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What are Mosquito-Borne Diseases?

Eastern, Western, Venezuelan, and St. Louis Equine Encephalitis and West Nile viruses cause damage to the central nervous system and will eventually lead to death. Dengue is a serious disease that presents as a high fever, flu-like symptoms, and bleeding from the nose. It is fatal in some cases. Malaria causes the red blood cells to burst, which creates a hemorrhagic fever that is usually fatal. Yellow fever is similar to Malaria and Dengue but the skin and eyes turn yellow due to liver damage, which may lead to death if not treated right away.

West Nile Virus, Eastern, Western, Venezuelan, and St. Louis Equine Encephalitis are all mosquito-borne viruses that can affect the central nervous system of any animal it infects, including humans. Dengue, Malaria, and Yellow Fever are also transmitted by mosquitos, but attack other parts of the body such as the blood and intestinal tract. Each of these diseases has their own set of symptoms, but they are all quite serious and should be treated as soon as possible by an equine veterinary professional.

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Symptoms of Mosquito-Borne Diseases in Horses

The symptoms of mosquito-borne diseases depend on the disease.

Eastern, Western, Venezuelan, and St. Louis Encephalitis

  • High fever
  • Depression
  • Changes in behavior
  • Impaired vision
  • Muscle twitching
  • Circling
  • Head pressing
  • Inability to swallow
  • Paralysis
  • Convulsions
  • Death

West Nile Virus

  • High body temperature
  • Nausea and vomiting
  • Loss of appetite
  • Extreme muscle aches
  • Weakness and fatigue
  • Swollen lymph nodes
  • Head pressing
  • Trouble getting up
  • Paralysis of rear legs
  • Seizures
  • Coma
  • Death


  • High body temperature
  • Muscle pain
  • Fatigue
  • Appetite loss
  • Nosebleeds
  • Breathing difficulty
  • Vomiting blood
  • Encephalitis (abnormal behavior, head pressing, confusion)
  • Convulsions
  • Shock
  • Death


  • Chills
  • Fever
  • Nausea and vomiting
  • Abdominal pain
  • Diarrhea
  • Respiratory distress
  • Renal failure
  • Liver failure
  • Shock
  • Death

Yellow Fever

  • Fever
  • Chills
  • Severe headache
  • Back pain
  • Body aches
  • Nausea
  • Fatigue
  • Weakness
  • Jaundice (yellow skin and eyes)
  • Abnormal bleeding
  • Shock
  • Multiple organ failure and death

Causes of Mosquito-Borne Diseases in Horses

Mosquitos are the main cause of all of these diseases. However, there are ways to prevent these pests such as:

  • Frequently changing water in horse trough
  • Make sure there is no standing water anywhere (old tires, swimming pools, plant pots)
  • Change bedding often
  • Keep storm drains and gutters free of standing water

Diagnosis of Mosquito-Borne Diseases in Horses

If you suspect that your horse has any of these mosquito-borne diseases, consult a veterinarian right away. No matter which condition your horse may have, the best chance for successful treatment is early detection. Your veterinarian will need to get your horse’s medical history and immunization records and do a physical examination, including vital signs, palpation, and auscultation of all vital organs and muscles. Ultrasound may be helpful in finding infestations of any of these viruses, but is usually inconclusive. 

Laboratory tests are the best way to get a definitive diagnosis of mosquito-borne diseases. General blood work such as a complete blood count (CBC), serum biochemical analysis, and blood cultures are commonly done right away. The veterinarian will also perform tests specifically for whichever virus is suspected such as virus-specific IgM, neutralizing antibodies, and liver enzymes. The most common findings are increased white blood count, creatinine, and liver enzymes as well as low iron (anemia). In addition, a urinalysis will likely show protein and/or blood in the urine.

Treatment of Mosquito-Borne Diseases in Horses

The treatment depends on the type of mosquito born disease your horse has, but will generally include medication, fluid and oxygen therapy, and other supportive measures.  

Eastern, Western, Venezuelan, and St. Louis Equine Encephalitis and West Nile Virus

While there is no medication to cure these diseases, supportive treatment will be done in the hospital. This treatment will include intravenous (IV) fluids, oxygen therapy, ventilator, antibiotics, and round the clock observation.


Medications for malaria include chloroquine, quinidine, quinine, mefloquine, artemether-lumefantrine, atovaquone-proguanil, clindamycin, and doxycycline. Supportive treatment will also be needed such as IV fluids, oxygen, and observation.

Yellow Fever

There is no drug that cures those with yellow fever but supportive treatment in the hospital is needed. Those horses that survive will be immune forever.


The best way to treat Dengue is with intravenous (IV) fluids and anti-inflammatory medication such as NSAIDS.

Recovery of Mosquito-Borne Diseases in Horses

The outcome of mosquito-borne diseases is varied, depending on the type, but most are not good. West Nile Virus, Eastern, Western, Venezuelan, and St. Louis Equine Encephalitis all have high mortality rates of between 75% and 90%. Dengue has a mortality rate of between 20% and 30% with treatment. There is a 20% to 50% mortality rate in those with Yellow Fever. Malaria has a mortality rate of 45% to 50%. With prompt treatment, your horse has a chance of surviving, but treatment has to be done by a professional.