What Insect Causes Sleeping Sickness?


Equine encephalomyelitis, also known as sleeping sickness, affects the nervous system of horses leading to confusion, mobility problems, drowsiness, depression and sometimes death. The viral disease is passed to horses through mosquito bites. Three types of the virus have been identified with the Eastern version being fatal in almost 90 percent of horse infections.

Sleeping Sickness Transmission

Several species of mosquitoes are responsible for transmitting the virus to horses. These species include the Aedes, Anopheles, Culex and Culeseta. However, horses are not the viruses primary target. Instead, mosquitoes usually infect birds, such as ducks or pigeons, with the virus. In the birds, the virus causes a blood infection that generally produces no negative symptoms. Instead, the reproduced virus waits in the bird’s blood for another mosquito that will transmit the virus into another bird where it can reproduce again. Unfortunately, horses sometimes end up in the middle of these exchanges, and the virus enters their body where it can cause serious symptoms, including:

  • Fever.
  • Depression.
  • Vision problems.
  • Muscle twitching.
  • Circling.
  • Head pressing.
  • Problems swallowing.
  • Convulsions.
  • Paralysis.

Because the virus affects horses differently, the viral transmission ends with the horse.

Types of Equine Encephalomyelitis

Three types of this disease have been identified: the Western, the Eastern, and the Venezuelan. All three types are transmitted via mosquitoes. As the names suggest, Western equine encephalomyelitis occurs more frequently west of the Mississippi River while Eastern equine encephalomyelitis happens more often east of the Mississippi. The Venezuelan variety typically occurs in South America. However, all three types can appear anywhere in North or South America. The mosquito vector differs depending on which subspecies are present in that environment. For example, the Western equine encephalomyelitis version of the disease is transmitted in Manitoba, Canada by the Culex tarsalis mosquito. In Connecticut, research on the Eastern equine encephalomyelitis version found that 68 percent of the 122 cases studied were transmitted by the Culiseta melanura mosquito.

Prevention and Treatment

No specific treatment is available for these viruses once a horse is infected. Instead, horses are given anti-inflammatory drugs and fluids to care for the symptoms. If your horse survives the disease, he may have continued impairment for the rest of his life. Because of the seriousness of the disease, veterinarians recommend vaccinating horses yearly. Because mosquitoes transmit the disease, reducing mosquito breeding grounds near horse stables or pastures also can decrease the risk of infection.

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