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.