter the United States through 

importation of infected equids from endemic 

countries or countries with recent introduction and 

spread. The greatest risk for ASHV introduction is 

associated with imported zebras, donkeys, or

equids with partial immunity, including vaccinated 

equids. Infected animals can serve as a source of 

virus, which midges can then pass to non-infected 

animals. Other possible routes of entry include:

o Importation of infective animal products such 

as equine serum, semen, and equine-origin 

biological products from AHSV-affected 

countries. 

o Introduction of infected vectors, specifically 

midges, via airplanes or ships.

o Introduction of infected vectors carried on the 

wind from areas of infection.

Report suspicious cases

AHS is an internationally reportable disease. If you 

see an equid that you believe might have AHS, 

contact your veterinarian. Suspected cases should

also be reported to your State animal health official. 

For more information, contact:

The Center for Epidemiology and Animal Health 

vs.ceah@usda.gov

(970) 494-7200