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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
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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