Arboviral encephalitis refers to a group of diseases caused by viruses transmitted primarily through mosquito bites. The term "arbovirus" stands for arthropod-borne virus, indicating that these viruses rely on arthropods, such as mosquitoes, as vectors for transmission. These viruses are all single-stranded RNA viruses and enveloped, meaning they have a lipid membrane surrounding their genetic material. Encephalitis itself is the inflammation of the brain, which can range from mild to severe depending on whether the virus crosses the blood-brain barrier.
In the United States, the most common arboviral encephalitis viruses belong to three main families: Flaviviridae, Togaviridae, and Bunyaviridae. The Flaviviridae family includes West Nile virus and St. Louis encephalitis virus. Both viruses primarily circulate among birds, with mosquitoes acting as vectors that transmit the virus from bird to bird. Humans are incidental hosts and do not contribute to further spread. West Nile virus is the most prevalent arboviral encephalitis cause in the U.S., often reported during summer months, while St. Louis encephalitis is relatively rare.
The Togaviridae family includes Eastern and Western equine encephalitis viruses. These viruses are named for their impact on horses ("equine"), which, like humans, are dead-end hosts and do not contribute to transmission cycles. Birds serve as the natural reservoir, with mosquitoes facilitating spread. Eastern equine encephalitis is notable for having the highest mortality rate among these viruses, although infections are rare. Western equine encephalitis is even less common.
The Bunyaviridae family includes California encephalitis virus and La Crosse encephalitis virus, named after regions where outbreaks have occurred. Unlike the others, their natural reservoirs are small mammals such as squirrels and chipmunks. This group generally causes the lowest mortality among arboviral encephalitides.
These viruses are zoonotic, meaning they normally circulate in animal hosts and are transmitted to humans via mosquito vectors. Humans and horses are considered dead-end hosts because the virus does not replicate sufficiently in them to continue the transmission cycle. The severity of symptoms depends on whether the virus crosses the blood-brain barrier. If it does not, infected individuals may experience mild, cold-like symptoms or remain asymptomatic. However, if the virus invades the central nervous system, it can cause encephalitis characterized by fever, headache, nausea, altered mental status, weakness, paralysis, and potentially death.
Diagnosis of arboviral encephalitis relies on detecting viral RNA through reverse transcription polymerase chain reaction (RT-PCR) or identifying specific antibodies in the blood. RT-PCR is particularly useful because these viruses have RNA genomes. Currently, there is no specific antiviral treatment for these infections; management is supportive, including hydration, respiratory support if needed, and prevention of secondary infections.
Prevention focuses on minimizing mosquito exposure. Effective strategies include using insect repellents containing DEET, which is the most proven active ingredient for repelling mosquitoes, employing mosquito netting, and eliminating standing water where mosquitoes breed. No vaccines are currently available for human use against these arboviral encephalitis viruses, making personal protective measures essential.
