The Buzz About EEE and WNV

Fun Fact: According to The Historical Medical Library of The College of Physicians of Philadelphia, horses and other animals were used to produce antitoxins in a variety of diseases, such as diphtheria, tetanus, and scarlet fever.

Recommended Vaccinations

The core vaccines recommended by AAEP are Eastern Equine Encephalitis (EEE),  Western Equine Encephalitis (WEE), Tetanus, Rabies, and West Nile Virus. These vaccinations protect against diseases that occur year after year – by way of soil, insects or local wildlife; every horse is at risk, regardless of location or lifestyle. These vaccinations are safe, effective, widely available, and they prevent diseases that have a high mortality rate and/or no effective treatment. Additionally, the rabies vaccination helps protect human health and lives. Other vaccines may be recommended by your veterinarian depending on your horse’s specific needs/situation.

Who Can Vaccinate?

Vaccinations are available for over the counter purchase by anyone. However, it is strongly recommended that a veterinarian administer vaccines. If your horse was an adverse reaction associate with the vaccination or develops the disease while up-to-date on vaccines, the vaccine company will cover the cost of diagnostics and treatment when those vaccines were administered by a licensed veterinarian.

Vaccinating for Disease Prevention

Vaccination easily ranks as one of the single most important things you do to protect your horse’s health. Effectiveness of vaccines can be seen in human health history. Human diseases such as Smallpox and Polio have been almost 100% eradicated due to the use of vaccines; the last endemic case of Smallpox in the world occurred in Somalia in October 1977. While not all vaccines are as effective as these examples, most have significantly decreased the amount of disease.

Eastern Equine Encephalitis

 A Brief History

The virus Eastern Equine Encephalomyelitis is thought to have been the case of EEE in North America since 1831. It did not receive its name until a major outbreak occurred in horses  \in the coastal areas of Delaware, Maryland, New Jersey, and Virginia in 1933. Additional outbreaks occurred in Virginia and North Carolina in 1934 and 1935, respectively. Mosquitoes were first determined to be the carriers of EEE in 1934, and in 1950 it was discovered that birds can be a reservoir host for the virus.

Transmission

• Transmitted by mosquitoes or other biting insects
• Birds act as reservoirs for the virus; mosquitoes and other biting insects then carry the pathogen from infected birds and transmit it to horses when they bite
  • The majority of wild birds infected with the virus will exhibit no clinical signs
•  A horse affected with EEE is not contagious and poses no risk to other horses, humans or birds.

Incubation Period: 3-7 days

Carrier Status: Infected horses CANNOT transmit the disease to other horses, horses do not replicate enough virus to pass on to another mosquito

Severity: High; morbidity rate in horses infected with EEE is 75-95%

Clinical Signs

• Depression and anorexia without fever when initially infected
• Moderate to high fever 102.5-104.5F
• Lack of appetite, lethargy/drowsiness
• Neurologic signs- Onset of neurologic disease is frequently sudden and progressive
  • Periods of hyperexcitability, apprehension and/or drowsiness
  • Fine tremors and fasciculations of the face and neck muscle
  • Convulsions
  • Cranial nerve paralysis– facial paralysis and weakness of the tongue are very common
  • Head tilt, droopy lip, muzzle deviation
  • Weakness, ataxia, and dysmetria (incoordination) in one or all limbs o
  • Complete paralysis of one or more limbs
• Colic
• Recumbency (inability to stand)
• Death

Diagnosis: Made by a veterinarian using combination of clinical signs, history and bloodwork

Treatment

• There is no cure for EEE
• Supportive care is administered in horses which show clinical signs

Prognosis

• Poor. Horses infected with EEE do not often survive
• Morbidity rate is 75-95% and death usually occurs within 2-3 days of onset of signs

Prevention

• Keep all horses up to date on vaccinations!
  • Initial vaccination with booster, followed by yearly revaccination
  • More frequent boosters (i.e. twice yearly) are recommended in areas with year-round mosquito seasons and in endemic areas
• Practice vector control management on your facility to reduce risk of transmission from insects.
  • Use insect repellents frequently; re-apply after rain
  • Keep horses in at night when possible, and apply insect repellent
  • Eliminate or minimize standing water, eliminate brush piles, gutters, old tires and litter
  • Remove all equipment in which standing water can collect
• Biosecurity: There are no recommended biosecurity protocols nor do restrictions need to be placed on affected or recovered animals as they pose no risk of infection to other horses.

West Nile Virus

WNV is a mosquito-borne virus that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord) in humans and horses. Mosquitoes acquire the virus from infected birds and transmit it to the dead end host In 1999, WNV was seen for the first time in New York. Since then, over 25,000 cases of WNV encephalitis have been reported in U.S. horses. Horses represent 96.9% of all reported non-human mammalian cases of WNV disease.

Transmission

• Transmitted by mosquitoes or other biting insects
• Birds act as reservoirs for the virus; mosquitoes and other biting insects then carry the pathogen from infected birds and transmit it to horses when they bite
• A horse affected with WNV encephalitis is not contagious and poses no risk to other horses, or birds

Incubation Period: 7-10 days

Carrier Status

• Infected horses CANNOT transmit the disease to other horses.
• The virus can only be transmitted to a horse via an insect vector

Severity: Medium. Up to 40% of infected horses are euthanized or die as a result of infection

Clinical Signs and Symptoms

• Depression and anorexia without fever when initially infected
• Mild low-grade fever (101.5-103.5F) in about 25% of affected horses
• Lack of appetite, lethargy/drowsiness
• Neurologic signs
• Paralysis
  • Facial paralysis and weakness of the tongue are very common
• Recumbency (inability to stand)
• Incoordinationin one or more limbs
• Weakness
• Colic
• Death

Diagnosis

• Made by a veterinarian
• Use a combo of clinical signs, history and bloodwork

Treatment

• There is no cure for West Nile Virus
• Supportive care is administered in cases which show clinical signs

Prognosis

• Horses showing clinical signs of the disease develop in up to 39% of horses infected
• Some infected horses never show clinical signs of the disease and horses who survive usually make a full recovery
  • Approximately 40% of horses will have residual neurologic deficits after recovering
• However, up to 40% of infected horses are euthanized or die if they become recumbent

References

• https://aaep.org/sites/default/files/Documents/Outside%20Linked%20Documents/DiseaseFactsheet_WNV.pdf
• https://aaep.org/sites/default/files/Documents/Outside%20Linked%20Documents/DiseaseFactsheet_EEE1.pdf
• https://equusmagazine.com/management/core-vaccines-25417
• https://www.aphis.usda.gov/publications/animal_health/content/printable_version/fs_eastern_equine_enceph.pdf
• https://cwhl.ahdc.vet.cornell.edu/disease/eastern-equine-encephalitis#collapse3
• https://extension.psu.edu/west-nile-virus-a-threat-to-horses
• https://www.vet.k-state.edu/vhc/services/equine/timely-topics/wnv-fact-sheet.pdf
• https://www.historyofvaccines.org/content/horse-serum
• http://www.equinediseasecc.org/disease-information
• https://aaep.org/guidelines/vaccination-guidelines