Arboviral encephalitis

Arboviral encephalitis: Inflammation of the brain (encephalitis) caused by infection with an arbovirus, a virus transmitted by a mosquito, tick or another arthropod. Infection of vertebrates, including humans, occurs when an infected arthropod feasts upon them for a blood meal.

There are a number of types of arboviral encephalitis. Those that occur in the US include LaCrosse (LAC) encephalitis, eastern equine encephalitis (EEE), western equine encephalitis (WEE), and St. Louis encephalitis (SLE). All of these are transmitted by mosquitoes.

Another arbovirus, Powassan (POW), transmitted by ticks, is a minor cause of encephalitis in the northern U.S. The arbovirus that causes Venezuelan equine encephalitis (VEE) is usually found in Central and South American but has ventured into the US.

Many other types of arboviral encephalitis occur throughout the world. Most of these diseases are problems only for those individuals traveling to countries where the viruses are endemic. They include Japanese encephalitis (JE), tick-borne encephalitis (TBE), Murray Valley encephalitis (MVE) and, most notorious of all, the West Nile virus (WNV) which causes West Nile encephalitis, also known as West Nile fever.

All arboviral encephalitides are zoonotic, being maintained in complex life cycles involving a nonhuman primary vertebrate host and a primary arthropod vector. These cycles usually remain undetected until humans encroach on a natural focus, or the virus escapes this focus via a secondary vector or vertebrate host as the result of some ecologic change.

Humans and domestic animals can develop clinical illness but usually are “dead-end” hosts because they do not produce significant viremia, and do not contribute to the transmission cycle. Many arboviruses that cause encephalitis have a variety of different vertebrate hosts and some are transmitted by more than one vector. Maintenance of the viruses in nature may be facilitated by vertical transmission (e.g., the virus is transmitted from the female through the eggs to the offspring).

Most cases of arboviral encephalitis occur from June through September (in the Northern hemisphere) when arthropods are most active. In milder (i.e., warmer) parts of the US, where arthropods are active late into the year, cases can occur into the winter months.

The majority of human infections are asymptomatic or may result in a nonspecific flu-like syndrome. Onset may be insidious or sudden with fever, headache, myalgia (muscle ache), malaise and occasionally prostration. Infection may, however, lead to encephalitis, with a fatal outcome or permanent neurologic sequelae. Fortunately, only a small proportion of infected persons progress to frank encephalitis.

Antibiotics are not effective for treatment and no effective antiviral drugs have yet been discovered for the arboviruses. Treatment is supportive, attempting to deal with problems such as swelling of the brain, loss of the automatic breathing activity of the brain and other treatable complications like bacterial pneumonia.

There are no commercially available human vaccines for these US diseases. There is a Japanese encephalitis vaccine available in the US. A tick-borne encephalitis vaccine is available in Europe. An equine vaccine is available for EEE, WEE and Venezuelan equine encephalitis (VEE).

Arboviral encephalitis can be prevented in two major ways: personal protective measures and public health measures to reduce the population of infected mosquitoes. Personal measures include reducing time outdoors particularly in early evening hours, wearing long pants and long sleeved shirts and applying mosquito repellent to exposed skin areas. Public health measures often require spraying of insecticides to kill juvenile (larvae) and adult mosquitoes.