Determinants of Anopheles seasonal distribution patterns across a forest to periurban gradient near Iquitos, Peru.

As part of a field ecology study of arbovirus and malaria activity in the Amazon Basin, Loreto Department, Peru, we collected mosquitoes landing on humans at a forest site and inside and outside of residences and military barracks at periurban, rural, and village sites. We collected 11 Anopheles spp. from these four sites. An. darlingi, the principal malaria vector in the region, accounted for 98.7% of all Anopheles spp. collected at Puerto Almendra. Peaks in landing activity occurred during the December and April collection periods. However, the percent of sporozoite-positive Anopheles spp. was highest 1-2 months later, when landing activity decreased to approximately 10% of the peak activity periods. At all sites, peak landing activity occurred about 2 hours after sunset. These data provide a better understanding of the taxonomy, population density, and seasonal and habitat distribution of potential malaria vectors within the Amazon Basin region.

Characterization of spatial repellent, contact irritant, and toxicant chemical actions of standard vector control compounds.

A previously described modular high-throughput screening system was used to characterize the spatial repellent, contact irritant, and toxicant chemical actions of 14 compounds historically used or under investigation for vector control. The response of F1-F4 Aedes aegypti (Thailand strain) to various concentrations of 4 organochlorines (chlordane, DDT, dieldrin, methoxychlor); 4 pyrethroids (alphacypermethrin, cypermethrin, deltamethrin, permethrin); 3 organophosphates (chlorpyrophos methyl, fenitrothion, malathion); 2 carbamates (bendiocarb, propoxur); and 1 pyrazole (chlorfenapyr) were evaluated. Results show chemicals exert different combinations of contact irritant, spatial repellent, and toxic actions. This is true even within the same chemical class. These actions can be ordered for each chemical based on the testing dose at which the specific response is elicited. Data also indicate that behavioral responses to spatial repellent and contact irritant actions are separate (or independent) from the toxic action of a compound. Results from pyrethroid and DDT assays also show chemicals can induce behavior-modifying actions, such as contact irritancy and spatial repellency, which will reduce man-vector contact, despite evidence of insecticide resistance within the test population. These findings support previous laboratory and field studies showing man-vector contact and disease transmission are routinely interrupted by spatial repellent and contact irritant actions of common public health insecticides. Studies similar to that presented here can be used as baseline evidence for expected vector responses and support best approaches for more detailed behavioral research.

Seasonal distribution, biology, and human attraction patterns of mosquitoes (Diptera: Culicidae) in a rural village and adjacent forested site near Iquitos, Peru.

This study was conducted as part of a field-ecology study of arboviral and malarial activity in the Amazon Basin, Loreto Department, Peru, to determine the relative abundance, species diversity, and seasonal and vertical distributions of potential mosquito vectors. Mosquitoes were captured either by volunteers using mouth aspirators while mosquitoes attempted to land on the collectors or in dry ice-baited ABC light traps. Anopheles darlingi, the principal malaria vector in the region, was the most commonly captured anopheline mosquito in Puerto Almendra village (99%) while landing on humans, with a mean of 37.1 mosquitoes captured per 24-h period, representing nearly one half of all mosquitoes collected. An. darlingi human landing activity began shortly after sunset, peaked at 2000-2100 hours, and declined gradually until sunrise. This species readily entered houses, because 51% of the An. darlingi captured by paired collectors, stationed inside and outside houses, were captured indoors. Human landing collections provided a more accurate estimate of human attraction of An. darlingi, capturing 30 times as many as co-located dry ice-baited ABC light traps. In contrast, eight times as many Culex (Melanoconion) species, including known arbovirus vectors, were captured in light traps as by co-located human collectors. Despite being located within 300 m of the village collection site, only a few Anopheles species were captured at the forest collection site, including only 0.1 An darlingi/ 24 h, thus indicating that An. darlingi activity was directly associated with the rural village. These data provide a better understanding of the taxonomy, population density, and seasonal distribution of potential mosquito vectors of disease within the Amazon Basin region and allow for the development of appropriate vector and disease prevention strategies that target vector populations.

A new classification system for the actions of IRS chemicals traditionally used for malaria control.

Knowledge of how mosquitoes respond to insecticides is of paramount importance in understanding how an insecticide functions to prevent disease transmission. A suite of laboratory assays was used to quantitatively characterize mosquito responses to toxic, contact irritant, and non-contact spatial repellent actions of standard insecticides. Highly replicated tests of these compounds over a range of concentrations proved that all were toxic, some were contact irritants, and even fewer were non-contact repellents. Of many chemicals tested, three were selected for testing in experimental huts to confirm that chemical actions documented in laboratory tests are also expressed in the field. The laboratory tests showed the primary action of DDT is repellent, alphacypermethrin is irritant, and dieldrin is only toxic. These tests were followed with hut studies in Thailand against marked-released populations. DDT exhibited a highly protective level of repellency that kept mosquitoes outside of huts. Alphacypermethrin did not keep mosquitoes out, but its strong irritant action caused them to prematurely exit the treated house. Dieldrin was highly toxic but showed no irritant or repellent action. Based on the combination of laboratory and confirmatory field data, we propose a new paradigm for classifying chemicals used for vector control according to how the chemicals actually function to prevent disease transmission inside houses. The new classification scheme will characterize chemicals on the basis of spatial repellent, contact irritant and toxic actions.

An update on the potential of north American mosquitoes (Diptera: Culicidae) to transmit West Nile Virus.

ABSTRACT Since first discovered in the New York City area in 1999, West Nile virus (WNV) has become established over much of the continental United States and has been responsible for >10,000 cases of severe disease and 400 human fatalities, as well as thousands of fatal infections in horses. To develop appropriate surveillance and control strategies, the identification of which mosquito species are competent vectors and how various factors influence their ability to transmit this virus must be determined. Therefore, we evaluated numerous mosquito species for their ability to transmit WNV under laboratory conditions. This report contains data for several mosquito species not reported previously, as well as a summary of transmission data compiled from previously reported studies. Mosquitoes were allowed to feed on chickens infected with WNV isolated from a crow that died during the 1999 outbreak in New York City. These mosquitoes were tested approximately 2 wk later to determine infection, dissemination, and transmission rates. All Culex species tested were competent vectors in the laboratory and varied from highly efficient vectors (e.g., Culex tarsalis Coquillett) to moderately efficient ones (e.g., Culex nigripalpus Theobald). Nearly all of the Culex species tested could serve as efficient enzootic or amplifying vectors for WNV. Several container-breeding Aedes and Ochlerotatus species were highly efficient vectors under laboratory conditions, but because of their feeding preferences, would probably not be involved in the maintenance of WNV in nature. However, they would be potential bridge vectors between the avian-Culex cycle and mammalian hosts. In contrast, most of the surface pool-breeding Aedes and Ochlerotatus species tested were relatively inefficient vectors under laboratory conditions and would probably not play a significant role in transmitting WNV in nature. In determining the potential for a mosquito species to become involved in transmitting WNV, it is necessary to consider not only its laboratory vector competence but also its abundance, host-feeding preference, involvement with other viruses with similar transmission cycles, and whether WNV has been isolated from this species under natural conditions.

A novel high-throughput screening system to evaluate the behavioral response of adult mosquitoes to chemicals.

A modular and novel assay system for rapid mass screening of chemical compounds for contact irritant and spatial repellent actions against adult mosquitoes is described. The responses of Aedes aegypti to various concentrations of 3 topical repellents, deet, Bayrepel, and SS220, were evaluated. At treatment concentrations > or = 25 nmol/cm2 of SS220, mosquitoes exhibited significant contact irritant (escape) and spatial repellent (movement away from the chemical source) responses, whereas, a 10-fold increase in the treatment concentration of deet and Bayrepel was required to produce similar responses. The novel bioassay system detected contact irritancy and spatial repellency activity with reproducible results and provided baseline data for determining minimum effective concentrations for other chemicals. The system is compact in size, easy to decontaminate, and requires only a minute quantity of chemical compound.

Seasonal distribution, biology, and human attraction patterns of culicine mosquitoes (Diptera: Culicidae) in a forest near Puerto Almendras, Iquitos, Peru.

This study was conducted as part of a field ecology study of arboviral activity in the Amazon Basin, Peru, to determine the taxonomy, frequency, seasonal, and vertical distributions of potential mosquito vectors. In addition, the relative efficiency of human-landing collections and dry ice-baited Centers for Disease Control (CDC)-type light traps was determined for collecting mosquitoes. A total of 70 species of mosquitoes from 14 genera were collected from June 1996 through December 1997 at a forested site near Puerto Almendras, approximately 20 km west-southwest of Iquitos, Peru. Three species [Psorophora (Janthinosoma) albigenu (Peryassu), Ochlerotatus (Ochlerotatus) fulvus (Wiedemann), and Ochlerotatus (Ochlerotatus) serratus (Theobald)] accounted for 70% of all mosquitoes captured in human-landing collections. Overall, biting activity occurred throughout the 24-h cycle but was higher during the daytime, primarily because of large populations of two day-biting species, Ps. albigenu and Oc. serratus. Oc. fulvus was active throughout the 24-h cycle but was more frequently collected during the evening. Oc. fulvus, Ps. albigenu, Culex (Melanoconion) pedroi Sirivanakarn & Belkin, and a mixture of Culex (Melaonoconion) vomerifer Komp, and Culex (Melanoconion) gnomatos Sallum, Huchings & Ferreira, accounted for 73% of the mosquitoes captured during darkness) by human collectors. In general, Ochlerotatus spp. and Psorophora spp. were more commonly captured in human-landing collections, whereas most Culex spp. were more frequently collected in the dry ice-baited CDC-type light traps. In general, mosquito populations were lowest from June through August when river levels were at their lowest. Two large population peaks occurred in November-December and in February-March as a result of "flood water" mosquito populations (e.g., Ps. albigenu). These data provide a better understanding of the taxonomy, population density, and seasonal distribution of potential mosquito vectors within the Amazon Basin region and allow for the development of appropriate vector and disease prevention strategies.

Experimental transmission of St. Louis encephalitis virus by Ochlerotatus j. japonicus.

Ochlerotatus japonicus japonicus a newly discovered nonindigenous mosquito species in North America, and a colonized strain of Culex pipiens were compared for their vector competence for St. Louis encephalitis virus (SLE). Infection rates in Oc. j. japonicus were 0-33% after feeding on chickens with viremias between 10(4.1) and 10(4.7) plaque-forming units (PFU)/ml of blood. In comparison, infection rates were 12-94% for Cx. pipiens that fed on the same chickens. When fed on chickens with viremias between 10(5.3) and 10(5.6) PFU/ml of blood, infection rates for Oc. j. japonicus and Cx. pipiens were similar, 96% and 100%, respectively. After 12-14 days of extrinsic incubation at 26 degrees C, all 34 infected Oc. j. japonicus had a disseminated infection. In contrast, only 23 (43%) of 54 infected Cx. pipiens had a disseminated infection after feeding on the same chickens. If they developed a disseminated infection, both species efficiently transmitted (> or = 87%) SLE. Estimated transmission rates at viral doses sufficient to infect both of the tested species were 29-84% for Oc. j. japonicus and 30-50% for Cx. pipiens. Because of its continued geographic expansion, field and laboratory evidence incriminating it as a vector of the closely related West Nile virus, and its ability to transmit SLE in the laboratory, Oc. j. japonicus should be considered as a potential enzootic or epizootic vector of SLE.

Laboratory transmission of La Crosse virus by Ochlerotatus j. japonicus (Diptera: Culicidae).

Ochlerotatus j. japonicus, a recent introduction to the United States, was studied to determine its capability to serve as a vector of La Crosse (LAC) virus. A field-collectedpopulation of Ochlerotatus triseriatus, the primary vector of LAC virus, was similarly tested for comparison. After Oc. j. japonicus ingested virus fromhamsters with viremias of 10(3.6-5.4) plaque-forming units (PFU)/ml of blood, its estimated transmission rates were 35-88%. These rates were slightly lower than, though similar to, those for Oc. triseriatus, 75-100%. Viral titers in Oc. j. japonicus peaked at approximately 10(5.5) PFU/ mosquito about 7 d after ingesting blood meal in which the concentration of LAC virus was PFU/ml of blood; virus had disseminated from the midgut in 100% (8/8) of these specimens. These data, combined with the close association between the habitats of Oc. j. japonicus and Oc. triseriatus and the reported expansion of the range of this newly discovered species in the eastern United States, indicate that Oc. j. japonicus could function as an additional vector of LAC virus.

Experimental vertical transmission of West Nile virus by Culex pipiens (Diptera: Culicidae).

Despite the detection of West Nile (WN) virus in overwintering Culex pipiens L. in New York in February 2000, the mechanism by which this virus persists throughout the winter to initiate infections in vertebrate hosts and vectors the following spring remains unknown. After a blood meal, parous mosquitoes generally do not survive until spring and gonotrophic dissociation occurs in only a small percentage of the population. To investigate vertical transmission as a means of viral survival during interepizootics, we intrathoracically inoculated Cx. pipiens and Aedes albopictus (Skuse) with WN virus and subsequently tested their F1 progeny for the presence of virus. Among the Cx. pipiens, we recovered virus from two of 1,417 adult progeny that had been reared at 18 degrees C for a minimal filial infection rate (MFIR) of approximately 1.4/1,000 and four of 1,873 adult progeny reared at 26 degrees C (MFIR = 2.1/1,000). The mean titer of the positive pools was 10(5.6) plaque-forming units (PFU)/ml (=10(5.9) PFU/mosquito for positive mosquitoes) of virus. Overall, the MFIR was approximately 1.8/1,000 for Cx. pipiens. Although reports indicate that Ae. albopictus vertically transmit various viruses in the Japanese encephalitis virus complex, we did not detect WN virus in any of > 13,000 F1 progeny of WN virus-inoculated specimens. Female Cx. pipiens that are vertically infected during the late summer season and then survive the winter could serve as a source of WN virus to initiate an infection cycle the following spring.

Experimental transmission of eastern equine encephalitis virus by Ochlerotatus j. japonicus (Diptera: Culicidae).

We evaluated the potential for Ochlerotatus j. japonicus (Theobald), a newly recognized invasive mosquito species in the United States, to transmit eastern equine encephalitis (EEE) virus. Aedes albopictus (Skuse) and Culex pipiens (L.) were similarly tested for comparison. Ochlerotatus j. japonicus and Ae. albopictus became infected and transmitted EEE virus by bite after feeding on young chickens 1 d after they had been inoculated with EEE virus (viremias ranging from 10(7.0-8.7) plaque-forming units [PFU]/ml of blood). No Cx. pipiens (n = 20) had detectable levels of virus 14 d after feeding on an EEE-virus infected chicken with a viremia of 10(8.1) PFU per ml of blood. Depending on the viral titer in the donor chicken, infection rates ranged from 55-100% for Oc. j. japonicus and 93-100% for Ae. albopictus. In these two species, dissemination rates were identical to or nearly identical to infection rates. Depending on the viral titer in the blood meal, estimated transmission rates ranged from 15 to 25% for Oc. j. japonicus and 59-63% for Ae. albopictus. Studies of replication of EEE virus in Oc. j. japonicus showed that there was an "eclipse phase" in the first 4 d after an infectious blood meal, that viral titers peak by day 7 at around 10(5.7) per mosquito, and that virus escaped the mid-gut as soon as 3 d after the infectious blood meal. These data, combined with the opportunistic feeding behavior of Oc. j. japonicus in Asia and the reported expansion of its range in the eastern United States, indicate that it could function as a bridge vector for EEE virus between the enzootic Culiseta melanura (Coquillett)-avian cycle and susceptible mammalian hosts.

Vector competence of Culex tarsalis from Orange County, California, for West Nile virus.

To evaluate the vector competence of Culex tarsalis Coquillett for West Nile virus (WN), females reared from larvae collected in Huntington Beach, Orange County, CA, were fed on 2-3-day-old chickens previously inoculated with a New York strain (Crow 397-99) of WN. The Cx. tarsalis mosquitoes were efficient laboratory vectors of WN, with estimated transmission rates of 81% and 91% for mosquitoes that ingested 10(6.5) or 10(7.3) plaque-forming units of WN/mL of blood, respectively. Based on efficiency of viral transmission and the role of this species in the transmission of the closely related St. Louis encephalitis virus, Cx. tarsalis should be considered a potentially important vector of WN in the western United States.

Vector competence of three North American strains of Aedes albopictus for West Nile virus.

To evaluate the potential for North American (NA) Aedes albopictus to transmit West Nile virus (WN), mosquito strains derived from 3 NA sources (Frederick County, Maryland, FRED strain; Cheverly, MD, CHEV strain; Chambers and Liberty counties, Texas, TAMU strain) were tested. These strains were tested along with a previously tested strain from a Hawaiian source (OAHU strain). Mosquitoes were fed on 2- to 3-day-old chickens previously inoculated with a New York strain (Crow 397-99) of WN. All of the NA strains were competent laboratory vectors of WN, with transmission rates of 36, 50, 83, and 92% for the FRED, CHEV, OAHU, and TAMU strains, respectively. The extrinsic incubation period for WN in Ae. albopictus held at 26 degrees C was estimated to be 10 days. Based on efficiency of viral transmission, evidence of natural infection, bionomics, and distribution, Ae. albopictus could be an important bridge vector of WN in the southeastern USA.