Mosquito-borne diseases pose significant health risks to horses. These diseases are caused by viruses or parasites transmitted through mosquito bites when they feed on a horse’s blood.

In horses, the most common mosquito-borne illnesses are West Nile Virus and Eastern, Western, and Venezuelan Equine Encephalitis. These viruses affect the brain and are sometimes fatal.

Symptoms of mosquito-borne diseases vary, but may include changes in behavior and mental capacity such as difficulty controlling the face, head, or mouth, seizures, fever, and coma.

Young and old horses, horses in areas with greater mosquito activity, and unvaccinated horses are at greater risk. There is no treatment available for equine encephalitis or West Nile viruses in horses. Supportive care is recommended.

The best way to prevent mosquito-borne diseases is to vaccinate horses and reduce mosquito exposure. Strategies to control mosquito populations include removing standing water, applying mosquito repellent, keeping stables well ventilated, and avoiding turnout during high mosquito activity.

Mosquito-Borne Illness in Horses

Mosquitoes are a type of flying insect that feed on the blood of many different animals, including horses. When a mosquito feeds on an animal with a contagious, blood-borne disease, it can transfer the infectious agent (i.e. the pathogen) to the next animal it bites. This ability to transfer diseases between hosts makes mosquitoes a vector for blood-borne illness in horses.

Mosquitoes are particularly complex as vectors, since they can transmit diseases between host species. Equine mosquito-borne illnesses are transmitted to horses from mosquitoes who have bitten infected birds. [1]

Examples of mosquito-borne diseases that affect horses include: [1]

  • West Nile Virus (WNV): A viral infection that can cause severe neurological disorders.
  • Eastern Equine Encephalitis (EEE): A viral disease known for its high mortality rate in horses.
  • Western Equine Encephalitis (WEE): A viral infection similar to EEE but generally less severe.
  • Venezuelan Equine Encephalitis (VEE): A viral disease that can cause outbreaks of encephalitis.
  • Saint Louis Encephalitis (SLE): Primarily affects humans but can also infect horses.
  • Japanese Encephalitis (JE): Similar to SLE and found throughout Asia.

Diagnosis is based on blood work, urinalysis, PCR tests, cerebral fluid analysis (spinal tap), and confirmed with postmortem tissue analysis. When confirmed, cases of mosquito-borne diseases must be reported to appropriate local animal health protection services.

Types of Mosquitoes

Mosquitoes belong to the Culicidae family of insects, and there are over 3000 different species worldwide. There are three species of mosquito that are dangerous to horses: [1]

  • Anopheles
  • Culex
  • Aedes

Culex are carriers of equine encephalitis and West Nile viruses. [1] Anopheles and Aedes are carriers of equine encephalitis viruses only. [1]

In each species, it is the female that bites and draws blood to nourish her eggs. [1] The mosquito is attracted to its victims by exhaled breath, body odor, movement, and changes in temperature. [1] When an infected mosquito pierces the skin with its mouth parts, it injects the virus into the horse’s blood stream. [1]

Mosquito Life Cycle

The mosquito’s life cycle has four stages: egg, larva, pupa, and adult. All mosquitoes complete the majority of their life cycle in standing water. [1]

  • Eggs: Eggs are laid one at a time on the surface of water or damp soil and depending on the species, will stay separate or be stuck together in rafts of 200 to 300 eggs. Most eggs hatch into larvae within 24 to 48 hours.
  • Larvae: Mosquito larvae live in water and breathe at the surface. They feed on microorganisms and bits of organic matter. They molt 4 times until they are almost half an inch (1.25 cm) long. The fourth molt is when they become pupae.
  • Pupae: The pupae do not feed, but take in oxygen through two tubes in their bodies. They live for between 1 and 4 days, floating on the surface of the water. Once their development is complete, they emerge as adult mosquitoes.

Equine Encephalitis Viruses

The equine encephalitis viruses are all similar in their structure and effect on horses, with slight distinctions depending on where they originated from.

These viruses are considered life-threatening as they directly impact the brain and central nervous system, and currently there is no available treatment for any of them.

Horses are typically dead-end hosts of these viruses, which means that mosquitoes feeding on infected horses typically do not contract enough virus to cause disease in the next animal they feed on. Infected horses also do not pass these diseases on to other horses or to humans through direct contact.

Eastern Equine Encephalitis

Eastern Equine Encephalitis (EEE) is transmitted to horses and humans by mosquitoes, but is not transmitted from horses to other animals directly. [1][2]

EEE is relatively rare and is most commonly found in the United States in areas east of the Mississippi River, although it has been found as far west as Texas. [3]

The incubation period for EEE is 3 to 14 days. [2][8] This means that after a horse is bitten by an infected mosquito, it can take anywhere from three to fourteen days for symptoms of the disease to appear.

The prognosis for horses with EEE is extremely poor with 75 to 95% mortality. [3] Vaccinated horses have a greater chance of survival. [4] Horses that survive often have permanent deficits in their mental and physical capacity. [3]

Western Equine Encephalitis

Western Equine Encephalitis (WEE) is also transmissible from mosquitoes to horses and humans, and again horses do not transmit this virus directly to other animals. [5]

WEE has not been reported in the United States for more than two decades. [3] The incubation period for WEE is 2 days to 3 weeks. [3]

The prognosis for horses with WEE is fair with 20 to 40% mortality. [3] This disease is not as severe as EEE or VEE and long-term neurologic deficits are less likely. [3]

Venezuelan Equine Encephalitis

Similar to WEE, Venezuelan Equine Encephalitis (VEE) is also transmitted by mosquitoes to horses and humans. [5]

Horses are amplifying hosts of VEE. [5] Amplifying hosts pose no risk of disease transmission to other animals through direct contact. However, infected animals can carry a high enough viral load that they can transmit the virus back to mosquitoes, who can then spread the virus further. [5]

VEE is most commonly found in Central and South America and occasionally in Mexico. [6] It has not been in reported in the United States since 1971. [6]

The incubation period for VEE is 2 to 4 days. [3]

The prognosis for horses with VEE is poor with a mortality rate of 40 to 90%. [3] Horses that survive often have permanent deficits to their mental and physical capacity. [3]

West Nile Virus

West Nile Virus (WNV) is a different type of virus from the equine encephalitis viruses, but the transmission routes and risks are the same. [7] Horses are dead-end hosts of WNV, which means they pose no risk of infection to other horses or to humans. [7]

WNV is considered endemic in the United States, Canada, Mexico, and the Caribbean with yearly outbreaks during the mosquito season. [7]

The incubation period for WNV in horses is 7 to 10 days. [7]

The prognosis for horses with WNV is fair with a mortality rate of 20 to 44%. [5] Horses that survive typically have no mental or physical deficits after 6 months. [5]

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Symptoms of Mosquito-Borne Viruses

The symptoms of Eastern, Western, and Venezuelan Equine Encephalitis and West Nile viruses are variable and non-specific. This means that each horse has a different constellation of symptoms and none of the symptoms can confirm a diagnosis. [3]

Symptoms of these mosquito-borne viruses in horses include: [2][5]

Neurologic Symptoms

Alongside the above symptoms, neurologic symptoms may develop suddenly once the virus has infected the brain. [2] Neurologic symptoms include:[2][5][6][8][9]

  • Fearfulness
  • Excitability
  • Drowsiness
  • Circling
  • Head pressing
  • Teeth grinding
  • Inability to Swallow
  • Excessive sweating
  • Changes in behavior
  • Stiffness
  • Tremors or twitches in the face and neck
  • Convulsions
  • Seizures
  • Blindness
  • Paralysis of the face
  • Weakness of the tongue
  • Changes to the shape of the muzzle
  • Head tilt
  • Poor balance/prioperception
  • Weakness
  • Incoordination
  • Paralysis of the limbs
  • Colic
  • Inability to stand (recumbency)
  • Coma
  • Sudden death

Risk Factors

Eastern, Western, and Venezuelan Equine Encephalitis and West Nile viruses are found in North, South, and Central America. [5] All breeds of horses are at equal risk. [5] Young horses and old horses may be at greater risk. [4][8] There does not appear to be variation due to sex. [5]

All of the following factors impact the risk of mosquito-borne illness in horses: [1][4][5][7][8]

  • Vaccination status: Unvaccinated or improperly vaccinated horses are at greater risk than vaccinated horses.
  • Climate and travel: Horses that live or travel to areas where mosquito-borne diseases are prevalent have higher rates of exposure.
  • Seasonal insect activity: Cases are more prevalent in the summer months when mosquitoes are more active. Horses that live in milder regions where mosquitoes are active for a longer period of the year experience more exposure.
  • Lack of insect control: Horses in areas where mosquito control methods are not deployed or are poorly deployed have a greater risk
  • Lifestyle: Horses that spend greater time outside, especially at times of the day when mosquitoes are active, are more at risk.
  • Local environment: Horses that live in areas with high humidity such as near swamps are at increased risk

Diagnosis

Diagnosis of Eastern, Western, Venezuelan Equine Encephalitis and West Nile viruses in horses is based on: [3][5][7]

  • Blood tests
  • Urinalysis
  • Analysis of cerebral fluid (“spinal tap”)
  • PCR tests

Diagnosis is confirmed with postmortem analysis of brain tissues. [3][5]

Handling infected tissues puts humans at risk for contracting disease. Protective clothing must be worn and, since these diseases share symptoms with rabies, a rabies protocol must be in place during the postmortem examination. [3][5][7]

Treatment

There are no treatments for EEE, WEE, VEE, or WNV in horses. [5] Supportive care is recommended to keep the horse hydrated and comfortable while the disease runs its course. [5][6]

EEE, WEE, VEE, and WNV are all reportable diseases and must be reported to local animal health protection services when they are suspected. [3][7] It is also necessary to report the presence of dead birds around the same facilities as infected horses. [6]

Prevention

Vaccines are available to protect horses from Eastern, Western, Venezuelan Equine Encephalitis and West Nile Virus. [3][7] Boosters are required annually and throughout the period of the year where mosquitoes are active. [1]

Controlling mosquito populations is also recommended. [1] In some areas widespread control of mosquito populations is offered through aerial spraying or truck spraying programs. [1]

Individuals are also encouraged to control mosquito populations near their horses by: [1][6]

  • Removing all sources of standing water
  • Refreshing trough water twice weekly
  • Applying mosquito repellent
  • Keeping horses indoors when mosquitoes are out
  • Placing fans to keep the air moving around the horse

Summary

Horses that are bitten by infected mosquitoes are in danger of contracting Eastern, Western, and Venezuelan Equine Encephalitis and West Nile Virus. These viruses can infect the brain, leading to physical and mental changes.

There are no treatments for these diseases, but prevention is possible with vaccination and vigilant control of mosquito populations.

  • Symptoms are variable and non-specific and include changes to the physical and mental capacity of the horse
  • Diagnosis is based on blood work, urinalysis, PCR tests, cerebral fluid analysis, and confirmed with postmortem tissue analysis
  • The prognosis is fair to extremely poor depending on the specific disease, and in certain cases survivors do not recover full mental or physical capacity
  • Mosquitoes are active during certain times of the year and day, therefore protecting the horse by applying mosquito repellent or keeping them indoors in well-ventilated spaces during these periods is recommended
  • Horses are dead-end or amplifying hosts of mosquito-borne illnesses, which means that they do not pose a direct threat to other horses or to humans when they are infected.
  • Eastern, Western, and Venezuelan Equine Encephalitis and West Nile Virus are reportable diseases. Horses suspected of being infected must be reported to the appropriate local animal health protection services

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References

  1. Mosquitoes, in External Parasite and Vector Control Guidelines. American Association of Equine Practitioners
  2. Eastern Equine Encephalitis (EEE) From EDCC. Equine Disease Communication Center, 2017
  3. ARBOVIRUSES. American Association of Equine Practitioners, 2017.
  4. Eastern Equine Encephalitis (EEE) from AAEP. American Association of Equine Practitioners, 2023.
  5. Blackwell’s Five-Minute Veterinary Consult Clinical Companion
  6. Disease Alert: Equine Encephalitis (EEE/WEE/VEE), Vector-Borne Equine Encephalitides, 2024.
  7. West Nile Virus. American Association of Equine Practitioners, 2017.
  8. Reed, S. M. et al., Equine internal medicine, 3rd ed. St. Louis, MO: Saunders Elsevier, 2010.
  9. Sellon, D. C. and Long, M. T., Eds., Equine infectious diseases, Second edition. St. Louis, Missouri: Saunders/Elsevier, 2013.