Laboratory diagnosis of yellow fever infections in humans, alouatta monkeys and haemagogus mosquitoes in Trinidad, 1978-1980

The yellow fever epidemic in Trinidad began with an epizootic in alouatta monkeys in November, 1978. Activity was detected by virus isolation from Alouatta monkeys and from Haemagogus mosquitoes between this date and July 1980. First reports of monkey deaths centered around the Guayaguayare forests of South Trinidad, following which the epizootic spread west to the Moruga area, and north through the Biche Forest reserve, eventually reaching the north-west Chaguaramas peninsula. Surveillance of febrile persons attending clinics and hospitals was intensified and 18 confirmed cases were identified, from 14 of whom yellow fever virus was isolated. In four cases, diagnoses were made on the basis of liver pathology only. Paired sera were obtained from 10 of 11 persons who survived, and in each case serological conversions were obtained by haemagglutination inhibition and mouse neutralization tests. Yellow Fever virus was isolated from 16 of 32 Alouatta monkeys and from 19 of 174 pools of Haemagogus mosquitoes. Two systems were used for virus isolation: the Aedes cell line AP-61 and suckling mice. The former proved to be more sensitive, detecting virus in 11.7 per cent of 725 specimens inoculated whereas suckling mice detected virus in 7.9 per cent of 724 specimens. In 11 instances virus was detected in an animal or mosquito pool by AP-61 but not initially by mouse inoculation whereas the converse was true in only one case.

A clinico-pathological study of yellow fever with demonstration of the virus in the human liver

The clinical histopathological and serological findings in three patients who developed yellow fever after entering the Guayaguayare forest in south-east Trinidad in January and February 1979 are described in this report. The patients were all previously healthy young males and the clinical features of the disease varied from fulminant viral hepatitis with hepato-renal failure to a self-limiting anicteric viral illness. The first patient died, and histopathological examination of the liver was done post-mortem. Needle biopsy of the liver was done on case 2 which presented with the clinical picture of infective hepatitis, and on case 3 with the features of an anicteric viral illness. The histopathological hallmarks of midzonal necrosis, granular eosinophilic degeneration (Councilman bodies) and fatty change in the parenchymal cells of the liver were seen in three cases. The electron microscopic study demonstrated the presence of the yellow fever virus in the liver of the patient who died.

Entomological activities during the yellow fever epidemic in Trinidad, 1978-1980

An epidemic of sylvan yellow fever occurred in Trinidad during the period 1978 to 1980. Entomological activities were initiated at various localities where monkey (Alouatta spp.) mortalities were reported. A total of 5,468 Haemagogus janthinomys was collected and tested which resulted in the isolation of 17 strains of yellow fever virus. Two strains of the virus were isolated from a total of 49 Haemagogus leucocelaenus and these constituted the first isolations of yellow fever virus from this species of mosquito from Trinidad. Attempts to demonstrate transovarial transmission of yellow fever virus from adults feared from eggs and larvae collected in the field were unsuccessful. However, from two locations virus isolations were made at the end of rainy season (January) and at the beginning of the successive rainy season (June).

Studies on yellow fever in inter-epizootic period in Trinidad

Following an outbreak of yellow fever in Trinidad in 1978-1979, a study was undertaken to investigate the jungle reservoirs and vectors of yellow fever virus in the inter-epizootic period, to seek evidence of natural transovarial transmission of yellow fever virus in Haemagogus collected. These were made four months after the last detection of virus in that area of the island and could not positively be attributed to transovarial transmission. None of the 42,815 progeny of wild mosquitoes yielded virus. No yellow fever virus was isolated from 34,300 mosquitoes other than Haemagogus nor from 15 vertebrates collected in areas of former virus activity. Good progress was made in optimizing the use of the AP-61 cell line resulting in rapid isolation and identification of dengue and yellow fever viruses from Caribbean countries.

The 1988-1990 epizootic of yellow fever in Trinidad: virological aspects

Sylvatic yellow fever virus activity occurred in Trinidad between December, 1988 and January, 1990. It was first detected in the Trinity Hills area of Guayaguayare in the south eastern forest of the island and subsequently spread west to Moruga and north to Fishing Pond. Surveillance activities in these areas resulted in the collection of over 18,000 mosquitoes and seven monkeys which were referred to the CAREC laboratories for viral investigation. Six pools of mosquitoes yielded yellow fever virus in December, 1988 and January, 1989. Isolations were made from Haemagogus janthinomys (five pools) and Sabethes chloropterus (one pool). Extensive collections in other areas failed to yield virus. Three Alouatta monkeys proved to be infected with yellow fever. These were all Alouatta sp. and were submitted to the laboratory in February and March of 1989 and January of 1990. Thorough virological investigation of febrile persons, with and without liver involvement, showed no evidence to spread to the human population either before or during the epizootic. This was attributed to the high level of immunization achieved in the population following the yellow fever outbreak of 1978-1980.

Sylvatic yellow fever activity in Trinidad 1988-1989

Of a total of 18,068 mosquitoes (361 pools) collected in south-eastern Trinidad forests from December, 1988 to May, 1989, 47 species belonging to 14 genera were identified. Five yellow fever virus isolates were made from Haemagogus janthinomys and one from Sabethes chloropterus. All the other pools of mosquitoes examined were negative for the virus. The mosquito isolates were made in December and January. In addition, in late February and early March, two infected howler monkeys (Alouatta sp.) were detected. Since March, despite continued surveillance, no yellow fever virus has been detected in mosquitoes or monkeys. There has been no reported human infection.

Yellow fever studies in Trinidad, 1988-1989: attempts to demonstrate Transovarial Transmission of yellow fever virus in Haemagogus mosquitoes collected in nature

Subsequent to an epizootic of yellow fever (YF) in monkeys and the isolation of yellow fever (YF) virus from sylvatic mosquitoes in December, 1988 and January, 1989 in south-east Trinidad, ovitrapping for mosquito eggs was started in January, 1989, in an attempt to demonstrate transovarial transmission of the virus. Multi-paddle ovitraps were used. From a total of 52,632 eggs collected in various parts of the country, the following species of mosquitoes were reared: Haemagogus janthinomys (54 per cent ), Hg. leucocelaenus (32 per cent ), Aedes insolitus (12 per cent ), Hg. equinus (1 per cent ). When mosquito eggs were removed from the hard board paddles then vacuum-treated for hatching, the process proved very economical in storage space. Most eggs were obtained at the start of the wet season, four to five months after the isolation yellow fever virus from wild caught adult mosquitoes and monkey carcasses. When adult mosquitoes reared from eggs were processed for yellow fever virus, no virus was detected from any of the mosquito pools to indicate that transovarial transmission had occurred.