African horse sickness (AHS) is a viral infection transmitted by biting midges. [1] Up to 90% of horses infected with the virus die after initial infection. [1] Due to international trade and climate change, there is a risk of AHS spreading into countries previously considered “AHS free”.

There are three major forms of African horse sickness: the pulmonary form, the cardiac form, and horsesickness fever. Horses can also develop a mixed form which has combined symptoms of pulmonary and cardiac disease. Common symptoms of AHS include fever, difficulty breathing, swelling of the head, and fluid leaking from the nostrils.

Veterinarians can diagnose AHS on a blood test or by submitting tissues after performing a necropsy. There is no known treatment for AHS, and most horses die shortly after developing symptoms of disease.

Prevention involves a combination of quarantine practices, vaccination, and reducing exposure to biting midges. Many countries that are “AHS-free” also have import restrictions to reduce the risk of AHS entering the country.

Causes of African Horse Sickness

AHS is caused by an orbivirus called African horse sickness virus. [1] The virus is endemic (regularly occurs) within eastern and central Africa. [1] Estimates suggest that AHS causes around $95 million USD in economic impact in sub-Saharan Africa each year. [2]

Outbreaks occur occasionally in northern and southern Africa, the Middle East, Asia, and countries around the Mediterranean Sea. [1]

African horse sickness is not contagious, meaning that it does not spread through direct contact with an infected animal. [3] Biting midges are the primary vector for AHS, meaning they transmit the virus by feeding on an infected animal, then pass it to the next animal they feed on. [1]

Zebras and donkeys are reservoirs for the infection, as they can carry the virus without developing severe symptoms. [1] Large populations of donkeys or zebras are a common source of the virus, where it is picked up by midges then passed to horses. [1]

After infection occurs, the virus reproduces in endothelial cells, the cells that line blood vessels. [1] This causes damage and injury to blood vessels throughout the body, which can lead to organ damage. [1]

Risk Factors

Risk factors for developing AHS include: [1][4]

  • Exposure to zebras or donkeys
  • Living in an area where AHS is endemic
  • Warm, moist environment increasing midge populations
  • Periods of drought followed by heavy rains increasing midge populations

Other Species

Dogs are the only non-equid species that can develop symptoms after exposure to AHS infection. [4] Dogs are usually exposed when eating infected horse meat. [4] Some dogs may die due to AHS infection. [4]

Humans do not develop symptoms of AHS after exposure to infected biting midges. [4]

Symptoms

Symptoms of African horse sickness develop between 5-7 days after infection. [1] There are three major “forms” of AHS, with different symptoms: [1]

  • Pulmonary form
  • Cardiac form
  • Horsesickness fever

Some horses develop a “mixed” form of AHS where they develop symptoms of both cardiac and pulmonary disease. [1]

The factors determining which type of AHS develops are largely unknown. [1] Factors such as the strain of the virus, pre-existing immunity, and strength of the horse’s immune response may impact which form of AHS a horse develops. [1]

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Pulmonary Form

The pulmonary form of AHS is called “dunkop” in Afrikaans. [1] This form of disease primarily affects the lungs and respiratory tract. [1]

Symptoms include: [1]

  • Fever
  • Difficulty breathing
  • Rapid breathing
  • Coughing
  • Fluid leaking from the nostrils
  • Profuse sweating

Horses with dunkop typically develop symptoms suddenly, and death usually occurs within 30 minutes to a few hours after breathing difficulties begin. [1] Fewer than 5% of horses recover from a dunkop episode. [1]

Cardiac Form

Dikkop“, or the cardiac form of AHS, affects the heart. [1] The virus causes damage to the heart muscle, preventing it from producing strong enough contractions to distribute blood around the body. [1] This causes fluid accumulation in the tissues, including the lungs. [1]

Symptoms of dikkop include: [1]

  • Swelling of  the face and head including the indentations above the eyes, the eyelids, lips, cheeks, and tongue
  • Difficulty breathing
  • Difficulty swallowing
  • Blue gums
  • Small pinpoint bleeds on the gums
  • Severe colic

The symptoms of dikkop are usually less severe than dunkop. [1] However this form is still highly fatal, with over 50% of horses with dikkop dying within 4-8 days after symptoms develop. [1] In horses that recover, the swellings subside over 3-8 days. [1]

Horsesickness Fever

Horsesickness fever most commonly occurs in horses previously vaccinated for AHS. [1] This form is also most common in donkeys and zebras. [1]

In this form of AHS, the affected animal develops a fever over 4-5 days, then recovers. [1] Owners typically notice no symptoms of horsesickness fever. [1]

Symptoms that may be present include: [1]

Diagnosis

A diagnosis of AHS typically relies on observation of symptoms. [1] However, the symptoms of AHS can be similar to other diseases, such as: [1][3]

Reportable Disease Finding

Since AHS is a reportable disease in many countries, any horse showing symptoms possibly associated with AHS must be reported to government authorities. [1] To confirm the diagnosis in these cases, the veterinarian can submit blood for virus isolation, which identifies the AHS virus or its genetic material using specialized techniques. [1]

Many horses with AHS die before diagnostic testing can take place. [1] In these horses, necropsy (an autopsy performed on an animal) to identify changes in the internal organs can help determine a diagnosis. [1]

Common findings on necropsy include: [1]

  • Fluid within the lungs and thorax
  • Swelling of the tissues under the skin and between muscles
  • Small hemorrhages on many internal organ surfaces
  • Pale-gray areas of the heart muscle indicating damage

The pathologist conducting the necropsy can submit tissues for virus isolation or examine samples under a microscope to confirm the diagnosis. [1]

Treatment

There is no specific treatment available for AHS. [1]

Treatment involves supportive care and stall rest, as any amount of exercise can potentially lead to death. [1] Horses that survive AHS infection require at least 4 weeks of stall rest after symptoms resolve, then a slow return to their previous level of performance. [1]

Prognosis

Mortality rates of AHS in horses ranges between 50-95%. [3] The pulmonary, cardiac, and mixed forms are the most fatal types of AHS infection, and typically occur in horses who have not been previously exposed to the virus. [3]

Conversely, horsesickness fever has a good prognosis and rarely results in death. [3] This form is most common in donkeys, zebras, and horses vaccinated against AHS. [3]

Prevention

As there is no known treatment for AHS, prevention is critical in areas where AHS infection is common. [4]

Prevention has three major focuses: [4]

  • Quarantine practices
  • Insect control
  • Vaccination

Quarantine Practices

Quarantine practices are crucial to prevent infected animals from spreading the disease to new locations. [4]

The fundamental principles of quarantine and biosecurity aim to prevent: [5]

  • Movement of infected horses to non-infected zones
  • Introduction of non-infected horses to infected zones

Quarantine practices can apply to a single property, a region, or even entire countries. [5] Many countries are “AHSV-free” and have strict import practices to prevent infected horses from entering the country. [5]

Quarantine Timeline

After identifying an infected animal, the entire property is placed under quarantine and all equids on the property are tested for AHS. [6] No horses should leave or enter the quarantine zone to prevent spread of disease. [6]

Any infected horses are placed in an insect-proof environment until their infection resolves. [5] The remaining non-infected horses should have their temperature taken daily to identify any new infections quickly. [1]

In many countries, AHS is a reportable disease requiring government involvement after confirming a diagnosis. [5] Government officials enforce quarantine protocols and carry out any legally required procedures for disease control. [5] Many “AHSV-free” countries require euthanasia of all infected horses to eliminate the risk of disease spread to other animals. [4]

Import Restrictions

Import restrictions are a form of quarantine used to keep a disease out of a country or region. These restrictions apply to any horse coming from an AHS-affected area and entering a non-affected area. [5]

Import restrictions typically include: [5]

  • Isolation of the horse in an insect-proof environment for 14-40 days
  • Repeated tests for the virus that return a negative result
  • No symptoms of disease present before transporting the horse

Countries and regions considered AHSV-free include: [7]

  • Most of North and South America
  • Most of continental Europe
  • United Kingdom
  • Australia
  • New Zealand

Reducing Midge Exposure

The bite of an infected midge is necessary to transmit the disease, so many prevention strategies focus on reducing midge exposure. [4]

Strategies to reduce midge exposure include: [4]

  • Stabling horses during the night when midges are most active
  • Placing fine mesh screens over doors and windows in stabling areas
  • Applying insecticides in and around stables and to the horses
  • Preventing leaks in taps or water pipes that can create a suitable soil habitat for midge development
  • Application of larvicides to damp soil areas to prevent midge development
Always inform your veterinarian of all animals who may be exposed to insecticides before treating your horse. Some species are highly sensitive to insecticides, particularly cats.

 

Vaccination

Vaccination is recommended to reduce the risk of AHS infection for horses in endemic areas. [1][8] Studies show that around 16% of vaccinated horses in endemic areas develop AHS if infected, however they usually only develop mild or no symptoms. [9]

In some cases, vaccines are used in outbreak scenarios to reduce the number of horses affected by the disease. [1]

Currently, the only vaccines available for AHS are live attenuated vaccines, made of a modified form of the AHS virus that cannot cause disease. [3] Veterinarians recommend annual vaccination of all horses in areas where AHS is endemic. [6]

Limitations of the Vaccine

Although veterinarians successfully use the AHS vaccine to prevent disease in endemic areas and mitigate outbreaks, the vaccine has several limitations that prevent more widespread use. [8]

Limitations include: [4][5][8][10]

  • Possibility of the live attenuated vaccine virus mutating back to an infectious state
  • Vaccinated horses return a positive result on some tests used to diagnose AHS
  • The vaccine is only available in Africa
  • Only some strains of virus are present in the vaccine, which may limit protection
  • Several vaccination rounds are necessary to achieve full immunity in horses
  • Can damage the growing fetus in pregnant mares

Research is ongoing to develop new types of vaccines to address some of these concerns. [4][8]

Global Spread of African Horse Sickness

Climate change is encouraging the spread of biting midge populations into new habitats. [5] This spread may result in AHS entering countries previously considered AHSV-free. [5]

In particular, Culicoides imicola, one of the major midge species carrying AHS, is spreading north into many areas of Europe. [5] There is already evidence that these midge species are transmitting bluetongue virus, a similar virus that affects ruminants, into new populations within Europe. [11]

A similar situation has not yet been identified for African horse sickness. [11]

International trade poses an additional concern for AHS spread, as South America, south east Asia, Australia, and some parts of the United States are climatically suitable for survival of C. imicola. [11]

There is evidence that international trade introduced new species of mosquito into Europe, however movement of C. imicola specifically has not been confirmed. [11]

Finally, international movement of horses also poses a risk for AHS spread. [11] Horses carrying the virus that enter new countries with biting midge species may introduce the virus into that insect population. [11]

Laboratory studies show that Culicoides sonorensis, one of the major midge species in the United States, can transmit African horse sickness if exposed. [11]

Summary

African horse sickness is a viral infection primarily found in sub-Saharan Africa that causes high mortality in affected horses.

  • AHS spreads primarily through biting midges
  • There are three main forms: the pulmonary form, the cardiac form, and horsesickness fever
  • Symptoms include fever, difficulty breathing, swelling of the head, and fluid leaking from the nostrils
  • Diagnosis typically involves a blood test or submission of tissues
  • There is no known treatment for AHS
  • Prevention involves quarantine of affected animals, movement restrictions into unaffected areas, vaccination, and reducing exposure to biting midges

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References

  1. Sellon. D. C. and Long. M. T., Eds., Equine Infectious Diseases. Second edition. Saunders/Elsevier, St. Louis, Missouri. 2013.
  2. Redmond. E. F. et al., Economic Assessment of African Horse Sickness Vaccine Impact. Equine Veterinary Journal. 2022.View Summary
  3. Stern. A., African Horse Sickness. Compendium. 2011.
  4. Mellor. P. S. and Hamblin. C., African Horse Sickness. Veterinary Research. 2004.
  5. Zientara. S. and Lecollinet. S., African Horse Sickness: -EN- -FR- Peste Équine -ES- Peste Equina. Revue Scientifique et Technique de l’OIE. 2015.
  6. Morales-Briceño. A., African Horse Sickness: A Practice Update. Microbiology Research International. 2021.
  7. African Horse Sickness. WOAH – World Organisation for Animal Health.
  8. Carpenter. S. et al., African Horse Sickness Virus: History, Transmission, and Current Status. Annual Review of Entomology. 2017.View Summary
  9. Weyer. C. T. et al., African Horse Sickness in Naturally Infected, Immunised Horses. Equine Veterinary Journal. 2013.View Summary
  10. Dennis. S. J. et al., African Horse Sickness: A Review of Current Understanding and Vaccine Development. Viruses. Multidisciplinary Digital Publishing Institute. 2019.View Summary
  11. Robin. M. et al., African Horse Sickness: The Potential for an Outbreak in Disease-Free Regions and Current Disease Control and Elimination Techniques. Equine Veterinary Journal. 2016.View Summary