Simulation of blacklegged tick (Acari:Ixodidae) population dynamics and transmission of Borrelia burgdorferi.

A model (LYMESIM) was developed for computer simulation of blacklegged tick, Ixodes scapularis Say, population dynamics and transmission of the Lyme disease agent. Borrelia burgdorferi Johnson. Schmid, Hyde, Steigerwalt & Brenner, LYMESIM simulates the effects of ambient temperature, saturation deficit, precipitation, habitat type, and host type and density on tick populations. Epidemiological parameters including host infectivity, tick infectivity, transovarial transmission, and transstadial transmission are included in the model to simulate transmission of the Lyme disease spirochete between vector ticks and vertebrate hosts. Validity of LYMESIM was established by comparing simulated and observed populations of immature I. scapularis on white-footed mice. Peromyscus leucopus, (Rafinesque), at 2 locations in Massachusetts. Validity also was indicated by comparisons of simulated and observed seasonality of blacklegged ticks in New York, Massachusetts, Florida, and Oklahoma-Arkansas. Further model validity was shown by correlation between simulated and observed numbers of immature ticks engorging on white-footed mice at 3 sites in Massachusetts. The model produced acceptable values for initial population growth rate, generation time, and 20-yr population density when historical meteorological data for 16 locations in eastern North America were used. Realistic rates of infection in ticks were produced for locations in the northeastern and northcentral United States. LYMESIM was used to study the effect of white-footed mouse and white-tailed deer, Odocoileus virginianus (Zimmerman), densities on tick density and infection rates. The model was also used to estimate tick density thresholds for maintenance of B. burgdorferi.

Simulation of management strategies for the blacklegged tick (Acari: Ixodidae) and the Lyme disease spirochete, Borrelia burgdorferi.

A computer model (LYMESIM) was developed to simulate the effects of management technologies on populations of the blacklegged tick, Ixodes scapularis Say, and the Lyme disease spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner in eastern North America. Technologies considered in this study were area-wide acaricide, acaricide self-treatment of white-footed mice and white-tailed deer, vegetation reduction, and white-tailed deer density reduction. Computer simulations were run with normal weather patterns for coastal Connecticut and New York. Results showed that area-wide acaricide, vegetation reduction, or a combination of these technologies would be useful for short-term seasonal management of ticks and disease in small recreational or residential sites. Acaricide self-treatment of deer appears to be the most cost-effective technology for use in long-term management programs in large areas. Simulation results also suggested that deer density reduction should be considered as a management strategy component. Integrated management strategies are presented that could be used in pilot tests and operational tick and tick-borne disease programs.

A review of ultralow-volume aerial sprays of insecticide for mosquito control.

This review of research on ultralow-volume (ULV) aerial sprays for mosquito control is a component of an Aerial SPray EXpert system (ASPEX). Topics include application volume, adulticiding, larviciding, droplet size, and meteorology. The review discusses the efficacy of ULV aerial sprays against many important pest and vector species of mosquitoes in a wide range of locations and habitats in the USA and in some countries of Asia, Africa, and the Americas. Nine conclusions were drawn from this review. 1) ULV applications are as effective for mosquito control as highly-diluted, water-based sprays. 2) More acres can be sprayed per aircraft load with the ULV method than with dilute sprays. 3) High-altitude ULV sprays using wide or stacked swaths could be used in emergencies if wind speed and direction data at appropriate altitudes are available to accurately place the spray. 4) Successful adult mosquito control can be achieved in dense foliage or open housing with ULV aerial sprays, but doses of insecticide must be increased. 5) ULV aerial application of mosquito larvicides can be used successfully in large areas. 6) The optimum droplet size for adult mosquito control is 5-25 microns volume median diameter (VMD). 7) For mosquito adulticiding, near optimum atomization of ULV sprays is achieved with flat-fan nozzles oriented straight down or slightly forward for high-speed aircraft (> or = 150 mph) or rotary atomizers on slow-speed aircraft (< 150 mph). 8) Optimum atomization minimizes paint spotting. 9) Maximum adult mosquito control is achieved just after sunrise and just before sunset with 2-10-mph crosswinds.

An operational perspective on measuring aerosol cloud dynamics.

Three areas are discussed in this paper: 1) U.S. Air Force Reserve/USDA bioassays to determine the effective swath width of ultra-low volume (ULV) aerial applications conducted with the C-130 Modular Aerial Spray System (MASS), 2) the use of aerial spray computer models to predict spray offset distance and their use as a substitute for field testing, and 3) a demonstration on an aerial spray expert system called ASPEX being developed at the U.S. Air Force Reserve Aerial Spray Branch.

A simulation model of the epidemiology of urban dengue fever: literature analysis, model development, preliminary validation, and samples of simulation results.

We have developed a pair of stochastic simulation models that describe the daily dynamics of dengue virus transmission in the urban environment. Our goal has been to construct comprehensive models that take into account the majority of factors known to influence dengue epidemiology. The models have an orientation toward site-specific data and are designed to be used by operational programs as well as researchers. The first model, the container-inhabiting mosquito simulation model (CIMSiM), a weather-driven dynamic life-table model of container-inhabiting mosquitoes such as Aedes aegypti, provides inputs to the tranmission model, the dengue simulation model (DENSiM); a description and validation of the entomology model was published previously. The basis of the transmission model is the simulation of a human population growing in response to country- and age-specific birth and death rates. An accounting of individual serologies is maintained by type of dengue virus, reflecting infection and birth to seropositive mothers. Daily estimates of adult mosquito survival, gonotrophic development, and the weight and number of emerging females from the CIMSiM are used to create the biting mosquito population in the DENSiM. The survival and emergence values determine the size of the population while the rate of gonotrophic development and female weight estimates influence biting frequency. Temperature and titer of virus in the human influences the extrinsic incubation period; titer may also influence the probability of transfer of virus from human to mosquito. The infection model within the DENSiM accounts for the development of virus within individuals and its passage between both populations. As in the case of the CIMSiM, the specific values used for any particular phenomenon are on menus where they can be readily changed. It is possible to simulate concurrent epidemics involving different serotypes. To provide a modicum of validation and to demonstrate the parameterization process for a specific location, we compare simulation results with reports on the nature of epidemics and seroprevalence of antibody in Honduras in low-lying coastal urbanizations and Tegucigalpa following the initial introduction of dengue-1 in 1978 into Central America. We conclude with some additional examples of simulation results to give an indication of the types of questions that can be investigated with the models.

Integrated management strategies for Amblyomma americanum (Acari: Ixodidae) on pastured beef cattle.

Data on tick control and knowledge of the damage caused to beef cattle by tick feeding were incorporated into a computerized dynamic life table model (LSTSIM) for Amblyomma americanum (L.). Simulations were made to determine economically feasible, 5-yr integrated pest management (IPM) strategies for A. americanum in forage areas utilized by Bos taurus, Bos indicus, and crossbred cattle (B. taurus x B. indicus). The effects of host resistance, pasture rotation, habitat conversion, topical acaricides, systemic acaricides, and the area-wide application of acaricides to pastures on populations of parasitic female ticks were simulated as individual control technologies and as components of multiple-factor IPM strategies. The most effective, single-factor control strategy for A. americanum in beef cattle forage areas is the use of tick-resistant B. indicus cattle. Pasture rotation combined with area-wide acaricide applications was the only economically feasible IPM strategy for B. indicus cattle and reduced tick densities by 89% after 5 yr. Thirteen economically feasible IPM strategies were identified for use with B. taurus cattle. Of these, the most efficacious was pasture rotation in May combined with systemic or topical acaricide applications. Other strategies included systemic acaricides with area-wide acaricide applications to pastures, topical acaricides with area-wide acaricide applications, pasture rotation with habitat conversion and topical acaricides, and pasture rotation with habitat conversion, topical acaricides, and area-wide acaricide applications. Each technology reduced tick densities on B. taurus cattle by > 80% over a 5-yr period. Nine IPM strategies were economically feasible for use with crossbred cattle. Of these, pasture rotation combined with systemic or topical acaricide applications reduced the number of female ticks on cattle by > 84%; pasture rotation combined with habitat conversion reduced tick numbers by 77%. The most effective nonacaricide-based IPM strategy for B. taurus and crossbred cattle was pasture rotation combined with habitat conversion. No acaricide-free IPM strategy was economically feasible for use with B. indicus cattle.

Dynamic life table model for Aedes aegypti (Diptera: Culicidae): analysis of the literature and model development.

The container-inhabiting mosquito simulation model (CIMSiM) is a weather-driven, dynamic life table simulation model of Aedes aegypti (L.). It is designed to provide a framework for related models of similar mosquitoes which inhibit artificial and natural containers. CIMSiM is an attempt to provide a mechanistic, comprehensive, and dynamic accounting of the multitude of relationships known to play a role in the life history of these mosquitoes. Development rates of eggs, larvae, pupae, and the gonotrophic cycle are based on temperature using an enzyme kinetics approach. Larval weight gain and food depletion are based on the differential equations of Gilpin & McClelland compensated for temperature. Survivals are a function of weather, habitat, and other factors. The heterogeneity of the larval habitat is depicted by modeling the immature cohorts within up to nine different containers, each of which represents an important type of mosquito-producing container in the field. The model provides estimates of the age-specific density of each life stage within a representative 1-ha area. CIMSiM is interactive and runs on IBM-compatible personal computers. The user specifies a region of the world of interest; the model responds with lists of countries and associated cities where historical data on weather, larval habitat, and human densities are available. Each location is tied to an environmental file containing a description of the significant mosquito-producing containers in the area and their characteristics. In addition to weather and environmental information, CIMSiM uses biological files that include species-specific values for each of the parameters used in the model. Within CIMSiM, it is possible to create new environmental and biological files or modify existing ones to allow simulations to be tailored to particular locations or to parameter sensitivity studies. The model also may be used to evaluate any number and combination of standard and novel control methods.

Dynamic life table model for Aedes aegypti (diptera: Culicidae): simulation results and validation.

The container-inhabiting mosquito simulation model (CIMSiM) is a weather-driven, dynamic life table simulation model of Aedes aegypti (L.) and similar nondiapausing Aedes mosquitoes that inhabit artificial and natural containers. This paper presents a validation of CIMSiM simulating Ae. aegypti using several independent series of data that were not used in model development. Validation data sets include laboratory work designed to elucidate the role of diet on fecundity and rates of larval development and survival. Comparisons are made with four field studies conducted in Bangkok, Thailand, on seasonal changes in population dynamics and with a field study in New Orleans, LA, on larval habitat. Finally, predicted ovipositional activity of Ae. aegypti in seven cities in the southeastern United States for the period 1981-1985 is compared with a data set developed by the U.S. Public Health Service. On the basis of these comparisons, we believe that, for stated design goals, CIMSiM adequately simulates the population dynamics of Ae. aegypti in response to specific information on weather and immature habitat. We anticipate that it will be useful in simulation studies concerning the development and optimization of control strategies and that, with further field validation, can provide entomological inputs for a dengue virus transmission model.

New version of LSTSIM for computer simulation of Amblyomma americanum (Acari: Ixodidae) population dynamics.

A previous version of Lone Star Tick Simulation Model (LSTSIM) for a wildlife ecosystem was revised and expanded to include a beef cattle forage area and improved handling of tick-host-habitat interactions. Relationships between environmental and biological variables were also refined in the new version. General validity of the revised model was established by comparing simulated and observed host-seeking populations of Amblyomma americanum (L.) at five geographic locations, three in Oklahoma and two in Kentucky-Tennessee. Additional validity was indicated from comparisons of simulated and observed seasonality of lone star ticks at one location in Kentucky. The model produced acceptable values for initial population growth rate, generation time, and 15-yr population density when historical weather files for 14 locations in the United States were used. The model of A. americanum population dynamics was used to study the relationship between tick density and density of white-tailed deer, Odocoileus virginianus (Zimmerman), and cattle. The revised model can be used for additional simulation studies on effects of tick control technologies and integrated management strategies.

Computer simulation of Babesia bovis (Babes) and B. bigemina (Smith & Kilborne) transmission by Boophilus cattle ticks (Acari: Ixodidae).

A computer model was developed to simulate the processes involved in transmission of the cattle fever parasites Babesia bovis (Babes) and Babesia bigemina (Smith & Kilborne) between cattle and Boophilus ticks. The model of Babesia transmission was combined with a dynamic life history model for population dynamics of the tick vectors, Boophilus microplus (Canestrini) and B. annulatus (Say). Epidemiological parameters and relationships in the model include the reduction in fecundity of infected ticks, rate of transovarial transmission, effect of cattle type and inoculation rate on infectivity of cattle, variation of infected cattle recovery rate with age of infection, inoculation rate, and species of parasite. Some parameters in the model were fitted by iterative simulations to produce realistic rates of Babesia infection in larval ticks. Comparisons of simulated and reported epidemiological data from one location in Australia indicated a reasonable level of validity for the model. Theoretical tick density thresholds for maintenance of Babesia in cattle and for inoculation of greater than or equal to 99.5% calves were determined by iterative simulations at 10 locations with B. microplus and six locations with B. annulatus. The model and transmission thresholds can serve as the basis for further simulation studies on strategies for control or eradication of babesiosis.

Computer simulation of Boophilus cattle tick (Acari: Ixodidae) population dynamics.

A comprehensive computer model was developed for simulation of the population dynamics of the cattle ticks, Boophilus microplus (Canestrini) and B. annulatus (Say). The model is deterministic and based on a dynamic life table with weekly time steps. The model simulates the effects of major environmental variables, such as ambient temperature, saturation deficit, precipitation, type of pasture, type of cattle, and cattle density on Boophilus cattle tick population dynamics. General validity of the model is established by comparing simulated and observed yearly densities of standard female ticks/host/day. B. microplus population comparisons were made for a series of years using weekly weather data from two locations in Queensland, Australia. The model also produced acceptable values for initial population growth rate, generation time, and 3-yr population density when historical weather at 7 locations in Australia and 23 locations in the Americas were used. This model provides a framework for the study of Babesia transmission by Boophilus ticks, and can be used to study the effects of control technologies and to develop more efficient and environmentally acceptable eradication strategies for Boophilus ticks.

Computer simulation of Rocky Mountain spotted fever transmission by the American dog tick (Acari: Ixodidae).

A computer model was developed for simulation of the transmission of Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever (RMSF), by the American dog tick, Dermacentor variabilis (Say). The model of RMSF was combined with a model for population dynamics of the American dog tick and included simulation of infection and transmission of rickettsiae between ticks and host mammals and transmission of RMSF to humans. The model simulated the effects of biotic and environmental variables such as weather, host density, habitat, transovarial transmission, fecundity of infected ticks, and infectivity level of ticks and mammals. Some parameters in the model were fitted by iterative simulations to produce realistic rates of R. rickettsii infection in adult ticks and small and medium-sized mammal hosts. Parameters also were fitted to yield the historical average number of RMSF cases for Virginia. Comparisons of the simulated and actual number of cases for nine other states indicated a reasonable level of validity for the model. A theoretical tick density threshold of 252 unfed adult ticks/ha for transmission of RMSF was determined from a relationship between rate of transmission to humans and density of ticks. The transmission threshold can be used for additional modeling efforts to study the effects of management technologies on tick densities and RMSF human cases. The model can serve as a framework for modeling other tick-borne diseases such as Lyme disease, babesiosis, and heartwater.

Computer simulation of management strategies for American dog ticks (Acari: Ixodidae) and Rocky Mountain spotted fever.

Computer models were developed to simulate the effects of management technologies on populations of the American dog tick, Dermacentor variabilis (Say), principal vector of Rocky Mountain spotted fever (RMSF) in eastern North America. The technologies modeled were area-wide acaricide application, acaricide-food-baited tubes for self-treatment by small mammals, dipping of dogs in acaricides, acaricide-impregnated plastic dog collars, reduction of small mammal host populations (host management), and removal of vegetation that protects free-living tick stages (vegetative management). Submodels for each of these technologies were incorporated into a model (ADTSIM) for the population dynamics of the tick and RMSF transmission. Comparisons of simulated and observed data were used to verify reasonable accuracy of the submodels. Repetitive simulations were made to identify levels and timing of each control method (alone or combined) required to reduce tick populations below a RMSF transmission threshold of 252 unfed adults/ha. Eight to 30 acaricide applications, depending on acaricide and percentage of population treated, were needed during a 10-yr period to reduce densities of ticks below the threshold. The baited-tube method, host management, and vegetative management (depending on level and frequency of treatment) also were capable of reducing tick density below the threshold. However, acaricide-impregnated plastic dog collars did not reduce tick density below the threshold unless at least 50% of the hosts of adult ticks were domestic dogs. Integrated strategies were developed for management of ticks and RMSF in six selected states. These strategies reduced numbers of human cases of RMSF 90% or more by year 20 by maintaining tick densities between 100 and 252 unfed adults/ha.

Computer simulation of population dynamics of the American dog tick (Acari: Ixodidae).

A comprehensive computer model was developed for simulation of the population dynamics of the American dog tick (ADT), Dermacentor variabilis Say, in North America. The model simulates the effects of major environmental variables, such as ambient temperature, saturation deficit, kind of habitat, and host density, on ADT population dynamics in ecosystems with small mammals as hosts for immature ticks and medium-sized mammals or domestic dogs as hosts for adult ticks. General validity of the model was established by comparisons between simulated and actual population densities for a series of years at locations in Virginia, Maryland, and Massachusetts using actual weekly weather data for each year as a model input. Using historical-average weather data for 11 locations within the known geographic range of ADT and 3 locations outside this range, the model produced acceptable values for initial population growth rate and generation time, as well as realistic equilibrium population densities and seasonal activity patterns. This model can be used as a framework for additional modeling efforts to simulate the transmission of Rocky Mountain spotted fever and to study various strategies for management of ADT populations.

The effectiveness of permethrin and deet, alone or in combination, for protection against Aedes taeniorhynchus.

Field tests were conducted to compare the degree of protection from bites by the mosquito Aedes taeniorhynchus (Wiedemann) provided by wearing clothing treated with permethrin [(3-phenoxyphenyl)methyl (+/-) cis/trans 3-(2-dichloroethenyl)2, 2-dimethylcyclopropanecarboxylate] with that provided by applying deet (N,N-diethyl-m-toluamide) to exposed skin or by applying deet and wearing the treated clothing. Human test subjects were exposed to natural populations of mosquitoes for a 9-hour daytime period (total of 8 days/treatment) while using one or both protection methods. Unprotected test subjects were also exposed for short periods each day as a check to determine the overall biting rate of mosquitoes. The combined use of both protection methods was the most effective treatment in preventing bites, resulting in an average of 1.5 bites/9-hour day, compared with 53.5 and 98.5 bites on subjects protected only with treated clothing or deet, respectively, and 2,287 bites (extrapolated) on subjects who wore untreated clothing during the same time period. Measurements also indicated that the toxic effect of permethrin reduced biting rates by greater than 90% within the immediate area where subjects wore permethrin-treated uniforms for 9 hours.

Computer simulation of the effectiveness of male-linked translocations for the control of Anopheles albimanus Wiedemann.

A deterministic simulation model was used to establish the potential value of releasing male-linked translocation heterozygotes as a control measure for Anopheles albimanus Wiedemann. Theoretical population reductions exceeding 90% were obtained within 90 and 120 days after releases at initial ratios of 5 translocation males (TM): 1 normal male (NM) and 1 TM: 1 NM, respectively. Additional simulations emphasized the importance of the need for a method that would eliminate females from the release material. Releases containing 15% females were less effective than those with none. When a malaria subroutine was included in the model, the calculations showed that all the theoretical releases greatly reduced the number of malaria-infective females and therefore would have a profound effect on transmission of the disease. The number of malaria-infective females present was eliminated completely when only translocation males were released; however, a small number were present when the releases contained 15% females. Male-linked translocation males required longer periods of time to bring about population control than males that were completely sterile.