Vector competence of Aedes vexans (Diptera: Culicidae) for West Nile virus and potential as an enzootic vector.

Vector competence of Aedes vexans (Meigen) and Culex pipiens pipiens L. (Diptera: Culicidae) for West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was compared. Infection rates of both species were similar 14 d after feeding on chickens, with WNV titers ranging from 10(4.2) to 10(8.7) plaque-forming units (PFU)/ml. Median infectious doses and 95% confidence intervals (CI) were 10(6.0(5.8, 63)) and 10(5.7(5.4, 5.9)) PFU for Ae. vexans and Cx. p. pipiens, respectively. WNV transmission was not observed in Ae. vexans that fed on chickens with WNV titers < 10(5.0) PFU/ml, in contrast to a mean (95% CI) transmission rate of 7(2,18)% for Cx. p. pipiens. Mean WNV transmission rates for Ae. vexans and Cx. p. pipiens were 13(7,21)% and 10(5,19)%, respectively, after feeding on chickens with WNV titers of 10(5.3 +/- 0.1) and 10(5.7 +/- 0.1) PFU/ml, and 31(25,37)% and 41(30,53)% after feeding on chickens with WNV titers > or = 10(6.1 +/- 0.1) PFU/ml. Time postinfection (p.i.) significantly influenced WNV transmission by Ae. vexans as indicated by a nearly 10-fold increase in transmission rate between days 7 and 14 p.i. Mean WNV load expectorated with saliva ofAe. vexans was 10(2.4(2.1, 2.7)) PFU, and it was not significantly affected by the titer of chickens on which they originally fed or time p.i. These data indicate that vector competence of the primarily mammalophilic Ae. vexans, which also feeds on birds, approaches that of Cx. p. pipiens for WNV. Because peridomestic mammals, such as cottontail rabbits, squirrels, and chipmunks, develop WNV titers infective for Ae. vexans, this species may play a significant role in WNV enzootic cycles.

The potential of Aedes triseriatus (Diptera: Culicidae) as an enzootic vector of West Nile virus.

The susceptibility of Aedes triseriatus (Say) (Diptera: Culicidae) to low levels of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was determined and compared with that of Culex pipiens L. to assess the likelihood of its participation in an enzootic cycle involving mammals. Ae. triseriatus and Cx. pipiens were exposed to WNV by feeding on baby chickens with WNV serum titers ranging from 10(4.1 +/- 0.1) to 10(8.6 +/- 0.1) plaque-forming units (PFU)/ml and from 10(4.1 +/- 0.1) to 10(7.0) PFU/ml, respectively. Infection rates and 95% confidence intervals (CIs) of 8% (4, 14) and 25% (15, 38) occurred in Ae. triseriatus and Cx. pipiens after feeding on chickens with WNV titers of 10(4.1 +/- 0.1) PFU/ml and increased to 65% (49, 79) and 100% (72, 100) in Ae. triseriatus and Cx. pipiens after feeding on chickens with titers of 10(7.1 +/- 0.1) PFU/ml. The mean infection rate of Ae. triseriatus ranged from 97% (84, 100) to 100% (79, 100) after feeding on chickens with WNV titers of > or = 10(8.2) PFU/ml. The infectious dose (ID)50 values for Ae. triseriatus and Cx. pipiens were 10(6.5) (6.4, 6.7) and 10(4.9) (4.6, 5.1) PFU/ml, respectively. The combined estimated transmission rate of Ae. triseriatus at 14 and 18 d after feeding on chickens with a mean WNV titer of 10(8.6 +/- 0.1) PFU/ml was 55%. Although Ae. triseriatus is significantly less susceptible to WNV than Cx. pipiens, the susceptibility of Ae. triseriatus to WNV titers < 10(5.0) PFU/ml and its ability to transmit WNV suggest that Ae. triseriatus has the potential to be an enzootic vector among mammalian populations.

Geographic distribution of ticks (Acari: Ixodidae) in Iowa with emphasis on Ixodes scapularis and their infection with Borrelia burgdorferi.

In Iowa, public concern regarding Lyme disease has increased markedly over the last decade. In response to these concerns, a statewide surveillance program was initiated in 1990 based on ticks received by the Department of Entomology at Iowa State University. Ticks were received from health care professionals, state government agencies, and the general public. A total of 5,343 ticks from all 99 Iowa counties were identified during the 12 years of this study. Dermacentor variabilis was the most numerous species, followed by Amblyomma americanum, and Ixodes scapularis. Dermacentor variabilis were distributed statewide, but A. americanum came primarily from southern Iowa counties. Prior to 1996, most I. scapularis came from counties along the Mississippi River. In the last 5 years, I. scapularis have been received from several counties in central and western Iowa and there is molecular evidence of infection with Borrelia burgdorferi in a substantial percentage of ticks. All I. scapularis were tested for the presence of B. burgdorferi. During the 12 years of this study, the presence of B. burgdorferi in I. scapularis varied from a low of zero percent in 1991-1995 to 18% in 1996. On average, fewer than 10% of all ticks examined per year were I. scapularis. In the 2000 tick season, the number of I. scapularis per year increased to 22% of submissions. This species further increased to 36.6% of ticks received in 2002.

Detection of human blood in the bat tick Carios (Ornithodoros) kelleyi (Acari: Argasidae) in Iowa.

The argasid tick Carios (Ornithodoros) kelleyi Cooley & Kohls is a common ectoparasite of bats and has been found in massive numbers in homes with associated bat colonies in eastern Iowa. This tick feeds nearly exclusively on bats in nature. Several inhabitants of infested homes complained of "bug bites" at night while asleep that may have resulted in erythematous, edematous, urticaric skin lesions and constitutional signs and symptoms. We provide laboratory evidence that a single, engorged C. kelleyi nymph contained host blood from a human female. The clinical implications of our findings are intriguing but unclear.

Evaluation of the efficacy of lambda-cyhalothrin applied by three spray application methods for emergency control of Aedes aegypti in Costa Rica.

An extended duration formulation of lambda-cyhalothrin (Icon CS) applied as either an ultra-low volume (ULV) or thermal fog spray from a new hand-held sprayer (Twin-Fog) or as a low-volume spray (LV) from a backpack mist blower against Aedes aegypti was evaluated in Costa Rica. Spray applications were made at the front door for 1 min or to each room for 15 sec for the ULV and LV, and thermal fog applications were made to houses in separate blocks for each treatment. The efficacy and duration of effectiveness of the spray was determined from sentinel caged mosquito mortality and mosquito collections from within houses using hand-held, battery-powered aspirators. Sentinel caged mosquito mortality in both open and sequestered locations was 97-100% for the ULV and thermal fog spray treatments, with control mortality less than 2%. Both ULV applications (front door and each room) provided 3 wk of significant control (P < 0.05) based on adult Ae. aegypti house collections.

Attraction of Anopheles (Diptera: culicidae) to volatile chemicals in Western Kenya.

Anopheles gambiae s.l. and Anopheles funestus Giles are the primary vectors of malaria in East Africa. Identification of host-location olfactory cues may increase trap sensitivity for vector control and surveillance programs. Solid-state army miniature light traps were operated near sleeping humans in huts at night without lights and augmented with the potential attractants: L-lactic acid, Limburger cheese volatiles, hexanoic acid, and carbon dioxide. Mosquito response varied between species and gender. Female An. funestus exhibited a greater response to traps baited with L-lactic acid in combination with carbon dioxide than carbon dioxide alone in two different experiments.

Detection of dengue virus neutralizing antibodies in bats from Costa Rica and Ecuador.

Neutralizing antibodies for dengue virus serotypes 1 and 2 and serotypes 2 and 3 were detected in 1998 in 12 of 53 (22.6%) and 3 of 10 (30.0%) bats sampled in Costa Rica and Ecuador, respectively. Dengue is a consistent health problem in the two Costa Rican communities in which bats were sampled. The high percentage of bats with neutralizing antibodies to dengue virus in these two Costa Rican communities suggests that bats may become infected with dengue virus. This appears to be the case in Costa Rica and Ecuador.

The abundance and seasonal distribution of Culex mosquitoes in Iowa during 1995-97.

The abundance and seasonal distribution of Culex mosquitoes were monitored by measuring oviposition activity during the summers of 1995, 1996, and 1997. Five species of culicine mosquitoes laid egg rafts in ovitraps. Egg rafts of 4 Culex species-Cx. restuans (54.98%), Cx. pipiens (25.41%), Cx. salinarius (12.18%), and Cx. tarsalis (0.14%)-constituted more than 92% of the total egg rafts collected. Culiseta inornata (0.1%) was the only other species to lay viable egg rafts in the ovitraps. A small percentage (7.19%) of egg rafts did not hatch; thus identification was not possible. Compared with New Jersey light trap data in a nearby area, the abundance of Cx. tarsalis and Cs. inornata was markedly underestimated with ovitraps. These data may also reflect differences in the specific location of the 2 trap sites rather than attractiveness of ovitraps for Cx. tarsalis and Cs. inornata. In general, Cx. restuans oviposition activity began in late May, and it was the dominant Culex species through June. After July 1, the number of egg rafts laid by Cx. restuans decreased continually until the end of the summer. Culex pipiens and Cx. salinarius oviposition began in early June and increased gradually during the summer. Over the course of a summer, there were about twice as many Cx. pipiens as there were Cx. salinarius. A crossover in the number of Cx. pipiens and Cx. restuans egg rafts occurred in late July or early August, depending on the year. By late August each year, Cx. pipiens was the most abundant species. At this time, Cx. restuans and Cx. salinarius populations were similar, but each was about half of the Cx. pipiens population. Differences in the abundance of all 3 species could not be explained by changes in ambient temperature (both minimum and maximum) or relative humidity, either within or among years.

Longevity and spontaneous flight activity of Culex tarsalis (Diptera: Culicidae) infected with western equine encephalomyelitis virus.

The longevity of an Iowa strain of Culex tarsalis Coquillett fed blood meals containing 2 concentrations of western equine encephalomyelitis virus from Iowa (WEE-7738) was compared with that of Cx. tarsalis fed blood without virus. Females exposed to 4.7-5.0 log TCID50 per mosquito of WEE-7738 did not live as long as mosquitoes exposed to 2.7-3.0 log TCID50 per mosquito or controls. Only 1% of mosquitoes fed blood containing the higher virus concentration survived to day 18 after exposure. However, 13% of mosquitoes fed blood with the lower virus titer and 19.5% of the controls were still alive on day 18 after exposure. Flight activity scores of Cx. tarsalis infected with 4.7-5.0 log TCID50 per mosquito of WEE-7738 were 27.5% lower, and there were 26.1% fewer spontaneous flights than noninfected controls from days 6-11 after infection. After day 8 after infection, infected Cx. tarsalis had 37.1% lower activity scores and 40.0% fewer spontaneous flights than noninfected controls. Virus infection did not affect how long a mosquito flew in a 24-h period (the daily flying time) or the duration of individual flights. The spontaneous flight activity pattern (circadian rhythm) of infected mosquitoes was identical to those of controls. Both infected and noninfected mosquitoes began spontaneous flight activity at 2000-2100 hours (CST) and were active throughout the entire dark phase of the 24-h cycle. Although mosquitoes were active throughout the night, there was a burst or peak of activity between 2200 and 2300 hours when the complete dark cycle began. These results indicate that the adverse effect of WEE infection on longevity and spontaneous flight activity of Cx. tarsalis may decrease vectorial capacity of Cx. tarsalis for WEE.

Vector competence of Culex tarsalis (Diptera: Culicidae) from Iowa for a sympatric strain of western equine encephalomyelitis (WEE-7738) virus.

Experiments were designed to evaluate the vector competence of Culex tarsalis Coquillet from an area (Sioux City) where Cx. tarsalis is most abundant in Iowa for western equine encephalomyelitis virus (WEE-7738). WEE-7738 was isolated from Aedes trivittatus (Coquillet) collected in Ames, IA, in 1977. Infection rate, dissemination rate, multiplication efficiency, and transmission rate were determined for this virus in the SC strain of Cx. tarsalis. SC strain of Cx. tarsalis was as susceptible to WEE-7738 as Californian strains of Cx. tarsalis were to BFS1703 strain of WEE; OID50 of SC Cx. tarsalis was 2.63 log TCID50 per mosquito and OID50 of Californian strains of Cx. tarsalis were 2.0-4.1 log PFU per mosquito. However, transmission of WEE-7738 (4.2%) by the SC strain of Cx. tarsalis was lower than those (10-60%) reported in other studies.

Relationship between weights of the engorged nymphal stage and resultant sexes in Ixodes scapularis and Dermacentor variabilis (Acari: Ixodidae) ticks.

The relationship between weights of the nymphal stage and their resultant sexes in Ixodes scapularis Say and Dermacentor variabilis (Say) was examined under laboratory conditions. Nymphs were fed to repletion on laboratory white mice, weighed individually, and held until they molted. Fully engorged I. scapularis nymphs that subsequently molted to females weighed between 3.8 and 6.4 mg (n = 64). Those nymphs that molted to males weighed between 2.0 and 3.2 mg (n = 57). Body weights of engorged nymphs that became females were significantly greater than those of nymphs that became males (P < 0.05). Similarly, the range of body weights of engorged D. variabilis nymphs that subsequently became females was 9.1-15.1 mg (n = 32), whereas those nymphs that molted to males weighed between 3.9 and 13.0 mg (n = 39). Body weights of engorged nymphs that became females were significantly greater than those of nymphs that became males (P < 0.05). This suggests that nymphs of both species that become females presumably imbibe more blood than those that became males.

Attraction of Culex pipiens pipiens (Diptera: Culicidae) to flower volatiles.

Methylene chloride extracts of flowers from 4 plants (common milkweed, Asclepias syriaca L; ox-eye daisy, Leucanthemum vulgare Lamarck; Canada goldenrod, Solidago canadensis L.; and yarrow, Achillea millefolium L.) were tested in a dual-port olfactometer for attractiveness to female northern house mosquitoes, Culex pipiens pipiens L. Fifty milligrams of extract was used for each test, which was equivalent to the amount of extract from flowers of 0.58 common milkweed inflorescences, 1.75 ox-eye daisy flower heads, 0.33 Canada goldenrod plants, or 3.14 yarrow plants. Significantly more Cx. pipiens responded to the common milkweed flower extract than to a control. Ox-eye daisy, Canada goldenrod and yarrow flower extracts were not significantly attractive to this species. A headspace sample of the common milkweed extract was collected by solid phase microextraction and analyzed by gas chromatography-mass spectrometry. The most abundant compounds in the headspace sample were 2-phenylethanol and benzyl alcohol. Analysis of the common milkweed extract and standards of 2-phenylethanol and benzyl alcohol by gas chromatography-flame ionization detection confirmed that these compounds were present in the extract. Cx. pipiens did not respond to synthetic blends of these 2 compounds when tested in a dual-port olfactometer.

Survival of Ixodes scapularis (Acari: Ixodidae) exposed to cold.

The cold hardiness of Ixodes scapularis Say unengorged larvae, engorged larvae, unengorged nymphs, engorged nymphs, and unengorged adults was evaluated. Ticks were exposed to cold for 2 or 8 h at a range of temperatures. Likelihood ratio tests and LT50 estimates were used to evaluate cold hardiness. Likelihood ratio tests indicated that stage and engorgement effects were significant with the exception of the stage effect between engorged larvae and engorged nymphs. LT50 estimates indicated that unengorged nymphs were the most cold-hardy, followed by engorged nymphs, unengorged adults, and both unengorged and engorged larvae. Sex of adult ticks had no statistically significant effect on cold hardiness. Unengorged larvae and unengorged nymphs were tested for evidence of a cold-hardening response. No larvae or nymphs exposed to a cold pretreatment and subsequent cold treatment displayed rapid cold-hardening.

Arbovirus surveillance in Iowa, USA, during the flood of 1993.

During the summer of 1993, mosquitoes were collected by dry ice-baited CDC light traps from July through September in 12 different cities in Iowa. In all, 169,907 mosquitoes belonging to 17 different species were collected. A total of 2,013 pools were processed for arbovirus isolation, from which 59 arbovirus isolates were obtained: 41 Flanders (FLA), 16 trivittatus (TVT), one Cache Valley (CV), and one Turlock (TUR). Supplementary sentinel chicken and human data are also included. In spite of the increase in larval habitats and elevated mosquito populations, there was not an increase in virus transmission.

Response of Iowa mosquito populations to unusual precipitation patterns as measured by New Jersey light trap collections.

New Jersey light trap data from 1993, a year with unusually high amounts of precipitation, and 1994 were compared with mean light trap counts for the previous 8-10 years in each of 6 locations. The study took place in Iowa. In 1993, Aedes trivittatus and Culex tarsalis populations were unchanged. Anopheles punctipennis and Culex pipiens complex populations were significantly higher, and Aedes vexans populations increased in all 6 sites, significantly so in 2 sites. Species composition was unchanged. In 1994, populations of each of these species were not significantly different from the long-term means. Aedes vexans populations were above average in all sites and populations of the remaining species were below average.

Internal ophthalmomyiasis and treatment by laser photocoagulation: a case report.

The clinical diagnosis, treatment management, and epidemiology of a case of unilateral posterior ophthalmomyiasis in a university farm manager are reported. Subretinal tracking was prominent, leading to vision abnormalities, which initially prompted the patient to seek medical evaluation. Identification of the organism based on study of photographs was judged to be a first-instar larva of a Cuterebra (rodent bot fly), although identification of first-instar (stage) myiasis-producing fly larvae is impossible. Inasmuch as the patient was a herdsman, it is likely that the larva is of the genus Hypoderma (cattle grub) or possibly Gasterophilus (horse bot). Laser treatment was effectively used to destroy and immobilize the organism. Subsequently, the patient's health remains satisfactory, and his vision has improved. This case is reported to confirm increasing frequency of ophthalmomyiasis and to improve awareness of its features. Early recognition of this condition, when tumors and other conditions remain suspect, would preclude unnecessarily invasive surgical procedures, including enucleation.

Serotonin depletion affects blood-feeding but not host-seeking ability in Aedes triseriatus (Diptera: Culicidae).

The host-seeking and blood-feeding abilities of Aedes triseriatus (Say) were evaluated after treatment with the amine depleting drugs alpha-methyl-tyrosine (AMT) and alpha-methyl-tryptophan (AMTP). High-performance liquid chromatography with electrochemical detection (HPLCED) was used to demonstrate that AMTP depleted serotonin levels, whereas AMT reduced dopamine levels. The host-seeking ability (evaluated with an olfactometer) of Ae. triseriatus was not altered by treatment with either AMT or AMTP. Blood-feeding success was reduced significantly by either oral administration or injection of AMTP. Mosquitoes treated with AMTP responded positively when placed in close proximity to a host (rabbit), but fewer treated than untreated mosquitoes fed to repletion or fed at all. Those feeding to repletion took longer to do so. No differences in feeding ability were observed between control and AMT-treated mosquitoes. These observations indicate that serotonin plays a role in modulating blood-feeding activities of Ae. triseriatus.

Studies on the biology of Ixodes dammini in the upper midwest of the United States.

The seasonal activity pattern of Ixodes dammini was monitored in south-central Wisconsin in 1989 and 1990. Immature tick activity and abundance were assessed by small mammal trapping (732 Peromyscus leucopus examined) and supplemented with flagging. Tick survival and molting times were evaluated by maintaining ticks in environmental chambers at field sites. Results indicate the biology of I. dammini in the upper midwest is similar to that previously reported from the eastern United States. One notable difference was that seasonal larval activity appeared broader (late April through October) and peaked earlier (on 9 July 1989 mean was 7.7 larvae/mouse; on 9 June 1990 mean was 7.3 larvae/mouse). Nymphal activity on P. leucopus peaked in May of both years (mean = 2.5 in 1989; mean = 1.8 in 1990). Bloodfed females placed in the field in early May 1990 oviposited in early June, and larvae emerged by early August. All bloodfed stages successfully overwintered at field sites.

Rates of Borrelia burgdorferi infection in Ixodes dammini (Acari: Ixodidae) in southwestern Wisconsin.

Ixodes dammini Spielman, Clifford, Piesman & Corwin collected from a study site in southwestern Wisconsin were examined for Borrelia burgdorferi infection. Adult spirochete infection rates varied seasonally (38.1%, spring 1990; 60.3%, fall 1990; 41.2%, spring 1991) and were similar to or higher than rates reported in various studies from the northeastern United States. Statistical comparison of seasonal infection rates indicated a significant difference between the fall 1990 adult population and the subsequent overwintered population of the same cohort (spring 1991). Additionally, a significant decrease in the intensity of spirochete infection was observed in the overwintered adult sample.

Comparison of four membranes for artificially bloodfeeding mosquitoes.

Four species of mosquitoes, Aedes aegypti, Ae. triseriatus, Culex pipiens and Cx. tarsalis, were allowed access to defibrinated rabbit blood via 4 different membranes and a standard membrane feeder. Natural animal skins (mouse or quail) were the most effective. More than 50% of the Aedes mosquitoes fed within 5 min and approximately 90% fed within 20 min on either mouse or quail skin. Culex species fed best on quail skin, but the difference in feeding on quail skin and sheep intestine was not significant at 10 or 20 min. In general Culex mosquitoes fed less readily on natural animal skins than Aedes.