1. Biology and behavior of the dengue vector mosquito, Aedes aegypti

Aedes aegypti, the yellow fever mosquito, is also an important vector of four dengue virus serotypes. These viruses cause more human morbidity and mortality than any other arthropod-borne viral disease. Each year over 2.5 billion people are at risk of infection. Recent epidemics have affected millions, and approximately 250,000-500,000 people worldwide suffer the severe consequences of infection. No vaccine is currently available for dengue and there is no cure. Consequently, efforts to reduce morbidity and mortality of dengue infection have focused on the mosquito vector. My research explores a variety of hypothesis related to the blood feeding, mating and ovipositions behavior, of this species as well as aspects of survival and control within the context of vector-borne diseases transmission.
Ae. aegypti is found throughout most tropical countries of the world. It is one of the few examples of a truly domesticated mosquito. It relies on humans to survive, as we provide water filled containers as a primary breeding site, shelter in the form of homes and we also provide it with an ideal blood source. In previous work, we demonstrated a fitness advantage for this mosquito to feed on human blood vs. other types of blood (Fig.1) (Harrington et al. 2001). In further studies in Thailand, we have used DNA fingerprinting techniques borrowed from forensic science to determine what individuals are fed on within a community. Most of us realize that certain people are fed on more than others. Our research supports this conclusion and provides us with information about who is at greatest risk of dengue infection within a community (Harrington et al. in manuscript).

FIG 1. Cumulative Net Replacement Rate (R0) (x-axis) plotted against blood feeding day for Ae. aegypti fed on different host blood diets (from Harrington et al. 2001).

Other work on Ae. aegypti has focused on the survival and dispersal of this species in its natural habitat.This species does not tend to fly far. A large series of mark-release-recapture studies over several years have supported this conclusion (Harrington et al. 2005). Most mosquitoes were recaptured less than 100 m from their release site (Fig 2).
 

FIG 2. Dispersal of Ae. aegypti by age since emergence over all seasons and villages in Thailand February 2000 – July 2001 (from Harrington et al. 2005).

Mosquito survival is one of the most important parameters influencing vectorial capacity. In order to be infectious, mosquitoes must survive the extrinsic incubation of the pathogen from the time they take an infectious blood meal. This time interval can vary depending on the ambient temperature, but with DEN virus it may take 12 or more days. We can easily measure mosquito survival in the laboratory, but in the natural field setting this is a difficult question to answer. We have conducted a variety of studies to address patterns of survival and aging of Ae. aegypti in the field (Harrington et al. 2001, Harrington et al. submitted). One major challenge to our ability to determine survival and mortality patterns of mosquitoes is our lack of methods to determine mosquito age. Recently my colleagues and I have developed age grading methods based on surface cuticular hydrocarbon profiles. We have tested these on field populations in large scale mark-release-recapture studies (Gerarde et al. 2004) and on wild populations (Scott et al. in manuscript).
Past and current collaborators on many of these studies include Dr. Thomas Scott (University of California Davis), Dr. John Edman, now retired from UC Davis, Dr. James Jones, US and Dr. Russell Coleman (ARMY-AFRIMS), Dr. Sangvorn Kitthawee (Mahidol University), and Dr. John Clark (University of Massachusetts, Amherst).

2.Genetic strategies to control dengue viruses

This project is part of a large collaboration bringing together scientists with lab and field expertise.  The purpose of the study is to develop and test genetic-control strategies eliminating the most important vector-borne infections: malaria and dengue. Annually, these previously intractable diseases impact over 520 million lives causing over 3 million deaths and immeasurable suffering in over 200 countries worldwide.

We are conducting studies of mating biology, behavior and overall fitness of Ae. aegypti at Cornell including investigations of male sperm capacity (Fig. 3) and mating behavior.

FIG 3. Ae. aegypti sperm (magnified 200x).  
 
Another major area of is field site establishment and characterization of local vector populations.  For more information on this project and FAQ go to http://stopdengue.hs.uci.edu/.

3. Host feeding preferences and insecticide resistance status of epidemic mosquito vectors of West Nile virus in New York State.

 

 

 

 

 

 

 

Little research has been conducted on human blood feeding patterns of important mosquito vectors of arboviruses in New York State.Using newly designed vegetation aspirators, we conducted a sampling program over 2 years in parks and public areas of New York City and Upstate New York to evaluate human and horse blood feeding of putative West Nile vectors. Analyses were conducted with antibody sandwich ELISA (Fig 4) and amplification of species specific cytochrome b sequences. Analysis of this large scale project with over mosquito blood meals is on-going.
Seroconversion of sentinel chickens has not been useful for predicting West Nile out breaks in the northeast. This suggests that the blood feeding preferences of important epidemic WNV vectors may differ and more work needs to be done on optimizing sentinel traps (Darbro and Harrington 2006).
A critical need for West Nile control programs is information on current insecticide resistance status of mosquito vectors. This information can be used to plan control programs and reduce WNV virus transmission to humans. We investigated resistance status of mosquitoes in two major urban areas of upstate New York (Paul et al. 2005).Our results indicate that more efforts should be made to gather historical baseline data and monitor mosquito resistance through time.
Past and current collaborators include: Dr. Jeff Scott, Dr. Amy Glaser, Dr. Ed Dubovi (Veterinary Diagnostic Laboratory), and Dr. Andre Dhondt (Department of Ecology and Evolutionary Biology and Cornell Laboratory of Ornithology).

 

FIG 4. Antibody-sandwich ELISA showing human blood meals from mosquitoes in blue.

4. Climate effects, West Nile virus vector development, and transmission risk (NOAA/OGP).

Although daily weather and seasonal to inter-annual climatic variability influence mosquito vector biology and risk of vector-borne disease, this information is not readily employed in disease control programs. The reasons for this disconnect between climatic information and vector management are: (1) accurate relationships between climate and infectious disease are most likely dependent upon local scale parameters that have not been related to regional climate data and (2) interaction among climatologists, entomologists, public health and vector control professionals has not been integrated at the level at which information can be developed, validated, and readily incorporated into mosquito management plans. In this study we have gathered the expertise of climatologists, entomologists, social science/risk analysis experts and public health/vector control professionals. We propose to develop a system for predicting and monitoring risk of mosquito vectors, West Nile virus (WNV) transmission, and human health risk that will be readily usable by public health professionals for decision-making. This system will provide a mechanism for early warning of WNV risk and serve as a model for other existing and future vector-borne disease risks for which competent vectors are already present in the United States. These risks include Rift Valley fever, Japanese Encephalitis and Ross River virus. In order to develop, refine, and validate the system we will focus our efforts on New York State with the intent to make the system adaptable to any region. We hypothesize that a few key climate factors influence and drive WNV transmission dynamics and these key factors can be modeled to accurately predict the risk of WNV transmission to people.
The specific aims of the proposed study are to (1) determine the date of spring emergence and generational development rates for epidemiologically important mosquito vector species in New York State, (2) incorporate mosquito development, population abundance, and virus replication/amplification rates for WNV into a climate-based model, (3) determine the types of information needed for decision-making as well as the optimal format and means of delivery that would be most useful to technical staff and decision makers, and (4) develop a final product, to be utilized by public health and vector control professionals, that interfaces a decision support and risk communication program with the climate-forecasting model. This final product will be tested and validated in future years side-by-side with conventional decision making tools by health departments and vector control programs.

By directly addressing and overcoming the reasons for why previous models have failed, the unique group of collaborators assembled for this project will gather the data needed to build realistic, validated, and effective models for predicting vector activity and human health risk.

 

Collaborators on this project include:

Art DeGaetano (Northeast Climate Center, Cornell)

Katherine McComas (Department of Communication, Cornell)

Bryon Backenson, (Arthropod-Borne Disease Program, New York State Department of Health)

Richard Petit ( Vector Control Section, Onondaga County Health Deptment)

Wayne Gall (Arthropod-borne Disease Program, Western Region, NYS Department of Health)

Scott Campbell (Arthropod-Borne Disease Laboratory, Suffolk County Department of Health Services)

Dominick Ninivaggi (Division of Vector Control, Suffolk County Department of PublicWorks)

5.Integrated Pest Management for West Nile Virus Mosquitoes in Peridomestic Settings (USDA/CREES).

Unfortunately, no studies have been conducted to determine the actual benefit of removing mosquito-breeding sites from residential property in New York State (Fig 5). Furthermore, absolutely no information is available on key container types involved in mosquito production. Research has been conducted with tropical container breeding mosquitoes such as Ae. aegypti. Results of these studies have demonstrated the existence of key mosquito producing container types and efforts have been made to focus on eliminating standing water in these sources. Often source reduction efforts must be done on a spatially large scale in order to be effective, but this may depend on the dispersal rates of mosquito species and variation among geographic strains. No reliable information has been published on dispersal rates for potential West Nile virus mosquito vectors in New York State. A recent study of Cx. pipiens resistance in New York (Paul et al. 2005) however, suggests that populations may be more localized than previously thought.

The overall goal of our research is to provide extremely reliable and highly effective information to the public concerning integrated mosquito management and West Nile virus protection. Source reduction of mosquito breeding habitat is commonly recommended to reduce exposure to mosquito vectors, yet no published studies exist that document whether this practice effectively reduces West Nile virus risk to homeowners. Our objectives are to: (1) systematically survey, identify, and characterize artificial mosquito breeding sites in peridomestic areas; (2) identify key vector mosquitoes that utilize artificial containers in these peridomestic settings; (3) evaluate the relationship between actual artificial habitats, mosquito production and West Nile virus risk; and (4) extensively disseminate results of our research to homeowners, Cornell Cooperative Extension Educators, master gardeners, and public health departments through educational materials and the medical entomology extension web site (http://www.entomology.cornell.edu/MedEnt/index.html).

FIG 5.Mosquito larvae in a sample from outdoor containers.