Arthropods are groups of animals with hard exoskeletons, segmented bodies and jointed limbs, such as mosquitoes and ticks. A virus transmitted by mosquitoes or ticks to an animal or person while taking a blood meal is known as an arbovirus.

Several arboviruses, usually named for the area in which they were discovered, can cause arthropod-borne encephalitis infection in humans. Such infected mosquitoes or ticks that serve as vectors (carriers or transmitters) are a growing health concern.

A mosquito feeding on an encephalitis-infected animal becomes a vector of the disease for the remainder of its lifespan. Once infected, the animal briefly exhibits very high levels of the virus in its blood until immunity develops and recovery can occur. The mosquito then infects the next animal on which it feeds, thus passing the virus to more animals. Birds, horses, rabbits, chipmunks, bats, skunks, cats, squirrels and other small mammals living in areas with high mosquito populations typically serve as hosts to encephalitis and other related diseases.

This feeding cycle usually continues without considerable harm to either the vector or host, and without reaching humans. Mosquitoes tend to bite birds and other small mammals first, with humans being bitten as strictly a second choice. However, certain unusual conditions, such as environmental disasters or inclement weather, can increase the risk of human infection by increasing infected mosquito populations in a given area.

The most common causes of arthropod-borne encephalitis through arboviruses are:

• Japanese encephalitis (JE) virus. Although vaccines are available, JE is the most lethal and common arboviral (mosquito or tick borne) encephalitis virus worldwide. The Centers for Disease Control and Prevention (CDC) estimates that 30,000 to 50,000 cases of JE occur each year in Asia, with approximately 15,000 of those resulting in death. However, an average of only about one case per year of JE virus is reported in Americans who live or travel to Asia. Approximately 30 percent of JE virus survivors retain severe neurological impairment. JE virus is prevalent in many tropical areas of Asia and the Indian subcontinent, with epidemics occurring most frequently in the summer months. Water birds tend to be the primary host. However, some agricultural animals (e.g., pigs) can also host the disease.
  
  
• West Nile virus. A relative newcomer in the United States, West Nile virus first appeared in America in 1999. West Nile virus is more commonly found in parts of Africa, Asia, Europe and the Middle East. Like with most other forms of encephalitis, birds serve as the main animal hosts. However, the disease can also be spread by humans during childbirth, breastfeeding, organ transplant and blood transfusions. Encephalitis caused by this arbovirus can be severe in older adults and/or those with compromised immune systems. In some cases, the virus may only cause fever and may not involve the nervous system (as meningitis and encephalitis do). Nonetheless, people infected with West Nile virus may experience chronic health problems such as fatigue, headaches, depression and tremors, according to a recent study. And patients with relatively benign West Nile fever can be just as likely to experience these long-term health consequences as those who are hospitalized with the more severe West Nile virus-related illnesses meningitis or encephalitis, according to the same study. In 2006, about 4,200 cases of infection by this virus were reported to the CDC, but fewer than 1,500 of them involved meningitis or encephalitis.
  
  
• Lyme disease. Transmitted by ticks typically found in rural and wooded areas of North America, Canada and Europe, Lyme disease can cause encephalitis in rare instances.
  
  
• Tick-borne encephalitis (TBE) virus. TBE virus is unique in that it is produced by three closely affiliated viruses: the western European subtype, the Siberian subtype and the far eastern subtype. A variety of host mammals and tick vectors throughout these regions maintain TBE virus through different natural cycles. Humans tend to be exposed to TBE virus in warmer areas during the spring and summer, as well as throughout the Mediterranean during fall and winter. In rare cases, infection with TBE virus can occur through the consumption of unpasteurized dairy products from sheep, goats or cows.
  
  
• Colorado tick fever virus. Occasionally, people stricken with Colorado tick fever virus develop either encephalitis or meningitis (inflammation of the membranes lining the brain). The Colorado tick fever virus is transmitted via the wood tick in parts of the western United States and Canada. Due to the mountainous geography of the region, wood ticks tend to live in elevations ranging from 4,000 to 10,000 feet (1,219 to 3,048 meters). Colorado tick fever virus can be spread between the months of March and September. However, peak months for transmission tend to be from April to June.
  
  
• Western equine encephalitis (WEE) virus. The WEE virus infects mainly horses in the central and western plains areas of North America. Flooding of these areas is common and is thought to precipitate outbreaks of WEE during hotter summer months. Significant outbreaks of WEE were common in this region during the 1950s and 1960s. However, a variety of factors have reduced outbreaks substantially since that time, including improved horse vaccination methods and declining horse populations. Very few adults infected with the WEE virus actually develop encephalitis.  However, death or serious complications (e.g., brain damage) can occur in very rare cases, most notably in small children and infants.