ABSTRACT
Arboviruses, namely, viruses that are transmitted by mosquitoes, ticks and other arthropods, are dependent on separate replication cycles in insect and vertebrate hosts and transmission to susceptible vertebrate hosts by the insect vector. As a consequence, there are several vulnerable points in the life cycle of arboviruses and marked ecological and atmospheric constraints on the abundance of the vector. Some of these factors can be tracked by indicators including remote sensing devices on satellites. Despite the apparent fragility of the life cycle and the improbable and undeniable biological success of the arboviruses, it has been adopted by over 500 viruses in six virus families that blanket the world. Furthermore, arboviruses are ecological opportunists, expanding or jumping into new territories when facilitating factors allow.
Human infection by arboviruses is not fundamental to the viral life cycle. Often it results from infection with a different vector than the one that maintains the virus cycle in small mammals or birds. Humans are usually dead-end hosts because insufficient viremia is achieved to allow transmission by the vector. The effect in humans is highly variable, with some arboviruses having a high ratio of inapparent to apparent infections and/or producing only mild undifferentiated febrile illness, while others may produce highly lethal hemorrhagic disease or encephalitis.
This chapter addresses most important arboviral infections that affect the central nervous system with the exception of dengue and Zika, which will be covered in the separate chapters.
Abstract for Arthropod-borne virus encephalitis
Arboviruses are viruses that are transmitted by mosquitoes, ticks, and other arthropods. These viruses are dependent on transmission to susceptible vertebrate hosts by an insect vector following separate replication cycles in the insect and vertebrate hosts. There are at least 500 identified arboviruses in 6 separate families. Their importance is highlighted by relatively contemporary epidemics including those of West Nile virus and Zika virus and, more recently, in the United States, eastern equine encephalitis. Risk factors for acquiring these infections is generally reflective of those circumstances that increase exposure to the disease vector, such as inadequate screens and lack of air-conditioning with respect to mosquito-borne disease and skin exposure in tick-infested regions for tick-borne disease. Most often, the infection remains inapparent or presents as a nonspecific illness, but devastating neurological disease may occur. Neurological manifestations vary with the etiological agent. A broad spectrum of neurological disorders may be observed ranging from microcephaly in the newborn from in utero infection with the Zika virus and destruction of fetal neurons to Guillain–Barre syndrome. Commonly, these infections present with an otherwise nondescript meningitis, meningoencephalitis, and encephalitis clinically indistinguishable from one another. Vaccines have been developed for several of the arboviruses, but with the exception of Japanese encephalitis, have not been approved for use in humans.
