Eastern equine encephalitis (EEE) is a devastating and sometimes fatal infection of the brain caused by Eastern equine encephalitis virus (EEEV). Although the disease is rare, 2019 has seen a notable spike in incidence. So far this year, more than 30 human cases of EEE have been reported, far exceeding the number seen in any year in the last half-century. The increased frequency of the virus lends urgency to the implementation of preventive measures and of accurate and timely diagnostics.

EEE and Animals: Birds, Mosquitoes, Horses, Humans and More

EEEV is transmitted by the bite of an infected mosquito. As suggested by its name, EEEV can infect horses, causing fever, behavioral changes and other symptoms of encephalitis. Infection is often deadly for the horse. However, horses (like humans) are dead-end hosts for EEEV; that is, the level of viremia in their blood is generally insufficient to infect mosquitoes that bite them, so they do not serve as a source for ongoing spread of the disease.

Instead, wild birds are the main reservoir for EEEV. Infected birds are usually asymptomatic but have high levels of viremia, such that mosquitoes that feed on them can transmit the virus. Fortunately, the species of mosquito that most often feeds on birds, Culiseta melanura, generally avoids humans; only when a mosquito belonging to a genus that is less particular about its victims feeds on a human after taking a blood meal from an infected bird is the virus able to leave the bird-mosquito cycle. (Although EEEV infection is asymptomatic in most birds, emus infected with the virus suffer from a fatal bloody diarrhea and can transmit EEEV directly to humans who come in contact with infected blood or stool.)

EEE is generally found in a limited geographic distribution, with most US cases of EEE occurring in Massachusetts, New York, Michigan, North Carolina, and Florida. This distribution reflects the preference of Culiseta melanura mosquitoes for hardwood swamps found in these regions.

In most people, EEEV infection appears to cause a non-specific, self-resolving febrile illness; fewer than 5% of those infected with EEEV go on to develop encephalitis. People who don’t develop encephalitis are rarely diagnosed with the virus, because they are not usually ill enough to undergo testing for specific pathogens. In rare cases, however, EEEV infection can progress to cause a severe, life-threatening meningoencephalitis characterized by high fever, seizures, encephalopathy, and coma. Cerebrospinal fluid typically shows a neutrophilic pleocytosis, which is unusual among viral causes of meningoencephalitis, while brain MRI findings include abnormalities in the basal ganglia and thalamus. Many therapies have been used in patients with EEE, including intravenous immunoglobulin (IVIg) and steroids, but there are insufficient data to determine whether any of these treatments improve outcomes in EEE, and supportive care remains the cornerstone of therapy.

The prognosis for patients with EEE is poor; around one-third of affected patients die and the majority of survivors are left with mild to severe permanent neurological sequelae. While the mortality rate in children infected with EEE is similar to that in adults, outcomes are generally better for those who survive, with around one-third of children regaining full neurological function. 

Why Is There So Much EEE This Year?

The incidence of EEE follows an interesting cyclical pattern. Most years, around 5-7 cases of neuroinvasive disease are reported in humans in the US, but every 10-20 years the number of annual cases doubles for a few years in a row; the last such increase in frequency occurred in 2010-2012. So far, 2019 has had the highest number of cases on record, with more than 30 cases reported already. Most cases each year occur in late summer.

Although the reasons for the cyclical nature of EEE occurrence are not fully understood, the pattern may reflect the introduction of new viral strains by migrating birds. The incidence of EEEV is monitored both by reporting of human EEE cases to state public health departments and also by monitoring of mosquitoes. In Massachusetts, for example, mosquito traps are set throughout the state and pools of mosquitos (groups of 10-50 insects from a certain geographical site; Figure 2) are tested for the presence of EEEV. In a typical year, EEE is detected in only a few mosquito pools in the state, if any; so far in 2019 it has been detected in more than 400 pools.

How (and why) Is EEE Diagnosed?

In patients with EEE encephalitis, a diagnosis is usually made by detection of IgM antibodies against the virus in serum or CSF. This testing can be performed at state public health laboratories or at the Arbovirus Diagnostic Laboratory at the CDC, and is also offered by many commercial reference laboratories. An IgM capture ELISA is usually the first step in the testing process; if this is positive, a plaque reduction neutralization test (PRNT) can be used to confirm the presence of EEEV-specific neutralizing antibodies and to distinguish antibodies to EEEV from antibodies to other alphaviruses such as Chikungunya virus and Venezuelan equine encephalitis virus. In the PRNT, patient serum is mixed with a sample of the virus to be identified (in this case EEEV). If neutralizing antibody to the virus is present in the serum, it will bind to and inactivate the virus. The sample is then added to a monolayer of cells; if the virus has been inactivated by antibodies in the serum, it will be prevented from infecting the cells in the monolayer and the number of “plaques” (areas of infected cells) will be reduced. The CDC also offers specialized post-mortem testing, including nucleic acid amplification, immunohistochemistry, and viral culture, for cases where a suspected diagnosis of EEE remains unverified at the time of death.

It would be reasonable to wonder what the purpose is of testing for EEEV when we have no specific therapies to offer patients. There are, however, several important benefits to the identification of EEEV in patients with encephalitis.

• Discontinuation of unnecessary antimicrobials. Almost all patients who are hospitalized with acute onset meningoencephalitis (fevers, headache, confusion, changes in mental status, seizures, etc.) are started empirically on broad antimicrobial therapy, which usually includes acyclovir, an antiviral drug that treats herpes simplex virus, a common cause of encephalitis that is devastating if left untreated. All of these antimicrobials have side effects and risks (acyclovir, for example, can be toxic to the kidneys), and broad-spectrum antibacterial therapy contributes to the development of resistance. Once a diagnosis of EEE is made, these agents can be stopped (unless there is concern for a secondary or additional infectious process), thus minimizing unnecessary toxicities.
• Providing information for family and caregivers. The sudden onset of neuroinvasive EEE can be devastating for families and friends of infected patients, who watch their loved one abruptly lose alertness, coordination, and the ability to communicate. Given its generally poor prognosis in adults, the diagnosis of EEE may not be encouraging to families, but it is informative. Knowing what is causing a loved one’s symptoms can help caregivers and family members understand what the course of illness may be like, to learn how the infection was contracted, and to focus on supporting the patient and themselves rather than waiting on a diagnosis.
• Supporting epidemiological surveillance. State public health departments track cases of EEE closely, and use this information, along with testing of mosquito pools and sentinel animals, to assess the risk of EEE in individual towns and counties. These data are essential for mitigation efforts including aerial mosquito spraying and advisories to people living in communities at high risk of EEE to avoid outdoor activities during dusk hours when mosquitoes are most active.

EEE is a devastating illness. Because there are no specific treatments, prevention is key. While a EEEV vaccine for horses is available, there is at present no commercially available vaccine for humans; instead, preventive measures center around the avoidance of mosquito bites through application of insect repellant, wearing appropriate clothing, and staying indoors after dusk. Because mosquitoes lay eggs in standing water, eliminating any such breeding grounds (e.g. by draining empty buckets or flowerpots) is also important for reducing mosquito populations.

Fortunately, even in years like 2019 when incidence of EEE reaches record-high levels, it is still a very rare disease. Advice and perspective about the virus can be helpful to anxious families and communities. While we don’t yet have a specific treatment for EEEV or other arboviruses, the medical community still has an important role to play in caring for patients and their loved ones by providing a prompt diagnosis, clear information and communication, and supportive care.

The above represents the views of the author and does not necessarily reflect the opinion of the American Society for Microbiology.