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An outbreak of dengue type 2 infection occurred in the Pacific island Kingdom of Tonga in 1974 and an outbreak of dengue type 1 occurred there in 1975. The 1974 outbreak was characterized by relatively mild clinical disease with few hemorrhagic manifestations, a low attack rate, and relatively low viremia levels. The 1975 outbreak was characterized by relatively severe disease with frequent hemorrhagic manifestations and a high attack rate. The differences between the outbreaks could not be attributed to differences in abudance of, or susceptibility to infection of, mosquito vectors or to the prior immune status or other characteristics of the human population. It appeared that a difference in viral virulence was the most likely explanation.

Patients with confirmed dengue infection were studied in an attempt to elucidate the cause of the eruption of dengue fever. Punch biopsies of lesional skin were obtained and processed for the presence of viral antigen and immune globulins by the direct fluorescent antibody test. Another biopsy specimen and the acute sera were processed for virus isolation. Although virus was isolated from the serum of two patients, isolation attempts from the biopsy specimens of all four patients were negative. Fluorescent antibody studies revealed no evidence of either viral antigen or immune globulins in lesional skin. Although limited, the data suggest that the maculopapular rash of dengue fever may be caused by some mechanism other than direct viral infection of the skin or involvement of immune globulins.

Thirty-four strains of Asian and Pacific mosquitoes belonging to 22 species of 7 genera were compared for oral and/or parenteral susceptibility to infection with 1 or more strains of each of the 4 dengue serotypes. Surprisingly, several species of common man-biting Aedes were much more susceptible to oral infection with each of the 4 dengue serotypes than was Aedes aegypti. These species included Aedes albopictus and members of the scutellaris group of the subgenus Stegomyia found on South Pacific islands. Mosquito strains and species relatively susceptible to 1 dengue serotype usually were relatively susceptible to the others also. Almost all species of Aedes tested were uniformly susceptible to parenteral infection with the dengue viruses but, with the exception of a species of Tripteroides, species of all other genera were comparatively resistant to that mode of infection. Dengue viruses usually replicated to about the same extent in orally-infected mosquitoes as they did in parenterally-infected specimens of the same species. Seventeen species of mosquitoes of 7 genera also were tested for parenteral susceptibility to infection with Japanese encephalitis virus. With the possible exception of 2 species of Anopheles, the virus replicated to about the same degree in all species tested and achieved levels considerably higher than did any of the dengue viruses in the same mosquito strain and species held under the same conditions.

A large urban population of Aedes aegypti in Jakarta, Indonesia was studied for one year to determine whether it was homogeneous in terms of susceptibility to dengue viruses and whether seasonal changes in susceptibility to dengue viruses occurred. Mosquitoes from several districts in Jakarta showed a low but homogeneous susceptibility to dengue 2 virus from November 1975 to April 1976. In June 1976, increased susceptibility to dengue 2 virus was observed among some of the subpopulations of Ae. aegypti, and higher infection rates and increased variation were observed among these mosquitoes during the rest of the study period. Correlation with confirmed DHF cases in Jakarta was discussed.

Prior laboratory studies and field observations suggested that it might be possible to reduce the size of the population of, or eliminate, Aedes polynesiensis by the introduction of Aedes albopictus. The former mosquito is the principal vector of nonperiodic filariasis caused by Wuchereria bancrofti and the latter is a closely related species refractory to the development of human filariae. The practicability of such competitive displacement was studied by a field trial on a remote coral atoll where there was an established population of A. polynesiensis. Three strains of Al albopictus were liberated at separate localities on the atoll and their fate was followed for 4 years. One strain disappeared within 12 months after release and the other two disappeared within 48 months. It was not clear whether A. albopicuts failed to become established because the strains were unsuitable, the general environment was inappropriate, or A. polynesiensis was present in such numbers that A. albopictus rarely succeded in mating with its own species.

Formulations of temephos (Abate) and Bacillus thuringiensis var. israelensis (B.t.i.) on corncob and dried coconut husk carriers were tested for slow-release insecticide properties against Aedes aegypti larvae in Puerto Rico. Granular formulations of 5% and 10% temephos gave continuous larval control in used automobile tires for 27 to 124 days and 34 to 162 days, respectively. Temephos on radial sections of corncob and coconut husk chips gave good larval control in tires for 27 to 63 and 61 to 134 days, respectively, depending on the size of the carrier. Small (2-3 g) and large (3-5 g) coconut husk chips tested in 167 liter drums provided continuous control for 55 to 105 days, respectively. Granular formulations of B.t.i. controlled Ae. aegypti in tires for 19 to 33 days, and B.t.i. briquets exhibited larvicidal activity in large containers for 26 to 78 days.