The first human case of Eastern equine encephalitis in New York State since 2015 was reported in Ulster County last month.
Eastern equine encephalitis, also known as EEE and “Triple E,” is an alphavirus that spreads to humans and several animals such as horses and emus through the bite of infected mosquitoes. EEE is most common from late spring into early fall, peaking in summer months when mosquitoes are most active.
Human and animal cases are both rare. An average of 11 human cases in the US occur annually, though outbreaks exceeding 15 people have occurred in the past. As of Oct. 14, 15 cases have been reported across eight states, including New York.
Among horses, 112 cases were reported across the U.S. in 2021, including five in New York.
According to Prof. Elisha Frye, population medicine and diagnostic sciences, 18 equine cases were diagnosed in New York State as of Oct. 2. 15 of the cases were diagnosed at Cornell University’s Animal Health Diagnostic Center.
“This is definitely the most Triple E cases I've seen in one season,” Frye said.
EEE is endemic in North, South and Central America and the Caribbean, particularly around freshwater hardwood swamps in the Atlantic and Gulf coast states and in the Great Lakes regions.
Birds living within the swamps are the reservoir of EEE, which is spread by mosquitoes that feed on primarily birds, such as Culiseta melanura.
For humans and horses to become infected with EEE, bridge vectors must bite an infected bird and then bite a human or horse to spread infection. For EEE, bridge vectors are any mosquito species that can transmit a virus from wild birds to uninfected mammals. Common bridge vectors include Aedes sollicitans, Coquillettidia perturbans and Ochlerotatus canadensis.
Humans and horses are dead-end hosts of EEE. This means that the virus cannot be spread between humans and horses and to mosquitoes that bite infected humans and horses.
According to Prof. Laura Harrington, entomology, who is also the director of the Northeast Regional Center for Excellence in Vector Borne Diseases, the ecology of EEE is not yet fully understood.
“I would say that there's still a lot we don't know about the disease dynamics,” Harrington said. “Birds are moving [EEE], … but, in recent years, they've found reptiles and amphibians that are infected in the wintertime. So some people are thinking maybe it's overwintering.”
Overwintering refers to the maintenance of the virus in ecosystems in temperate regions. The virus may be overwintering in other animals and mosquitoes, allowing the virus to remain in the environment.
Human cases of EEE are detected through passive surveillance, where laboratories and health departments report cases when they come up. EEE is a nationally notifiable disease, meaning that all detected cases must be reported to the Centers of Disease Control and Prevention.
In New York, all equine cases are also mandatory to report.
EEE is considered a select agent, according to Frye. Select agents are any biological agents or toxins that pose a severe threat to human, animal or plant health.
Mosquitoes are also tested for EEE. In known EEE hotspots, mosquitoes will be collected early in the mosquito breeding season. A mosquito pool, which is formed by combining mosquitoes by species, location and collection date, is tested to identify if there is disease within the mosquitoes.
“This year, the mosquito populations were about standard, but there were many more mosquito pools ... positive for EEE,” Harrington said. “That was a good early warning that this was going to be a bad year and something needed to be done.”
According to State Health Commissioner Dr. James McDonald, mosquitoes carrying EEE have been detected within 15 counties across New York State this year.
Anyone can become infected with EEE, but people younger than 15 and older than 50 are more likely to develop severe illness. Many people who are infected with EEE do not have any symptoms. The majority of symptomatic cases experience fever, chills, joint pain and muscle pain that lasts for one to two weeks. While only five percent of cases develop meningitis or encephalitis, an estimated 30 percent of these cases die, and many survivors experience lifelong neurological problems after recovery.
Horses can also be asymptomatic or have mild symptoms from EEE. Horses diagnosed with EEE often show signs of depression, fever and lack of energy. Horses often present severe neurological symptoms, including paralysis, tremors, seizures and weakness. An estimated 90 percent of horses presenting neurological symptoms from EEE will die.
There is currently no vaccine available for humans. The best way to reduce EEE cases is to prevent mosquito bites.
Personal prevention includes using Environmental Protection Agency approved DEET insect repellent, wearing long sleeves and long pants while outdoors and avoiding being outdoors during dusk and nighttime when mosquitoes are typically most active. People should also drain and dump out any standing water in yards to reduce mosquito breeding.
While personal prevention is currently the best method for mosquito-borne diseases, reducing mosquito populations has been immensely difficult. As temperatures rise and precipitation increases, vector-borne diseases may become more common and widespread as vectors can survive for longer and travel into new regions.
“Unfortunately, the strategies we use have been in place for decades, and they're not working. So we need to get smart about things,” Harrington said.
Better measures could include interventions that target mosquito behaviors to reduce breeding and developing a vaccine that could be protective for people at risk, according to Harrington.
While there is no vaccine currently available for humans, there is an effective vaccine for horses.
“Vaccinating your horse is very important,” Frye said. “The cases that we see here are from under-vaccinated or not vaccinated horses.”
According to the American Association of Equine Practitioners, the EEE vaccine is a core vaccine for all horses in the U.S. All horses should be vaccinated in the spring before mosquito season and must be revaccinated annually.
Following the detection of a human case, Governor Kathy Hochul has mobilized ongoing statewide efforts to prevent cases. This includes providing access to insect repellent, education campaigns and spraying efforts until November 30.
Taylor Rijos can be reached at tlr65@cornell.edu.
