INTRODUCTION

Arthropod-borne encephalitis viruses represent a significant public health problem throughout most of the world. These viruses, which belong to the families Flaviviridae, Togaviridae, Bunyaviridae, and Reoviridae, are usually highly adapted to particular reservoir hosts and are spread from animal to animal via the bite of an infected arthropod, usually a specific mosquito or tick species (table 1).

This topic will review the major characteristics of most of the arthropod-borne viral encephalitides. General issues related to viral encephalitis, including clinical manifestations, cerebrospinal fluid findings, distinction from postinfectious encephalitis and meningitis, and an approach to the patients with suspected central nervous system infection are discussed separately. (See "Viral encephalitis in adults".)

TRANSMISSION CYCLE

The mosquito or tick becomes infected when feeding on the blood of the viremic animal. The virus then replicates in the mosquito or tick tissues, ultimately infecting the salivary glands. The mosquito or tick transmits the virus to a new host when it injects infective salivary fluid while taking a blood meal.

The natural animal hosts of these viruses usually remain unaffected and viral circulation generally remains undetected until one of the following occurs:

• Humans encroach on the natural enzootic focus
• Environmental or other conditions that favor substantial amplification in the primary vector-host cycle cause a sufficient number of vectors to become infected so that the human risk is substantially increased
• The virus escapes the primary cycle via a secondary vector or vertebrate host, thereby bringing infected, human-biting vectors in close proximity to human habitation