Introduction

Eastern equine encephalitis, or EEE, is a very rare viral infection that causes inflammation of the brain (encephalitis) and spinal cord (myelitis). In spite of being rare, EEE is an extremely serious and often lethal infection. It may also be known as triple E or sleeping sickness. The name eastern equine encephalitis is given because it occurs mostly in the Eastern US and commonly affects horses (equine).

It is transmitted through the bite of a mosquito that has already been infected with the eastern equine encephalomyelitis virus (EEEV). EEE can also occur in humans. The animals that are affected by it include mammals (e.g. horses), amphibians, birds, and reptiles. 

You can get infected with EEEV if you live, visit or work in regions where this virus is abundant. In terms of age, if you are older than 50 years or younger than 15 years old, then you are at the greatest risk of having severe EEE after being infected with the virus.

Most of the cases of EEE are found in the southeast region of the US, and the mosquito Culiseta melanura is the predominant species that transmit this disease (other species of mosquito can also spread it). EEE has a very high mortality rate in both horses and humans and therefore it is considered a very serious mosquito-transmitted infection. Horses that have not been vaccinated (non-immune horses) develop this disease after being infected with EEE virus. Horses between the ages of six months to two years are at the most risk of developing EEE after infection with the virus.

Signs of EEE in horses

After an infected mosquito feeds on the non-vaccinated horse, the EEE virus circulates throughout the body and then invades the central nervous system (CNS), which includes the brain and spinal cord. This attack on the CNS causes symptoms such as depression, seizures, blindness, and staggering.

Horses aged between six months and two years are the most vulnerable and develop EEE after being bitten by the infected mosquito. Survival rates in affected horses are very low and most of the horses that develop eastern equine encephalitis (EEE) die within several days of infection.

The following signs and symptoms occur after development of EEE in horses:

• Fever
• Depression
• Lack of appetite
• Inability to swallow
• Blindness
• Staggering
• Aimless walking (usually in circles)
• Head pressing
• Ataxia
• Paralysis
• Seizures
• Death (in about 80% of cases)

Diagnosis of EEE

Your veterinarian will collect blood samples from your horse and perform tests that give results within 2 to 5 days. Cerebrospinal fluid (CSF) may also be collected to accurately diagnose EEE in your horse. CSF analysis involves taking a sample of CSF from the spine of your horse and then performing tests on it in a laboratory.¹

Diagnosis is usually done based on the positive serology results after enzyme-linked immunosorbent assay (ELISA) testing on both the blood sample (more specifically, serum) and/or cerebrospinal fluid. A polymerase chain reaction (PCR) test can also be done on CSF. These tests show whether the virus or antibodies against the causative virus are present in the fluid (serum or CSF) or not.¹

Treatment of EEE in horses

There is no cure available that eradicates eastern equine encephalitis in horses. However, supportive treatment is given to horses that show signs and symptoms of EEE to help them live comfortably. This supportive care includes administration of intravenous fluids, proper nutrition and anti-inflammatory drugs.

The prognosis for horses that have been diagnosed with eastern equine encephalitis (EEE) is very poor. The mortality rates are very high and as many as 90% of horses die 2 to 3 days after being infected. Those horses which do survive have to face permanent brain damage.

How does eastern equine encephalitis virus spread?

Eastern equine encephalitis does not spread from person to person, person to animal, or animal to person. As this disease is not contagious, the infected individuals do not need to be quarantined.

The spread of EEE is through the bite of a specific species of infected mosquitoes (e.g. Culex, Aedes, Coquillettidia) that feed on mammals (horses and humans) as well as birds. Wild birds act as natural stores of EEE virus and by feeding on these birds, mosquitoes can obtain the virus and then transmit it to mammals (humans and horses).

Prevention of EEE in horses

Due to EEE being likely to be fatal, the best course of action is to prevent your horses from developing it. The following are the preventative measures you can take so your horses stay protected from EEE:

• Make sure to get your horses vaccinated at least twice annually. In case of an outbreak, it is advisable to get vaccine boosters. Vaccination is the key step in the prevention of this lethal disease. Contact your veterinarian to make up a proper schedule so that your horses are vaccinated on time
• Eliminate any and all sources of standing water (a breeding habitat for mosquitoes) in your pastures, and reduce mosquito populations
• Spray your horses with insect repellents regularly. Make certain that the sprays are long-acting by asking your veterinarian
• Do not let your horses go outside during dusk or dawn as these are the peak times for mosquitoes to feed
• Switch on the fans inside the barns or shelters and open the windows to produce a breeze
• Keep mosquito-eating fish in water troughs, ponds and tanks

FAQs

What is Eastern equine encephalitis (EEE)?

Eastern equine encephalitis (EEE) is a very serious and often fatal viral infection that causes the inflammation of the brain as well as the spinal cord of horses. Other than horses, it can also affect other animals like mammals (including humans), amphibians, birds, and reptiles.

What are the symptoms of EEE in horses?

Signs and symptoms of EEE in horses include fever, uncoordinated movements, head pressing, staggering, blindness, depression, convulsions, paralysis, and death. 

How is EEE transmitted in horses?

EEE is caused by the EEE virus that is transmitted from infected birds to horses via a mosquito bite. The best preventive measure other than vaccination is to reduce the mosquito population surrounding your horses and thus reduce their mosquito exposure.

How can my horses get infected with the EEE virus?

EEE virus is transmitted to the horses through the bite of an infected mosquito. This mosquito may be a member of the species Culex, Aedes, Coquillettidia, or Culiseta melanura. Therefore, if you are residing in regions with an abundance of such mosquitoes, your horses are vulnerable to developing EEE.

How can I protect my horse from EEE?

Vaccination is the first measure towards keeping your horses safe from EEE. Other ways of minimising the chances of infection include decreasing mosquito population and exposure, for example, by changing drinking water frequently and eliminating any standing water sources. 

What to do if my horse gets EEE?

If you suspect that your horse might have been infected with EEE virus, then contact your veterinarian. You should also observe other horses to determine if they are affected or not. Your veterinarian will do a blood test and the result usually comes out within 2 to 5 days. After EEE has been diagnosed, treatment is usually limited and mainly focuses on making your horse comfortable. 

Is there a treatment for EEE in horses?

Treatment is not specific as there has not been any antiviral discovered against EEE virus as of yet. Supportive care is provided and this includes fluid therapy, proper nutrition, and anti-inflammatory medications.

What is the prognosis for horses with EEE?

Prognosis for horses with EEE is very poor and about 90% of horses having EEE die within several days of infection.

Can a horse with EEE spread this disease?

No, infected horses cannot spread EEE to other animals or humans, it can only spread through the mosquitoes that have fed on infected birds. There is not enough EEE virus in the system of affected horses to be transferred to mosquitoes. 

Is there any vaccine available against EEE for horses?

Yes, a vaccine is available as a preventative measure to protect your horses from getting EEE. EEE virus causes this disease in non-vaccinated horses, so getting your horses vaccinated will keep them safe from EEE.

Summary

Eastern equine encephalitis, also called triple E, EEE, or sleeping sickness, is an extremely rare but also very serious and often lethal viral infection that is most commonly found in horses but may also occur in humans and animals including amphibians, reptiles, and birds. The causative virus, EEE virus, is transferred from infected birds to mosquitoes when they feed on these birds.

Wild birds are commonly infected and the mosquitoes that carry this virus from birds to horses are from the species of Culex, Aedes, Coquillettidia, and Culiseta melanura. This virus (EEE virus) attacks the CNS of horses and causes symptoms like high-grade fever, head pressing, walking in circles, depression, staggering, blindness, convulsions, and death. Mortality rates are very high in this disease and about 90% of horses having EEE die within several days of showing clinical signs.

The horses that survive have permanent brain damage. Diagnosis is done by your veterinarian who takes a sample of your horse’s blood or cerebrospinal fluid and performs tests on them. Treatment is usually supportive as there has not been any antiviral medication discovered against EEE virus.