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October 4, 1999 Overview of the North American Outbreak Following earlier media reports of an outbreak of encephalitis in New York City, USA, which was originally thought to be due to infection with the St. Louis encephalitis virus, more sophisticated laboratory studies have identified the aetiologic agent to be another Group B flavivirus. At this time the US Centres for Disease Control and Prevention [Fort Collins, Colorado] has determined that the causative agent is a West Nile-like virus. Definitive confirmation of West Nile virus as the causative agent will only be available once sequencing studies have been completed. As at September 28, a total of 17 confirmed and 20 probable human cases with 4 fatalities have been documented in the New York City outbreak. Most of the severe clinical cases and all of the fatalities have occurred among older persons [CDC's MMWR of October 1, 1999]. This West Nile-like virus has also been identified in dead birds in New York State as well as in mosquitoes and birds in southern areas of the State of Connecticut. The outbreak in New York was successfully controlled through extensive aerial and ground applications of mosquito adulticides and larvicides. Important Facts West Nile [WN] virus is member of the antigenic complex of flaviviruses [Group B arthropod-borne viruses] which includes the Murray Valley, St. Louis and Japanese B Encephalitis viruses. It is known to occur naturally in Africa; the Middle East, where it has been most studied in Egypt; parts of Europe; the former USSR; India and Indonesia. This disease has never before been recognised in the western hemisphere, and it is possible that international travel of infected persons or the importation of infected birds may have played a role in its introduction into North America. It is also possible that a WN-like virus may exist as a separate antigenic group in this geographically distant region. WN virus is not spread directly from person to person or from birds to people, but is transmitted through the bite of an infected mosquito, as described below. WN fever is a zoonosis that is transmitted from birds to man and other mammals by ornithophilic [bird-feeding] mosquitoes. WN virus has a very broad host range. The main vertebrate hosts are birds and the results of antigenic and genomic analyses have been interpreted to indicate intercontinental exchange of WN virus strains by migrating birds. Birds have a high and prolonged viraemia, which enables them to serve as a source of infection for the arthropod vector. Its vectors are known to include mosquito species of the genera Culex [pipiens, quinquefasciatus], Aedes, and Anopheles, as well as soft [Argas] and hard [Hyalomma] tick species. However, Culex mosquitoes appear to be the dominant vector. In the current American outbreak, WN-like virus has been isolated from adult Culex pipiens and Aedes vexans mosquitoes. Based on studies conducted in Egypt, a hypothesis was formulated that the virus is maintained during the winter months by a slow cycle involving the mosquito C. pipiens. Arthropod-borne [arboviral] infections of man range in their severity from completely sub-clinical but immunising to fatal illnesses of a few days duration. The incubation period is 5 to 15 days. The typical case of WN is characterised by fever, headache, backache, generalized myalgia and anorexia. A maculopapular or roseolar rash occurs in approximately half of the cases. Occasionally, in less than 15% of infections, aseptic meningitis and meningo-encephalitis may result. Inapparent and mild infections are common, especially in children. Viraemia was usually detectable up to 10 days after onset of illness. For the most part, the clinical syndrome produced by WN is very similar to that of classic dengue fever. Implications for the Caribbean The potential for spread of this West Nile-like virus from North America to the Caribbean and South America via migrating infected birds is a real possibility. While birds migrating southwards to over-winter may fly along many different routes or flyways, two routes are important for the Caribbean. The Mississipi flyway describes a flight-path over Florida, West Cuba, the Bahamas and Jamaica into Central America, while the Atlantic flyway is a path along the Caribbean chain of islands into South America. The following points are pertinent from a Caribbean perspective:
Please note: For laboratory diagnosis of WN infection, serum, CSF and/or autopsy specimens should be collected, stored and transported according to the guidelines already disseminated for meningitis/encephalitis (viral). [See the Communicable Disease Manual (draft version) that was circulated at the June 1999 Meeting of Caribbean National Epidemiologists and Laboratory Directors.] Please feel free to call us at 868-622-2152 for any clarification or further inquiries. Please notify our laboratories in advance if clinical specimens are being referred to CAREC. For answers to additional questions on the West Nile Encephalitis refer to the Centers for Disease Control and Prevention (CDC) Website
Dr. Merle J Lewis |
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Caribbean Epidemiology Centre Page last modified 05 October, 1999 |
