Host choice and West Nile virus infection rates in blood-fed mosquitoes, including members of the Culex pipiens complex, from Memphis and Shelby County, Tennessee, 2002-2003

Occurrence of avian Plasmodium and West Nile virus in Culex species in Wisconsin

The occurrence of multiple pathogens in mosquitoes and birds could affect the dynamics of disease transmission. We collected adult Culex pipiens and Cx. restuans (Cx. pipiens/restuans hereafter) from sites in Wisconsin and tested them for West Nile virus (WNV) and for avian malaria (Plasmodium). Gravid Cx. pipiens/restuans were tested for WNV using a commercial immunoassay, the RAMP WNV test, and positive results were verified by reverse transcriptase-polymerase chain reaction. There were 2 WNV-positive pools of Cx. pipiens/restuans in 2006 and 1 in 2007. Using a bias-corrected maximum likelihood estimation, the WNV infection rate for Cx. pipiens/restuans was 5.48/1,000 mosquitoes in 2006 and 1.08/1,000 mosquitoes in 2007. Gravid Cx. pipiens or Cx. restuans were tested individually for avian Plasmodium by a restriction enzyme-based assay. Twelve mosquitoes were positive for avian Plasmodium (10.0%), 2 were positive for Haemoproteus, and 3 were positive for Leucocytozoon. There were 4 mixed infections, with mosquitoes positive for > 1 of the hemosporidian parasites. This work documents a high rate of hemosporidian infection in Culex spp. and illustrates the potential for co-infections with other arboviruses in bird-feeding mosquitoes and their avian hosts. In addition, hemosporidian infection rates may be a useful tool for investigating the ecological dynamics of Culex/avian interactions.

Blood feeding patterns of mosquitoes: random or structured?

Background

The foraging behavior of blood-sucking arthropods is the defining biological event shaping the transmission cycle of vector-borne parasites. It is also a phenomenon that pertains to the realm of community ecology, since blood-feeding patterns of vectors can occur across a community of vertebrate hosts. Although great advances in knowledge of the genetic basis for blood-feeding choices have been reported for selected vector species, little is known about the role of community composition of vertebrate hosts in determining such patterns.

Methods & Results

Here, we present an analysis of feeding patterns of vectors across a variety of locations, looking at foraging patterns of communities of mosquitoes, across communities of hosts primarily comprised of mammals and birds. Using null models of species co-occurrence, which do not require ancillary information about host abundance, we found that blood-feeding patterns were aggregated in studies from multiple sites, but random in studies from a single site. This combination of results supports the idea that mosquito species in a community may rely primarily on host availability in a given landscape, and that contacts with specific hosts will be influenced more by the presence/absence of hosts than by innate mosquito choices. This observation stresses the importance of blood-feeding plasticity as a key trait explaining the emergence of many zoonotic mosquito transmitted diseases.

Discussion

From an epidemiological perspective our observations support the idea that phenomena promoting synchronization of vectors and hosts can promote the emergence of vector-borne zoonotic diseases, as suggested by observations on the linkages between deforestation and the emergence of several human diseases.

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.

Avian host-selection by Culex pipiens in experimental trials

Evidence from field studies suggests that Culex pipiens, the primary mosquito vector of West Nile virus (WNV) in the northeastern and north central United States, feeds preferentially on American robins (Turdus migratorius). To determine the contribution of innate preferences to observed preference patterns in the field, we conducted host preference trials with a known number of adult female C. pipiens in outdoor cages comparing the relative attractiveness of American robins with two common sympatric bird species, European starling, Sternus vulgaris and house sparrow, Passer domesticus. Host seeking C. pipiens were three times more likely to enter robin-baited traps when with the alternate host was a European starling (n = 4 trials; OR = 3.06; CI [1.42–6.46]) and almost twice more likely when the alternative was a house sparrow (n = 8 trials; OR = 1.80; CI = [1.22–2.90]). There was no difference in the probability of trap entry when two robins were offered (n = 8 trials). Logistic regression analysis determined that the age, sex and weight of the birds, the date of the trial, starting-time, temperature, humidity, wind-speed and age of the mosquitoes had no effect on the probability of a choosing a robin over an alternate bird. Findings indicate that preferential feeding by C. pipiens mosquitoes on certain avian hosts is likely to be inherent, and we discuss the implications innate host preferences may have on enzootic WNV transmission.

Vertebrate hosts and phylogenetic relationships of amphibian trypanosomes from a potential invertebrate vector, Culex territans Walker (Diptera: Culicidae)

The blood meals of field-collected female Culex territans (Diptera: Culicidae) were concurrently assayed for the presence of trypanosomes and for vertebrate host identification. We amplified vertebrate DNA in 42 of 119 females and made positive identification to the host species level in 29 of those samples. Of the 119 field-collected Cx. territans females, 24 were infected with trypanosomes. Phylogenetic analysis placed the trypanosomes in the amphibian portion of the aquatic clade of the Trypanosomatidae. These trypanosomes were isolated from Cx. territans females that had fed on the frog species Rana clamitans, R. catesbeiana, R. virgatipes, and Rana spp. Results support a potential new lineage of dipteran-transmitted amphibian trypanosomes may occur within the aquatic clade. The frequency in which female Cx. territans acquire trypanosomes, through diverse feeding habits, indicates a new relationship between amphibian trypanosomes and mosquitoes that has not been examined previously. Combining Trypanosoma species, invertebrate, and vertebrate hosts to existing phylogenies can elucidate trypanosome and host relationships.

Humoral immunity to West Nile virus is long-lasting and protective in the house sparrow (Passer domesticus)

The house sparrow (Passer domesticus) is a common and abundant amplifying host of West Nile virus (WNV) and many survive infection and develop humoral immunity. We experimentally inoculated house sparrows with WNV and monitored duration and protection of resulting antibodies. Neutralizing antibody titers remained relatively constant for >or= 36 months (N = 42) and provided sterilizing immunity for up to 36 months post-inoculation in 98.6% of individuals (N = 72). These results imply that immune house sparrows are protected from WNV infection for multiple transmission seasons. Additionally, individuals experiencing WNV-associated mortality reached significantly higher peak viremia titers than survivors, and mortality during acute infection was significantly higher in caged versus free-flight sparrows. A better understanding of the long-term immunity and mortality rates in birds is valuable in interpreting serosurveillance and diagnostic data and modeling transmission and disease dynamics.

Seasonal blood-feeding behavior of Culex tarsalis (Diptera: Culicidae) in Weld County, Colorado, 2007

Studies on Culex tarsalis Coquillett in Colorado have shown marked seasonal variation in the proportion of blood meals from birds and mammals. However, limitations in the specificity of antibodies used in the precipitin test and lack of vertebrate host availability data warrant revisiting Cx. tarsalis blood feeding behavior in the context of West Nile virus (WNV) transmission. We characterized the host preference of Cx. tarsalis during peak WNV transmission season in eastern Colorado and estimated the relative contribution of different avian species to WNV transmission. Cx. tarsalis preferred birds to mammals each month, although the proportion of blood meals from mammals increased in July and August. The distribution of blood meals differed significantly across months, in part because of changes in the proportion of blood meals from American robins, a preferred host. The estimated proportion of WNV-infectious vectors derived from American robins declined from 60 to 1% between June and August. The majority of avian blood meals came from doves, preferred hosts that contributed 25-40% of the WNV-infectious mosquitoes each month. Active WNV transmission was observed in association with a large house sparrow communal roost. These data show how seasonal patterns in Cx. tarsalis blood feeding behavior relate to WNV transmission in eastern Colorado, with the American robin contributing greatly to early-season virus transmission and a communal roost of sparrows serving as a focus for late-season amplification.

Microsatellite characterization of subspecies and their hybrids in Culex pipiens complex (Diptera: Culicidae) mosquitoes along a north-south transect in the central United States

Mosquitoes in the Culex pipiens complex, Cx. p. pipiens L. and Cx. p. quinquefasciatus Say are morphologically similar and important vectors of West Nile and St. Louis Encephalitis viruses in the United States. The subspecies differ with respect to overwintering strategies, with Cx. p. pipiens entering diapause in response to winter conditions and Cx. p. quinquefasciatus lacking this ability, yet they hybridize when found in sympatry. Specimens (n = 646) were collected using gravid traps set along a transect from New Orleans, LA, to Chicago, IL. Microsatellite markers were used to: genetically characterize subspecies and hybrids, determine the degree and extent of hybridization to better define the hybrid zone, and examine the efficacy of hybrid detection between microsatellites and a single-gene assay based on the acetylcholinesterase.2 gene (HotAce.2). The results support the presence of two distinct genetic entities, with abroad, stable hybrid zone in between. Admixture analyses classified >40% of individuals as hybrids. Allelic richness was markedly different at the northern and southern ends of the transect, and there was a significant isolation by distance effect. The hybrid zone seems to be wider and extends further to the south than previous work indicated, and as a result, we propose new boundaries compared with those indicated by a previous study. Microsatellites detected more hybrids than the HotAce.2 assay, although the latter assay remains useful as a screening tool. We suggest that the parental subspecies and the hybrid zone are maintained by heterosis combined with selection for diapause at northern latitudes.

Identification of mosquito bloodmeals using mitochondrial cytochrome oxidase subunit I and cytochrome b gene sequences

Primer pairs were designed and protocols developed to selectively amplify segments of vertebrate mitochondrial cytochrome oxidase subunit 1 (COI) and cytochrome b (Cyt b) mtDNA from the bloodmeals of mosquitoes (Diptera: Culicidae). The protocols use two pairs of nested COI primers and one pair of Cyt b primers to amplify short segments of DNA. Resultant sequences are then compared with sequences in GenBank, using the BLAST function, for putative host identification. Vertebrate DNA was amplified from 88% of our sample of 162 wild-caught, blood-fed mosquitoes from Oregon, U.S.A. and GenBank BLAST searches putatively identified 98% of the amplified sequences, including one amphibian, seven mammalian and 14 avian species. Criteria and caveats for putative identification of bloodmeals are discussed.

Temporal and spatial synchrony of Culex territans (Diptera: Culicidae) with their amphibian hosts

Culex territans Walker (Diptera: Culicidae) larvae share nonpolluted freshwater habitats with amphibians, which are bloodmeal hosts of their adults. To examine synchrony of Cx. territans with amphibian species, 10 larval habitat sites were sampled weekly from March to November of 2004. Cx. territans larvae were temporally and spatially associated with the green frog, Rana clamitans Latrielle. We predicted that if the preferred hosts were abundant at low temperatures, Cx. territans might be able to digest bloodmeals at those same temperatures. Using the thermal heat summation model, 192.3 d above 3.9 degrees C were needed to complete the gonotrophic cycle. This is the lowest thermal minimum reported for a Nearctic species of mosquito. Using this model, we calculated that the first larvae of Cx. territans field collected on 6 May 2004 were the progeny of females that bloodfed during the last week of March or first week of April. We conclude that Cx. territans has physiological mechanisms that allow them to take advantage of early season bloodmeal sources.

Blood feeding patterns of potential arbovirus vectors of the genus culex targeting ectothermic hosts

Reptiles and amphibians constitute a significant portion of vertebrate biomass in terrestrial ecosystems and may be important arbovirus reservoirs. To investigate mosquito preference for ectothermic hosts, feeding indices were calculated from data collected in Tuskegee National Forest, Alabama, USA. Four mosquito species fed upon ectothermic hosts, with Culex peccator and Cx. territans feeding primarily upon ectotherms. These two species appeared to target distinct species with little overlap in host choice. Culex peccator was a generalist in its feeding patterns within ectotherms, and Cx. territans appeared to be a more specialized feeder. Six of eleven ectotherm species fed upon by Cx. territans were fed upon more often than predicted based upon abundance. Spring peepers were highly preferred over other host species by Cx. territans. Blood meals taken from each host species varied temporally, with some hosts being targeted fairly evenly throughout the season and others being fed upon in seasonal peaks.

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.

Nestling passerines are not important hosts for amplification of West Nile Virus in Chicago, Illinois

Nestling birds have been hypothesized to be important hosts for mosquito-borne arboviruses, but the role of nestlings for West Nile virus (WNV) amplification remains unclear. We sampled open-cup and cavity-nesting passerines in Chicago, Illinois, an area of intense WNV transmission, to determine infection rates in nestlings and mosquitoes, and to test whether mosquitoes are attracted to nesting birds. Analysis of Culex pipiens mosquito populations demonstrated WNV amplification to high mosquito infection rates during both years of the study near the locations where nestlings were sampled. Nevertheless, of 194 nestlings representing 12 species, only one 8-day-old house wren was positive for WNV RNA, and only one 10-day-old mourning dove was seropositive for antibodies to WNV, but at a low titer (1:20). The number of mosquitoes captured in nest box traps and control traps was not significantly different. These combined results suggest that nestling passerines play no evident role in WNV amplification and transmission in the Chicago area.

Temperature, viral genetics, and the transmission of West Nile virus by Culex pipiens mosquitoes

The distribution and intensity of transmission of vector-borne pathogens can be strongly influenced by the competence of vectors. Vector competence, in turn, can be influenced by temperature and viral genetics. West Nile virus (WNV) was introduced into the United States of America in 1999 and subsequently spread throughout much of the Americas. Previously, we have shown that a novel genotype of WNV, WN02, first detected in 2001, spread across the US and was more efficient than the introduced genotype, NY99, at infecting, disseminating, and being transmitted by Culex mosquitoes. In the current study, we determined the relationship between temperature and time since feeding on the probability of transmitting each genotype of WNV. We found that the advantage of the WN02 genotype increases with the product of time and temperature. Thus, warmer temperatures would have facilitated the invasion of the WN02 genotype. In addition, we found that transmission of WNV accelerated sharply with increasing temperature, T, (best fit by a function of T(4)) showing that traditional degree-day models underestimate the impact of temperature on WNV transmission. This laboratory study suggests that both viral evolution and temperature help shape the distribution and intensity of transmission of WNV, and provides a model for predicting the impact of temperature and global warming on WNV transmission.

Transmission dynamics and changing epidemiology of West Nile virus

West Nile virus (WNV) is a flavivirus that is maintained in a bird-mosquito transmission cycle. Humans, horses and other non-avian vertebrates are usually incidental hosts, but evidence is accumulating that this might not always be the case. Historically, WNV has been associated with asymptomatic infections and sporadic disease outbreaks in humans and horses in Africa, Europe, Asia and Australia. However, since 1994, the virus has caused frequent outbreaks of severe neuroinvasive disease in humans and horses in Europe and the Mediterranean Basin. In 1999, WNV underwent a dramatic expansion of its geographic range, and was reported for the first time in the Western Hemisphere during an outbreak of human and equine encephalitis in New York City. The outbreak was accompanied by extensive and unprecedented avian mortality. Since then, WNV has dispersed across the Western Hemisphere and is now found throughout the USA, Canada, Mexico and the Caribbean, and parts of Central and South America. WNV has been responsible for >27,000 human cases, >25,000 equine cases and hundreds of thousands of avian deaths in the USA but, surprisingly, there have been only sparse reports of WNV disease in vertebrates in the Caribbean and Latin America. This review summarizes our current understanding of WNV with particular emphasis on its transmission dynamics and changing epidemiology.

Host-seeking heights, host-seeking activity patterns, and West Nile virus infection rates for members of the Culex pipiens complex at different habitat types within the hybrid zone, Shelby County, TN, 2002 (Diptera: Culicidae)

Host-seeking heights, host-seeking activity patterns, and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) infection rates were assessed for members of the Culex pipiens complex from July to December 2002, by using chicken-baited can traps (CT) at four ecologically different sites in Shelby County, TN. Host-seeking height was assessed by CT placed at elevations of 3.1, 4.6, and 7.6 m during one 24-h period per month. Host-seeking activity was assessed by paired CT placed at an elevation of 4.6 m. Can traps were sampled at one 10-h daytime interval and at seven 2-h intervals during the evening, night, and morning. Cx. pipiens complex mosquitoes accounted for 87.1% of collected mosquitoes. Culex (Melanoconion) erraticus (Dyar & Knab) accounted for 11.9% of specimens. The average number of Cx. pipiens complex mosquitoes collected per 24-h CT period from July to September was lowest at a rural middle income site (1.7), intermediate at an urban middle income site (11.3), and highest at an urban low income site (47.4). Can traps at the forested site failed to collect Cx. pipiens complex mosquitoes. From July to September at urban sites, Culex pipiens pipiens L. was the rarest of the three complex members accounting for 11.1-25.6% of specimens. At the rural site, Culex pipiens quinquefasciatus Say was the rarest member of the complex. Cx. p. pipiens was not collected after September. Mean abundance of Cx. pipiens complex mosquitoes was higher in traps at 7.6 m than in traps at 4.6 m. Abundances at 3.1 m were intermediate and not significantly different from abundances at the other heights. Initiation of host-seeking activity was associated with the end of civil twilight and activity occurred over an extended nighttime period lasting 8-10 h. All 11 WNV-positive mosquitoes were Cx. pipiens complex mosquitoes collected from urban sites in traps placed at elevations of 4.6 and 7.6 m. Infection rates were marginally nonsignificant by height. Infection rates, host-seeking heights, and activity patterns were not significantly different among members of the Cx. pipiens complex.