Assessing the prevention effectiveness of local Lyme disease control.

The effectiveness of any public health intervention is determined by its theoretical efficacy and by the level of engagement of the target population. A computer simulation model and basic epidemiologic concepts were used to estimate the effectiveness of interventions for preventing Lyme disease in a hypothetical community. The process for estimating numbers of Lyme disease cases prevented by each intervention is described. This assessment compares the effectiveness of alternative community-based prevention strategies, illuminates the limitations and distributive effects of interventions, and helps clarify available prevention options for community residents.

A critical review of ultralow-volume aerosols of insecticide applied with vehicle-mounted generators for adult mosquito control.

This review of ultralow-volume (ULV) ground aerosols for adult mosquito control includes discussion on application volume, aerosol generators, droplet size, meteorology, swath, dispersal speed, assay methods, insecticide efficacy, and nontarget effects. It summarizes the efficacy of ULV insecticidal aerosols against many important pest and disease-bearing species of mosquitoes in a wide range of locations and habitats in the United States and in some countries of Asia and the Americas. Fourteen conclusions were drawn from the review. 1) ULV ground aerosol applications of insecticide are as efficacious against adult mosquitoes as high- or low-volume aerosols. 2) ULV aerosols with an optimum droplet size spectrum can be produced by several types of nozzles including vortex, pneumatic, and rotary. Droplet size of a particular insecticide formulation is dependent primarily on nozzle air pressure or rotation speed and secondarily on insecticide flow rate. 3) Label flow rates of insecticide for ULV aerosol application can be delivered accurately during routine operations with speed-correlated metering systems within a calibrated speed range, usually not exceeding 20 mph. 4) The most economical and convenient method of droplet size determination for ULV aerosols of insecticide is the waved-slide technique. 5) The efficacy of ULV ground aerosols against adult mosquitoes is related to droplet size because it governs air transport and impingement. The optimum droplet size for mosquito adulticiding is 8-15 microns volume median diameter (VMD) on the basis of laboratory wind-tunnel tests and field research with caged mosquitoes. 6) In general, ULV aerosols should be applied following sunset when mosquitoes are active and meteorological conditions are favorable for achieving maximum levels of control. Application can be made during daytime hours when conditions permit, but rates may have to be increased. The critical meteorological factors are wind velocity and direction, temperature, and atmospheric stability and turbulence. 7) Maximum effective swaths are obtained with aerosols in the optimum VMD range during favorable meteorological conditions in open to moderately open terrain. The insecticide dosage must be increased in proportion to increased swath to maintain the same level of mosquito control. 8) Dispersal speed within a range of 2.5-20 mph is not a factor affecting efficacy if insecticide rate and optimum droplet size are maintained. 9) The results of caged mosquito assays are comparable with reductions in free-flying natural populations. 10) The field efficacies of mosquito adulticides applied as ULV ground aerosols are predictable from the results of laboratory wind-tunnel tests. 11) Results of field tests in open to moderately open terrain during favorable meteorological conditions indicated that ULV insecticidal aerosol application rates producing 90% or more control of Anopheles, Culex, and Psorophora spp. are below or approximately equal to maximum United States Environmental Protection Agency label rates. Against some Aedes spp., some pyrethroid insecticides must be synergized to produce 90% control at label rates. 12) Results of field tests in residential areas with moderate to dense vegetation and in citrus groves or other densely wooded areas showed that insecticide rates of ULV ground aerosols must be increased 2-3-fold to obtain 90% or more control of adult mosquitoes. However, the maximum rates on some insecticide labels would have to be increased to allow higher application rates. 13) Applications of ULV ground aerosols of insecticide in accordance with label directions following sunset do not pose a serious threat to humans, nontarget beneficial animals, or automotive paints. 14) Some aerosol generators operated at high RPM levels exceed the OSHA 8-h hearing hazard criteria of 90 dBA and may require hearing protectors for operators.

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.

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.

Management of lone star ticks (Acari: Ixodidae) in recreational areas with acaricide applications, vegetative management, and exclusion of white-tailed deer.

A project on management of lone star ticks, Amblyomma americanum (L.), at Land Between the Lakes, a Tennessee Valley Authority recreational area in Kentucky-Tennessee, during 1984-1988, demonstrated the effectiveness and economics of three control technologies. Acaricide applications (chlorpyrifos at 0.28 kg [AI]/ha), vegetative management (mowing and removal of 40% overstory and 90-100% of midstory, understory, and leaf litter), and host management (white-tailed deer exclusion from a 71-ha campground with a single-line fence) provided 75, 70, and 64% mean controls of all life stages of the lone star tick, respectively. Combinations of acaricide applications + vegetative management, acaricide applications + host management, and acaricide applications + vegetative management + host management produced 94, 89, and 96% mean control of all life stages, respectively. The costs of acaricide applications (two per year), vegetative management (two mowings per year), and white-tailed deer exclusion (single-line fence) were $45, $150, and $30/ha/yr, respectively. Results of this project are used to design management strategies that could be considered for use against lone star ticks in recreational areas.

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.