Susceptibility of selected rodent species from Colorado to Borrelia burgdorferi.

To determine the susceptibility of some common Colorado (USA) rodent species to Borrelia burgdorferi, pregnant Peromyscus maniculatus, Tamias minimus, and Spermophilus lateralis were trapped in May 1990 and kept in quarantine until their young were old enough to be used in the experiment. Six to eight 8-wk-old individuals of each of the Colorado species and, for comparison, eight laboratory raised P. leucopus were subcutaneously inoculated with > or = 10(5) spirochetes in 0.1 ml in July 1990. Tissue specimens were collected for isolation from these animals through April 1991. Spirochetes were isolated from blood, ear, bladder, kidney, spleen, liver, and eye in Barbour-Stoener-Kelly (BSK) medium from P. maniculatus, P. leucopus and T. minimus. Spirochetes were isolated from at least one tissue from all of these animals and no isolations were obtained from any of the S. lateralis. Thus, three of the four rodent species tested are susceptible to, and could harbor, B. burgdorferi.

Comparative reproduction and nonparasitic development of Boophilus microplus and hybridized Boophilus ticks (Acari: Ixodidae) under natural field conditions in subtropical south Texas.

The reproductive biology and nonparasitic development of Boophilus microplus (Canestrini) and hybridized Boophilus ticks (B. annulatus (Say) male x B. microplus female) held under natural field conditions in south Texas throughout the year were compared. Comparisons between the two types of ticks indicated that the ovipositional biology (percentage of ovipositing females and number of eggs laid) of the females favored hybrid ticks during some months and B. microplus ticks during other months. However, on a year long basis, there was virtually no difference in the percentage of ovipositing females or in the number of eggs deposited by hybrid females as compared to B. microplus. The duration of each nonparasitic development period (preoviposition period, incubation period of eggs, and larval longevity) showed that both types of ticks had very similar developmental and survival rates during the year. Generally the difference in duration of each of the nonparasitic parameters was < 8 days, leading to a high degree of synchrony of the nonparasitic developmental rates between the two types of ticks. On the other hand, egg hatchability of hybrid ticks was consistently lower than pure-strain B. microplus throughout the year with significantly lower hatch rates occurring in April, July, August, October, and November. Thus, results obtained on percentage of ovipositing females, number of eggs laid, preoviposition period, incubation period of eggs, and larval longevity provide encouragement for the possible use of sterile hybrid males as a means of eliminating a native B. microplus population. In contrast, results of the egg hatchability of the two types of ticks indicate that the selective advantage afforded to B. microplus could have an adverse affect on the success of a sterile hybrid male program by making the number of hybrid ticks necessary to eliminate a native population prohibitively high.

Experimental infection of the eastern chipmunk (Tamias striatus) with the Lyme disease spirochete (Borrelia burgdorferi).

Lyme disease, caused by the spirochete Borrelia burgdorferi, is endemic in the northeast, north-central, and Pacific coastal states of the United States. The eastern chipmunk (Tamias striatus) is found throughout the disease-enzootic regions except along the Pacific coast, and may serve as an important reservoir host in some locations. To characterize their potential as a host, 11 adult chipmunks were inoculated with 10(5) spirochetes from strains of B. burgdorferi isolated from Peromyscus leucopus and Ixodes scapularis in a hyperendemic area of Westchester County, New York (USA). All inoculated chipmunks became infected. Spirochetemias were detected by isolating spirochetes in Barbour-Stoenner-Kelly media in eight of eight chipmunks, and lasted for 2 to 5 days. Spirochetes were isolated from the ears of all animals, starting at 1 wk and for < or = 4 mo, and from various internal organs at 133 days post-inoculation. Laboratory-reared larval I. scapularis ticks became infected with spirochetes after feeding on two of the inoculated chipmunks.

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.

Survival of larvae of Boophilus annulatus, Boophilus microplus, and Boophilus hybrids (Acari: Ixodidae) in different temperature and humidity regimes in the laboratory.

The survival period for larvae of Boophilus annulatus (Say), Boophilus microplus (Canestrini) and hybridized Boophilus ticks was determined by exposure to various combinations of temperature (20, 25, 30 and 35 degrees C) and relative humidity (32, 63, 75, 84 and 97% RH) in the laboratory. Results indicated that within a given temperature and RH regime, there was no difference (P greater than 0.05) in larval survival among the three species tested, indicating that these ticks respond similarly over a wide range of temperature and RH combinations. Larval survival in all three species was longest (P less than 0.05) at 20 degrees C and either 84 or 97% RH. With each increase in temperature at the 84 and 97% RH treatment levels, there was a corresponding significant (P less than 0.05) decrease in larval survival. When the temperature reached 35 degrees C at all humidities or when the RH was 63% or less at all temperatures, the mean larval survival period was 43 days or less in all cases and little difference (P greater than 0.05) was observed among the treatment regimes included. Results suggest that at a RH of 75% and more, the temperature is the determining factor in larval survival, whereas at a RH of 63% and less the RH is the determining factor in larval survival, regardless of temperature.

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.

Effects of prolonged exposure at low temperature on Boophilus microplus (Acari: Ixodidae).

The effects of prolonged exposure at 12 degrees C on oviposition were determined for Boophilus microplus (Canestrini) females. The percentage of females ovipositing remained high (greater than or equal to 80%) for exposure times of less than or equal to 30 d, but declined to 0.0 and 6.7% when females were held for 90 and 105 d, respectively. Duration had no effect on the preoviposition period after females were returned to optimum temperature (25 degrees C). The number of eggs laid per female declined at each exposure interval between 15 d, where no significant adverse effect on egg deposition was noted, and 90 d, where no eggs were laid. The hatch of eggs derived from exposed females showed a similar decline as the exposure time increased. A strong negative relationship between exposure time and egg viability was observed for eggs deposited at optimum temperature and then exposed to low temperature at intervals of 0 to 105 d. Larval longevity also showed a substantial negative relationship to exposure time. Results of the investigation and differences noted to results with B. annulatus (Say) also are discussed.

Efficacy and stability of wettable powder amitraz in field and laboratory studies against Boophilus annulatus (Acari: Ixodidae) in south Texas.

A study was done at the USDA-ARS, Cattle Fever Tick Research Laboratory, Mission, Tex., to determine the efficacy of a 50% wettable powder (WP) amitraz formulation applied as a whole-body spray in a standard dip vat, and in a laboratory bioassay against Boophilus annulatus (Say) on cattle. A study also was done at the King Ranch in Kleberg County, Tex., to determine the stability of 50% WP amitraz in a dip vat under South Texas conditions Cattle were infested with all parasitic life stages of B. annulatus and were sprayed or dipped with a concentration of 0.025% amitraz. As determined by calculations of the index of reproduction, the whole-body spray treatment provided 86% control of the ticks and the dip treatment provided 99.8% control. Laboratory bioassay results compared favorably with those obtained with the dip vat treatment. Amitraz WP settled very rapidly in the freshly charged ranch vat. However, as more cattle were dipped and the vat became polluted with dirt and excrement, settling occurred much more slowly. Overall, amitraz remained stable in the vat during the test period.

Suitability of white-tailed deer as hosts for cattle fever ticks (Acari: Ixodidae).

White-tailed deer (Odocoileus virginianus L.) were evaluated as an alternate host for the cattle fever tick, Boophilus annulatus (Say). The total of adult female ticks recovered and associated reproductive parameters were compared between deer and bovine hosts. Significantly fewer ticks were recovered from deer than from cattle; this was attributed to the grooming behavior of the deer. The mean weight of ticks taken from deer (243.5 mg) was significantly less than that of ticks recovered from cattle (344.5 mg) with about 15% of the ticks from deer prematurely detached. Egg mass weight from the ticks recovered from deer (129.4 mg) was significantly less than that of ticks from cattle (172.1 mg). A significant difference was observed between the percentage hatch of the eggs of ticks from deer (63.1%) and ticks from cattle (76.7%). The index of reproduction was about 20 times greater for the ticks from the cattle than for the ticks from the deer. When the larval progeny of the ticks recovered from deer were reared on cattle, significantly fewer adult females were recovered than when larvae of ticks recovered from cattle were reared on cattle. This indicates a possible reduction in viability. All other parameters were comparable between the two groups. Thus, deer are biologically suitable hosts for B. annulatus, but significantly fewer ticks complete engorgement and those that complete engorgement have reduced fecundity.

Population estimation of the horse fly, Tabanus abactor (Diptera: Tabanidae), in northcentral Oklahoma.

A mark-recapture technique was used to estimate daily populations of host-seeking Tabanus abactor Philip. Daily population estimates were calculated for 31 and 37 d for areas of 2.1 km2 and 4.6 km2 for 1982 and 1983, respectively. In 1982, population estimates ranged from 22,767 to 2,125,322 flies/2.1 km2 per d with a mean of 467,277 flies/d. In 1983, population estimates ranged from 48,962 to 2,794,389 flies/4.6 km2 per d with a mean of 759,616 flies/d. The estimated average number of host-seeking females per hectare was 2,225 (0.22 flies/m2) and 1,651 (0.17 flies/m2) for 1982 and 1983, respectively. Although daily population estimates fluctuated greatly, primarily because of fluctuation in recapture rates, seasonal trends during both years were similar. The estimates calculated in this study appear to be representative of the population of T. abactor in northcentral Oklahoma based on previous studies determining seasonal abundance.

Cage design for the confinement of deer and goats infested with ectoparasites.

The design for a cage for the confinement of white-tailed deer (Odocoileus virginianus) and goats (Capra sp.) for use in ectoparasite studies is presented. The cage was successfully used for the infestation of these hosts with Boophilus annulatus and may be useful for host/parasite studies using similar hosts and other ectoparasites. The cage will accommodate mature hosts with adequate space for normal grooming activity. A detailed description of materials and methods for the assembly of the cage is provided in the text.