Heterogeneous feeding patterns of the dengue vector, Aedes aegypti, on individual human hosts in rural Thailand.

BACKGROUND: Mosquito biting frequency and how bites are distributed among different people can have significant epidemiologic effects. An improved understanding of mosquito vector-human interactions would refine knowledge of the entomological processes supporting pathogen transmission and could reveal targets for minimizing risk and breaking pathogen transmission cycles. METHODOLOGY AND PRINCIPAL FINDINGS: We used human DNA blood meal profiling of the dengue virus (DENV) vector, Aedes aegypti, to quantify its contact with human hosts and to infer epidemiologic implications of its blood feeding behavior. We determined the number of different people bitten, biting frequency by host age, size, mosquito age, and the number of times each person was bitten. Of 3,677 engorged mosquitoes collected and 1,186 complete DNA profiles, only 420 meals matched people from the study area, indicating that Ae. aegypti feed on people moving transiently through communities to conduct daily business. 10-13% of engorged mosquitoes fed on more than one person. No biting rate differences were detected between high- and low-dengue transmission seasons. We estimate that 43-46% of engorged mosquitoes bit more than one person within each gonotrophic cycle. Most multiple meals were from residents of the mosquito collection house or neighbors. People ≤ 25 years old were bitten less often than older people. Some hosts were fed on frequently, with three hosts bitten nine times. Interaction networks for mosquitoes and humans revealed biologically significant blood feeding hotspots, including community marketplaces. CONCLUSION AND SIGNIFICANCE: High multiple-feeding rates and feeding on community visitors are likely important features in the efficient transmission and rapid spread of DENV. These results help explain why reducing vector populations alone is difficult for dengue prevention and support the argument for additional studies of mosquito feeding behavior, which when integrated with a greater understanding of human behavior will refine estimates of risk and strategies for dengue control.

Age-dependent survival of the dengue vector Aedes aegypti (Diptera: Culicidae) demonstrated by simultaneous release-recapture of different age cohorts.

The assumption that mosquito survival remains constant and that it is independent of age was tested with free-ranging Aedes aegypti (L.) (Diptera: Culicidae) by using a "synthetic cohort" mark-release-recapture method. Mosquito age cohorts (1-2, 3-4, and 19-20 d) were released simultaneously in homes in rural Thai villages during dry and rainy seasons (2002). Significant age-dependent effects were detected during the dry and rainy seasons. More young than older mosquitoes were recaptured (1-4 versus 5-20 d). The best predictive fit for recaptures was obtained using a quadratic function of age. Our results provide the first field-based evidence for age-dependent Ae. aegypti mortality in which the death rate increases with advancing age and highlight the need for research on ecological and epidemiological aspects of this process.

Transmission potential of the human head louse, Pediculus capitis (Anoplura: Pediculidae).

BACKGROUND: Millions of people are infested by head lice every year. However, louse transfer between hosts is not well-understood. Our goals were to determine: (1) which stages were most likely to disperse and why, (2) the likelihood of fomites transmission, and (3) if host blood gender affects louse development. METHODS: Various life stages of lice at differing densities were permitted to cross over a 15-cm hair bridge placed between two artificial blood-feeding arenas. Louse transfer caused by hot air movements, combing, toweling, and passive transfer to fabric was investigated. The ability of lice to oviposit on different foreign substrates and the hatching potential of eggs intermittently incubated for 8 h/night on a host were likewise investigated. Louse in vitro development following feeding on human female or male donor blood was compared. RESULTS: Adult lice were the most likely to disperse. Neither population density nor hunger significantly affected dispersal tendencies. Lice were dislodged by air movement, combs and towels, and passively transferred to fabric within 5 min. Females oviposited on a variety of substrates and 59% of eggs incubated for 8 h/night hatched after 14-16 days. There was no survivorship difference between lice artificially fed on female vs. male blood. CONCLUSIONS: Adult lice are the most mobile, indicating that they are most likely to initiate new infestations. Although head-to-head contact may be the primary route of transmission, less direct routes involving fomites may play a role and need further evaluation. Blood-borne factors do not appear to cause any gender-biased host preference.

Dispersal of the dengue vector Aedes aegypti within and between rural communities.

Knowledge of mosquito dispersal is critical for vector-borne disease control and prevention strategies and for understanding population structure and pathogen dissemination. We determined Aedes aegypti flight range and dispersal patterns from 21 mark-release-recapture experiments conducted over 11 years (1991-2002) in Puerto Rico and Thailand. Dispersal was compared by release location, sex, age, season, and village. For all experiments, the majority of mosquitoes were collected from their release house or adjacent house. Inter-village movement was detected rarely, with a few mosquitoes moving a maximum of 512 meters from one Thai village to the next. Average dispersal distances were similar for males and females and females released indoors versus outdoors. The movement of Ae. aegypti was not influenced by season or age, but differed by village. Results demonstrate that adult Ae. aegypti disperse relatively short distances, suggesting that people rather than mosquitoes are the primary mode of dengue virus dissemination within and among communities.

West Nile virus in California.

West Nile virus (WNV) was first isolated in California during July 2003 from a pool of Culex tarsalis collected near El Centro, Imperial County. WNV transmission then increased and spread in Imperial and Coachella Valleys, where it was tracked by isolation from pools of Cx. tarsalis, seroconversions in sentinel chickens, and seroprevalence in free-ranging birds. WNV then dispersed to the city of Riverside, Riverside County, and to the Whittier Dam area of Los Angeles County, where it was detected in dead birds and pools of Cx. pipiens quinquefasciatus. By October, WNV was detected in dead birds collected from riparian corridors in Los Angeles, west to Long Beach, and through inland valleys south from Riverside to San Diego County. WNV was reported concurrently from Arizona in mid-August and from Baja, Mexico, in mid-November. Possible mechanisms for virus introduction, amplification, and dispersal are discussed.

Relative abundance of two cuticular hydrocarbons indicates whether a mosquito is old enough to transmit malaria parasites.

Temporal changes in the cuticular hydrocarbons of female Anophelesstephensi (Liston) (Diptera: Culicidae) were quantified using gas-liquid chromatography with flame-ionization detection. The ratio of two prominent hydrocarbons, nonacosane (C29) and hentriacontane (C31), was found to change significantly with respect to mosquito age over a period of 15 d. A regression model was developed using this ratio, C29/C31 = 3.96 - 1.63 log (age), and prediction intervals, based on a 12-d developmental interval necessary for females to transmit malaria, were generated using confidence levels for one-sided tests. The model predicted that females that had a C29/C31 ratio of 2.6 or greater were only 10% probable to be old enough to transmit malaria, whereas females with ratios of 1.8 or less were 90% probable.

Home remedies to control head lice: assessment of home remedies to control the human head louse, Pediculus humanus capitis (Anoplura: Pediculidae).

As the frequency and level of pediculicide resistance increases throughout the world, the need for novel solutions to control pediculosis has intensified. The development and registration of new pesticides has become so costly that many chemical companies are unwilling to pursue it and health-care providers now face a serious lack of new commercial pediculicides. Many infested people resort to using "home-remedy" approaches that have not been scientifically tested. In this article, we examined the potential value of six purportedly effective "home remedies" (vinegar, isopropyl alcohol, olive oil, mayonnaise, melted butter, and petroleum jelly) to treat head louse infestations and the likelihood of drowning lice by water submersion. Results indicated that only the application of petroleum jelly caused significant louse mortality but no treatment prevented lice from laying eggs. Most home remedy products did little to kill eggs, despite prolonged exposure. Petroleum jelly caused the greatest egg mortality, allowing only 6% to hatch. It was extremely difficult to drown lice, despite extended periods (i.e., 8 hr) of water submersion, suggesting that killing lice by depriving them of oxygen is inefficient. None of the home remedy products we surveyed was an effective means of louse control. This suggests that when treatment failure occurs, an increased amount of time and effort should be focused on alternative chemical pediculicides and/or manual louse removal (i.e., combing) rather than using any of these products.

Identification of the people from whom engorged Aedes aegypti took blood meals in Florida, Puerto Rico, using polymerase chain reaction-based DNA profiling.

We used polymerase chain reaction-based DNA profiling to construct allelic profiles for residents and visitors of 22 houses in Florida, Puerto Rico, and human DNA from blood meals in Aedes aegypti that were collected in those homes. Complete profiles were obtained for < or = 2 days after blood ingestion. Eighteen percent of the meals came from two different people. There was no evidence of meals from > or = 2 people. Eighty percent of the meal sources were identified, > 70% were taken from residents of the collection house, and > 90% were from residents of the study community. Across the community, feeding was non-random with a bias towards young adults and males. Three people accounted for 56% of the meals. Our results confirm that multiple feeding on different people is an important component in the role of Ae. aegypti in dengue virus transmission and help explain the spatial distribution of dengue cases in a previous epidemic in Florida, Puerto Rico.

Estimation and comparison of mosquito survival rates with release-recapture-removal data.

Methods for the estimation and comparison of survival rates are considered when data arises from a release of individuals followed by a sequence of recaptures, with recaptured individuals removed from the population. It is shown that commonly used methods based on linear regression of the log of recapture numbers versus time can lead to substantial errors if individuals are removed from the population. A general nonlinear regression approach is proposed combined with bootstrap techniques for obtaining confidence intervals and tests of hypotheses. Simulations demonstrate that these techniques perform well using data from an Aedes aegypit L. mark-release-recapture study in Thailand.

Lack of manipulation of Rhodnius prolixus (Hemiptera: Reduviidae) vector competence by Trypanosoma cruzi.

Behavioral implications of Trypanosoma cruzi Chagas infection in Rhodnius prolixus Stål were observed. Feeding and defecation behaviors of infected versus uninfected insects were assessed on an artificial membrane-feeding system and on live guinea pigs. Based on a defecation index, fifth instars were the most efficient vectors, followed by adult females, fourth instars, and adult males. Bugs fasted for longer periods (5-6 mo) took smaller blood meals but defecated significantly earlier than bugs fasted for shorter periods (2-3 mo). Multiple blood feeding, degree of fasting, life stage, T cruzi infection, and gender affected the vector potential of R. prolixus. Our data indicate that T. cruzi and R. prolixus have not coevolved to facilitate the transmission of T. cruzi, which suggests that this parasite-host relationship may be relatively recent.

Parasite polymorphism may serve to enhance fitness in different host environments.

Phenotypic expression of trans-sialidase (TS) by Trypanosoma cruzi (Silvio strain) is restricted to a subpopulation (20-30%) of broad trypomastigotes (TS+ parasites), while the larger subpopulation of slender trypomastigotes lacks TS expression (TS- parasites). Rhodnius prolixus nymphs were infected with fractionated T. cruzi parasite populations (Silvio strain). Bugs ingesting TS- parasites produced significantly higher parasite loads than bugs ingesting either TS+ parasites or unfractionated populations. The addition of 1.0 microg of TS monoclonal antibody (TCN-2)/ml of blood meal to TS+ populations increased parasite populations to levels comparable to those of TS- populations. In contrast, the addition of exogenous TS to TS- parasite blood meals significantly reduced parasite loads to levels comparable to bugs ingesting TS+ parasites. These results suggest that T. cruzi trypomastigote polymorphism may serve to enhance survival in different host environments.

Further evaluation of Indoor resting boxes for aedes aegypti surveillance.

Several types, locations and intervals of artificial resting boxes were evaluated for their ability to attract Aedes aegypti males and females resting in houses in Thailand. Overall, 34% of both male and female Ae. aegypti captured resting inside houses were collected from the resting boxes. Boxes with black cloth strips covering the entrance attracted the same number of males and females as boxes without strips. There was no significant difference between the mean number of females collected in small resting boxes, with an inside surface area of about one-fifth the surface area of the larger boxes, and the larger boxes. However, significantly, fewer males were collected in the smaller box. The length of time during which the box was placed in the house prior to sampling, either a 15-17-hour interval from evening to the next morning or a 3-4- hour interval in the morning, had no effect on the number of mosquitoes collected. The position where the box was located in the house had more effect on the number of females resting in the box than it did on the number of males. Boxes placed in the dark corners of the house attracted more resting females than those placed in the lighted, open areas, usually in the middle of the house; however, the differences were not significant. The findings further confirm that resting boxes are practical for use in routine sampling of Ae. aegypti inside houses. Small boxes can be transported conveniently and are as efficient for collecting females as larger boxes. Boxes can be placed in lighted or dark areas of a house for as little as 3-4 hours to sample the Ae. aegypti house population.