Latest Buzz on Mosquito-borne Infections in the U.S.

“I have to tell you, I’ve never been as sick in my life. Ever. By far, this is the worst I’ve ever been with an illness.”

Dr. Anthony Fauci, commenting on his West Nile virus infection, August 24, 2024

“About a third of people who develop encephalitis from [Eastern Equine Encephalitis virus] die from the infection, and survivors can suffer lifelong mental and physical disabilities.”

Associated Press, August 28, 2024

Mosquito-borne infections continue to capture much of the infectious disease-related news in the U.S. (See, e.g., “As mosquito-borne illnesses spread, here’s how to tell West Nile, dengue and EEE apart,” ABC News, September 3, 2024; “Are Mosquito-Borne Viruses Becoming More Common and Severe? A Clinical Virologist Explains,” Associated Press, September 7, 2024). Yet, it’s difficult for most of us to keep straight all the mosquito species and diseases they cause. In this week’s Germ Gems post, I focus on the two causes of mosquito-borne viral encephalitis that pose the biggest threat to Americans, namely West Nile virus (WNV) and Eastern Equine Encephalitis (EEEV).

WNV infections. WNV is a single-stranded RNA virus that is transmitted primarily by Culex species mosquitoes. The first cases of WNV infections in humans in the U.S. were reported 25 years ago in New York City. Today, it is by far the most serious mosquito-borne viral encephalitis in the U.S. Yet, there is no specific treatment for WNV and no vaccine for humans to prevent WNV infections. (Although I reviewed the topic of WNV infections in my September 20, 2023 Germ Gems post: “America: Get West Nile Virus on Your Radar Screen,” I will repeat some salient points in this post.)

Like many vector-borne infections, WNV is a zoonotic pathogen, that is the virus can infect and be carried by animals. The virus has been found in more than 250 different bird species. Some birds of the order Passeriformes, such as sparrows, are reservoirs for the virus. This means that when a sparrow is bitten by an infected mosquito, the WNV doesn’t kill the sparrow but grows steadily and harmlessly inside the bird where it can be picked up and transmitted via mosquitoes to other creatures, including humans.

Birds belonging to the family Corvidae, such as crows, ravens and blue jays, are dead-end hosts which means the WNV doesn’t just grow inside these birds, it kills them. In addition to corvids, humans, horses, and at least 24 other mammal species are dead-end hosts for WNV.

According to the Centers for Disease Control and Prevention, about 20% of people who are infected with WNV develop a fever and flu-like illness, with fatigue and weakness sometimes persisting for months. Less than 1% of infected people develop neuroinvasive disease, usually encephalitis (infection/inflammation of the brain parenchyma) or meningitis (infection/inflammation of the covering of the brain). Occasionally, acute flaccid paralysis resembling poliomyelitis or tremors and muscle rigidity, as see in Parkinson’s disease, occur.

The number of cases and deaths due to WNV varies from year to year. In 2023, 47 states reported 2,406 WNV cases (more than half were neuroinvasive), and Colorado was the hardest hit with 50 deaths due to WNV.

One of the reasons WNV has been in the news recently is because Dr. Anthony Fauci, the former director of the National Institute of Allergy and Infectious Diseases (NIAID), was hospitalized in August for one week with a severe case of West Nile fever. While an estimated 70-80% of human WNV infections are subclinical or asymptomatic, severe and fatal cases of encephalitis threaten mainly those with a compromised immune system and—with apologies to Dr. Fauci—this includes people over 70 with age-related immunosenescence.

Eastern Equine Encephalitis (EEE or “Triple E”). EEE was first recognized in the U.S. in 1831 when over 70 horses in Massachusetts died of encephalitis. In 1933, EEEV was first isolated from a horse with encephalomyelitis. Then in 1938, the first human cases were identified. Subsequently, the EEEV, a single-stranded RNA virus belonging to the Togaviridae family of viruses, was described.

EEEV is spread by the bite of a mosquito. Its principal mosquito vector, Coquilleta perturbans, breeds in cattail marshes, hence its moniker, the “cattail mosquito.” Most cases of EEE occur in eastern or Gulf Coast states.

The symptoms of EEE are similar to those of WNV disease and include fever, headache, vomiting, diarrhea, seizures, behavioral changes, and drowsiness. As is the case of WNV infections, there is no treatment for EEE other than supportive care. 

EEE is exceedingly rare. According to the medical literature, the average number of human cases of EEE reported annually in the U.S. is eight. (In 2019, an unusually high number of 38 cases were reported.)

 Although EEEV is nowhere near as big a threat as is WNV, the EEEV is highly virulent with a mortality rate of about 30%, and many survivors are left suffering with permanent neurological damage.  This year’s first case of EEE was in August in a New Hampshire resident who died from the infection. (This was New Hampshire’s first case of EEEV infection in a decade.) The second case this year was in an eighty year old man in Massachusetts. The EEEV was recently isolated from horses in both states.

Prevention of WNV and EEEV infections. Horse lovers likely know that there are highly effective vaccines that prevent WNV and EEEV infections in horses as well as another very effective vaccine given to horses that prevents Western Equine Encephalitis, a disease not seen in humans since the late 1990s. Unfortunately, there are no vaccines that prevent mosquito-borne viral infections in humans.

Evidently, the lack of a substantial commercial interest has stood in the way of getting a  vaccine to prevent WNV in humans. And I’m afraid that now that Dr. Fauci has retired from the NIAID, the interest of the National Institutes of Health in supporting development of a WNV vaccine will be harder to pique. 

Until such time that a vaccine is available for human use, the best strategy for preventing  mosquito-borne encephalitis is to avoid the mosquito vector. This does not mean curbing outside activities such as walking, hiking or, as in Dr. Fauci’s case, gardening. Instead, take preventive measures to avoid the pesky mosquito vector. Use insect repellents any time you’re out-of-doors; schedule outdoor activities to avoid the hours from dusk to dawn; wear long-sleeved clothing; repair damaged window and door screens; remove standing water from areas around your home. Then enjoy the glorious fall weather.