St. Louis Encephalitis New York Style
Special Report
As of september 15, 1999, the cdc had confirmed 11 cases of St. Louis Encephalitis (SLE) acquired in New York City with an additional 65 cases under investigation.1 The two most recently confirmed cases were in a 15-year-old and a 38-year-old, each of whom were stable or recovering. Nine of the 11 confirmed cases were in adults aged 58-87 years while the other two were ages 38 and 15 years. Eight of the confirmed cases were in residents of Queens, two were from the South Bronx, and one was from Brooklyn.
Three of the 11 patients with confirmed SLE had died and each of the three was an octogenarian resident of the borough of Queens. Malathion spraying of the entire city by helicopters and ground spraying with resmethrine during the dusk and dawn periods of peak activity of the relevant Culex mosquitoes was started on September 3rd and will continue until the first hard frost.
SLE is a flavivirus composed of at least three distinct, albeit antigenically indistinguishable, genotypes.2 It is widely distributed in the western and eastern United States and South America and is also found in Central America and the Caribbean. The virus is trasmitted by Culex mosquitoes after amplification in passerine birds; common ones in New York City include house sparrows, pigeons, blue jays, grackles, starlings, and robins. The mosquito believed to be involved in the New York City outbreak is most likely Culex pipiens pipiens, which is usually found near domestic habitats; larva develop in polluted groundwater while adults may rest in open house foundations and urban storm sewers.
SLE transmission in the western United States is usually rural and occurs perennially at a low, but relatively persistent, level while in the eastern United States, transmission is predominantly urban and is periodic, causing outbreaks after prolonged intervals within an area during which there is limited endemic transmission. As a consequence of these contrasting patterns of transmission, seroprevalence rates in the general population are lower in the east than in the west.
Comment by Stan Deresinski, MD, FACP
Only approximately 0.3% of infections are symptomatic, with the frequency of symptomatic infection increasing with age, varying from approximately 0.125% in children younger than 10 years of age to 1.2% in individuals older than 60 years of age.3 The incubation period is estimated to be 4-21 days.2 Clinical illness may range from a mild, self-limited, influenza-like illness to predominant neurologic involvement manifested as meningitis or potentially fatal meningoencephalitis. Cranial nerve palsies occur in approximately one-fourth of those with neurological disease. Severe life-threatening neurological involvement is more frequent in the elderly than in younger subjects. The fatality rate is reported to be 17% in patients with encephalitis.
Recovery of the virus from cerebrospinal fluid (CSF) is seldom successful. Most diagnoses are based on serological tests; cross-reactivity with other flaviviruses may be problematic in some patients. There is no available antiviral therapy with demonstrated efficacy. Recombinant interferon-alpha has, however, been demonstrated to have some activity in a murine model of infection.4
The spraying undertaken in New York is an attempt to interrupt the mosquito-bird amplification cycle of the virus by reducing the adult mosquito population. Large- scale application of larvicidals can be subsequently considered. Surrounding communities are at risk since birds, some infected, travel.
According to The New York Times,5 these areas may, however, be at lesser risk than New York City: "Unlike suburban areas surrounding the city in Connecticut, New Jersey and Long Island, New York does not routinely spray to kill mosquitoes, and does not set traps to keep track of local mosquito populations, leaving city officials without crucial information about viral patterns. When the encephalitis was confirmed last week, the city had to call in experts from Long Island and even borrow malathion, the pesticide sprayed over the weekend, from Suffolk County’s Department of Vector Control, the agency that battles disease-spreading pests." New York City’s mosquito-control efforts had a budget of $120,000 last year, with two full-time employees and two more available in the summer. In comparison, "Los Angeles County, whose population is 9.2 million compared with New York City’s 7.4 million, has a vector-control program with a $6 million budget and about 100 employees." Thus, this outbreak is another example of the consequence of governmental neglect of preventive health services.
References
1. ProMED-mail Archives. http://www.healthnet.org/ programs/promed-hma/9909/msg00111.html.
2. Monath TP, Tsai TF. Flaviviruses. In: Richman DD, Whitley RJ, Hayden FG, eds. Clinical Virology. New York, NY: Churchill Livingstone; 1997:1139-1146.
3. Luby JP, et al. The epidemiology of St. Louis encephalitis in Houston, Texas, 1964. Am J Epidemiol 1967;86:584-597.
4. Brooks TJ, Phillpotts RJ. Interferon-alpha protects mice against lethal infection with St. Louis encephalitis virus delivered by the aerosol and subcutaneous routes. Antiviral Res 1999;41:57-64.
5. Wilgoren J. New York Mosquito Control is Weak and Late, Experts Say. The New York Times. September 8, 1999; Metropolitan desk, section B:A1.
Which of the following regarding St. Louis Encephalitis (SLE) is correct?
a. SLE is a flavivirus.
b. SLE is transmitted by ticks.
c. SLE is more severe in children than in the elderly.
d. SLE involves pigs as an intermediate host.
The mosquito Culex pipiens pipiens:
a. is believed to be involved in the recent New York City outbreak of SLE.
b. is found near domestic habitats.
c. transmits the SLE virus after amplification in passerine birds.
d. All of the above
New York City routinely sprays to kill mosquitoes and sets traps to keep track of local mosquito populations.
a. True
b. False
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