Introduction

Eastern equine encephalitis virus (EEEV) is an arbovirus, a group of conditions that are transmitted to humans via the bite of an infected insect such as a mosquito or a tick. This virus is responsible for causing one of the most severe arboviral encephalitides, an inflammation of the brain, in America. EEEV is currently considered an emerging disease due to its considerable increase in the wider population; in the USA, there have been six to eight cases annually reported.^1.2 In addition, there has been the suggestion that EEEV may be a bioterrorism weapon due to its airborne transmission.³ The case fatality of this virus is 30%; however, 50% of survivors experience a neurologic sequelae.⁴

Etiology and epidemiology of EEEV

Eastern equine encephalitis virus, first identified in humans in 1938, is an arbovirus from the Togaviridae family, genus alphavirus. They are maintained by a cycle between birds and predominantly mosquitoes associated with freshwater hardwood swamps. EEEV is transferred via mosquitoes that have been infected and undergo blood feeding to passerine (tree-perching) birds and opportunistic mammals, reptiles, or amphibians. This virus can escape its usual reservoir to infect dead-end hosts, which are organisms where viremia levels, the amount of virus in the blood, accumulate to a high enough amount to allow transmission to other feeding mosquitoes; dead-end hosts are organisms like humans and swine. It is important to note that cross-transmission happens unpredictably, and changes in weather and global warming may affect its incidence. In addition, it is also important to highlight that EEEV can be transmitted via other methods; for example, in 2017, there was a reported transmission via organ transplant; however, this was the first of its type.^1,5

Regarding its incidence, as mentioned previously, there is an annual mean incidence of eight cases in the US, where infants (less than 5 years of age) and the elderly (more than 60 years of age) are at a higher risk of acquiring the condition. In addition, the incidence of neuroinvasive disease is two times higher in men.

Moreover, since EEEV became a nationally notifiable condition, it became monitored by ArboNET - a national arboviral disease surveillance system developed since 2003.⁸

Recognition 

According to the guidelines on the Centres for Disease Control and Prevention website, anyone with a fever or an acute neurologic illness who has been recently exposed to mosquitoes, organ transplants, or potentially blood transfusions, particularly in the summer months, should be reported. In addition, it is also important to rule out other possible conditions that would commonly cause encephalitis and aseptic meningitis, such as other arboviruses, during clinical evaluation.

Symptoms 

Most people infected with eastern equine encephalitis do not develop symptoms, and the ones that do there is a time difference between the bite of the infected mosquito and the onset of the illness - this can range from four to ten days.

EEE can cause symptoms such as a febrile illness or a neurologic disease, including meningitis (an infection of the membranes that surround the brain and spinal cord) or encephalitis (an infection of the brain). However, the type of illness depends on the age of the individual affected as well as other factors.

Febrile illness is characterised by:

• Fever
• Chills
• Body aches
• Joint pain

It normally lasts one to two weeks, and the majority of individuals do a full recovery with no involvement of the central nervous system.

Neurologic disease is characterised by:

• Fever
• Headache
• Vomiting
• Diarrhea
• Seizures
• Behavioural changes
• Drowsiness
• Coma

However, as mentioned, there is a differential onset depending on the age of the affected individual. In infants, neurologic disease occurs soon after onset, whilst in elderly individuals, encephalitis may occur several days after systemic illness. In addition, statistics state that a third of all people with encephalitis due to EEEV pass away, usually 2 to 10 days after the onset of symptoms, but it can also occur later. The people who manage to recover are left with long-term physical or mental impairments, which can range from mild brain dysfunction to severe intellectual impairment, personality disorders, seizures, paralysis, and cranial nerve dysfunction. If these impairments are severe, then individuals might require long-term care, leading up to death.

Diagnosis 

Healthcare professionals, once individuals present themselves to the hospital, may diagnose EEEV infection based on the following:

• Signs and symptoms
• History of living in or travelling to an area where the EEE virus is known to circulate
• History of possible exposure to mosquitoes that can carry the EEE virus
• Laboratory testing of blood or spinal fluid

Treatment 

• Currently, there is no available treatment for eastern equine encephalitis; therefore, clinical management is merely supportive
• Patients who experience severe meningeal symptoms usually require pain control for headaches, antiemetric therapy combined with rehydration for associated nausea and vomiting
• Patients with encephalitis require close monitoring for the development of elevated intracranial pressure, seizures, and inability to protect their airways

Prevention and control 

Currently, there are no vaccines available to prevent an infection of EEEV. The only way to prevent it is by reducing the chance of being exposed to infected mosquitoes; this includes the usage of insect repellent, wearing long-sleeved shirts and pants, treating clothing and gear with 0.5% permethrin, and taking steps to control mosquitoes indoors and outdoors. In addition, due to the possibility of EEEV being transmitted via organ transplantation and most likely blood transfusion, individuals who have been recently diagnosed with EEEV should refrain from donating blood for 120 days (four months) after their illness.^1,4

Summary

• Eastern Equine Encephalitis (EEE) remains a formidable public health concern, characterised by its severe impact on both humans and animals
• The virus, belonging to the arbovirus group, is primarily transmitted through the bite of infected mosquitoes, with freshwater hardwood swamps serving as a key ecological reservoir
• Early recognition of EEE is essential for timely intervention 
• The Centres for Disease Control and Prevention recommends suspicion and reporting for individuals with fever or acute neurologic illness, especially those exposed to mosquitoes during the summer months or recipients of organ transplants or blood transfusions
• Symptoms of EEE range from febrile illness to severe neurologic disease, with varying onset times based on age and other factors
• The disease's significant case fatality rate and the potential for long-term neurologic sequelae highlight the urgency of prevention and control measures
• While there is currently no specific treatment for EEE, supportive care is crucial, particularly for patients with severe meningeal or encephalitic symptoms
• Prevention strategies focus on reducing exposure to infected mosquitoes, emphasising the use of insect repellent, protective clothing, and mosquito control measures.
• As of this moment, there is no available vaccine for EEE
• As we continue to grapple with this challenging virus, it is paramount to invest in research, public awareness, and preventive measures to mitigate the impact of EEE on individuals and communities
• In the absence of a vaccine, a comprehensive approach to mosquito-borne illness prevention remains our best defence against Eastern Equine Encephalitis

FAQs

What is the difference between eastern and western equine encephalitis?

Eastern Equine Encephalitis is endemic in the eastern U.S. with recurrent outbreaks, while Western Equine Encephalitis has virtually disappeared in North America, with circulation persisting mainly in South America.^7,8

What is the mortality rate for eastern equine encephalitis?

EEE fatality can reach 50–75% in severe outbreaks, though most reviews estimate an overall case-fatality of ~30%.⁵