Gary G. Clark

Dengue and dengue haemorrhagic fever

Summary The incidence and geographical distribution of dengue have greatly increased in recent years. Dengue is an acute mosquito-transmitted viral disease characterised by fever, headache, muscle and joint pains, rash, nausea, and vomiting. Some infections result in dengue haemorrhagic fever (DHF), a syndrome that in its most severe form can threaten the patients life, primarily through increased vascular permeability and shock. The case fatality rate in patients with dengue shock syndrome can be as high as 44%. For decades, two distinct hypotheses to explain the mechanism of DHF have been debated—secondary infection or viral virulence. However, a combination of both now seems to be the plausible explanation. The geographical expansion of DHF presents the need for well-documented clinical, epidemiological, and virological descriptions of the syndrome in the Americas. Biological and social research are essential to develop effective mosquito control, medications to reduce capillary leakage, and a safe tetravalent vaccine.

Longitudinal studies of Aedes aegypti (Diptera : Culicidae) in Thailand and Puerto Rico : Blood feeding frequency

Abstract We used a histologic technique to study multiple blood feeding in a single gonotrophic cycle by engorged Aedes aegypti (L.) that were collected weekly for 2 yr from houses in a rural village in Thailand (n = 1,891) and a residential section of San Juan, Puerto Rico (n = 1,675). Overall, mosquitoes from Thailand contained significantly more multiple meals (n = 1,300, 42% double meals, 5% triple meals) than mosquitoes collected in Puerto Rico (n = 1,156, 32% double meals, 2% triple meals). The portion of specimens for which frequency of feeding could not be determined was 31% at both sites. We estimated that on average Ae. aegypti take 0.76 and 0.63 human blood meals per day in Thailand and Puerto Rico, respectively. However, frequency of multiple feeding varied among houses and, in Puerto Rico, the neighborhoods from which mosquitoes were collected. In Thailand 65% of the mosquitoes fed twice on the same day, whereas in Puerto Rico 57% took multiple meals separated by ≥ 1 d. At both sites, the majority of engorged specimens were collected inside houses (Thailand 86%, Puerto Rico 95%). The number of blood meals detected was independent of where mosquitoes were collected (inside versus outside of the house) at both sites and the time of day collections were made in Puerto Rico. Feeding rates were slightly higher for mosquitoes collected in the afternoon in Thailand. Temperatures were significantly higher and mosquitoes significantly smaller in Thailand than in Puerto Rico. At both sites female size was negatively associated with temperature. Rates of multiple feeding were associated positively with temperature and negatively with mosquito size in Thailand, but not in Puerto Rico. Multiple feeding during a single gonotrophic cycle is a regular part of Ae. aegypti biology, can vary geographically and under different climate conditions, and may be associated with variation in patterns of dengue virus transmission.

Texas Lifestyle Limits Transmission of Dengue Virus

Urban dengue is common in most countries of the Americas, but has been rare in the United States for more than half a century. In 1999 we investigated an outbreak of the disease that affected Nuevo Laredo, Tamaulipas, Mexico, and Laredo, Texas, United States, contiguous cities that straddle the international border. The incidence of recent cases, indicated by immunoglobulin M antibody serosurvey, was higher in Nuevo Laredo, although the vector, Aedes aegypti, was more abundant in Laredo. Environmental factors that affect contact with mosquitoes, such as air-conditioning and human behavior, appear to account for this paradox. We conclude that the low prevalence of dengue in the United States is primarily due to economic, rather than climatic, factors.

Sterile-Insect Methods for Control of Mosquito-Borne Diseases: An Analysis

Effective vector control, and more specifically mosquito control, is a complex and difficult problem, as illustrated by the continuing prevalence (and spread) of mosquito-transmitted diseases. The sterile insect technique and similar methods control certain agricultural insect pest populations in a species-specific, environmentally sound, and effective manner; there is increased interest in applying this approach to vector control. Such an approach, like all others in use and development, is not a one-size-fits-all solution, and will be more appropriate in some situations than others. In addition, the proposed release of pest insects, and more so genetically modified pest insects, is bound to raise questions in the general public and the scientific community as to such a methods efficacy, safety, and sustainability. This article attempts to address these concerns and indicate where sterile-insect methods are likely to be useful for vector control.

West Nile virus activity in Latin America and the Caribbean.

OBJECTIVES West Nile virus (Flavivirus: Flaviviridae; WNV) has spread rapidly throughout the Caribbean Basin since its initial detection there in 2001. This report summarizes our current knowledge of WNV transmission in tropical America. METHODS We reviewed the published literature and consulted with key public health officials to obtain unpublished data. RESULTS West Nile virus infections first appeared in human residents of the Cayman Islands and the Florida Keys in 2001, and in apparently healthy Jamaican birds sampled early in 2002. Serologic evidence of WNV infection in 2002 was detected in horses, chickens and resident free-ranging birds in Guadeloupe, the Dominican Republic, and eastern Mexico. In 2003, WNV spread in Mexico and northern Central America, and serologic evidence was detected in the Bahamas, Puerto Rico and Cuba. In 2004, the first serologic evidence of WNV activity in South American ecosystems surfaced in September-October in Colombia and Trinidad, where domestic animals circulated WNV-neutralizing antibodies. CONCLUSIONS The sparse reports of equine, human and avian disease in Latin America and the Caribbean is puzzling. Isolates are needed to evaluate viral attenuation or other possible explanations for reduced disease burden in tropical ecosystems.

Community involvement in the control of Aedes aegypti

In the past 15 years, there has been a dramatic resurgence of dengue and dengue hemorrhagic fever worldwide, with increased frequency of epidemics and geographic expansion of both the mosquito vectors and the viruses. The reasons for this resurgence are not well understood, but include uncontrolled and unplanned urbanization, increased movement of people and viruses by airplane and lack of effective control of Aedes aegypti, the principal mosquito vector of dengue viruses. The recommended method for Ae. aegypti control during the past 20 years has been ultra-low volume (ULV) application of insecticides, a method which targets the adult mosquito. Lack of efficacy of the ULV approach led to a reevaluation of recommended strategies for prevention and control of epidemic dengue and ultimately, resulted in development and widespread use of community-based, integrated approaches to Ae. aegypti control. This chapter reviews the use of community participation for controlling Ae. aegypti via larval source reduction and critically discusses programs in four countries from the standpoint of effectiveness and sustainability. It is concluded that a combination of vertically structured centralized and community-based approaches should provide short-term success as well as long-term sustainability.

Ecological factors influencing Aedes aegypti (Diptera: Culicidae) productivity in artificial containers in Salinas, Puerto Rico.

Abstract We investigated the effects of environmental factors and immature density on the productivity of Aedes aegypti (L.) and explored the hypothesis that immature populations were under nutritional stress. In total, 1,367 containers with water in 624 premises were studied in Salinas, southern Puerto Rico (May–July 2004). We counted 3,632 pupae, and most female pupae (70%) were in five of 18 types of containers. These containers were unattended and influenced by local yards’ environmental conditions. Pupal productivity was significantly associated with the number of trees per premise, water volume, and lower water temperatures. Larval and pupal abundance were larger in containers with leaf litter or algae. Pupal productivity and biomass of emerging females varied in containers with litter of different tree species. We found a significant and positive association between numbers of larvae and pupae of Ae. aegypti and a negative relationship between larval density and mass of emerging females. From multivariate analyses, we interpreted that 1) food limitation or competition existed in a number of containers; and 2) to a lesser extent, there was lack of negative larval density effects in containers with a larger water volume and lower temperature, where emerging females were not under nutritional stress. Corroborating evidence for food limitation or intraspecific competition effects came from our observations that females emerging in the field had an average body mass comparable with those females produced in the laboratory with the lowest feeding regime. Ae. aegypti larvae in Salinas are most likely influenced by resource limitation or competition and by rainfall in unmanaged containers in the absence of aquatic predators. Source reduction and improved yard management targeting unattended containers would eliminate most Ae. aegypti productivity and removal or control of shaded, larger containers would eliminate the production of the largest emerging mosquito females in the study area.

Analysis of Survival of Young and Old Aedes aegypti (Diptera: Culicidae) from Puerto Rico and Thailand

Abstract It generally is assumed that the daily probability of survival of wild adult mosquitoes is independent of age. To test this assumption we conducted mark-release-recapture studies in Puerto Rico and Thailand to determine if estimated daily survival rates between two different age cohorts of the dengue vector Aedes aegypti (L.) were the same. Survivorship was estimated with nonlinear regression analysis using bootstrapping to obtain estimates of errors. Initial recapture success of the younger cohort was greater than the older cohort at both locations. Our analysis revealed a significantly greater survival rate for the younger cohort of females in Puerto Rico, and no significant differences between age cohorts in Thailand. For comparison, a traditional approach for analyzing these type of data, linear regression of log-transformed captures over time (exponential model), was used to calculate the probability of daily survival based on slopes of linear regression lines for recaptured mosquitoes. With this method, the estimated daily survival rate of older females (13–23 d old) was significantly greater than survival of younger ones (3–13 d old) in Puerto Rico and Thailand. In addition, short-range movement of mosquitoes was observed in Puerto Rico; maximum dispersal distance detected was 79 m. Survival rates of adult Ae. aegypti may be age-dependent and nonlinear regression analysis is a sensitive approach for comparing patterns of mosquito survival based on mark, single release, multiple recapture data.

Susceptibility of Aedes aegypti, Culex quinquefasciatus Say, and Anopheles quadrimaculatus Say to 19 pesticides with different modes of action.

Abstract To access the relative potency of pesticides to control adult mosquitoes, 19 pesticides with various modes of action were evaluated against Aedes aegypti, Culex quinquefasciatus Say, and Anopheles quadrimaculatus Say. On the basis of 24-h LD50 values after topical application, the only pesticide that had higher activity than permethrin was fipronil, with LD50 values lower than permethrin for 107-, 4,849-, and 2-fold against Ae. aegypti, Cx. quinquefasciatus Say, and An. quadrimaculatus Say, respectively. Abamectin, imidacloprid, spinosad, diazinon, and carbaryl showed slightly lower activity than permethrin (<20-fold). However, bifenazate showed very low activity against the three mosquito species tested, with LD50 values higher than permethrin for >1000-fold. On the basis of 24-h LD50 values, Cx. quinquefasciatus was the least susceptible species to nine pesticides tested (DNOC, azocyclotin, chlorfenapyr, carbaryl, spinosad, imidacloprid, diazinon, abamectin, and permethrin), whereas Ae. aegypti was the least susceptible species to six pesticides tested (dicofol, amitraz, propargite, hydramethylnon, cyhexatin, and diafenthiuron), and An. quadrimaculatus was the least susceptible species to four pesticides tested (bifenazate, pyridaben, indoxacarb, and fipronil). Our results revealed that different species of mosquitoes had different susceptibility to pesticides, showing the need to select the most efficacious compounds for the least susceptible mosquito species to achieve successful mosquito control.

Topically Applied AaeIAP1 Double-Stranded RNA Kills Female Adults of Aedes aegypti

Abstract Aedes aegypti (L.) (Diptera: Culicidae) is the primary vector of both dengue and yellow fever. Use of insecticides is one of the primary ways to control this medically important insect pest. However, few new insecticides have been developed for mosquito control in recent years. As a part of our effort to develop new insecticides to control mosquitoes, an inhibitor of apoptosis protein 1 gene in Aedes aegypti (AaeIAP1) was targeted for the development of molecular pesticides. Herein, for the first time, we report that topically applied AaeIAP1 double-stranded RNA products are able to kill female adults of Ae. aegypti. Our results indicate that critical pathways or genes could be targeted to develop molecular pesticides for the control of medically important diseases vectors.