Grantee Research Project Results

Final Report: Ecology of Disease Vectors in the Northeastern USA

EPA Grant Number: X832719
Title: Ecology of Disease Vectors in the Northeastern USA
Investigators: Daniels, Thomas J , Falco, Richard C
Institution: Fordham University
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 2005 through September 30, 2006
Project Amount: $193,500
RFA: Targeted Research Grant (2005) RFA Text |  Recipients Lists
Research Category: Targeted Research

Objective:

Vector-borne diseases such as Lyme disease and West Nile virus encephalitis are among the most important public health threats facing the United States as we enter the new millennium. In 2004 alone, more than 20,000 cases of Lyme disease and nearly 2,500 cases of West Nile fever were reported in the US.

There is a trend for increasing vector-borne disease incidence over the last two decades. This increase has been influenced by the arrival of exotic infectious agents, like West Nile virus, from other parts of the globe. The emergence of new pathogens in an area puts a severe strain on diagnostic and prevention resources for local and state public health officials and highlights the need for continued surveillance and research into understanding how and why vector-borne diseases flourish and spread in the United States.

There were two scientific objectives of this project: (1) Continue an ongoing study on the population ecology study of black-legged ticks (Ixodes scapularis) to determine annual abundance of larval, nymphal, and adult stages in order to monitor population changes and fluctuations in local risk for Lyme disease, and (2) Continue a study of the species composition and larval habitats of mosquitoes in a West Nile virus-endemic area. Both of these studies are long-term ecological investigations of disease vectors that significantly impact public health in the lower Hudson Valley of New York and other areas of the northeastern and Midwestern US. Our long-term goal is to build on the work that was funded and incrementally add to the services that can be offered to public health officials and students.

Summary/Accomplishments (Outputs/Outcomes):

Tick Studies
Lyme disease is the most important vector-borne illness in the United States, with more than 100,000 cases reported from 48 states over the last ten years. Risk of Lyme disease is tied to the presence of black-legged ticks, Ixodes scapularis, the vector species, such that local changes in tick abundance from one year to the next often correlate well with reported case numbers over time. As black-legged ticks increase in number and expand into new areas, the risk of contracting either established or emerging tick-borne diseases will rise. This possibility has increased demand for knowledge of the ecology of ticks, accurate assessment of risk for tick-borne diseases, and reliable, environmentally sensitive methods to control ticks. For these reasons, it is imperative that vector-borne disease research efforts maintain an ecological focus to address the nature of exposure to vectors and their pathogens, and ultimately to develop environmentally sensitive control measures aimed at reducing risk. Nowhere is this work more important than in southern New York State, which is a mixing zone between urban and rural areas.

Study Site and Permanent Grid Surveillance. Field work was conducted at the Louis Calder Center of Fordham University, located 28 km north (41°8'N, 73°48'W) of New York City, in the village of Armonk, Westchester County, New York, USA. The habitat is characteristic of mixed deciduous forest. As part of a long-term study to monitor relative tick abundance patterns throughout the year, we conducted drag sampling, in which a 1 m² panel of white corduroy is pulled along the ground and over vegetation, on a "permanent" grid that was established in 1987. The grid, measuring 3,600 m² (60 m x 60 m) in area, was sampled 1 - 3 times weekly from late March through mid-December, as weather permitted. For each sample, 500 m² of area was dragged by randomly selecting transects through the study grid. Ticks found clinging to the cloth were removed with forceps, then stored for later identification and counting. Mean weekly densities (ticks per sq m) were then calculated and plotted to determine the seasonal distribution of each tick stage. Once peak abundance was attained for a particular life stage, population estimation sampling was initiated on a second grid located ~100 m distant. Routine drag sampling continued on the permanent grid while population estimation sampling is conducted strictly on the second grid. Peaks of tick activity were observed in spring, summer, and fall 2005 for nymphs, larvae, and adults, respectively.

Tick Population Estimation Studies. Two estimation methods were used during the study. A multiple mark-release-recapture technique, the Schnabel (1938) method, was used on host-seeking nymphal and adult ticks to estimate their respective population sizes. Because of the large numbers of larvae collected while drag sampling, and the high probability of mortality resulting from handling, mark-release-recapture techniques were deemed inappropriate for larval estimates. Therefore, population estimates for this stage relied on removal sampling in which counts were made of all larvae removed from each plot. Estimation studies were routinely initiated immediately after peak abundance was reached at the permanent grid site and tick numbers were stable or just beginning to decline. For those years in which population estimation sampling was delayed for more than one week after peak tick abundance because of weather, capture numbers were adjusted upward on the basis of drag data from the permanent grid site (percentage difference between tick abundance at peak and at the time the estimation study was begun).

Nymphs & Adults: Drag sampling was conducted on two plots totaling 1,400 m² site (600 and 800 m², respectively) within a 3,600 m² study area. Area dragged was increased to 2,000 m² for adult estimates conducted after 1990; the greater area was intended to increase the total number of ticks collected, including recaptures, and enhance statistical confidence given low adult densities. For each 100 m² quadrat sampled, drag cloths were checked every 20 m and all I. scapularis removed. Captured ticks were examined visually and, aided by a handheld magnifier with light, categorized as marked (recaptured) or unmarked. Ticks then were marked for the first time, or remarked, with paint pens and released at the capture site. Counts of new and recaptured ticks were recorded for each quadrat and summed for each of the two sampling areas. A density of 2.3 nymphs per sq m was estimated in 2005, followed in 2006 by 0.7 nymphs per sq m. Compared to previous years, these nymphal densities were high and low, respectively, and highlight the variability in tick numbers from one year to the next.

Larvae: Forty-five (45) circular plots, each 3.14 m² (1 m radius) in area, were randomly selected within the 1,400 m² nymphal sampling area. Each circular plot then was drag sampled once per day a minimum of 7 times over a 3 wk period. All larvae were removed with an adhesive tape lint-roller; tapes were placed in plastic bags, sealed, and identified by plot number. Counts of larvae that had been removed from each plot were conducted in the laboratory. Concurrent with removal sampling, an additional 45 plots were randomly selected twice weekly, on average. These represented control plots in which removal sampling was not affected by the number of ticks collected. A density of 27.1 larvae per sq m was estimated in 2005, followed in 2006 by 9.3 larvae per sq m. Compared to previous years, these larval densities were moderate. Again, they highlight the variability in tick numbers from one year to the next.

Interstadial Mortality. In general, the population of I. scapularis has remained relatively stable over time at our study site, suggesting that density-dependent regulatory factors are at work, at least during some portion of the life cycle. To test the hypothesis that density-dependent regulation occurs between larval and nymphal stages, a critical time that influences the number of potential disease vectors (nymphs are responsible for most disease transmission), we examined the relationship between larval density and subsequent larval-to-nymphal mortality. In those years when larval density was highest, the relative proportion that successfully became nymphs was lowest, effectively stabilizing the population. The same relationship was observed between the nymphal and adult stage. Reasons for this density-dependence are currently speculative and further investigation of this phenomenon is ongoing.

Shift in Seasonal Pattern of Activity. Based on permanent grid data, we determined the (standard) week of highest tick abundance for each life stage, for each year since 1988, and plotted them over time. For larvae, peak week varied by as much as four weeks among all years since 1988. To determine if there was a trend in peak abundance, a time series analysis was performed in which peak data were exponentially smoothed and plotted. The fitted data were used to forecast larval peak week five years in the future. Larval peak has remained essentially constant since the study began, suggesting that factors dictating larval activity have not changed appreciably in that time, though a slight trend toward earlier peak activity is clear. However, when the nymphal data were analyzed in the same manner, a distinct trend toward earlier peak activity was observed over time and this trend is forecast to continue for the near future. Reasons for this are currently under investigation.

Mosquito Studies
The emergence and rapid spread of West Nile Virus (WNV) in 1999, transmitted by mosquitoes commonly found in New York City and its suburbs, illustrate the importance of maintaining training, surveillance, and research programs focused on emerging vector-borne diseases. In the span of just two years, this newly introduced virus resulted in the deaths of nine New York residents and spread to more than 25 states. In 2007, eight years after the initial outbreak, all but one (Maine) of the 48 contiguous states reported either human cases (n = 43) or animal or mosquito infections (n = 4) (CDC 2007 - http://www.cdc.gov/ncidod/dvbid/westnile/Mapsactivity/surv&control07Maps.htm Exit ). The work conducted for this study provided a framework for investigating the role of mosquitoes as disease vectors in a landscape that encompasses a variety of habitats.

Tree Hole Use by Woodland Mosquito Species. This study was conducted in the summer of 2005 at the Louis Calder Center, the biological field station of Fordham University in Armonk, NY. Tree holes were identified and monitored within an approximately 30 hectare plot at the Center. All tree holes were marked and their GPS locations recorded. Tree holes were examined every other week during June, July, and August of 2005. All water was removed from the tree hole with a baster, mosquito larvae placed in separate containers, and the water replaced in the tree hole. Larvae were brought back to the laboratory, where they were reared to adulthood and identified. Mosquito density in tree holes was compared to that collected in 2003 and 2004.

Just twenty-four active tree holes were found within the 30 ha site in 2005. By comparison, more than three times as many tree holes containing water were found in 2004, and more than seven times as many were found in 2003. The paucity of mosquitoes found in tree holes in 2005 can be likely explained by less rain during July of that year, when tree holes were sampled. These results demonstrate the importance of rainfall as a factor in determining the population levels of mosquitoes that breed in small containers such as tree holes.

Surveillance of Adult Mosquito Population. Four trap sites were set up at the Calder Center (NE, NW, SE, and SW). Each site had two traps: (1) a CDC light trap baited with dry ice (the dry ice serves as a source for CO2, a mosquito attractant) and (2) a gravid trap. The latter traps consist of a plastic container partially filled with organically infused water, over which an inverted CDC trap is placed. Mosquitoes are sucked into the trap when they approach the water to lay eggs. CDC and gravid traps both collect adult mosquitoes from a variety of species, but the state of the mosquitoes targeted differ between the traps; CDC traps collect host-seeking mosquitoes that haven’t yet had a blood meal, while gravid traps attract mosquitoes ready to lay eggs.

The most abundant mosquito sampled from CDC traps was Cx. pipiens/restuans (24.7%), a common household mosquito. This was followed by Oc. canadensis (22.7%), a snowpool mosquito, and Ae. vexans (14.1%). Oc. japonicus, the invasive species, was also fairly abundant, accounting for 9.2% of all specimens collected. Oc. japonicus was the most prevalent species in gravid traps, accounting for 68.9% of mosquitoes collected, indicating that a breeding population of Oc. japonicus exists at the Calder Center study sites. This was more than twice the number of Cx. pipiens/restuans found, even though the substrate used in this trap was highly attractive to gravid Culex mosquitoes.

These results demonstrate that Oc. japonicus is established at this study site and is a potential competitor for local mosquito species. The fact that Oc. japonicus was more prevalent in gravid traps than Culex mosquitoes may indicate that the invasive species was out-competing the local species in the artificial container habitats in this experiment. More studies, both in the field and in the lab, are needed to determine the impact of Oc. japonicus on the breeding of local mosquito species, particularly Oc. triseriatus, a tree-hole mosquito with which it competes.

Daily Patterns in Host-seeking and Egg Laying. The two most productive trap sites at the Calder Center, based on adult trapping data, were selected for this study. Both CDC and gravid traps were set at each site and were operated for 24 hour periods, for a total of 12 trapping sessions. At the end of each session, mosquitoes were collected and taken to the laboratory for identification. Collection containers were replaced. Oc. japonicus was shown in this study to seek hosts primarily between 9:00PM and 4:00AM, when over 80% of the specimens were collected. This was similar to the host-seeking activity pattern of Cx. pipiens, a local night-biting mosquito. Considered a “day biter” in its native regions, the daily activity patterns of Oc. japonicus in the northeastern U.S., both host-seeking and oviposition, have not previously been studied. These results demonstrate that this mosquito may have a different behavior pattern in the northeastern US when compared to its native habitat in Asia. This may have important public health consequences if Oc. japonicus proves to be an effect transmitter of newly introduced arboviruses. Knowledge of the daily activity pattern of a vector is an important component of prevention and control plans. Such information can be used in timing spray applications to control adult mosquitoes as well as incorporated into personal protection advice to prevent being bitten. The gravid trap results demonstrated that Oc. japonicus egg-laying activity peaked in early evening, with over 80% of gravid adults collected between the hours of 6:00PM and 9:00PM. Culex egg laying, on the other hand, appeared to be more evenly distributed.

Use of Artificial Containers. Fifteen plastic buckets were distributed in wooded and edge habitats and each bucket was filled approximately three quarters of the way with rainwater. Buckets were lined with seed germination paper, which provided a substrate for container-breeding mosquitoes, such as those in the genus Ochlerotatus, to lay their eggs. Species of the genus Culex would also be able to lay eggs in the buckets, on the surface of the rainwater. Two to three large oak leaves were placed in each bucket to provide initial organic material. Buckets were left open to allow rain, leaf litter, and debris to enter, as would be the case for any other artificial container found in the suburban habitat. Buckets were examined twice weekly during July and August for a total of 19 sampling days. Pupae were collected, taken to the laboratory, and placed in emergence containers. Adult mosquitoes were identified to species after emergence. Oc. japonicus was the most abundant mosquito collected from artificial containers, accounting for over 90% of all larvae collected. Cx pipiens/restuans was the second-most abundant species collected at 6%. In addition, Oc. japonicus was collected during all 19 sampling periods while Cx. pipiens/restuans was found in 14 of the 19 sampling periods (73.7%). These results demonstrate that Oc. japonicus utilized the artificial containers as a breeding site over other species and that Oc. japonicus is a very efficient larval competitor in artificial containers in the northeast.

Journal Articles:

No journal articles submitted with this report: View all 1 publications for this project

Supplemental Keywords:

Risk assessment, human health, life history, entomology, vector-borne disease, tick, mosquito, Northeast, land, exposure, risk, health effects, pathogens, viruses, bacteria, habitat, life-cycle analysis, survey, biology, ecology, epidemiology, zoology, monitoring, analytical, surveys, measurement methods, Midwest, midatlantic, New York (NY),