Continued transmission of West Nile virus to humans in southeastern Romania, 1997-1998

West Nile virus in the western hemisphere

West Nile virus first appeared in the western hemisphere in 1999 in New York. Genetic analysis determined that the virus was introduced from the Mediterranean Basin. This review discusses the establishment of West Nile virus in the naïve environment of the northeastern USA, its ecology, epizootiology, pathology, prevention and prediction, as well as laboratory studies that have been conducted to elucidate the transmission cycle.

Balancing the risks: vector control and pesticide use in response to emerging illness

The competing public health concerns of vector-borne disease and vector control strategies, particularly pesticide use, are inherently subjective and difficult to balance. Disease response decisions must frequently be made in the absence of data or clear criteria. The factors to be weighed include the vector control measures versus those posed by the disease itself; short-term versus long-term disease management goals, specifically with regard to the issue of pesticide resistance; the need to distinguish among diseases of differing severity in making response choices; and the issue of pesticide efficacy. New York City's experience with West Nile virus has illustrated each of these issues. A framework for assessing the appropriate response to West Nile virus can serve to guide our response to likely new pathogens.

European experience with the West Nile virus ecology and epidemiology: could it be relevant for the New World?

A review of West Nile virus (WNV) and the epidemiology of West Nile fever (WNF) in Europe is presented. European epidemics of WNF reveal some general features. They usually burst out with full strength in the first year, but few cases are observed in the consecutive 1 to 2 (exceptionally 3) years, whereas smaller epidemics or clusters of cases only last for one season. The outbreaks are associated with high populations of mosquitoes (especially Culex spp.) caused by flooding and subsequent dry and warm weather, or formation of suitable larval breeding habitats. Urban WNF outbreaks associated with Culex pipiens biotype molestus are dangerous. Natural (exoanthropic, sylvatic) foci of WNV characterized by the wild bird-ornithophilic mosquito cycle probably occur in many wetlands of climatically warm and some temperate parts of Europe; these foci remain silent but could activate under circumstances supporting an enhanced virus circulation due to appropriate abiotic (weather) and biotic (increased populations of vector mosquitoes and susceptible avian hosts) factors. It is very probable that WNV strains are transported between sub-Saharan Africa and Europe by migratory birds. The surveillance system for WNF should consist of four main components: (1) monitoring of mosquito populations and their infection rate; (2) wild vertebrate surveys; (3) sentinel birds (domestic ducks rather than chickens); and (4) monitoring of human disease. In the case of persisting high risk of WNF for humans and equids in certain enzootic areas, immunization against WNF should be considered. For that purpose a commercially available, cross-protective vaccine against Japanese encephalitis could be used.

The West Nile virus:its recent emergence in North America

West Nile fever emerged in New York in the summer of 1999 when seven people, several horses and thousands of wild birds died. It was soon established that the human disease and the mortality of birds were related. Continued surveillance detected West Nile virus in mosquitoes, birds, horses, small mammals, bats and humans, and has shown its spread to several northeastern states. These events confirm the establishment of West Nile virus endemically in the United States.

West Nile virus in overwintering Culex mosquitoes, New York City, 2000

After the 1999 West Nile (WN) encephalitis outbreak in New York, 2,300 overwintering adult mosquitoes were tested for WN virus by cell culture and reverse transcriptase-polymerase chain reaction. WN viral RNA and live virus were found in pools of Culex mosquitoes. Persistence in overwintering Cx. pipiens may be important in the maintenance of WN virus in the northeastern United States.

West Nile outbreak in horses in southern France, 2000: the return after 35 years

On September 6, 2000, two cases of equine encephalitis caused by West Nile (WN) virus were reported in southern France (Hérault Province), near Camargue National Park, where a WN outbreak occurred in 1962. Through November 30, 76 cases were laboratory confirmed among 131 equines with neurologic disorders. The last confirmed case was on November 3, 2000. All but three cases were located in a region nicknamed "la petite Camargue," which has several large marshes, numerous colonies of migratory and resident birds, and large mosquito populations. No human case has been confirmed among clinically suspected patients, nor have abnormal deaths of birds been reported. A serosurvey has been undertaken in horses in the infected area, and other studies are in progress.

Evaluation of immunoglobulin M (IgM) and IgG enzyme immunoassays in serologic diagnosis of West Nile Virus infection

A unique urban encephalitis epidemic in Romania signaled the emergence of neurological infection due to West Nile (WN) virus as a novel public health threat in Eastern Europe and provided an opportunity to evaluate patterns of immunoglobulin G (IgG) and IgM reactivity in IgM capture and IgG enzyme-linked immunosorbent assays (ELISAs). WN virus infection was diagnosed serologically in 236 of 290 patients from whom acute serum or cerebrospinal fluid (CSF) samples were available. In 37% of serum samples and in 25% of CSF samples collected in the first week of illness, anti-WN virus IgM antibody was detected in the absence of virus-specific IgG. The switch to an IgG antibody response occurred after 4 to 5 days of illness and earlier in CSF than in serum. A specific humoral immune response was detected in the CSF before the serum in some patients for whom paired CSF and serum samples from the same day were available. IgM antibody in convalescent serum samples persisted beyond 2 months after the onset of illness in more than 50% of patients. ELISA optical density values and antibody concentrations were well correlated for both IgM and IgG immunoassays. Anti-WN virus IgM antibody in acute-phase samples did not cross-react significantly with flaviviruses in other antigenic groups.