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What are Mosquito Bites?

Mosquito bites can be irritating in any horse, but in hypersensitive horses, the allergic reaction can become more severe with repeated annual exposure, or with aging. While this allergy can be managed, viral diseases that can spread through a bite can be more challenging. Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), and West Nile virus (WNV) can all be deadly conditions that can progress from a mild fever to paralysis and death. Measures to prevent mosquito breeding, and to reduce your horse’s exposure to them can make all the difference in protecting your horse from these conditions.

When a mosquito bites your horse, there may be more happening than a simple irritation. If your horse is hypersensitive to the saliva of mosquitoes, he can have a severe allergic reaction that can lead to skin issues, hair loss, and discomfort. Your horse can also be at risk of contracting a deadly virus transmitted by the mosquito.

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Symptoms of Mosquito Bites in Horses

Symptoms of mosquito bites are generally seen seasonally in the spring and summer, although areas with milder winters can have problems year round. Signs of insect hypersensitivity are often seen after many seasons of exposure, and can worsen with each subsequent season. While lesions can become permanent, they usually heal in cooler weather, allowing hair to grow back. Symptoms include:

  • Red bumps
  • Itchiness that can become severe
  • Hair loss
  • Flaky skin
  • Hives
  • Skin inflammation
  • Papules or lesions
  • Crusting
  • Thick, leathery patches of skin
  • Rubbing that causes loss of fur, such as on the mane, or on the tail, giving a rat tail appearance 
  • Self-excoriation, or compulsive skin picking on areas of bites, such as flanks, withers, base of mane, tail head, rump, face, ears, and ventral abdomen
  • Discomfort when groomed or saddled
  • Increased coarseness of hair coat
  • Abnormal pigmentation of hair
  • Weight loss
  • Restlessness
  • Tail switching

Symptoms related to Eastern equine encephalitis, Western equine encephalitis, or West Nile virus include:

  • Mild to severe fever
  • Stiffness
  • Loss of appetite
  • Propulsive walking
  • Depression
  • Drowsiness, or sleeping for long periods at a time
  • Excitability
  • Aggression
  • Circling
  • Head tilt
  • Head pressing
  • Trembling limbs and facial muscles
  • Impaired vision
  • Blindness
  • Difficulty urinating or defecating
  • Vertigo 
  • Incoordination
  • Stumbling 
  • Aimless wandering
  • Paralysis in muzzle, tongue, larynx or pharynx
  • Grinding teeth
  • Inability to swallow
  • Droopy ear
  • Altered mental state
  • Weakness or paralysis of hind limbs
  • Unable to stand
  • Convulsions 
  • Complete paralysis
  • Lameness 
  • Coma
  • Death

Causes of Mosquito Bites in Horses

Once a mosquito bites a horse, it can either cause an allergic reaction or transmit a virus. Hypersensitivity to mosquito bites is caused by the antigens in the mosquito saliva that induce reactions in the affected horse. There may be a genetic component predisposing certain breeds to be hypersensitive, namely Welsh ponies, Shire horses, Icelandic ponies, and Friesians.

Contraction of a virus from a mosquito bite is due to the transfer of the togaviridae virus in the case of Eastern equine encephalitis or Western equine encephalitis.

Diagnosis of Mosquito Bites in Horses

To diagnose a case of hypersensitivity to mosquito bites, symptoms and the time of year they are seen are taken into account. Skin allergy testing can give a definitive diagnosis. Other tests can include an in vitro ELISA test, which can find the presence of antibodies, and the administration of mosquito antigen. The response to such treatment can be a positive sign of hypersensitivity in your horse, and may take a day to appear. Due to the similarity of other conditions, there may be testing to eliminate other possibilities, which may include skin biopsies.

Eastern and Western equine encephalitis can be diagnosed based on symptoms, the time of year, and the presence of the virus in other horses in the area. Tests to confirm the diagnosis are often used post mortem, and include examining blood samples for antigens, virus isolation from brain tissue or cerebro-spinal fluid, and a PCR test.

West Nile virus can be diagnosed with the IgM capture ELISA test, which can confirm an exposure to the virus within the last three months.

Treatment of Mosquito Bites in Horses

Treatment for insect bite hypersensitivity begins with repelling mosquitoes to prevent your horse from getting bitten throughout the insect season. Ways to manage this can include:

  • Beginning insect repellent measures before the beginning of insect season
  • Insect repelling sprays for your horse, preferably with sunscreen to increase longevity
  • Using insect repelling lotions, shampoos, body sprays, or wipe-on preparations on your horse’s skin to provide a barrier
  • Applying insecticide to rugs near stalls
  • Turning out your horse during the times of day mosquitoes are least active
  • Using box fans in stalls, or ceiling fans
  • Removing standing water and manure from barns or stalls
  • Environmental insect control
  • Using fine mesh screens sprayed with insect repellent in stables
  • Stabling at night
  • Using blankets and hoods that protect against mosquitoes when not stabled
  • Avoiding insect breeding by cleaning and draining ponds and water troughs frequently 
  • Keeping your horse at least 0.5 miles from mosquito breeding areas
  • Clearing gutters near barns and other buildings of leaves and debris
  • Using sticky traps or electric zappers to reduce insect populations near your horse
  • Using incandescent lights around the barn to keep mosquitoes away from your horse

To treat the allergic reaction in your horse, your veterinarian may prescribe anti-inflammatories. Topical or oral corticosteroids can also help to reduce inflammation. Antihistamines may be prescribed, though are often ineffective alone. Hyposensitization therapy through allergy shots can also be used. Immunotherapy specific to your horse’s allergy may be considered. Dietary supplementation of omega-3 fatty acids can be a good long term therapy in cases of allergies. Your veterinarian may also prescribe anti-pruritics to control the itching and skin irritation. Topical emollients can also help control itching, and can include oatmeal, baby oil, and aloe vera.

Treatment for Eastern or Western equine encephalitis is mostly through supportive care, as there is no known treatment specific for these viruses. If your horse has neurological symptoms, anti-inflammatories and anti-convulsants may be prescribed, along with fluid therapy. Heavy bedding, padded stalls, leg wraps or a padded helmet may be recommended to prevent any self-inflicted injuries. Other care can include electrolyte and nutrition support, DMSO and corticosteroids, and a specific antiviral treatment in the case of Western equine encephalitis. Vaccines for both viruses are available.

Treatment for the West Nile virus also includes anti-inflammatories, corticosteroids, fluids and other supportive care. A vaccine for WNV is available.

Recovery of Mosquito Bites in Horses

Recovery of hypersensitive horses can be good if properly managed through insect repellent strategies and supportive care. You may be sent home with medications to administer, or emollient based shampoos and creams to ease the discomfort of your horse. Symptoms are often seasonal, and monitoring your horse for signs can alert you to the need for supportive treatments. 

Horses suffering from exposure to Eastern or Western equine encephalitis may recover with a subsequent immunity for up to two years after the initial infection. However, they may also suffer permanent brain damage. Mortality rates for the two viruses vary between 20% to 100%. West Nile virus has a mortality rate of 30% to 40%. 

Prevent these deadly viruses by reducing your horse’s exposure to mosquitoes, and with the proper use of vaccines. Discuss with your veterinarian the best times in your area to vaccinate your horse.