The authors of the study, published in this week’s issue of The Lancet, conclude that for every diagnosed case of meningoencephalitis, there were likely to have been 140 other infections, including 30 individuals with an influenza-like illness.

In the summer of 1999, West Nile virus was recognized in the western hemisphere for the first time when it caused an epidemic of encephalitis and meningitis in New York City.

Intensive hospital-based surveillance identified 59 cases, including seven deaths in the region.

Farzad Mostashari and colleagues from the New York City Department of Health and the Centers for Disease Control and Prevention (CDC) did a household-based survey in October, 1999 (around six weeks after the peak of the outbreak).

They assessed more clearly the public-health impact of the epidemic, its range of illness, and risk factors associated with infection.

The investigators used a representative sample of households in an area of about 7.3 km2 at the outbreak epicenter.

Blood samples were tested for antibodies specific for West Nile virus. 677 individuals from 459 households took part in the survey. 19 (2.6%) were seropositive (ie, they had the virus confirmed by blood test); a third of these individuals (32%) reported a recent febrile illness, compared with 70 of 648 (11%) who were seronegative.

A febrile syndrome with fatigue, headache, muscle pain and painful joints was highly associated with seropositivity. By extrapolation from the 59 diagnosed meningoencephalitis cases, the investigators conservatively estimate that the New York outbreak consisted of 8,200 (range 3,500-13,000) West Nile viral infections, including about 1,700 febrile infections.

The risk of severe illness was higher among older people, with one case of meningoencephalitis for every 50 infections for those aged 65 years and over, compared with one case for every 300 infections for people aged less than 65 years.

Use of DEET-containing mosquito-repellent was protective of infection in those who spent 2 or more hours outdoors between dusk and dawn, the peak biting period for Culex mosquitoes. However, 70% of residents reported never using mosquito repellent, even after the outbreak was recognized.

Farzad Mostashari comments, “As West Nile virus becomes more established in the northeastern USA and threatens to extend its geographic range in the future, public health authorities and policymakers should be aware of the entire range of illness caused by West Nile virus.

"Physicians in communities at risk for West Nile virus disease outbreaks should consider infection with this virus in the differential diagnosis of unexplained summertime fevers, especially if accompanied by headache, muscle ache and joint pain.”

In an accompanying Commentary (p 254), Z. Hubálek from the Institute of Vertebrate Biology, Valtice, Czech Republic, compares and contrasts the New York outbreak with other epidemics, notably in central Europe.

He comments: “In conclusion, the good news for public health is that fewer than 1% of cases of West Nile fever develop into meningoencephalitis. The bad news is that more than 90% of the milder cases are not diagnosed correctly, and that the severity of symptoms and the fatality rate may vary from outbreak to outbreak. The factors influencing the variations should be investigated.”

[Contact: Dr. Farzad Mostashari, Dr. Z Hubálek]

27-Jul-2001