Eastern equine encephalitis (EEE) is a potentially deadly illness caused by a mosquito-borne virus. While infections in humans have been rare in the United States, an upsurge in reported cases this year has caused experts to wonder whether EEE could be the next Zika or West Nile.

The EEE virus is carried by mosquitoes—through mosquito bites, it can be transmitted to equines, such as horses or zebras, and to humans.

This virus has been present in the US for centuries, though it has rarely infected people.

However, if it does infect a person and the infection evolves into a severe form of the disease, EEE can be deadly.

Only a handful of these infections in humans had been reported throughout the US each year for the past few years.

According to data from the Centers for Disease Control and Prevention (CDC), last year there were six reported cases of EEE and five in 2017.

Until this year, the highest annual number of EEE cases in the country over the last decade had been 15, in 2012.

But as of November, this year has seen an upsurge in EEE cases in humans, including fatalities due to the illness. The CDC report that there have been "36 confirmed cases of [EEE] virus disease [...] this year, including 14 deaths."

This situation has made some specialists wonder whether the EEE virus may not become the next threat to public health, much like the Zika or West Nile viruses.

'A new era' for mosquito-borne viruses?

Recently, experts from the National Institute of Allergy and Infectious Diseases (NIAID) published a commentary in The New England Journal of Medicine describing the EEE virus and how researchers plan to address this potential threat.

In the article—the first author of which is Dr. David Morens—the experts place EEE in the context of recent mosquito-borne virus (arbovirus) outbreaks worldwide.

"In recent years, the Americas have witnessed a steady stream of other emerging or reemerging arboviruses, such as dengue, West Nile, chikungunya, Zika, and Powassan, as well as increasing numbers of travel-related cases of various other arboviral infections," they write, warning that:

"This year's EEE outbreaks may thus be a harbinger of a new era of arboviral emergences."

Part of what makes the EEE virus potentially dangerous for humans is that its symptoms are sometimes indistinguishable from those of other viral infections. Some individuals report no symptoms at all in the initial stages of infection.

The EEE virus takes 3–10 days to incubate inside a human host, and its—nonspecific—symptoms include fever, malaise, intense headaches, muscle aches, nausea, and vomiting.

Moreover, the specialists explain, EEE infections are difficult to diagnose with tests, as it is tricky to isolate the virus in samples of blood or spinal fluid. Yet, if neurologic symptoms of EEE do appear, these will be visible within approximately 5 days of infection.

And these, too, may be initially indistinguishable from the symptoms of viral meningitis.

"However, [after this period,] rapid clinical progression ensues," the experts write. "By the time definitive serologic diagnosis is possible, within a week after infection, neurologic damage may already have occurred."

"An estimated 96% of people infected with EEE [virus] remain asymptomatic; however, of those who have symptoms, 33% or more die, and most of the rest sustain permanent, often severe, neurologic damage," the specialists report.

Worrying lack of a prevention strategy

So what can we do in the event of an EEE outbreak? So far, not much, according to Dr. Morens and colleagues. Currently, no known antiviral drugs are safe and effective in the treatment of this viral infection.

For the time being, people who become infected will receive no more than "supportive treatment," according to the CDC.

Some researchers have experimented with fighting the virus using monoclonal antibodies—artificially created antibodies that can help boost the immune response to a given pathogen. However, though this approach has shown some promise, scientists have, at this point, only tested it in animals.

Moreover, the monoclonal antibody treatment only appears to be effective if the researchers administer it to the animals before they become infected with the EEE virus.

Dr. Morens and colleagues believe that finding a vaccine for EEE would be an effective method of prevention, and some research has already gone into this.

"However," they note, "there may not be strong incentives to proceed to advanced development and licensure because of the nature of the disease: Outbreaks are rare, brief, and focal, and they occur sporadically in unpredictable locations, making it difficult to identify an appropriate target population for vaccination."

This is why the NIAID specialists are calling for a nationwide strategy for preventing an EEE outbreak before it gets the chance to become a reality.

"In the absence of vaccines or specific treatments, state and local health departments can provide early warning of imminent human infections by surveilling equids, birds, and mosquitoes," the team advises. Yet, "Even these blunt prevention tools are continuously threatened by underfunding of public health efforts."

"Sadly, the [US] ability to control arboviral diseases is little better in 2019 than it was more than a century ago," Dr. Morens and colleagues warn.

"Though the best way to respond to these threats is not entirely clear, to ignore them completely and do nothing would be irresponsible," the specialists conclude.