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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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Discussion

Historical Emergence of Dengue and Adaptation to Peridomestic Vectors

Our findings support the hypothesis that endemic DENV-2 strains are more efficient than sylvatic strains at infecting the peridomestic DENV vectors Ae. aegypti and Ae. albopictus. The overall trend that endemic DENV-2 strains were consistently more efficient at infecting peridomestic Aedes mosquitoes than were sylvatic DENV-2 strains (p = 0.000) supports our central hypothesis. Our data and previous phylogenetic studies (11) suggest that the emergence of endemic DENV from sylvatic progenitor strains occurred in conjunction with the peridomestication of Aedes mosquitoes and virus adaptation to these anthropophilic vectors. Although we tested only DENV-2 strains, emergence of DENV serotypes l, 3, and 4 may also have been mediated by vector switching (from infecting sylvatic Aedes mosquitoes to Ae. aegypti and Ae. albopictus). Very few sylvatic DENV-1 and 4 strains are available (and none of DENV-3), which makes evaluating this hypothesis difficult.

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The four independent evolutionary DENV emergence events (DENV-1-4) suggest that adaptation of DENV to new vectors and hosts occurred repeatedly from 300 to 1,500 years ago in Asia or Oceania (11,26). Since Ae. aegypti is not thought to have inhabited these regions at that time, Ae. albopictus was probably the original human vector (12). The widespread importance of Ae. aegypti as a vector may have begun in the 1700s, as commercial and slave trade transported it from its African origin. DENV-2 was probably introduced into Africa from Asia-Oceania approximately 1,000 years ago (11). The hypothesis that Ae. albopictus was the original peridomestic vector was supported by our study; Ae. albopictus was more susceptible to endemic DENV-2 strains than Ae. aegypti. The greater overall susceptibility (regardless of geographic origin) of Ae. albopictus compared to Ae. aegypti (94% and 69%, respectively) suggests a higher degree of adaptation, representing longer historical contact with Ae. albopictus. Other studies with sympatric populations from Brazil show Ae. aegypti to be more susceptible than Ae. albopictus to endemic DENV-2 (19,20).

Risk for Dengue in the United States

When the vectorial capacity of a mosquito for an arbovirus is considered, many factors come into play, including mosquito survivorship, density, proportion of infected mosquitoes that are feeding, extrinsic incubation period, vector susceptibility, and density of susceptible hosts (27). We used vector susceptibility in this study as a measure not only of epidemiologic importance but also of the extent of adaptation of a virus to its vector. However, the full competence of a vector is established not only by its ability to become infected but also by its ability to transmit a pathogen. This feature is what gives vector competence its epidemiologic importance. In our study, transmission potential was estimated from dissemination rates because previous studies have suggested that mosquitoes are capable of transmitting DENV as long as the virus is able to disseminate from the midgut into the hemocoel (i.e., there is no evidence of a salivary gland infection barrier) (18). Mosquitoes that have a disseminated infection were therefore assumed to be capable of transmission.

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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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