International Notes Yellow Fever Virus Activity -- Trinidad and Tobago

In December 1988, the Ministry of Health, Trinidad and Tobago, and the Caribbean Epidemiology Center (CAREC) received reports of howler monkeys (Alouatta sp.) dying in the Trinity Hills area of the southeastern forests of Trinidad. Surveillance of monkeys in forested areas is a component of the Ministry's ongoing surveillance of sylvatic yellow fever. Field and laboratory investigations did not establish specific evidence of yellow fever in either of two dead monkeys, one decomposing and one freshly shot. However, mosquito surveillance using human landing collections (mosquitoes collected as they landed on humans) yielded two isolates of yellow fever virus from 29 pools of Haemagogus janthinomys collected in the forests between December 8 and January 19. Three additional yellow fever isolates were detected in the same pools.

Surveillance for sick or dying monkeys and of forest mosquitoes was intensified after dying monkeys were reported. Initially, health-care facilities near forested areas were alerted to intensify fever surveillance and to encourage unimmunized persons to be vaccinated; subsequently, however, immunization coverage has been expanded island-wide. In Trinidad and Tobago, although intensified fever surveillance for human cases was implemented when reports of dying monkeys were confirmed, no human cases have been detected. As of February 1, 1989, no yellow fever virus has been isolated from monkeys. A program of local spraying of adulticides, identification and elimination of peridomestic breeding sites for Aedes aegypti, and application of larvicides has been initiated in villages near the forest edge. Although virus activity remains confined to one forested area, the Ministry of Health has made a public statement advising visitors to forested areas to be immunized.

No evidence of virus activity in the island of Tobago exists, and the port areas and airports of both islands are free of Ae. aegypti. Yellow fever virus has never been isolated from Ae. aegypti or mosquitoes in Tobago, and the monkeys and mosquitoes associated with sylvatic yellow fever are not found there.

Through CAREC, national epidemiologists and Pan American Health Organization offices of Caribbean countries, along with the World Health Organization (WHO), have been informed of the situation. Reported by: M Lewis, RD Deen, Acting Chief Medical Officer, Ministry of Health, Trinidad and Tobago. J Hospedales, B Hull, S Rawlins, Caribbean Epidemiology Center, Port of Spain, Trinidad and Tobago. Div of Vector-Borne Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Yellow fever is enzootic in certain sylvatic locations in South America; however, mechanisms underlying the periodic occurrence of epizootic activity in Trinidad are not fully understood--in particular, it is unknown whether the virus is periodically reintroduced from enzootic areas of South America or is maintained in the forest between outbreaks (1,2). Laboratory studies have shown that transovarial transmission of the virus in mosquitoes is possible (2), but in the western hemisphere, the virus has not been demonstrated in mosquitoes collected from sylvatic locations. Collections of ova, larvae, and adult mosquitoes in the affected forest have been planned to investigate the possibility of vertical transmission (2).

Human cases in Trinidad were last confirmed in 1979 when 18 cases were acquired in sylvatic locations. None of these 18 patients were immunized, and no evidence was found of urban spread by Ae. aegypti. That outbreak initially was detected in the Trinity Hills area. Following the outbreak in 1979, 80% of the population greater than 1 year of age was immunized in 1979 and 1980. In addition, an ongoing routine immunization program directed at 1-year-old children was implemented in 1979.

Most persons immunized in 1979 remain protected, and the routine immunization of children born after 1979 has resulted in a 70% level of immunity in children less than 10 years of age. Efforts have been directed toward full immunization coverage of persons who live or work in or near forests where virus activity is occurring. The high level of immunity that exists in the urban population of Trinidad and Tobago reduces the possibility that a sylvan outbreak could develop into an urban epidemic.

CDC recommends yellow fever vaccination for travelers greater than 9 months of age who leave urban areas (3). Since a yellow fever epizootic has not been established, a change in this recommendation for yellow fever vaccine in travelers to Trinidad is not indicated. A yellow fever vaccine certificate is not required for entry into Trinidad and Tobago except for travelers greater than 1 year of age coming from areas designated by WHO as currently infected with yellow fever. References

1. Downs WG. The known and the unknown in yellow fever ecology and epidemiology. Ecol Dis 1982;1:103-10. 2.Aitken THG. Yellow fever: evolution of ideas concerned with demonstrating the natural occurrence of transovarial transmission of virus in mosquitoes. Bull Soc Vector Ecol 1988;13:85-96. 3.CDC. Health information for international travel, 1988. Atlanta: US Department of Health and Human Services, Public Health Service, 1988:57; HHS publication no. (CDC)88-8280.

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