Eastern equine encephalitis (EEE), also called triple E and sleeping sickness, is a viral disease caused mainly by the Eastern equine encephalitis virus (EEEV). Most infections in humans are asymptomatic, but about 5% of the time the infection progresses to severe neuroinvasive disease. Symptoms typically appear 3–10 days after being bitten by an infected mosquito and initially include fever, headache, nausea, vomiting, fatigue, muscle pain, and joint pain. Neurological symptoms usually appear a few days later and include altered mental state, encephalitis, photophobia, seizures, paralysis, and loss of consciousness and coma. The case fatality rate is 30–75% depending on age, with disease severity greatest in young children and the elderly. About 50 to 90% of survivors experience long-term neurological complications that range from minor to severe. EEE is most common in horses, in which the disease carries a 70–90% case fatality rate and permanent brain damage for survivors.
Most human cases are caused by EEEV. Traditionally, four lineages of EEEV were recognized: I, II, III, and IV. Lineage I corresponds to EEEV and the other lineages are classified as a different virus: Madariaga virus (MADV). EEEV is found in North America, the Caribbean, and Central America, and MADV is found in Central America and South America. While both EEEV and MADV cause disease in horses, it is very rare for MADV to cause disease in humans. EEEV and MADV are single-stranded, positive-sense RNA viruses of the genus Alphavirus in the family Togaviridae. Alphaviruses are sorted into Old World alphaviruses and New World alphaviruses, and considered arthritogenic (affecting the joints) or encephalitic (affecting the brain). EEEV and MADV are New World encephalitic alphaviruses. Among encephalitic alphaviruses, EEEV causes the most severe disease in humans.
EEEV is maintained in nature in an enzootic cycle between natural reservoirs of the virus and mosquitos that feed on the blood of those animals. In North America, passerine birds are the main reservoirs of the virus, and Culiseta melanura is the main enzootic vector. In South America, rodents and marsupials may be reservoirs of MADV, and Culex mosquitos of the subgenus Melanoconion are likely the main enzootic vectors. The disease is occasionally transmitted to mammals and other non-reservoir species by other species of mosquitos, called bridge vectors. These mosquitos feed on the blood of both avian and mammalian hosts and include Coquillettidia perturbans and various species of the Aedes, Anopheles, and Culex genera. Humans, horses, and other incidental carriers of EEEV are considered dead-end hosts because they cannot transmit the virus back to mosquitos.
EEE is usually diagnosed by using enzyme-linked immunosorbent assay (ELISA) to test for anti-EEEV antibodies in serum or cerebrospinal fluid. The results of ELISA are then verified with plaque reduction neutralization tests. Other methods such as viral cultures and nucleic acid amplification assays may be used post-mortem. Neuroimaging and electroencephalogram (EEG) tests are useful for identify the severity of disease. There are no specific antiviral drugs used to treat EEE, so treatment is supportive in nature and includes corticosteroids, anti-convulsant drugs, intravenous fluids, tracheal intubation, and fever-reducing drugs. Physical therapy, occupational therapy, and speech therapy are often needed during the recovery process. Prevention methods include insecticides, larvicides, and eliminating mosquito breeding sites. A vaccine that protects against EEEV, but not MADV, is available for horses.
EEE was first recorded during an outbreak in horses in Massachusetts, USA in 1831. EEEV was first isolated from horse brains and linked to EEE during another outbreak in 1933. The first documented human cases were in 1938 in Massachusetts, and isolation from mosquitos first came in 1949 from Cq. perturbans and then in 1951 from Cs. melanura. The disease occurs along the eastern side of the Americas, mainly in the USA in states bordering the Atlantic Ocean, Gulf of Mexico, and Great Lakes. Fewer than ten human cases occur in a typical year, usually in close proximity to hardwood freshwater swamps and marshes where Cs. melanura and other vectors lives. Periodic outbreaks occur in years following years with heavy rainfall, likely due to creating a favorable environment for Cs. melanura. Outbreaks in horses usually precede those in humans, so an increase in cases in horses may be predictive of an upcoming human outbreak.
Signs and symptoms In most cases, infection with Eastern equine encephalitis virus (EEEV) is self-limiting with no symptoms. In about 5% of cases, though, the virus invades the central nervous system, where it causes Eastern equine encephalitis (EEE), also called triple E or sleeping sickness. This disease is severe and carries with it a high likelihood of death or long-term neurological complications for survivors. Neuroinvasive disease is most likely to occur for people under the age of 15 and over the age of 50. Symptoms usually appear 3–10 days after being bitten by an infected mosquito, but may appear up to three weeks later.
The early phase of EEE has a median duration of about 5 days but ranges from 0 to 28 days in duration. Acute symptoms include fever headache, nausea, vomiting, chills, fatigue, lethargy, malaise, muscle pain, joint pain, and swelling of lymph nodes. Usually within a few days, central nervous system involvement becomes apparent due to the emergence of neurological symptoms such as alterations in mental state, heightened irritability and agitation, personality changes, confusion, encephalitis, convulsions, seizures, paralysis, and loss of consciousness and coma. Localized neurological symptoms may occur as well, such as paralysis of cranial nerves, facial drooping, myoclonic jerks, and weakness in specific parts of the body, or on one side of the body. A stiff neck is indicative of infection in the meninges and meningitis.
Most seizures that occur are generalized, and most of these are tonic-clonic, followed by twitching. Focal seizures are less common, and complex seizures are the least common. In severe cases, status epilepticus may occur, in which there are prolonged seizures or multiple seizures consecutively without regaining consciousness between them. Elevated heart rate (tachycardia), elevated respiration rate (tachypnea), language difficulties (aphasia), and a strong desire to sleep (drowsiness or somnolence) may occur. Stupor or coma occur in most cases. Fluid may accumulate in the brain (cerebral edema), particularly in young children. Other possible symptoms include shock, which may be refractory, flaccid paralysis, difficulty speaking due to muscle weakness (dysarthria), discomfort to bright lights (photophobia), and diarrhea.
Virology
EEE is caused primarily by Eastern equine encephalitis virus (EEEV) and secondarily by Madariaga virus (MADV). EEEV and MADV belong to the genus Alphavirus in the family Togaviridae. Alphaviruses are often sorted into Old World alphaviruses and New World alphaviruses, the latter of which includes EEEV and MADV. They are also sorted into arthritogenic (affecting the joints) or encephalitic (affecting the brain). Unlike arthritogenic alphaviruses, encephalitic alphaviruses are associated with mortality, especially EEEV, which is the most severe alphavirus to affect humans.
Genome and structure The genome of EEEV comprises a positive-sense, single-stranded RNA strand about 12 kilobases in length. The genome starts with a non-coding region, followed by a single open reading frame (ORF) that encompasses two-thirds of the genome and encodes non-structural proteins. A second ORF encodes all structural proteins, separated from the non-structural ORF by a non-coding region. The structural ORF is followed by a non-coding region, and the genome then ends with a polyadenylated tail. The non-structural proteins are nsP1, nsP2, nsP3, and nsP4. The structural proteins are the capsid protein (CP), E3, E2, 6K/TF, and E1.
The functions of each protein are described hereafter. nsP1 is methyltransferase and guanyltransferase. nsP2 is a helicase and protease, and it interferes with the host immune response. nsP3 is a phosphoprotein, hydrolase, and interacts with host factors, including interfering with the host immune response. nsP4 is an RNA-dependent RNA polymerase and terminal adenylyltransferase. CP is the capsid protein and forms a protective shell around the genome. E2 and E1 are glycoproteins that form spikes on the surface of the viral envelope. E2 binds to cellular receptors. E1 mediates fusion of the viral envelope with the membrane of host endosomes. E3 is involved in translocating viral polyproteins, stabilizing E2-E1 spikes, and inhibiting premature membrane fusion. 6K has viroporin activity during budding and TF is a virulence factor.
The body (virion) of EEEV is about 70 nanometers in diameter and icosahedral in shape. It consists of the genome of the virus surrounded by a shell made of capsid proteins, called the capsid. The capsid is surrounded by a lipid membrane called an envelope, which contains spikes embedded in it that emanate from the surface of the envelope.
Life cycle The replication cycle of EEEV begins when the E2 spike protein binds to receptors on the surface of a cell. The virion is then taken into the cell via endocytosis and stored in an endosome. A decrease in pH causes the viral envelope to fuse with the endosome, which empties the capsid into the host cell's cytosol. The capsid then disintegrates and releases viral RNA into the cytosol. The positive-sense genome is then used as messenger RNA (mRNA) by host cell ribosomes to translate the non-structural ORF to create a polyprotein that contains the non-structural proteins, P1234. A stop codon exists between the genes for nsP3 and nsP4, so many polyproteins are P123 without nsP4. For P1234 polyproteins, nsP2 uses its protease activity to self-cleave the polyprotein into P123 and nsP4. nsP4 then transcribes the positive-sense RNA genome to produce a negative-sense strand. Negative-sense RNA forms a duplex with positive-sense RNA and serves as a template for the synthesis of full-length positive-sense RNA for progeny viruses.
Synthesis of negative-sense RNA is followed by cis-cleavage of nsP1 and P23, the latter of which is cleaved in trans into nsP2 and nsP3. This causes a shift from producing primarily negative-sense RNA to primarily positive-sense RNA since nsP1, nsP2, and nsP3 are involved in the synthesis of positive-sense strands. Structural polyproteins are translated from sub-genomic mRNA. Usually, a CP/E3/E2/6K/E1 polyprotein is produced, but sometimes a ribosomal frameshift occurs that produces CP/E3/E2/TF without E1. These polyproteins are then cleaved by both viral and host proteases to produce the individual structural proteins. The spike proteins are translated in the endoplasmic reticulum (ER) and Golgi apparatus, inserted into the ER, and then translocated to the plasma membrane. Once sufficient capsid proteins exist, they assemble into capsids with viral RNA to form nucleocapsids in the cytosol. Capsids then bud from the membrane to obtain an envelope and leave the cell.
Lineages