Dengue emergence and adaptation to peridomestic mosquitoes

Emerging Infectious Diseases,  Oct, 2004  by Abelardo C. Moncayo,  Zoraida Fernandez,  Diana Ortiz,  Mawlouth Diallo,  Amadou Sall,  Sammie Hartman,  C. Todd Davis,  Lark Coffey,  Christian C. Mathiot,  Robert B. Tesh,  Scott C. Weaver

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Vector Susceptibility

Artificial blood meals consisting of 1% sucrose, 20% fetal bovine serum, 5 mmol ATE 33% PBS-washed sheep blood cells, and 33% MEM were used for mosquito susceptibility determinations. Multiple cohorts of 30 to 50 mosquitoes were offered blood meals incubated at 37[degrees]C in a water-jacketed membrane feeder (23). After 1 h of feeding, engorged mosquitoes were sorted from unengorged ones, and a sample of the blood meal was assayed to determine the virus titer. Fully engorged mosquitoes were incubated for 14 days at 27[degrees]C with a 12:12 light-dark cycle. Then, legs were detached from cold-anesthetized mosquitoes and assayed to determine the dissemination rate of the virus from the midgut into the hemocoel (mosquito legs include hemolymph, which is believed to mediate infection of the salivary glands). Bodies were assayed to determine the overall infection rate. Blood meal titers were determined by IFAs on C6/36 cells (see above) of samples collected immediately after mosquito feeding.

Because mosquito infections caused by artificial blood meals are inefficient compared to those using viremic hosts, we used the highest virus titers available (6.5-10.0 [log.sub.10] TCI[D.sub.50]/mL) from cell culture passages for our experiments. To interpret our data as conservatively as possible, infection and dissemination rates for endemic strains were only compared with rates from the same or higher blood meal titers for sylvatic strains. Infection and dissemination differences were tested for significance with chi-square and Fisher exact tests with the SPSS (Chicago, IL) Base 11.5 statistical package. When no significant differences were observed among endemic or sylvatic strains tested, data were pooled for each group.

Results

Aedes aegypti Susceptibility

After ingestion of artificial blood meals containing 6.5-8.0 [log.sub.10] TCI[D.sub.50]/mL of endemic DENV-2, infection and dissemination rates in Ae. aegypti mosquitoes from Galveston, Texas, were 86.5%-100.0% and 90.6%-78.9%, respectively (Table 2). Infection and dissemination rates after exposure to sylvatic viruses were more variable but lower, ranging from 11.4% to 69.4% and from 0% to 64.4% after ingestion of 8.0 to 10.0 [log.sub.10] TCI[D.sub.50]/mL of DENV. Even after the (lowest) infection rate data for strain 1407 were removed from the pooled analysis because they were significantly different than those for the other sylvatic strains, both infection (p < 0.0001) and dissemination (p = 0.01) rates were different between endemic and sylvatic strains (Table 2).

Ae. aegypti from Santa Cruz, Bolivia, had lower infection and dissemination rates than Galveston populations after being exposed to endemic DENV strains. Infection rates were 40.9%-48.1% and dissemination rates were 76.9%-77.8% with blood meal titers of 9.5 [log.sub.10] TCI[D.sub.50/mL (Table 3). Infection and dissemination rates in Bolivian mosquitoes exposed to sylvatic viruses were also lower than those of Ae. aegypti from Galveston, and sylvanic strain infection rates in Bolivian mosquitoes were lower (p = 0.015), ranging from 16.7% to 27.3%, than those of endemic strains; dissemination rates were not significantly different (p = 0.663).