Showing papers in "Journal of Medical Entomology in 1993"

Dynamic Life Table Model for Aedes aegypti (Diptera: Culicidae): Analysis of the Literature and Model Development

Abstract: 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.

Blood-Feeding Patterns of Aedes aegypti (Diptera: Culicidae) Collected in a Rural Thai Village

Abstract: Aedes aegypti (L.) were collected by aspiration once each week from in and around houses in a rural village in Chachoengsao Province. Thailand, during May 1990 to June 1991. Of the 1,230 specimens analyzed with a sandwich ELISA, 73% reacted to one or more of the seven hosts which we tested. Eighty-eight percent (789/896) of all detectable meals were identified as being from a single host (human). Patent multiple meals (double and triple), of which one was always human, were detected in 7% (66/896) of the specimens. The probability of feeding on a human, either as a single host or in a patent mixed meal, was > 0.90 during 12 of the 14 mo of the study; this probability never dropped below 0.85. The probability of Ae. aegypti taking multiple blood meals during a single gonotrophic cycle varied widely from 0.0 to 1.0. The possibility is discussed, including epidemiological significance, that there are seasonal fluctuations in the frequency of multiple feeding by Ae. aegypti and that multiple feeding increases in concert with seasonal trends for dengue infections of humans in Thailand.

Dynamic Life Table Model for Aedes aegypti (Diptera: Culicidae): Simulation Results and Validation

Abstract: 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.

Conspecificity of the ticks Ixodes scapularis and I. dammini (Acari: Ixodidae).

Abstract: Reciprocal crosses between Ixodes dammini Spielman, Clifford, Piesman & Corwin from Massachusetts and Ixodes scapularis Say from Georgia produced offspring through the F3 generation when the experiment was discontinued. Reciprocal I. dammini × Ixodes pacificus Cooley & Kohls (California) and I. scapularis × I. pacificus crosses produced F1 progeny; however, all progeny were sterile. Assortative mating experiments between I. dammini and I. scapularis indicated that males and females of both species mated with the opposite sex of heterospecific or conspecific ticks when there was a choice. Conventional discriminant analysis of morphometric measurements of ticks from Georgia, North Carolina, Maryland, Massachusetts, and two populations of F1 hybrids indicated that there were recognizable differences. However, size-free (sheared) discriminant analysis indicated that these differences were largely size-dependent, with much overlap of the four eastern and two hybrid populations but no overlap with I. pacificus from California. Analysis of chromosomes (morphology and C band) indicated no differences between the Georgia and Massachusetts populations but showed a difference between them and the California population of I. pacificus. Analysis of isozymes showed that the genetic identity value for the Georgia and Massachusetts populations was within the normal range for conspecific populations, whereas the California population indicated congeneric but not conspecific relatedness to the Georgia and Massachusetts populations. Life cycle data collected under similar laboratory conditions showed no differences in length of feeding and molting periods among Georgia, Massachusetts, and California populations. These data and results of the work of other authors on tick host preferences and vector competence indicate that I. dammini is not a valid species separate from I. scapularis. Because the name Ixodes scapularis Say, 1821, has priority over the name Ixodes dammini Spielman, Clifford, Piesman & Corwin, 1979, I. dammini is relegated to a junior subjective synonym of I. scapularis (based on Article 23 of the International Code of Zoological Nomenclature).

Detection of multiple blood feeding in Aedes aegypti (Diptera: Culicidae) during a single gonotrophic cycle using a histologic technique

Abstract: We evaluated a histologic technique for its usefulness in detecting multiple blood feeding by Aedes aegypti (L.) in a single gonotrophic cycle. To standardize the procedure, we carried out a laboratory study in which 166 mosquitoes imbibed two blood meals at known intervals. Eighty percent (78/98) of the multiple meals were detected when the interval between meals was from 1 to < to = 24 h and the time from the second meal to fixation ranged from 0 to 12 hr. At intervals outside this range, only 34% (23/68) of the multiple meals were detected. Overall, 61% (101/166) of the double meals were detected. Examination of 96 engorged Ae. aegypti collected by aspiration from inside houses in San Juan, Puerto Rico, indicated that 50% had imbibed multiple meals. Most wild-caught mosquitoes took their last meal the day before capture, and most multiple feeders fed twice on consecutive days. A dark line of digested blood, or heme, around the first meal and a physical separation between meals were the most useful histologic parameters for detecting multiple feeding in wild Ae. aegypti. An association of multiple feeding with advanced stages of oocyte development suggests that, at the time of collection, most Ae. aegypti from the study site had fed twice in each gonotrophic cycle. We conclude that, although it is labor intensive, histologic examination is an appropriate technique for a longitudinal, community-wide survey of multiple feeding by Ae. aegypti.

Efficient Transmission of Tick-Borne Encephalitis Virus Between Cofeeding Ticks

Abstract: Most of the data on oral infections of ticks with tick-borne encephalitis virus have been derived from experiments using animals infected by syringe inoculation. To mimic the natural conditions of virus transmission, tick-borne encephalitis virus-infected Ixodes ricinus (Linnaeus) or Rhipicephalus appendiculatus Neumann adults (donors) were cofed with uninfected nymphs (recipients) of either tick species on uninfected guinea pigs. Two tick-retaining cells were attached to each guinea pig: cell 1 contained uninfected nymphs and virus-infected adults, and cell 2 contained uninfected nymphs. Following engorgement, 55% of I. ricinus nymphs and 65% of R. appendiculatus nymphs were shown to have acquired the virus while cofeeding with I. ricinus donor ticks. Similarly, 66% of R. appendiculatus recipient nymphs that cofed with R. appendiculatus virus-infected adults were infected. Some of the guinea pigs on which the ticks cofed were apparently nonviremic. The results indicate that efficient transmission of tick-borne encephalitis virus can occur between cofeeding ticks even when the host on which they feed does not develop a detectable viremia.

Variation in Attractiveness of Human Subjects to Malaria Mosquitoes (Diptera: Culicidae) in The Gambia

Abstract: During experimental hut trials to assess the efficacy of insecticide-treated bednets against malaria mosquitoes, we observed that human subjects varied consistently in their attractiveness to mosquitoes. Attractiveness was assessed by estimating the numbers of wild Anopheles gambiae Giles mosquitoes entering a hut in which a man was sleeping, and the numbers of human-bloodfed An. gambiae sensu lato collected from each hut each morning. Five trials were carried out at Wali Kunda in rural Gambia during 2.5 yr. During each 6-wk trial a man slept under a bednet in each of the six huts. Morning collections of mosquitoes from the room, enclosed verandas, and window traps of each hut provided estimates of the number of mosquitoes that had entered during the night. Blood meals were analyzed using an ELISA technique to identify those mosquitoes feeding on humans. Specimens were collected by field workers, not the subjects; therefore, sampling was independent of the subjects' ability to catch mosquitoes. Moreover, the trials were designed to measure the relative attractiveness of individual sleepers to mosquitoes, allowing for other sources of variation (i.e., among huts, bednets, nights, and day of the week). Attractiveness of men to mosquitoes differed significantly among individuals as indicated by the consistent differences between the numbers of mosquitoes entering each man's hut and the numbers feeding on each man. However, the two measures of attractiveness were apparently independent of each other: subjects who attracted consistently high numbers of vectors into their hut did not necessarily have high numbers of mosquitoes feeding on them. These findings support the view that some individuals within a community are at greater risk from mosquito-borne pathogens than others.

Host-Feeding Patterns of Aedes albopictus (Diptera: Culicidae) at a Temperate North American Site

Abstract: Precipitin tests and ELISA were used to investigate host-feeding patterns of 172 blood-fed Aedes albopictus (Skuse) collected at Potosi, MO, during the summers of 1989 and 1990. One hundred ten (64.0%) mosquitoes had fed on mammals, 29 (16.9%) on birds, and none on turtles or snakes. Thirty-three (19.2%) mosquitoes failed to react in all tests. Eighty-six (78.2%) of the 110 mammalian feeds were positive for lower taxa as follows: rabbit, 24.5%; deer, 14.5%; dog, 13.6%; human, 8.2%; squirrel, 7.3%; opossum, 4.5%; myomorph rodents other than Rattus, 3.6%; raccoon, 0.9%; and bovine, 0.9%. Positive feeds were not detected for the following mammals: cat (n = 99); horse (n = 95); Rattus (n = 84); and swine (n = 84). Fourteen (48.3%) of the 29 avian feeds were positive for lower taxa as follows: Passeriformes, 24.1%; Columbiformes, 17.2%; Ciconiiformes, 3.4%; and quail, 3.4%. These data, the first on host-feeding patterns for Ae. albopictus populations in the New World, indicate that Ae. albopictus is an opportunistic feeder that utilizes a wide variety of hosts and, therefore, has the potential to become involved in the transmission cycles of indigenous arboviruses.

Effect of Temperature on the Transmission of Western Equine Encephalomyelitis and St. Louis Encephalitis Viruses by Culex tarsalis (Diptera: Culicidae)

Abstract: The extrinsic incubation rate (inverse of the time in days from infection to median transmission) of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses by laboratory strains of Culex tarsalis Coquillett increased as a linear function of incubation temperatures from 10 to 30 degrees C. The estimated temperatures for zero transmission thresholds (intercept of the X axis) were 10.9 and 14.9 degrees C, and the number of degree days above these thresholds required for median transmission (inverse of the slope) was 67.6 and 115.2, respectively. Although the bodies of most Cx. tarsalis females remained infected and the WEE viral titer did not decrease significantly throughout the incubation periods at all temperatures, transmission rates by females incubated at 20 to 30 degrees C decreased markedly after peaking at 7-10 d after infection. In contrast, midgut escape and salivary gland infection barriers limited the transmission rates of SLE virus at all temperatures, but these rates did not decrease markedly as a function of incubation time, indicating that virus modulation did not occur. Degree-day models were used to calculate monthly changes in the duration of the extrinsic incubation period for WEE and SLE viruses in the San Joaquin and Coachella valleys based on mosquito temperatures estimated by combining nocturnal air and diurnal resting site temperatures. Temperatures in the San Joaquin Valley averaged 5 degrees C cooler than in the Coachella Valley, proportionately shortening the duration of the potential transmission season for WEE virus from 10 to 8 mo and for SLE virus from 8 to 5 mo, respectively.

Dispersal of the sand fly Lutzomyia longipalpis (Diptera: Psychodidae) at an endemic focus of visceral leishmaniasis in Colombia.

Abstract: Mark-release-recapture studies were carried out during 1990-1991 in El Callejon, Colombia, an endemic focus of American visceral leishmaniasis, to study the longevity, dispersal, and flight range of the principal vector, Lutzomyia longipalpis Lutz & Neiva. Several groups of wild-caught (n = 1,539) and laboratory-reared F1 (n = 2,208) sand flies were marked with fluorescent dusts and released. Recaptures at daytime resting sites, on animal bait, and in CDC light traps were made for 20 d following each release. From 2 to 9% of L. longipalpis were recaptured, a rate which differed between the sexes (7.7% male, 1.5% female). Overall, 49% of the L. longipalpis were recaptured between 0 and 50 m, 48% within 100 and 300 m, and nearly 3% at > or = 0.5 km from the release site. Sex differences in recapture site, distance flown, and direction were observed. Our results indicate that the dispersal behavior of peridomestic populations of L. longipalpis compares more closely with that of Old World sand fly species from similar habitats than to the sylvan Neotropical phlebotomines.

Multiple Blood Meals as a Reproductive Strategy in Anopheles (Diptera: Culicidae)

Abstract: Multiple blood meals within one gonotrophic cycle were taken readily at 6-24-hr intervals by nulliparous female Anopheles albimanus Wiedemann. Up to five blood meals were ingested and with each blood meal, more primary follicles matured as well as some secondary ones. This produced an irregular oviposition pattern that questioned the concept of gonotrophic concordance. The first blood meal initiated limited vitellogenesis and the maturation of few follicles. Instead, protein was diverted to the synthesis of maternal, extraovarian lipid and protein deposits; fecundity increased with successive blood meals. This pattern of protein and energy utilization may be explained in terms of the low caloric lipid and protein content of nulliparous females before the first blood meal. In An. albimanus, a critical female body size, approximately 0.25 cal of lipid per female, was necessary for the initiation of oogenesis; below this threshold at least two blood meals were required for follicle maturation. Less than 10% of the caloric input from a blood meal was utilized in the synthesis of ovarian protein and lipids, whereas a similar percentage was transferred to maternal deposits of protein and lipid. In nonoogenic females, a replete blood meal increased total body protein and lipid by 17 and 113%, respectively. Altogether, the efficiency of blood protein utilization was rather low, as indicated by losses of excretory nitrogen that regularly exceeded 75% of the input. Anopheles gambiae Giles and An. stephensi Liston also fed multiply, but fecundity was less affected. Instead, maternal deposits were synthesized from the blood meal in substantial amounts. In general, gonotrophic discordance also was found routinely in these anopheline species. The ratio of yolk protein to lipids varied inter- and intra-specifically, as well as among consecutive blood meals, indicating a considerable plasticity in the caloric distribution of these two yolk components.

Review of Chrysomya rufifacies (Diptera: Calliphoridae)

Abstract: The taxonomy, distribution, bionomics, and medical and veterinary importance of C. rufifacies are reviewed and summarized. Late instars are beneficial as predators of the maggots of pathogen-transmitting and myiasis-producing flies, but strains from parts of Australia, India, and Hawaii are harmful as secondary myiasis agents. The effects of invasions, possible competition, and displacement of native calliphorids are discussed. The developmental duration of the immature stadia at different temperatures are reviewed. The primary literature (274 papers) dealing with this fly is given and cross-referenced by subject to serve as a reference to research entomologists and governmental agencies involved in the control of this livestock parasite in the Americas and Australia.

Development of Baits for Tsetse Flies (Diptera: Glossinidae) in Zimbabwe

Abstract: Analysis of host-oriented behavior of tsetse flies, Glossina morsitans morsitans Westw. and G. pallidipes Austen, led to a 10- to 1,000-fold improvement in the cost effectiveness of baits for surveys and control. Baits now are used widely to replace air and ground broadcasting of insecticides. Principles of behavioral analysis are discussed, with emphasis on the need to: confirm that the measurements made are the measurements required; assess the probability of flies executing single specific actions in response to each component of the overall stimulus from baits; count not only the flies that do one thing, but also the number that do the alternative(s); and use objective sampling devices of measured efficiency. The relevance to research with other flies is considered. The need for new tools to study continuously the field behavior of individual flies is stressed.

Standard system for infecting ticks (Acari: Ixodidae) with the Lyme disease spirochete, Borrelia burgdorferi.

Abstract: A standard system for infecting ticks with the Lyme disease spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner is defined. Rodents infected via tick feeding or inoculation of tick homogenates were more infectious to ticks than rodents infected with culture-derived spirochetes. White laboratory mice were more infectious than hamsters. Three strains of B. burgdorferi (JD1, B31, and WI210) produced batches of infected ticks with > 80% infection when mice were infected with tick-derived material. Ixodes dammini Spielman, Clifford, Piesman & Corwin were 3.6x more efficient than I. pacificus Cooley & Kohls in acquiring and maintaining infection with two California strains of B. burgdorferi, originally isolated from I. pacificus.

Reduced Abundance of Immature Ixodes dammini (Acari: Ixodidae) Following Incremental Removal of Deer

Abstract: The abundance of immature Ixodes dammini Spielman, Clifford, Piesman & Corwin was monitored for 9 yr (1983–1991) before and during the controlled, limited hunting of white-tailed deer, Odocoileus virginianus (Zimmerman), at a coastal Massachusetts site. Deer abundance was reduced from an estimated 350 during 1985 to ≍60 during 1991. Although annual fluctuations were large, mean larval I. dammini abundance declined from 20.8 per white-footed mouse, Peromyscus leucopus (Rafinesque), during 1983–1986 before deer reduction to 10.3 per mouse following deer reduction (1987–1991). Similarly, mean nymphal I. dammini abundance declined from 2.7 per mouse before intervention to 1.6 per mouse after intervention. Immature I. dammini population fluctuations were not associated with those of P. leucopus. The total population of larvae infesting P. leucopus declined from 3,596 ticks before intervention to 1,535 ticks after intervention. Concurrently, the total nymphal population declined from 417 ticks before intervention to 187 ticks after intervention. The number of feeding adult female I. dammini on deer increased as deer density decreased.

Environmental temperature on the vector competence of Culex univittatus (Diptera: Culicidae) for West Nile virus.

Abstract: The effects of the extrinsic incubation temperature on the vector competence of Culex univittatus Theobald for West Nile (WN) virus were studied. A mean titer of 7.0 log10 CPD50/ml of mosquito suspension was reached in orally infected mosquitoes after 11, 15, and 16 d of incubation at 26 and 30 degrees C and at fluctuating temperatures in an outside cage (mean temperature, 23.5 degrees C), respectively. In contrast, 22 and 58 d were required to reach the same titers at 18 and 14 degrees C, respectively. Transmission rates of 100% were reached after 58 d (14 degrees C), 22 d (18 degrees C), and 15 and 16 d (30 degrees C and outside). Except at 30 degrees C, transmission rates fluctuated; e.g., at 18 degrees C from day 19, the transmission rate was 80-100%, whereas at 14 degrees C on day 36, the transmission rate was 60% and thereafter 20-100%. The maximum transmission rate occurred concurrently with maximum titers of virus secreted into capillary tubes during in vitro transmission attempts. Mosquito longevity increased as incubation temperature decreased and was maximum at 114 d at 14 degrees C. Mosquitoes that were transferred from 14 to 26 degrees C after 49 d subsequently oviposited, engorged on a pigeon, and transmitted virus, which indicated the possibility for overwintering of WN virus in adult Cx. univittatus. Vector competence at outside cycling temperatures was intermediate between that at 26 and 30 degrees C, indicating that incubation at 26 degrees C would give a fair reflection of the vector competence of Cx. univittatus during the summer near Johannesburg. Two human epidemics of WN virus are reevaluated in the light of these results; it is concluded that, in addition to abnormal rainfall, higher than normal temperatures were important factors for their occurrence.

Reduced Abundance of Ixodes scapularis (Acari: Ixodidae) with Exclusion of Deer by Electric Fencing

Abstract: To assess the effect of deer exclusion on populations of Ixodes scapularis Say (formerly I. dammini Spielman, Clifford, Piesman & Corwin) in the northeastern United States, host-seeking ticks and ticks on white-footed mice, Peromyscus leucopus (Rafinesque), were monitored inside and outside a wooded, residential deer exclosure (approximately 3.5 ha) in Lyme, CT, in 1991 and 1992. Another deer exclosure was added in Lyme (approximately 7.4 ha) during 1992. Additional sample sites at other residences served as secondary controls. A seven-wire, slanted, high-tensile electric deer fence was used at both areas. Larvae of I. scapularis were 81.5% (1991) and 97.8% (1992) less abundant within the exclosure than immediately outside the deer exclosures. Nymphs of I. scapularis were 47.4% (1991) and 55.8% (1992) less abundant within the deer fence. The effect on adult ticks was mixed. No difference in tick abundance was seen at the 3.5-ha site. However, larvae, nymphs, and adults were 100, 83.8, and 74.1% less abundant, respectively, in plots at the 7.4-ha exclosure > or = 70 m from the deer fence and isolated from woodlands outside the fence by lawns, driveways, and buildings. The recovery of larvae and nymphs of I. scapularis from mice captured within the deer exclosures indicates that infestations of nymphs and adults are probably, at least in part, a result of the movement of these rodents. Based upon the number of nymphs per 100 m2 infected with Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner, the causal agent of Lyme disease, there were 73 and 82% fewer infected nymphs within the deer exclosures in 1992 in comparison with the number of infected ticks outside the fence and at the secondary control sites, respectively. The exclusion of deer in conjunction with other tick control strategies in large areas could substantially reduce populations of I. scapularis and the risk of acquiring Lyme disease.

Reduced abundance of Ixodes scapularis (Acari: Ixodidae) and lyme disease risk by deer exclusion

Abstract: The effect of deer exclosures upon the numbers of immature Ixodes scapularis Say, the vector of Lyme disease in the eastern United States, was examined at five sites in Westchester County, NY. Study areas ranged in size from 6 to 101 ha where deer had been excluded for a period of 25 yr by > 2.4-m-high fencing that surrounded each site. A total area of 40,506 m2 was drag-sampled during the study to measure tick abundance. Nymphal densities (ticks per 1,000 m2) averaged 4.6 (range, 1.3-9.6) inside exclosures and 27.7 (range, 7.3-79.4) outside. Larval densities averaged 36.7 (range, 1.2-132.1) inside exclosures and 354.4 (range, 7.5-914.5) outside. Comparisons between exclosure sites and outside areas immediately adjacent to the exclosure fence, where deer had unrestricted access, revealed that exclosures had 83% fewer host-seeking nymphs and 90% fewer host-seeking larvae. Tick numbers inside exclosures did not always decline with increasing distance from the fence. There was no significant difference in the rate of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner infection for host-seeking ticks collected inside (20%, n = 50) exclosures compared with ticks collected outside (26%, n = 50) exclosures. Deer fencing may provide a means of significantly reducing the abundance of I. scapularis and the risk of Lyme disease in relatively large areas without the need to reduce or eliminate the deer population.

Reduction of nymphal Ixodes dammini (Acari: Ixodidae) in a residential suburban landscape by area application of insecticides.

Abstract: Seven formulations of three commercially available insecticides were evaluated for their ability to reduce the density of nymphal deer ticks (Ixodes dammini Spielman, Clifford, Piesman & Corwin) in a suburban residential area endemic for Lyme disease. In total, 101 residential properties in Mount Kisco, Westchester County, NY, were treated with one of the following: carbaryl emulsifiable concentrate (EC) at 0.6 or 1.1 kg AI/ha, carbaryl granular (GR) at 4.5 kg AI/ha, chlorpyrifos wettable powder (WP) at 0.6 or 1.1 kg AI/ha, chlorpyrifos GR at 1.1 kg AI/ha, or cyfluthrin EC at 0.1 kg AI/ha. All three chemicals were found to reduce the density of nymphs on treated properties significantly compared with untreated controls. Percentage of reduction of nymphs ranged from 67.9% for carbaryl EC 1.1 kg AI/ha to 97.4% for chlorpyrifos WP 1.1 kg AI/ha. There was no difference in the amount of reduction provided by each formulation of the same chemical, but carbaryl formulations were significantly less effective than chlorpyrifos or cyfluthrin. Nymphal densities after treatment remained low throughout the course of the study, indicating that a single well-timed spring application is sufficient to provide significant reduction of nymphs for an entire transmission season.

Spatial and temporal patterns of Ixodes scapularis (Acari: Ixodidae) in southeastern Connecticut.

Abstract: The spatial distribution and abundance of host-seeking Ixodes scapularis Say (formerly I. dammini) were studied in 10 residential sites in woodlands during 1989–1991. In total, 7,385 larval, 2,202 nymphal, 164 female, and 248 male I. scapularis were collected in Old Lyme, Lyme, East Haddam, and Chester, CT. The majority of host-seeking I. scapularis larvae (84.2%) and nymphs (73.5%) were collected within the woodland plots, whereas a large proportion of the adults were recovered from lawn (36.4%) and transitional areas between lawns and forest (20.9%). The majority (71.1% of 1,244) of all stages of I. scapularis on the lawn were recovered within 1 m of the lawn edge, particularly along woods and stone walls. In contrast, most of the I. scapularis (60.0% of 3,065) collected in the woods were >3 m from any defined edge and ticks were recovered throughout the forested plots. The abundance of nymphs on the lawns (1.0–29.4 nymphs per ha per sample) and in the woods (17.3–271.5 nymphs per ha per sample) varied significantly among the 10 residences during June and July. The average rate of infection by Borrelia burgdorferi, the causal agent of Lyme disease, in the nymphs was 14.2, 14.1, and 15.5% for 1989, 1990, and 1991, respectively. The abundance of I. scapularis nymphs in the forest was significantly higher in June 1991, than in 1989 or 1990. The risk of exposure to infected nymphs of I. scapularis varied spatially with the landscape and individual residence, and temporally.

Distribution of container-inhabiting Aedes larvae (Diptera: Culicidae) at a dengue focus in Thailand.

Abstract: A Thai village with dengue transmission was surveyed for Aedes aegypti (L.) and Ae. albopictus (Skuse) larvae at the end of the rainy season. All containers (1,819) in 186 households were surveyed, recording the presence of larvae, container type, level of water, kind of cover, and location. The number of positive containers was proportional to the total number of containers in a household. Standard water jars (≍200 liters) contributed 57% and small water jars (<100 liters) contributed 16% of the total infested containers. Each of the other 10 container types contributed ≤10% of the total infested containers and were not statistically different from each other. Containers inside houses were infested significantly more often than were those outdoors, those under elevated houses or roofs, or those in bathrooms. Unexpectedly, standard water jars located outdoors that were covered with commercial metal lids were infested significantly more often than were uncovered jars.

Seasonal abundance of stable flies and house flies (Diptera: Muscidae) in dairies in Alberta, Canada.

Abstract: Seasonal abundance of stable flies and house flies was studied at four dairies in southern Alberta, Canada, from May to October in 1989, 1990, and 1991. Stable flies were active from May to October in all years and showed population peaks in August and September. The weekly rate of change of stable fly populations was influenced by temperature and accumulated degree-days above 10 degrees C. The weekly rate of change of stable fly populations showed four peaks which were attributed to the emergence of an initial generation which had overwintered, followed by an additional three generations. Stable fly attacks on dairy cows occurred mainly from July through October, corresponding with the last two generations. House fly population abundance was much lower than stable fly populations and showed peaks in June, July, and September. Weekly changes in house fly abundance were not influenced by temperature and only weakly influenced by accumulated degree-days above 10 degrees C. Peaks in the weekly rate of change were associated with emergence of an initial, overwintering generation followed by four generations produced throughout the summer.

Different developmental strategies in two boreal blow flies (Diptera: Calliphoridae)

Abstract: The minimum developmental rates of Protophormia terraenovae (Robineau-Desvoidy) and Calliphora vomitoria (L.) were measured at four temperatures: 12.5, 23, 29, and 35 degrees C. Although both species are Holarctic in distribution and hemisynanthropic, there are important differences in the response of their developmental stages to various temperature regimes. The eggs and feeding larvae of P. terraenovae responded linearly to temperatures from 23 to 35 degrees C, but development was retarded at 12.5 degrees C. The feeding larvae took about 11 times longer to develop at 12.5 than at 23 degrees C, indicating poor cold adaptation. The converse is true of C. vomitoria, which took only 2 times as long at 12.5 than at 23 degrees C but failed to complete development at 29 and 35 degrees C. Our laboratory data and field studies of others suggest that, in nature, P. terraenovae and C. vomitoria prefer breeding in larger carcasses as a survival tactic and means of extending their distribution into colder regions.

Hypersensitivity of Horses in British Columbia to Extracts of Native and Exotic Species of Culicoides (Diptera: Ceratopogonidae)

Abstract: Six horses from British Columbia severely affected by Culicoides hypersensitivity, a seasonal dermatitis caused by the bites of Culicoides spp., were inoculated intradermally with extracts of six species or forms of Culicoides from British Columbia, United States, and Israel. Two native and four exotic species were thought to cause the disease in their own geographical area. The horses developed large welts within 20 min after injection of any of the six extracts, indicating an immediate (type I) reaction. The skin reactions caused by each extract peaked at or after 24 h, indicating an additional delayed (type IV) reaction. These reactions confirm that the reactions to the extracts were allergic in nature. The horses were physically irritated by each extract and developed transverse ridges in the injection area similar to those seen in natural cases of the disease, again indicating an allergic reaction. Previous studies showed that control horses injected with Culicoides extract showed only a small, short-term reaction, with no delayed component, no irritation, and no transverse ridge development, further indicating that the observed reactions were allergic in nature. The affected horses reacted to all extracts of Culicoides, despite the fact that they had not been previously exposed to most of the species, indicating that the allergen(s) was present in all the extracts tested.

Laboratory evaluation of repellents against four anopheline mosquitoes (Diptera: Culicidae) and two phlebotomine sand flies (Diptera: Psychodidae).

Abstract: Deet, the lactone CIC-4, and the piperidine compounds A13-37220 and A13-35765 were evaluated for initial repellency against laboratory-reared Anopheles albimanus Wiedemann, An. freeborni Aitken, An. gambiae Giles, An. stephensi Liston, and Phlebotomus papatasi (Scopoli) using a dose-response testing procedure on human volunteers. In addition, deet and CIC-4 were tested against Lutzomyia longipalpis (Lutz & Neiva). In general, the repellency of A13-37220, A13-35765, and CIC-4 was not markedly different from that of deet against each species tested; however, the different species varied greatly in response to the repellents. Overall, An. stephensi, L. longipalpis, and P. papatasi were the most sensitive, and An. albimanus the most tolerant species. The four repellents subsequently were tested against An. stephensi and An. albimanus to determine the duration of repellency. AI3-37220 provided effective (> 90%) protection against An. stephensi bites for 7 h, whereas deet, AI3-35765, and CIC-4 provided 6, 5, and 3 h of protection, respectively. Each of the four compounds provided < 1 h of protection against An. albimanus bites.

Maintenance by hares of European Borrelia burgdorferi in ecosystems without rodents.

Abstract: During May-October 1991 shrews (Sorex spp.), small rodents (Apodemus spp., Clethrionomys glareolus [Schreber]), and hares (Lepus spp.) were sampled near Stockholm, Sweden. Nymphal Ixodes ricinus (L.) derived from blood-engorged larvae collected from these mammals were investigated by phase-contrast microscopy and immunofluorescence assay (IFA) using the monoclonal antibody H6831 directed against the OspB protein of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner. Three rodent species, previously known as vertebrate reservoirs for B. burgdorferi in Switzerland, are competent reservoirs of Lyme borreliosis also in Sweden. The first data are presented showing Lyme disease reservoir competency of European insectivores (Sorex araneus L., S. minutus L.) and lagomorphs (Lepus europaeus Pallas, L. timidus L.). Lagomorphs are the only European vertebrates known to be both competent reservoirs for B. burgdorferi and a source of blood for all stages of I. ricinus. We propose that Lyme borreliosis can be maintained for prolonged periods on islands, without rodents and insectivores, if lagomorphs are present; and, in such ecosystems, fluctuations of lagomorph population levels may influence the numbers of B. burgdorferi-infected ticks and hence the risk of human Lyme disease infection.

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

Abstract: 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.

Relative importance of bird species as hosts for immature Ixodes dammini (Acari: Ixodidae) in a suburban residential landscape of southern New York State

Abstract: Abundance of birds and their tick parasites were estimated in a residential community located in Westchester County, NY, where Lyme disease is endemic. In total, 36 bird species (416 captures) were collected, of which 25 species (69%) were parasitized by ticks. Ixodes dammini Spielman, Clifford, Piesman & Corwin composed 96.4% of the 1,067 ticks found on birds. The bird species most heavily parasitized was house wren, Troglodytes aedon Vieillot (11.1 Ixodes dammini per bird). Relative density estimates of birds, using fixed circle radius counts, revealed dominance by the American robin, Turdus migratorius L. (29.3%), followed by the common grackle, Quiscalus quiscula (L.) (9.3%). Bird density estimates were combined with tick abundance data to calculate an importance value for each bird species as a host for immature I. dammini. The American robin was most important, accounting for 72.7% of all larval I. dammini found on birds, followed by the common grackle (8.2%) and the house wren (5.9%). Both the American robin and house wren are reservoir competent for the Lyme disease spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner and therefore may contribute to the risk of Lyme disease for humans. During August when larvae were most prevalent, 39% of the American robins and 70% of the common grackles were observed on lawns. These species are probable contributors to nymphal I. dammini populations found on lawns.

Distribution and seasonal and diurnal activity patterns of Eutrombicula alfreddugesi (Acari: Trombiculidae) in a forest edge ecosystem.

Abstract: Microclimatic and vegetative effects on the population size and activity patterns of larval Eutrombicula alfreddugesi Oudemans, 1910 (Acari: Trombiculidae), were investigated in Nebraska between latitude 40 degrees 0'0" N and 40 degrees 1'21" N. Larval population densities along a forest edge were greatest in areas of high relative humidity, moderate temperature, low incident sunlight, and increasing substrate vegetation. Although chigger populations existed throughout the forest edge, larger populations concentrated in short- to tall-grass transition zones. Chiggers were rarely found in the undergrowth beneath the tree canopy. Chigger activity correlated with a microclimatically driven diurnal rhythm. Activity was greatest during the late afternoon-early evening, between 1530 and 1930 hours (CDST). Larval activity dropped to low levels and remained so until sunrise; this period of reduced activity occurred between 1930 and 0530 hours. Small increases in larval activity occurred around sunrise (approximately 0600-0700 hours). Between about 0700 and 1530 hours, larval E. alfreddugesi were inactive and did not respond to normal sampling stimuli. Larval populations appeared in late April through early May, peaked in abundance in late June and early July, diminished through late summer, and disappeared in midautumn as the ground began to freeze.

Reported cases of human plague infections in the United States, 1970-1991.

Abstract: From 1970 to 1991, 295 indigenous cases and one imported case of human plague were reported in the United States. Eighty-two percent of the total indigenous cases occurred in New Mexico, Arizona, and Colorado. Ninety-three percent of these cases had onset in the months of April through November. Most cases (89%) presented as bubonic or septicemic plague, or both. Cases were reported more frequently in males (58%), and male mortality exceeded that of females (17 versus 11%). Ground squirrels were the most frequently implicated sources of infection in cases associated with flea bites, and domestic cats were found to play an increasingly important role in transmission of disease to humans during these decades.