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On Monday, the Texas Animal Health Commission (TAHC) released a statement advising all veterinarians and horse owners to be alert for possible signs of Eastern Equine Encephalitis Virus after several horses in Houston County – about 100 miles north of the city of Houston – were found to be infected.
Eastern equine encephalitis virus (EEE) is one of the rarest but most serious mosquito-borne diseases in the United States, according to the Texas Department of State Health Services (DHSH). Although it is most commonly transmitted to horses, other horses, and some birds, humans can also become infected in rare cases.
According to the Centers for Disease Control and Prevention, only a handful of cases are reported each year in the United States. However, approximately 30% of people infected with EEE die, and many survivors suffer lingering neurological problems. According to DHSH, there is currently a vaccine for horses, but not for humans. The last known case of EEE in humans in Texas occurred in 2001.
Loni Taylor, an epidemiologist and veterinarian at the Texas A&M University School of Public Health, said the vaccine for horses has proven so effective that there is no need for a vaccine for humans.
“As far as access to a potential vaccine for humans, there aren’t that many cases of this disease in humans,” she said. “I’m not saying that there aren’t people working on a potential vaccine, or that there may not be vaccines in other parts of the world where this vaccine has been worked on. It’s just not something that we’re recommending right now here in the U.S., because in humans, the standard is just to prevent mosquito bites, and that’s our best method of preventing this and many other diseases.”
Although some infected people never experience symptoms, the incubation period for EEE is typically 4 to 10 days and can lead to a variety of symptoms as the disease progresses.
According to DHSH, the following symptoms are commonly associated with EEE in humans:
- Fever
- chills
- Joint and muscle pain
- Headache
- Vomit
- Diarrhea
- Disorientation
- Cramps
- coma
EEE is an encephalitis disease that, according to Taylor, primarily affects the brain and causes inflammation there.
“It causes headaches, seizures, behavioral changes (and) drowsiness,” she said. “One of the names (of the symptoms) in horses in the old days was a type of ‘sleeping sickness,’ because they became very sleepy. Other signs we know in horses, just because they are more exposed, are the inability to swallow, pressing their head against a wall, shaking, staggering, running in circles, blindness (and) the inability to move their facial muscles.”
As with all other mosquito-borne diseases, the key to reducing transmission is to limit contact with mosquitoes and the number of mosquitoes, said a DHSH representative. It is recommended to use insect repellent containing DEET, wear long sleeves and pants – especially when outside at dawn and dusk – drain standing water to eliminate mosquito breeding grounds and ensure window screens are in good condition.
The latest arbovirus report from HSHS, which provides a weekly snapshot of mosquito surveillance programs across the state, shows that no mosquito swarms carrying EEE have been found so far this year. For comparison, more than 2,600 mosquito swarms carrying West Nile virus have been identified across the state this year.
“EEE is much rarer than West Nile virus, so I think people can be very comfortable with it,” Taylor said. “We see the virus whenever there are mosquitoes, and the more mosquitoes there are, the greater the chance that a horse will be bitten by a mosquito that may be infected. … Frankly, the reason it’s so rare is because the vaccine is very effective in horses.”
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One of the reasons the vaccine is so widely used in horses is because electrical and electronic alcohols can be much more deadly to horses than to humans, Taylor said.
“If horses have not been vaccinated, the mortality rate can be over 90%, so we strongly recommend getting horses vaccinated against it,” she said. “When I’ve been in areas where I’ve dealt with it, people have sometimes said they heard the word ‘horses’ in this virus and assumed they got it from a horse, but that’s not possible. Horses (and humans) are a dead-end host in our view, so you can’t transmit it through contact with a horse.”
According to the CDC, one human case each has been reported in Vermont, Massachusetts, New Jersey and Wisconsin.
On Tuesday, the Associated Press reported that a New Hampshire resident had died after testing positive for electrical and electronic equipment. The death came just days after the Massachusetts town of Plymouth, about 40 miles from Boston, announced it would close its outdoor facilities from dusk to dawn after a local horse became infected.
