Eastern equine encephalitis virus in mosquitoes and their role as bridge vectors

Eastern equine encephalitis virus (EEEV) is maintained in an enzootic cycle involving Culiseta melanura mosquitoes and avian hosts. Other mosquito species that feed opportunistically on mammals have been incriminated as bridge vectors to humans and horses. To evaluate the capacity of these mosquitoes to acquire, replicate, and potentially transmit EEEV, we estimated the infection prevalence and virus titers in mosquitoes collected in Connecticut, USA, by cell culture, plaque titration, and quantitative reverse transcription-PCR. Cs. melanura mosquitoes were the predominant source of EEEV (83 [68%] of 122 virus isolations) and the only species to support consistently high virus titers required for efficient transmission. Our findings suggest that Cs. melanura mosquitoes are primary enzootic and epidemic vectors of EEEV in this region, which may explain the relative paucity of human cases. This study emphasizes the need for evaluating virus titers from field-collected mosquitoes to help assess their role as vectors.

Vector-host interactions governing epidemiology of West Nile virus in Southern California

Southern California remains an important focus of West Nile virus (WNV) activity, with persistently elevated incidence after invasion by the virus in 2003 and subsequent amplification to epidemic levels in 2004. Eco-epidemiological studies of vectors-hosts-pathogen interactions are of paramount importance for better understanding of the transmission dynamics of WNV and other emerging mosquito-borne arboviruses. We investigated vector-host interactions and host-feeding patterns of 531 blood-engorged mosquitoes in four competent mosquito vectors by using a polymerase chain reaction (PCR) method targeting mitochondrial DNA to identify vertebrate hosts of blood-fed mosquitoes. Diagnostic testing by cell culture, real-time reverse transcriptase-PCR, and immunoassays were used to examine WNV infection in blood-fed mosquitoes, mosquito pools, dead birds, and mammals. Prevalence of WNV antibodies among wild birds was estimated by using a blocking enzyme-linked immunosorbent assay. Analyses of engorged Culex quinquefasciatus revealed that this mosquito species acquired 88.4% of the blood meals from avian and 11.6% from mammalian hosts, including humans. Similarly, Culex tarsalis fed 82% on birds and 18% on mammals. Culex erythrothorax fed on both birds (59%) and mammals (41%). In contrast, Culex stigmatosoma acquired all blood meals from avian hosts. House finches and a few other mostly passeriform birds served as the main hosts for the blood-seeking mosquitoes. Evidence of WNV infection was detected in mosquito pools, wild birds, dead birds, and mammals, including human fatalities during the study period. Our results emphasize the important role of house finches and several other passeriform birds in the maintenance and amplification of WNV in southern California, with Cx. quinquefasciatus acting as both the principal enzootic and "bridge vector" responsible for the spillover of WNV to humans. Other mosquito species, such as Cx. tarsalis and Cx. stigmatosoma, are important but less widely distributed, and also contribute to spatial and temporal transmission of WNV in southern California.

Studies on hibernating populations of Culex pipiens from a West Nile virus endemic focus in New York City: parity rates and isolation of West Nile virus

A 3-year study was undertaken to examine the parity status, survival, and prevalence of West Nile virus (WNV) in overwintering populations of Culex pipiens pipiens collected from a hibernaculum located in a WNV endemic region in New York City. Nearly 6,000 females were collected from December through April. Parity rates were highest among females collected in December and January, ranging from 12.3% to 21.9%, depending on the year. In each year of the study, the proportion of parous females declined significantly during the course of the winter; the percentage of parous females found in April ranged from 0.9% to 10%. Results provide unequivocal evidence that parous Cx. p. pipiens females from this region of the northeastern US enter hibernacula in the fall in comparatively high proportions not previously recognized for this species, and while these females experience significant mortality during the winter, some survived to April to emerge in the spring. The absence of any detectible blood remnants in overwintering females reaffirms that blood feeding does not occur among diapausing females during the winter. The possibility that a portion of the diapausing population may be autogenous as a result of hybridization with sympatric belowground populations of Cx. p. pipiens "form molestus" is discussed. A single isolation of WNV was obtained in Vero cell culture from a pool of 50 females collected on January 11, 2007, representing an infection prevalence of 0.07% in the overwintering population in 2007 (n = 1,370 mosquitoes, 33 pools). No isolations of WNV were made from mosquitoes collected in 2008 (n = 1,870 mosquitoes, 190 pools) or 2009 (n = 1,767 mosquitoes, 184 pools). Findings provide further evidence for local overwintering of WNV in diapausing Cx. p. pipiens, albeit at very low rates, consistent with the paucity of WNV-positive mosquitoes detected in June and early July despite the emergence of females from hibernacula in early May in this region.

Signaled avoidance in the eye withdrawal reflex of the green crab

Learning in a signaled avoidance procedure was studied in the eye withdrawal reflex of the green crab, Carcinus maenas. A puff of air to the eye, which causes eye retraction, was used as the unconditioned stimulus (US). A mild vibration on the carapace, which has no effect on untrained animals, was used as a warning (conditioned) stimulus (CS). Eye withdrawal during the CS led to the omission of the otherwise scheduled US. Acquisition was rapid, reaching about 75% avoidance after 30 trials. Extinction occurred slowly over the course of 40 CS-only trials. Yoked controls did not perform as well. The behavior of experimental animals in the avoidance procedure was found to be essentially identical to the performance of animals subjected to a classical conditioning paradigm in which CS responses had no effect on US presentation. Additional groups of animals were subjected to experiments in which (a) avoidance conditioning (60 trials) was followed by classical conditioning (40 trials) or (b) classical conditioning was followed by avoidance. The behavior of these groups was, again, essentially identical. The results suggest that there may be an underlying Pavlovian mechanism for the learned response, although the contribution of an operant process is not excluded. The results expand the range of invertebrate animals in which fundamental conditioning phenomena can be demonstrated, and may provide a neuronal model for learning in a signaled avoidance procedure.

Avian communal roosts as amplification foci for West Nile virus in urban areas in northeastern United States

West Nile virus (WNV) perpetuates in an enzootic transmission cycle involving Culex mosquitoes and virus-competent avian hosts. In the northeastern United States, the enzootic vectors, Cx. pipiens and Cx. restuans, feed preferentially on American robins (Turdus migratorius), suggesting a key role for this bird species in the WNV transmission cycle. We examined the role of American robin communal roosts as virus amplification foci in greater New Haven, Connecticut. Robin communal roosts were located by radio tracking. After mid-August, when most robins were using the roosts, Cx. pipiens and Cx. restuans fed often on robins and were significantly more infected with WNV at communal roosts than at non-roosting sites. We also identified 6.4% human-derived blood meals in Aedes vexans in communal roosts. Our results indicate that communal roosts act as late-season amplification foci facilitating transmission to humans because of high infection rates, high abundance, and feeding patterns of enzootic and bridge vectors.

Identification of bloodmeals in Anopheles quadrimaculatus and Anopheles punctipennis from eastern equine encephalitis virus foci in northeastern U.S.A

The host-feeding patterns of Anopheles quadrimaculatus Say and Anopheles punctipennis (Say) were examined in order to evaluate their potential contributions to the transmission of eastern equine encephalitis virus (EEEv) and other arboviruses in the northeastern U.S.A. Engorged mosquitoes of the two species were collected from EEEv foci in central New York (NY) and throughout New Jersey (NJ), and their bloodmeals were identified using a polymerase chain reaction (PCR)-based assay and sequencing portions of the mitochondrial cytochrome b gene. Analysis of 131 An. quadrimaculatus and 107 An. punctipennis from NY revealed that 97.7% and 97.2%, respectively, had acquired blood solely from mammalian hosts. Similarly, examination of 288 An. quadrimaculatus and 127 An. punctipennis from NJ showed 100% and 96.0%, respectively, contained mammalian-derived bloodmeals. Mosquitoes containing mixed bloodmeals from both avian and mammalian hosts were detected in 1.6% of An. quadrimaculatus from NY, and 2.8% and 4.0% of An. punctipennis from NY and NJ, respectively. White-tailed deer (Odocoileus virginianus) constituted the most common vertebrate host for these anopheline mosquitoes, accounting for 85.8-97.7% of all bloodmeals identified. The predominance of white-tailed deer as a source of bloodmeals supports enzootic amplification of deer-associated arboviruses in this region, including Jamestown Canyon, Cache Valley and Potosi viruses. One horse- and two human-derived bloodmeals were also detected in An. quadrimaculatus collected in NJ. Limited avian-derived bloodmeals were detected from mourning dove (Zenaida macroura), sharp-shinned hawk (Accipiter striatus) and house finch (Carpodacus mexicanus), mostly in mixed bloodmeals. Occasional feeding on avian hosts suggests that these mosquitoes may participate as epizootic-epidemic bridge vectors of EEEv from viraemic birds to mammalian hosts of concern, including horses and humans. An isolate of EEEv was recovered from the head and thorax of an An. punctipennis mosquito collected in NY.

Isolations of Jamestown Canyon virus (Bunyaviridae: Orthobunyavirus) from field-collected mosquitoes (Diptera: Culicidae) in Connecticut, USA: a ten-year analysis, 1997-2006

Jamestown Canyon virus (JCV) (Bunyaviridae: Orthobunyavirus) is a mosquito-borne zoonosis belonging to the California serogroup. It has a wide geographic distribution, occurring throughout much of temperate North America. White-tailed deer, Odocoileus virginianus are the principal amplification hosts, and boreal Aedes and Ochlerotatus mosquitoes are the primary vectors. A 10-year study was undertaken to identify potential mosquito vectors in Connecticut, quantify seasonal prevalence rates of infection, and define the geographic distribution of JCV in the state as a function of land use and white-tailed deer populations, which have increased substantially over this period. Jamestown Canyon virus was isolated from 22 mosquito species. Five of them, Ochlerotatus canadensis, Oc. cantator, Anopheles punctipennis, Coquillettidia perturbans, and Oc. abserratus were incriminated as the most likely vectors, based on yearly isolation frequencies and the spatial geographic distribution of infected mosquitoes. Jamestown Canyon virus was isolated from Oc. canadensis more consistently and from a greater range of collection sites than any other species. Frequent virus isolations were also made from Aedes cinereus, Aedes vexans, and Oc. sticticus, and new North American isolation records were established for Anopheles walkeri, Culex restuans, Culiseta morsitans, Oc. sticticus, Oc. taeniorhynchus, and Psorophora ferox. Other species from which JCV was isolated included C. melanura, Oc. aurifer, Oc. communis, Oc. excrucians, Oc. provocans, Oc. sollicitans, Oc. stimulans, Oc. triseriatus, and Oc. trivittatus. Jamestown Canyon virus was widely distributed throughout Connecticut and found to consistently circulate in a diverse array of mosquito vectors. Infected mosquitoes were collected from June through September, and peak infection rates paralleled mosquito abundance from mid-June through mid-July. Infection rates in mosquitoes were consistent from year to year, and overall virus activity was directly related to local mosquito abundance. Infected mosquitoes were equally distributed throughout the state, irrespective of land use, and infection rates were not directly associated with the abundance of white-tailed deer, possibly because of their saturation throughout the region.

Host-feeding patterns of potential mosquito vectors in Connecticut, U.S.A.: molecular analysis of bloodmeals from 23 species of Aedes, Anopheles, Culex, Coquillettidia, Psorophora, and Uranotaenia

We evaluated the blood-feeding patterns in several mosquito species that may serve as vectors of disease agents in the northeastern United States. Blood-fed mosquitoes were collected from 91 different sites throughout Connecticut over a 6-yr period (June-October 2002-2007), and the host-feeding patterns of 23 mosquito species representing six genera were examined by using a polymerase chain reaction-based assay and sequencing portions of the cytochrome b gene of mitochondrial DNA. This study was part of a statewide surveillance program and for some of the mosquito species a limited number of specimens were examined [e.g., Aedes communis (De Geer) (1), Anopheles barberi Coquillett (1), Uranotaenia sapphirina (Osten Sacken) (5)]. With the exception of Culex territans Walker that acquired bloodmeals from all four classes of vertebrates--birds, reptiles, amphibians, and mammals--all species of Aedes, Anopheles, Coquillettidia, Psorophora, and to a lesser degree, Uranotaenia, were found to feed predominately upon mammalian hosts. Fourteen mammalian species were identified as sources of blood, but the majority of feedings were taken from the white-tailed deer, Odocoileus virginianus. Human-derived bloodmeals were identified from 13 of the 23 mosquito species. Limited avian-derived bloodmeals were detected in Aedes canadensis (Theobald), Aedes cantator (Coquillett), Aedes cinereus Meigen, Aedes triseriatus (Coquillett), Aedes trivittatus (Coquillett), Coquillettidia perturbans (Walker) Cx. territans, Psorophoraferox (von Humboldt), and Ur. sapphirina. American robin, Turdus migratorius, was the most common source of avian blood, followed by a few other mostly Passeriformes birds. We conclude that the white-tailed deer serve as the main vertebrate host for these mammalophilic mosquitoes in this region of the United States. This feeding pattern supports enzootic amplification of arboviruses, including Jamestown Canyon, Cache Valley, and Potosi viruses that perpetuate in cervid hosts. Occasional feeding on avian hosts suggests that some of these mosquito species, such as Cq. perturbans, also could facilitate transmission of West Nile and eastern equine encephalitis viruses from viremic birds to mammalian hosts.

Tracking eastern equine encephalitis virus perpetuation in the northeastern United States by phylogenetic analysis

Epidemics and epizootics of eastern equine encephalitis virus (EEEV) occur sporadically in temperate regions where transmission is seasonal from late summer to early fall. These outbreaks may derive from virus that overwinters locally or perhaps results from reintroduction of virus from other sites. To evaluate these possibilities, we compared the phylogenetic relationships of EEEV isolates obtained from mosquitoes collected during statewide arbovirus surveillance in Connecticut, in addition to isolates from concurrent outbreaks in southern New Hampshire and upstate New York. In Connecticut, viral isolates grouped into temporally discrete clades by year of isolation or over 2 years of sampling. Two or more clades arose in 2000, 2001, 2003, 2004, and 2006, possibly the result of separate introduction events into the state, whereas viruses from upstate New York and New Hampshire segregated into single clades that persisted for 2 or more years. New Hampshire viruses shared recent common ancestry to those isolated in Connecticut suggesting viral dispersal among these regions. These results provide additional evidence for independent episodes of EEEV overwintering in northern foci.

Genetic relationships of Jamestown Canyon virus strains infecting mosquitoes collected in Connecticut

Jamestown Canyon virus (JCV) (family Bunyaviridae, genus Orthobunyavirus) is maintained in a mosquito-deer cycle and has been implicated in the etiology of meningitis and encephalitis with human cases reported from Ontario, Canada, Michigan, Connecticut, and New York. Despite the recognition of symptomatic cases in the northeastern United States, little is known about the genetic relationships of JCV variants circulating in this region. Accordingly, we compared the phylogenetic relationships of 56 JCV isolates from mosquitoes collected in Connecticut over a 40-year period to evaluate their evolutionary history and characterize patterns of genetic diversity in the state. We distinguished at least two major lineages in Connecticut on the basis of phylogenetic reconstruction of small (S), medium (M), and large (L) segment nucleotide sequences. Viruses representing each lineage infected a diverse group of mosquito species over multiple years of sampling and appeared to be geographically structured along an east-west axis. One of these lineages was detected in Connecticut from 1966 through 2006 with few mutational changes accumulating over time. Phylogenetic trees generated from portions of the M and L segments yielded different topologies from S segment sequences as three clades became consolidated into two. Although direct evidence for genetic exchange by reassortment was lacking among cocirculating strains in Connecticut, molecular trees from S, M, and L segments were incongruent, which suggests a distinct evolutionary history or process for each genomic segment. These results suggest that JCV variants are stably maintained in Connecticut where they infect a wide diversity of mosquito species.

Host feeding pattern of Culex quinquefasciatus (Diptera: Culicidae) and its role in transmission of West Nile virus in Harris County, Texas

The vertebrate hosts of 672 blood-engorged Culex quinquefasciatus Say, collected in Harris County, Texas, during 2005, were identified by nucleotide sequencing PCR products of the cytochrome b gene. Analysis revealed that 39.1% had acquired blood from birds, 52.5% from mammals, and 8.3% were mixed avian and mammalian blood meals. Most frequent vertebrate hosts were dog (41.0%), mourning dove (18.3%), domestic cat (8.8%), white-winged dove (4.3%), house sparrow (3.2%), house finch (3.0%), gray catbird (3.0%), and American robin (2.5%). Results are interpreted in conjunction with concurrent avian and mosquito West Nile virus (WNV) surveillance activities in Harris County. We conclude that Cx. quinquefasciatus is an opportunistic feeder and principal mosquito vector of WNV in this metropolitan area; however, transmission by other mosquito species or by other modes of infection, such as ingestion, must account for the high WNV infection rates among local blue jays and American crows.

A two-year evaluation of elevated canopy trapping for Culex mosquitoes and West Nile virus in an operational surveillance program in the northeastern United States

The effectiveness of CO2-baited Centers for Disease Control and Prevention miniature light traps elevated in the tree canopy (approximately 7.6 m) was compared with light traps placed at ground level (approximately1.5 m) and grass-sod infused gravid traps for collecting Culex pipiens, Culex restuans, and Culex salinarius and detecting West Nile virus (WNV) activity in an operational surveillance program that encompassed 12 ecologically diverse sites in Connecticut in 2004 and 2005. More than twice as many Cx. pipiens were collected on average in light traps suspended in the tree canopy than in either light or gravid traps placed at ground level. This difference was generally restricted to those collection sites where markedly greater numbers of Cx. pipiens were collected with all trapping methods but was not associated with site-specific urbanization indices. Culex restuans was not preferentially attracted to light traps suspended in the tree canopy. No differences in the overall abundance of this species were recorded with either of the 2 trapping procedures, but both light traps were more effective than the gravid traps. Culex salinarius was significantly more attracted to ground-based light traps than traps suspended in the tree canopy, while gravid traps were ineffective at all sites regardless of the level of urbanization or any other specific land-use characteristic. CO2-baited light traps placed in the tree canopy were generally superior to ground-based light traps for detecting WNV in Cx. pipiens. West Nile virus-infected females were collected more regularly, and the frequency of infected pools was significantly greater. Twofold higher minimum field infection rates (maximum likelihood estimation [MLE] = 6.7 vs. 3.0 per 1,000 mosquitoes) were also recorded from canopy collections of this species, and virus was detected in canopy-collected females several weeks before it was detected in collections from light traps at ground level. We conclude that the use of CO2-baited light traps placed in the tree canopy for targeted trapping of Cx. pipiens and subsequent detection of WNV are likely to yield better overall results than light traps placed at ground level in this region of the northeastern United States. The virus isolation data obtained from Cx. pipiens collected in gravid traps compared favorably both temporally and spatially with results from canopy trap collections. There were no significant differences in the overall frequency of WNV-infected pools or MLEs for Cx. pipiens, but fewer total WNV isolations were made from Cx. pipiens collected in the gravid traps and virus was detected more infrequently. Results reaffirmed the utility of gravid traps as effective surveillance tools for detection of WNV in Cx. pipiens in the northeastern United States. However, findings also demonstrated that CO2-baited light traps placed in the tree canopy provided more consistent results where weekly detection of virus amplification is a critical objective. The comparative effectiveness of ground- and canopy-based light traps for detection of WNV-infected Cx. restuans and Cx. salinarius was inconclusive owing to the limited number of virus isolations that were made from these species during the 2 years of study. However, WNV virus isolations were made several weeks earlier and more frequently from Cx. restuans collected in traps placed in the canopy rather than at ground level in 2004. Results support the view that ground-based light traps are more effective for detection of WNV in Cx. salinarius.

Molecular identification of blood-meal sources in Culiseta melanura and Culiseta morsitans from an endemic focus of eastern equine encephalitis virus in New York

Eastern equine encephalitis (EEE) virus perpetuates in an enzootic cycle involving ornithophilic mosquito vectors, principally Culiseta melanura (Coquillett) and avian amplification hosts. To better understand the role of Cs. melanura and Culiseta morsitans (Theobald) in the epizootiology of EEE virus, we collected blood-fed mosquitoes between 31 May and 15 October 2004 at two sites associated with an EEE virus focus in central New York and identified the source of vertebrate blood by nucleotide sequencing of polymerase chain reaction (PCR) products of the cytochrome b gene. Analysis of 484 Cs. melanura and 122 Cs. morsitans revealed that 94.2% and 86.9%, respectively, acquired blood solely from avian hosts. Blood meals derived exclusively from mammals were detected in 0.8% of Cs. melanura and 1.6% of Cs. morsitans. Individual mosquitoes containing mixed-blood meals from both avian and mammalian hosts were also detected in 5.0% of Cs. melanura and 11.5% of Cs. morsitans. Wood thrush constituted the most common vertebrate host for Cs. melanura (23.6%) and Cs. morsitans (30.9%), followed by American robin, song sparrow, ovenbird, red-eyed vireo, and common yellowthroat. Mammalian-derived blood meals were identified as white-tailed deer, horse, domestic cat, and eastern pipistrelle bat. There were three isolations of EEE virus from Cs. melanura and one from Cs. morsitans. These results suggest that wood thrush and a few other passerine birds may play key roles in supporting EEE virus transmission in the northeast and possibly throughout the geographic range of EEE in North America. The frequency of mammalian feedings also suggests that Cs. melanura and Cs. morsitans may play a role in the transmission of EEE virus to equines, in addition to maintaining enzootic transmission among avian hosts. We report the first isolation of arboviruses from mosquito vectors concomitant with the identifications of their blood meal sources.

A new genetic variant of La Crosse virus (bunyaviridae) isolated from New England

La Crosse virus (LACV) is found primarily in the Midwestern and Appalachian regions of the United States where it is a leading cause of mosquito-borne encephalitis in children. To determine whether the distribution of this virus extends further east into New England, we analyzed a bunyavirus that was isolated from a pool of eastern tree-hole mosquitoes, Ochlerotatus triseriatus (= Aedes triseriatus), collected from Fairfield, Connecticut (CT) in 2005. Nucleotide and encoded amino acid sequences from portions of the S, M, and L segments were more similar to the prototype strain of La Crosse virus than that of closely related snowshoe hare virus. Phylogenetic analysis of sequences from the M segment indicated that the CT isolate represents a distinct lineage of La Crosse virus, diverging earliest from other strains found in southeastern, central, and northeastern United States. Despite low sequence homology with other viral strains, the CT isolate was antigenically similar to the prototype strain of LACV by plaque-reduction neutralization tests with polyclonal and monoclonal antibodies. This represents the first isolation of LACV in New England to our knowledge and suggests long-term independent evolution of the CT isolate.

Host feeding patterns of Culex mosquitoes and West Nile virus transmission, northeastern United States

Culex salinarius is a bridge vector to humans, while Cx. pipiens and Cx. restuans are more efficient enzootic vectors.

To evaluate the role of Culex mosquitoes as enzootic and epidemic vectors for WNV, we identified the source of vertebrate blood by polymerase chain reaction amplification and sequencing portions of the cytochrome b gene of mitochondrial DNA. All Cx. restuans and 93% of Cx. pipiens acquired blood from avian hosts; Cx. salinarius fed frequently on both mammals (53%) and birds (36%). Mixed-blood meals were detected in 11% and 4% of Cx. salinarius and Cx. pipiens, respectively. American robin was the most common source of vertebrate blood for Cx. pipiens (38%) and Cx. restuans (37%). American crow represented <1% of the blood meals in Cx. pipiens and none in Cx. restuans. Human-derived blood meals were identified from 2 Cx. salinarius and 1 Cx. pipiens. Results suggest that Cx. salinarius is an important bridge vector to humans, while Cx. pipiens and Cx. restuans are more efficient enzootic vectors in the northeastern United States.

Isolations of Potosi virus from mosquitoes (Diptera: Culicidae) collected in Connecticut

Potosi virus (POTV) (Bunyaviridae: Orthobunyavirus) was first isolated from Aedes albopictus (Skuse) collected in Potosi, MO, in 1989, and subsequent isolations were reported from Illinois, Michigan, Ohio, and the Carolinas. To determine whether the distribution of this virus extends into the northeastern United States, we analyzed arboviruses acquired from mosquitoes collected in Connecticut from 1998 to 2004. In 2001, a bunyavirus was isolated from Aedes vexans (Meigen) that was different from other arboviruses known to occur in Connecticut by cross-neutralization and reverse transcription-polymerase chain reaction (RT-PCR) assays. Nucleotide and encoded amino acid sequences of a portion of the G2 envelope gene were 99 and 100% similar to POTV, respectively, yet distinct from indigenous strains of Jamestown Canyon (JCV), Cache Valley (CVV), and Trivittatus virus (TVTV). Viral isolates obtained from the statewide surveillance program were retested by RT-PCR coupled with restriction enzyme analysis to distinguish POTV from other bunyaviruses. POTV isolates, previously typed by neutralization, were correctly identified by RT-PCR; however, many isolates classified as JCV or CVV by enzyme-linked immunosorbent assay proved to be POTV by molecular assays. In total, 92 strains of POTV were isolated from 12 mosquito species in 2000, 2001, and 2003, whereas POTV was not detected in mosquitoes sampled during 1998, 1999, 2002, and 2004. Viral isolation rates were highest for Anopheles punctipennis (Say) (3.2-11.3 infection rate per 1,000 mosquitoes), whereas the greatest number of isolates came from Ochlerotatus trivittatus (Coquillett) (8-16 isolates). This finding represents the first detection of POTV in the northeastern United States where it infects a diverse array of mosquito species.

Selection for virulent dengue viruses occurs in humans and mosquitoes

Dengue is the most common mosquito-borne viral disease in humans. The spread of both mosquito vectors and viruses has led to the resurgence of epidemic dengue fever (a self-limited flu-like syndrome) and the emergence of dengue hemorrhagic fever (severe dengue with bleeding abnormalities) in urban centers of the tropics. There are no animal or laboratory models of dengue disease; indirect evidence suggests that dengue viruses differ in virulence, including their pathogenicities for humans and epidemic potential. We developed two assay systems (using human dendritic cells and Aedes aegypti mosquitoes) for measuring differences in virus replication that correlate with the potential to cause hemorrhagic dengue and increased virus transmission. Infection and growth experiments showed that dengue serotype 2 viruses causing dengue hemorrhagic fever epidemics (Southeast Asian genotype) can outcompete viruses that cause dengue fever only (American genotype). This fact implies that Southeast Asian genotype viruses will continue to displace other viruses, causing more hemorrhagic dengue epidemics.

Efficiency of dengue serotype 2 virus strains to infect and disseminate in Aedes aegypti

Dengue serotype 2 (DEN-2) viruses with the potential to cause dengue hemorrhagic fever have been shown to belong to the Southeast (SE) Asian genotype. These viruses appear to be rapidly displacing the American genotype of DEN-2 in the Western Hemisphere. To determine whether distinct genotypes of DEN-2 virus are better adapted to mosquito transmission, we classified 15 viral strains of DEN-2 phylogenetically and compared their ability to infect and disseminate in different populations of Aedes aegypti mosquitoes. Envelope gene nucleotide sequence analysis confirmed that six strains belonged to the American genotype and nine strains were of the SE Asian genotype. The overall rate of disseminated infection in mosquitoes from Texas was 27% for the SE Asian genotype versus 9% for the American genotype. This pattern of infection was similar in another population of mosquitoes sampled from southern Mexico (30% versus 13%). Together, these findings suggest that Ae. aegypti tends to be more susceptible to infection by DEN-2 viruses of the SE Asian genotype than to those of the American genotype, and this may have epidemiologic implications.

Risk of Lyme disease: perceptions of residents of a Lone Star tick-infested community

BACKGROUND

Lone Star ticks (Amblyomma americanum) have been suggested as a vector of the agent of Lyme disease (Borrelia burgdorferi sensu lato) in the USA, based on associations with an infection manifesting mainly as erythema migrans. In laboratory experiments, however, they failed to transmit B. burgdorferi sensu stricto.

METHODS

In this study, carried out from 1994 to 1996, we determined the seroprevalences of B. burgdorferi (1.2%), Ehrlichia chaffeensis (7%), E. phagocytophila (0%), Rickettsia rickettsii (0%), R. typhi (0%), Coxiella burneti (0%), Francisella tularensis (0%), and Babesia microti (0%) by standard serological methods for 325 residents (97% of the total population) of Gibson Island, coastal Maryland, USA, where 15% of the residents reported having had Lyme disease within a recent 5-year span.

FINDINGS

Of the 167 seronegative individuals who were followed up prospectively for 235 person-years of observation, only 2 (0.85%) seroconverted for B. burgdorferi. Of 1556 ticks submitted from residents, 95% were identified as Lone Star ticks; only 3% were deer ticks (Ixodes dammini), the main American vector of Lyme disease. B. burgdorferi s.s. infected 20% of host-seeking immature deer ticks, and borreliae ("B. lonestari") were detected in 1-2% of Lone Star ticks. Erythema migrans was noted in 65% of self-reports of Lyme disease, but many such reports indicated that the rash was present while the tick was still attached, suggesting a reaction to the bite itself rather than true Lyme disease. Sera from individuals reporting Lyme disease generally failed to react to B. burgdorferi or any other pathogen antigens.

CONCLUSION

The residents of Gibson Island had an exaggerated perception of the risk of Lyme disease because they were intensely infested with an aggressively human-biting and irritating nonvector tick. In addition, a Lyme disease mimic of undescribed etiology (named Masters' disease) seems to be associated with Lone Star ticks, and may confound Lyme disease surveillance. The epidemiological and entomological approach used in this study might fruitfully be applied wherever newly emergent tickborne zoonoses have been discovered.

Lone star tick-infecting borreliae are most closely related to the agent of bovine borreliosis

Although Borrelia theileri, the agent of bovine borreliosis, was described at the turn of the century (in 1903), its relationship with borreliae causing Lyme disease or relapsing fever remains undescribed. We tested the previously published hypothesis that spirochetes infecting Lone Star ticks (Amblyomma americanum) may comprise B. theileri by analyzing the 16S ribosomal DNAs (rDNAs) and flagellin genes of these spirochetes. B. theileri, the Amblyomma agent, and B. miyamotoi formed a natural group or clade distinct from but most closely related to that of the relapsing fever spirochetes. B. theileri and the Amblyomma agent were 97 and 98% similar at the nucleotide level within the analyzed portions of the 16S rDNA and the flagellin gene respectively, suggesting a recent divergence. The agent of bovine borreliosis might be explored as a surrogate antigen for the as-yet-uncultivatable Amblyomma agent in studies designed to explore the etiology of a Lyme disease-like infection associated with Lone Star ticks.

Differential susceptibility of Aedes aegypti to infection by the American and Southeast Asian genotypes of dengue type 2 virus

Outbreaks of dengue hemorrhagic fever have coincided with the introduction of the Southeast (SE) Asian genotype of dengue type 2 virus in the Western Hemisphere. This introduced genotype appears to be rapidly displacing the indigenous, American genotype of dengue 2 virus throughout the region. These field observations raise the possibility that the SE Asian genotype of dengue 2 is better adapted for vector transmission than its American counterpart. To evaluate this hypothesis, we compared the ability of viral strains of the SE Asian and American genotypes to infect, replicate, and disseminate within vector mosquitoes (Aedes aegypti). Viral strains of the SE Asian genotype tended to infect and disseminate more efficiently in mosquitoes than did variants of the American genotype. These differences, however, were observed solely in field-derived mosquitoes, whereas viral infection rates were virtually identical in the laboratory-adapted Rockefeller colony of Ae. aegypti. Our findings could provide a physiological basis for the contrasting patterns of dengue virus genotype transmission and spread. Such an understanding of functional differences between viral strains and genotypes may ultimately improve surveillance and intervention strategies.

Pyoderma gangrenosum. A diagnosis not to be missed

We describe a case of pyoderma gangrenosum which presented with severe wound breakdown after elective hip replacement. The patient was treated successfully with minimal wound debridement and steroids. This diagnosis should always be considered when confronted with an enlarging painful skin lesion which does not grow organisms when cultured and fails to respond to antibiotic therapy, especially if there are similar lesions in other sites. In patients who have a past history of pyoderma gangrenosum, prophylactic steroids may be indicated at the time of surgery or may be required early in the postoperative period.

Diversity of Babesia infecting deer ticks (Ixodes dammini)

To determine whether the presence of nonpathogenic piroplasms may confound field estimates of risk of Babesia microti infection, we identified sporozoites infecting the salivary glands of deer ticks (Ixodes dammini) by parallel microscopy and polymerase chain reaction assays. Piroplasms were evident in 14.4% of adult ticks from sites in the northcentral and northeastern United States. Of these, 83.3% contained DNA characteristic of Ba. odocoilei. This cervid piroplasm was detected in all of the sites examined and generally was more prevalent than was Ba. microti. Because deer ticks transmit both Ba. odocoilei and Ba. microti, estimates of pathogen prevalence based solely on microscopy may overestimate the risk of human babesiosis.

Duration of tick attachment required for transmission of granulocytic ehrlichiosis

Deer tick-transmitted pathogens such as Lyme disease spirochetes and babesiae appear to require a period of reactivation and replication during the tick's blood meal before it is able to infect a host. The duration of nymphal tick attachment that is required for transmission of the agent of human granulocytic ehrlichiosis (HGE) was determined by removing feeding ticks from mice at various time points. As with spirochetes and babesiae, ehrlichiae infected few mice when ticks were removed prior to 36 h of tick attachment. This "grace period" may serve as a modifying factor in the epidemiology of this newly emergent zoonosis and help physicians make informed decisions concerning management of tick bites in HGE-endemic areas.

A subtropical case of human babesiosis

This report constitutes the first well-documented case of symptomatic human babesiosis from a subtropical site, south of the 40th parallel. This paper describes the definitive identification of Babesia divergens infection in a splenectomized patient from the Canary Islands.

A new tick-borne encephalitis-like virus infecting New England deer ticks, Ixodes dammini

To determine if eastern North American Ixodes dammini, like related ticks in Eurasia, maintain tick-borne encephalitis group viruses, we analyzed ticks collected from sites where the agent of Lyme disease is zoonotic. Two viral isolates were obtained by inoculating mice with homogenates from tick salivary glands. The virus, which was described by reverse transcriptase polymerase chain reaction and direct sequencing of the amplification products, was similar to, but distinct from, Powassan virus and is provisionally named "deer tick virus." Enzootic tick-borne encephalitis group viruses accompany the agents of Lyme disease, babesiosis, and granulocytic ehrlichiosis in a Holarctic assemblage of emergent deer tick pathogens.

Serum-plasma separator in amylase assay

A modification of the Amylochrome method for amylase activity is described and evaluated. The modification consists of using Sure-Sep II Serum-Plasma Separators for the separation of the supernatant and precipitant in this procedure. Thirty patient sera and 20 control sera were run in duplicate with and without the separators. Comparison studies of the reference and separator methods show that the separator does not interfere with amylase activity, and there is excellent correlation (r = 0.99) between the methods. The time saved during centrifugation allows for easier and faster determination of amylase activity.