As mosquito-borne illnesses spread, here's how to tell West Nile, dengue and EEE apart

As mosquito season continues, public health officials in the U.S. have been tracking several different illnesses caused by the pesky flying insect.

Health officials are reporting at least eighteen deaths from West Nile virus in the U.S. this year, with three recent deaths recorded in Wisconsin and Illinois. There have been six cases of eastern equine encephalitis (EEE), including one person who died after testing positive for the virus, as well as dengue cases recorded in several states. The viruses that cause all three illnesses are transmitted to humans via mosquito bite.

"What we've seen is a rapid uptake in certain viruses that haven't been in the limelight in recent years. Both West Nile virus and 'triple E,' which is eastern equine encephalitis, have had an uptick, particularly post-COVID," Dr. Larry Han, an assistant professor of public health and health sciences at Northeastern University in Boston, told ABC News.

"There's also been a rise in dengue relative to what we've seen in recent years. And so, these 'triple threats,' you might say, have led to more attention and more worry among the general populace."

Experts who spoke with ABC News said some of the uptick is due to the "randomness" regarding which seasons have a greater number of cases than others, coupled with the general rise in mosquito-borne viruses during the summer and early fall months.

Here are some ways to tell the difference between West Nile, EEE, and dengue. However, if you're concerned you might have contracted any of these viruses, you should contact your doctor immediately.

How common are West Nile, EEE and dengue?

West Nile virus is the leading cause of mosquito-borne disease in the contiguous United States, according to the Centers for Disease Control and Prevention (CDC). It was first identified in the Western Hemisphere in August 1999 after people were diagnosed in New York City.

Dengue viruses are spread through bites from infected Aedes species mosquitoes, mostly found in tropical and subtropical regions of the world, according to the CDC. These mosquitoes are also responsible for spreading Zika and chikungunya viruses.

"It would be very rare to see dengue cases unless you lived in more tropical climates, so you might see this in Florida, some Southern states along the East Coast, but, for example, you would nearly never see this in Massachusetts or in the Midwest or West," Han said.

Comparatively, EEE is quite rare, with only a few cases reported in the U.S. every year. However, similar to dengue, most cases occur in the Eastern or Gulf Coast states, according to the CDC.

What are the symptoms?

The majority of people with West Nile virus do not present with symptoms, but about one in five will experience fever along with headaches, body aches, joint pain, diarrhea, vomiting, or a rash. Symptoms typically disappear in a week or less for most people but weakness and fatigue may last for weeks or months. For some people, however, the disease can progress to something far more concerning.

"About one out of 150 of those people that develop signs and symptoms can actually develop a more severe disease, like West Nile encephalitis, where we have the swelling of the brain that could potentially be fatal," Dr. Thomas Duszynski, director of epidemiology education at the Indiana University Richard M. Fairbanks School of Public Health, told ABC News.

Similarly, about one in four people who contract dengue develop symptoms lasting two to seven days, but symptoms are usually mild and may include fever, rash, nausea, vomiting, muscle pain, joint pain and bone pain.

Duszynski explained that most U.S. dengue cases are typically seen among people who contract it while traveling abroad, where dengue is endemic or where cases are more common, and who then bring the virus back to the U.S. Because dengue's symptoms are not specific to the virus, they're often confused with other illnesses, according to the CDC.

Most people who are infected with EEE show either mild symptoms or no symptoms at all, according to the CDC. However, severe cases typically present with fever, headache, chills and vomiting.

Like West Nile virus, EEE also can progress to a more severe condition, like encephalitis or meningitis, the latter of which is swelling of the membranes that surround the brain and spinal cord. Many survivors can experience ongoing neurologic problems, including convulsions, paralysis and intellectual disability. About 30% of encephalitis cases caused by EEE result in death.

"I think the mortality risk if one were to contract dengue or West Nile virus would be very low. For EEE, it's the opposite; it's very high," Han said.

Treatments and vaccines

There are no disease-specific treatments for any of the viruses. The CDC recommends rest, fluids, and over-the-counter medications to relieve symptoms. Patients with severe illness often need to be hospitalized and receive additional supportive treatments, such as intravenous fluids.

There also are no vaccines available for West Nile or EEE. However, a dengue vaccine is available in the U.S. and is FDA approved for children between ages 9 and 16 who have a laboratory-confirmed previous dengue virus infection and who live in areas where dengue is endemic.

The vaccine also is available in Puerto Rico and is part of the territory's routine childhood immunization schedule, the CDC said.

Dengue can be caused by dengue virus strains 1, 2, 3 or 4, with a person being infected multiple times over the course of their life, Duszynski told ABC News, so a vaccine may be able to help prevent infection from a different strain.

"If I got infected with one strain, if I get bit by this mosquito with the same strain, I'm probably going to be okay,' he said. "But it's those three other strains that are out there that … I could get infected with even though I had strain 1."

How to best protect yourself from infection

There are no differences when it comes to protecting yourself from dengue, EEE or West Nile virus, the experts told ABC News. Prevention methods include using effective mosquito repellents, wearing long-sleeve shirts and long pants when outside, and staying in places with air conditioning, when possible. The CDC also recommends emptying containers of still or stagnant water, to eliminate places where mosquitos can breed.

"The single deadliest [animal] to the human species has been the mosquito," Han said. "So, while we might think of sharks or we might think of lions or tigers, it's really the mosquito that has led to the devastation of the human population multiple times throughout history."

"I'm not saying that we should kill off all mosquitoes, because they play an important role in ecology, but there are various new techniques and strategies people are thinking about to combat mosquito-borne viruses and illnesses," Han continued. "I'm hopeful that we can make mosquito borne viruses less of a threat to humanity."