Emergence and re-emergence of viral diseases of the central nervous system

Abstract

Neurologic disease is a major cause of disability in resource-poor countries and a substantial portion of this disease is due to infections of the CNS. A wide variety of emerging and re-emerging viruses contribute to this disease burden. New emerging infections are commonly due to RNA viruses that have expanded their geographic range, spread from animal reservoirs or acquired new neurovirulence properties. Mosquito-borne viruses with expanding ranges include West Nile virus, Japanese encephalitis virus and Chikungunya virus. Zoonotic viruses that have recently crossed into humans to cause neurologic disease include the bat henipaviruses Nipah and Hendra, as well as the primate-derived human immunodeficiency virus. Viruses adapt to new hosts, or to cause more severe disease, by changing their genomes through reassortment (e.g. influenza virus), mutation (essentially all RNA viruses) and recombination (e.g. vaccine strains of poliovirus). Viruses that appear to have recently become more neurovirulent include West Nile virus, enterovirus 71 and possibly Chikungunya virus. In addition to these newer challenges, rabies, polio and measles all remain important causes of neurologic disease despite good vaccines and global efforts toward control. Control of human rabies depends on elimination of rabies in domestic dogs through regular vaccination. Poliovirus eradication is challenged by the ability of the live attenuated vaccine strains to revert to virulence during the prolonged period of gastrointestinal replication. Measles elimination depends on delivery of two doses of live virus vaccine to a high enough proportion of the population to maintain herd immunity for this highly infectious virus.

Fig. 1

Current geographic distribution of Japanese encephalitis virus. Source: World Health Organization.

Fig. 2

Map of the geographic areas of the world where rabies is considered a high risk. Red areas are high risk from contact with rabid dogs and orange areas are high risk from contact with infected bats and other wildlife. Gray and green areas are low or no risk. Source: World Health Organization.

Fig. 3

September 2009 map of countries reporting indigenous wild poliovirus (red) or imported wild poliovirus (orange). Source: World Health Organization.

Fig. 4

UNICEF estimates of measles vaccine coverage in July 2009. Red countries are below 50%, pink countries 50–79%, light blue countries 80–89% and dark blue countries greater than 90%. Source: World Health Organization.