Plasmodium transmission differs between mosquito species and parasite lineages.

Factors such as the particular combination of parasite-mosquito species, their co-evolutionary history and the host's parasite load greatly affect parasite transmission. However, the importance of these factors in the epidemiology of mosquito-borne parasites, such as avian malaria parasites, is largely unknown. Here, we assessed the competence of two mosquito species [Culex pipiens and Aedes (Ochlerotatus) caspius], for the transmission of four avian Plasmodium lineages (Plasmodium relictum SGS1 and GRW11 and Plasmodium cathemerium-related lineages COLL1 and PADOM01) naturally infecting wild house sparrows. We assessed the effects of parasite identity and parasite load on Plasmodium transmission risk through its effects on the transmission rate and mosquito survival. We found that Cx. pipiens was able to transmit the four Plasmodium lineages, while Ae. caspius was unable to transmit any of them. However, Cx. pipiens mosquitoes fed on birds infected by P. relictum showed a lower survival and transmission rate than those fed on birds infected by parasites related to P. cathemerium. Non-significant associations were found with the host-parasite load. Our results confirm the existence of inter- and intra-specific differences in the ability of Plasmodium lineages to develop in mosquito species and their effects on the survival of mosquitoes that result in important differences in the transmission risk of the different avian malaria parasite lineages studied.

A Survey of Aedes (Diptera: Culicidae) Mosquitoes in Tunisia and the Potential Role of Aedes detritus and Aedes caspius in the Transmission of Zika Virus.

The present study aimed to update the list of Aedes mosquito species occurring in Tunisia and to test the vector competence of Aedes (Ochlerotatus) caspius (Pallas) and Ae. (Ochlerotatus) detritus (Haliday), the locally most abundant and widespread species, to transmit Zika virus (ZIKV). In 2017-2018, mosquito larvae were collected from 39 different larval habitats in seven bioclimatic zones of Tunisia. The salinity and pH of each breeding site were measured. The survey revealed the presence of 10 Aedes species in Tunisia: Ae. (Stegomyia) albopictus (Skuse), Ae. (Ochlerotatus) berlandi (Séguy), Ae. caspius, Ae. detritus, Ae. (Finlaya) echinus (Edwards), Ae. (Finlaya) geniculatus (Olivier), Ae. (Acartomyia) mariae (Sergent and Sergent), Ae. (Ochlerotatus) pulcritarsis (Rondani), Ae. (Aedimorphus) vexans (Meigen), and Ae. (Fredwardsius) vittatus (Bigot). Of these 10 species, Ae. caspius and Ae. detritus were the most abundant in Tunisia. Aedes detritus and Ae. caspius larvae were reared until the imago stage under insectary conditions to test autogeny. The study showed that Ae. detritus is autogenous and stenogamous and Ae. caspius, anautogenous and eurygamous. Finally, the collected strains of these two species were experimentally infected with the Asian genotype of ZIKV, originally isolated from a patient in April 2014 in New Caledonia, to test their vector competence. Neither of these species was able to transmit ZIKV at 7 and 14 d postexposure. Further investigations are needed to test the competence of other Tunisian mosquito species that may be associated with ZIKV transmission.

Effects of host sex, body mass and infection by avian Plasmodium on the biting rate of two mosquito species with different feeding preferences.

BACKGROUND: The transmission of mosquito-borne pathogens is strongly influenced by the contact rates between mosquitoes and susceptible hosts. The biting rates of mosquitoes depend on different factors including the mosquito species and host-related traits (i.e. odour, heat and behaviour). However, host characteristics potentially affecting intraspecific differences in the biting rate of mosquitoes are poorly known. Here, we assessed the impact of three host-related traits on the biting rate of two mosquito species with different feeding preferences: the ornithophilic Culex pipiens and the mammophilic Ochlerotatus (Aedes) caspius. Seventy-two jackdaws Corvus monedula and 101 house sparrows Passer domesticus were individually exposed to mosquito bites to test the effect of host sex, body mass and infection status by the avian malaria parasite Plasmodium on biting rates. RESULTS: Ochlerotatus caspius showed significantly higher biting rates than Cx. pipiens on jackdaws, but non-significant differences were found on house sparrows. In addition, more Oc. caspius fed on female than on male jackdaws, while no differences were found for Cx. pipiens. The biting rate of mosquitoes on house sparrows increased through the year. The bird infection status and body mass of both avian hosts were not related to the biting rate of both mosquito species. CONCLUSIONS: Host sex was the only host-related trait potentially affecting the biting rate of mosquitoes, although its effect may differ between mosquito and host species.

Hyperparasitism of mosquitoes by water mite larvae.

Hyperparasitism of ectoparasitic water mite larvae on mosquitoes is still a neglected relationship and was investigated only in a few studies. We analysed 2313 female mosquitoes from six different sampling localities with regard to their degree of parasitism with water mite larvae. In total, we found 38 mosquito individuals parasitized by 93 water mite larvae, ranging from 1 to 12 larvae per mosquito. Water mite larvae detected are members of the two species Parathyas cf. barbigera (n = 92) and Arrenurus cf. globator (n = 1). Out of the analysed mosquitoes, individuals out of the species Aedes vexans, Anopheles claviger, Ochlerotatus communis, the Ochlerotatus cantans/annulipes group, Ochlerotatus cataphylla and Ochlerotatus sticticus were tested to be parasitized by water mite larvae. The highest prevalence was found within the species Oc. cataphylla (28.6 %) and Oc. cantans/annulipes (21.7 %). No water mite larvae were found, e.g. on individuals of Aedes cinereus, Coquillettidia richiardii, the Culex pipiens/torrentium group, Ochlerotatus caspius, Ochlerotatus dorsalis or Ochlerotatus punctor. All of the attachment sites were located between the neck and abdomen with the ventral thorax site being the most frequent one.

Haemoproteus infections (Haemosporida, Haemoproteidae) kill bird-biting mosquitoes.

Haemoproteus parasites (Haemosporida, Haemoproteidae) are widespread; some species cause severe diseases in avian hosts. Heavy Haemoproteus infections are often lethal for biting midges (Ceratopogonidae), which transmit avian haemoproteids, but there is no information regarding detrimental effect on other blood-sucking insects. We examined effects of Haemoproteus tartakovskyi (lineage hSISKIN1), Haemoproteus lanii (lineages hRB1and hRBS2) and Haemoproteus balmorali (lineage hCOLL3) on the survival of Ochlerotatus cantans, a widespread Eurasian mosquito. Wild-caught females were infected by allowing them to feed on naturally infected birds with light (0.01%) and high (3.0-9.6%) parasitaemia. Mosquitoes fed on uninfected birds were used as controls. Both experimental and control groups were maintained under the same laboratory conditions until 20 days post-exposure (dpe). Dead insects were counted daily and used for parasitological examination and PCR-based testing. No difference was discernible in the survival rate of control mosquitoes and those fed on meal with light parasitaemia. There was a highly significant difference in the survival rate between the control group and all groups fed on meals with high parasitaemia, with the greatest mortality reported 1-3 dpe. For 4 dpe, the percentage of survived control mosquitoes (88%) was 2.2-, 3.6- and 4-fold greater than that of groups fed on meals with high parasitaemia of H. balmorali, H. tartakovskyi and H. lanii, respectively. Numerous ookinetes were observed in the gut area and adjacent tissues located in the head, thorax and abdomen of infected insects 0.5-1 dpe. The migrating parasites damage organs throughout the entire body of mosquitoes; that is the main reason of mortality. To the end of this study, 46% of mosquitoes survived in control group, but the survival rates of experimental mosquitoes fed on meals with high parasitaemia were between 2.6- and 5.8-fold lower. This study indicates that widespread Haemoproteus infections are markedly virulent for bird-biting mosquitoes, which rapidly die after feeding on heavily infected blood meals.

Morphological and molecular characterization of a microsporidian parasite, Takaokaspora nipponicus n. gen., n. sp. from the invasive rock pool mosquito, Ochlerotatus japonicus japonicus.

A new genus and species of Microsporidia, Takaokaspora nipponicus n. gen., n. sp. is described from Ochlerotatus japonicus japonicus (Theobald) and Ochlerotatus hatorii (Yamada) based on light microscope and ultrastructural morphology, developmental features, transmission cycles and comparative sequence analyses of the small subunit ribosomal DNA (SSU rDNA). The microsporidium is both vertically and horizontally transmitted, exhibits dimorphic development alternating between diplokaryotic and monokaryotic stages and produces two morphologically distinct spores, one in larvae and another in adult females. Horizontal transmission of infection to larval mosquitoes occurs via direct oral ingestion of uninucleate spores that are produced in vertically-infected larval hosts. Development in horizontally-infected hosts is diplokaryotic following karyokinesis of uninucleate schizonts and binary fission to produce small (4.3µm × 2.0µm) membrane free, ovoid, binucleate spores that are confined to adult female reproductive tissues (ovariole sheath and oviducts). Vertical transmission of the microsporidium from adult females to larval progeny takes place via surface contamination of the egg (transovum). Microsporidian development in vertically-infected larvae is haplophasic with unpaired nuclei throughout, producing rosette-shaped sporogonial plasmodia contained within a thin non-persistent sporophorous vesicle and culminating in the formation of membrane free, uninucleate, conical spores (7.0µm×2.8µm). Development is confined to host fat body tissue which appears as swollen white masses in the thorax and selected segments of the abdomen causing larvae to appear abnormally distorted and results in death during the third and fourth instar stages. The SSU rDNA sequences obtained from the two morphologically identical microsporidia isolated from Oc. j. japonicus and Oc. hatorii were nearly identical and unique when compared with GenBank entries of all other mosquito-parasitic species. Phylogenetic trees constructed by Maximum Parsimony, Maximum Likelihood and bootstrap analyses using the Neighbor Joining search parameter yielded similar typologies. In each case, the novel microsporidium was the sister group to the clade containing Parathelohania species from Anopheles mosquitoes and the monotypic Novothelohania ovalae from Ochlerotatus caspius showing approximately 10-13% sequence divergence to those two genera providing strong support for establishment as a separate genus.

Development of a facile system for mass production of Brugia malayi in a small-space laboratory.

Brugia malayi is one of the important lymphatic filarial nematodes that cause elephantiasis and disability in humans in the Asian region. Mass production at any stage of this nematode in both small laboratory animal hosts and mosquito vectors is still necessary in order to continue various research aspects. This study elucidated on the use of nonblood feeding or the autogenous Ochlerotatus togoi (Thailand strain) and male Mongolian jird (Meriones unguiculatus) system. This has brought about a low-cost and highly-effective procedure for the mass production of blood containing microfilariae, infective (L3) larvae, and adults of B. malayi under nonanimal-blood-feeding insectary and small-space animal-house conditions. The highly-infective rates (human-heparinized blood, 86.67-93.33; swine-heparinized blood, 83.33-96.67; bovine-heparinized blood, 76.67-80; chicken-heparinized blood, 73.33-76.67) and parasite loads (human-heparinized blood, 10.58-12.36; swine-heparinized blood, 8.40-10.38; bovine-heparinized blood, 9.75-9.91; chicken-heparinized blood, 3.41-4.65) of autogenous O. togoi to B. malayi and high numbers of adults recovered from ten B. malayi-infected male jirds (total = 327, 16-52) are good supportive evidence. In addition, all special techniques required for succeeding in the establishment of a facile system regarding these matters are detailed.

Avian Plasmodium in Culex and Ochlerotatus Mosquitoes from Southern Spain: Effects of Season and Host-Feeding Source on Parasite Dynamics.

Haemosporidians, a group of vector-borne parasites that include Plasmodium, infect vertebrates including birds. Although mosquitoes are crucial elements in the transmission of avian malaria parasites, little is known of their ecology as vectors. We examined the presence of Plasmodium and Haemoproteus lineages in five mosquito species belonging to the genera Culex and Ochlerotatus to test for the effect of vector species, season and host-feeding source on the transmission dynamics of these pathogens. We analyzed 166 blood-fed individually and 5,579 unfed mosquitoes (grouped in 197 pools) from a locality in southern Spain. In all, 15 Plasmodium and two Haemoproteus lineages were identified on the basis of a fragment of 478 bp of the mitochondrial cytochrome b gene. Infection prevalence of blood parasites in unfed mosquitoes varied between species (range: 0-3.2%) and seasons. The feeding source was identified in 91 mosquitoes where 78% were identified as bird. We found that i) several Plasmodium lineages are shared among different Culex species and one Plasmodium lineage is shared between Culex and Ochlerotatus genera; ii) mosquitoes harboured Haemoproteus parasites; iii) pools of unfed females of mostly ornithophilic Culex species had a higher Plasmodium prevalence than the only mammophylic Culex species studied. However, the mammophylic Ochlerotatus caspius had in pool samples the greatest Plasmodium prevalence. This relative high prevalence may be determined by inter-specific differences in vector survival, susceptibility to infection but also the possibility that this species feeds on birds more frequently than previously thought. Finally, iv) infection rate of mosquitoes varies between seasons and reaches its maximum prevalence during autumn and minimum prevalence in spring.

Abortive long-lasting sporogony of two Haemoproteus species (Haemosporida, Haemoproteidae) in the mosquito Ochlerotatus cantans, with perspectives on haemosporidian vector research.

Haemoproteus spp. are cosmopolitan vector-born haemosporidian parasites, some species of which cause diseases in non-adapted birds. Recent polymerase chain reaction (PCR)-based studies have detected mitochondrial cytochrome b gene lineages of these Haemoproteus parasites in blood-sucking mosquitoes and speculated about possible involvement of these insects in transmission of avian haemoproteids. However, development of Haemoproteus lineages has not been documented in mosquitoes. We infected 304 individuals of Ochlerotatus cantans, a widespread Eurasian mosquito, with Haemoproteus tartakovskyi (lineage hSISKIN1) and Haemoproteus balmorali (lineage hROBIN1). Mosquitoes were allowed to take non-infected and infected blood meals and maintained in the laboratory until 17 days post-infection (dpi). They were tested for presence of sporogonic stages by microscopic and PCR-based methods. Microscopic examination revealed partial development of both parasites in the infected insects. Numerous ookinetes were seen in the gut area and adjacent tissues located in the head, thorax and abdomen of mosquitoes between 1 and 5 dpi. Numerous oocysts were seen in the midgut wall between 4 and 15 dpi; they were also present in the head and thorax of infected mosquitoes testifying to the active movement of ookinetes throughout the body. Oocysts degenerated between 11 and 17 dpi. Sporozoites were not seen in oocysts or mosquito salivary glands, indicating abortive sporogonic development at the oocyst stage. In accordance with microscopy data, PCR and sequencing revealed presence of the lineages hSISKIN1 and hROBIN1 in experimental mosquitoes as long as 15 and 17 dpi, respectively, demonstrating relatively long survival of Haemoproteus parasites in the resistant insects without DNA degeneration. The present study shows that PCR-based diagnostics should be carefully used in vector studies of haemosporidians because it detects parasites in insects for several weeks after initial infection, but does not distinguish abortive parasite development. Demonstration of infective sporozoites in insects is essential for definitively demonstrating the insects are vectors.

Peritrophic matrix formation and Brugia malayi microfilaria invasion of the midgut of a susceptible vector, Ochlerotatus togoi (Diptera: Culicidae).

The mosquito midgut is the first site that vector-borne pathogens contact during their multiplication, differentiation, or migration from blood meal to other tissues before transmission. After blood feeding, the mosquitoes synthesize a chitinous structure called peritrophic matrix (PM) that envelops the blood meal and separates the food bolus from the midgut epithelium. In this study, a systematic investigation of the PM formation and the interaction of Brugia malayi within the midgut of a susceptible vector, Ochlerotatus togoi, were performed using scanning electron microscopy (SEM). SEM analysis of the midguts dissected at different time points post feeding on a B. malayi-infected blood meal (PIBM) revealed that the PM was formed from 45 min PIBM and gradually thickened and matured during 8-18 h PIBM. The PM degraded from 24 to 72 h PIBM, when digestion was completed. The invasion process of the microfilariae was observed between 3 and 4 h PIBM. In the beginning of the process, only sheathed microfilariae interacted with the internal face of the PM by its anterior part, and then the midgut epithelium before entering the hemocoel, after that they exsheathed. Microfilarial sheaths lying within the hemocoel were observed suggesting that they may serve as a decoy to induce the immune systems of the mosquitoes to respond to the antigens on the sheaths, thereby protecting the exsheathed microfilariae. These initial findings would lead to further study on the proteins, chemicals, and factors in the midgut that are involved in the susceptibility of O. togoi as a vector of filariasis.

Risk of canine and human exposure to Dirofilaria immitis infected mosquitoes in endemic areas of Italy.

BACKGROUND: The occurrence of infections by Dirofilaria immitis in canine and human populations depends on several factors linked to both the definitive and intermediate hosts. Little data are available on the risk of human and dog exposure to D. immitis in endemic areas. Data collected on dog- and human-bait traps in endemic areas of north-eastern Italy were used to estimate the likelihood of a receptive host coming into contact with an infected vector. METHODS: From 1997 to 1999, mosquitoes were collected from three sampling sites of north-eastern Italy on D. immitis microfilaraemic dogs and on human baits. The bite/night/host rates were determined based on the number of feeding and probing mosquitoes on dogs and humans, respectively. The survival/mortality rates of different species of mosquitoes following the blood meal, and the rate of natural Dirofilaria infection in unfed specimens were estimated. The risk of exposure of dogs and humans to infected mosquito species was determined by combining the bite/host/night and the mosquito infection rates. RESULTS: A total of 1,165 mosquitoes were collected on human (n=815) and dog (n=350) baits with varying species composition (i.e., Culex pipiens, 87.3% and Ochlerotatus caspius, 11.6%). Overall, dogs were more attractive to Cx pipiens than humans (feeding rate 70.2% vs probing rate 25.9%). The highest bite/night/host rate was 84.0 for dogs and 26.5 for humans. Cx pipiens displayed a mortality rate of 76.3% within 13 days and Oc. caspius of 100% within two days following the infective blood meal. In addition, D. immitis DNA was detected in unfed Cx pipiens (infection rate of 0.26%-2.07%). The infection rate adjusted for mosquito mortality was 0.38%. Based on data collected, the contact between an infected mosquito and a host can occur as often as every four nights for D. immitis infected-mosquitoes in dogs and within two weeks for humans. CONCLUSIONS: Cx pipiens was confirmed as the most efficient natural vector of D. immitis in the studied area. In endemic areas, the risk of transmission can be very high for dogs and relevant for humans. Despite the increased awareness of veterinarians and owners on canine dirofilarioses, dogs from rural areas still maintain the natural life cycle of Dirofilaria spp., therefore acting as a source of infection to humans through vector bites.

Life cycle completion of parasite Ascogregarina taiwanensis (Apicomplexa: Lecudinidae) in non-native host Ochlerotatus japonicus (Diptera: Culicidae).

Ascogregarina taiwanensis (Lien and Levine), a protist gut parasite of Aedes albopictus (Skuse), is not known to complete its life cycle within the potential competitor species, Ochlerotatus japonicus (Theobald). In a laboratory cross infection study we demonstrated that A. taiwanensis completed its life cycle within Oc. japonicus and remained infectious. We also sampled cohabitating mosquito larvae in Mercer County, NJ, and based on ribosomal DNA sequence data, we determined that Oc. japonicus cohabitating with Ae. albopictus can become infected with A. taiwanensis.

Molecular and ultrastructural characterization of Andreanna caspii n. gen., n. sp. (Microsporida: Amblyosporidae), a parasite of Ochlerotatus caspius (Diptera: Culicidae).

A new genus and species of microsporidia, Andreanna caspii n. gen., n. sp. is described from the mosquito, Ochlerotatus caspius (Pallas) based on ultrastructural morphology, developmental characteristics, and comparative sequence analyses of the small subunit (SSU) ribosomal DNA (rDNA). Parasite development is confined to fat body tissue and infected larvae appear swollen with dull white masses within the thorax and abdomen. Meronts have diplokaryotic nuclei and are delineated by a simple plasmalemma contiguous with the host cell cytoplasm. Merogony occurs by synchronous binary division followed by cytokinesis. Diplokaryotic sporonts undergo meiosis and synchronous nuclear division forming sporogonial plasmodia with two, four and eight nuclei enclosed within a persistent sporophorous vesicle. Cytokinesis of sporogonial plasmodia results in the formation of eight uninucleate spores. The episporontal space of early sporonts is filled with a homogeneous accumulation of electron dense granular inclusions and ovoid vesicles of various dimensions, transforming into an interwoven matrix during the initial phase of sporogenesis. Spores are oval, uninucleate and measure 4.8+/-0.3 x 3.1+/-0.4 microm (fixed). The spore wall is 260 microm thick with an irregular exospore consisting of two layers (150-170 microm) and a thinner endospore (90-100 microm). The anchoring disk is well developed and is contiguous with a lamellar polaroplast that occupies the anterior third of the spore and possess more narrow lamellae on the posterior end. The polar filament is gradually tapered and arranged in a single row consisting of six coils ranging from 180 to 150 microm in diameter. The posterior vacuole (posterosome) is moderately sized and filled with a matrix of moderate electron density. Phylogenetic analysis of SSU rDNA from A. caspii and 30 other species of microsporidia including 11 genera parasitic in mosquitoes using maximum parsimony, neighbor joining and maximum likelihood methods showed A. caspii to be a sister group to the clade containing all of the Amblyospora species, including Culicospora, Edhazardia and Intrapredatorus, as well as Culicosporella and Hyalinocysta thus providing strong support for establishment of Andreanna as a separate genus.

Transmission dynamics of diurnally subperiodic lymphatic filariasis transmitted by Ochlerotatus (Finlaya) niveus in the Andaman & Nicobar Islands.

BACKGROUND & OBJECTIVE: In India diurnally subperiodic filariasis is prevalent only in the Nicobar district of Andaman and Nicobar Islands, with significant health problem. Sample surveys indicated that this form of filariasis is restricted to a small region of Nancowry group of islands with Ochlerotatus niveus as the vector. We therefore carried out a comprehensive study to assess the transmission dynamics of LF in Teressa island in Nicobar district in view of its control and evaluation of interventions. METHODS: Entomological studies were carried out for a period of twelve months covering all the seasons in the year, by means of man landing catches in Teressa Island, an endemic island for this form of filariasis. Parameters viz., the annual biting rate (ABR), annual infective biting rate (AIBR), annual transmission index (ATI), risk of infection index (RII) and annual transmission potential (ATP), which reflect the dynamics and intensity of transmission of filariasis, were estimated. Host efficiency of Oc. niveus was also assessed. RESULTS: The number of vectors biting a person in a year was estimated to be 21851, of which 107 were harbouring infective parasite. Risk of infection intensity was 0.02332. Every person in this study area was at the risk of receiving an estimated number of 22 infective stage larvae per year. The host efficiency index of Oc. niveus indicated that over 40 per cent of the microfilariae ingested were able to develop into infective stages. The ATP was 169 with evidence of year round transmission. The pattern of monthly transmission potential suggested that the intensity of transmission was high during summer months. INTERPRETATION & CONCLUSION: Perennial transmission of subperiodic Wuchereria bancrofti in the typical forest ecosystem was evident in Teressa Island with transmission parameters suggesting that summer is a high risk season for transmission. Personal protection measure is the method of choice to protect from the risk of infection, because of day biting, exophilic and exophagic behaviour of the vector and larvae are not amenable to larvicidal measures. An alternative method to control the transmission would be to use selective or periodic mass chemotherapy to reduce the parasite load in this community.

Ochlerotatus taeniorhynchus: a probable vector of Dirofilaria immitis in coastal areas of Yucatan, Mexico.

Mosquito collections were done on microfilaremic dogs, positive for Dirofilaria immitis (Leidy), for 15 consecutive nights in Celestun, Yucatan, southeastern Mexico, during January 2007. In total, 275 mosquitoes (3 male and 272 female) of five species were collected: Ochlerotatus taeniorhynchus (Wiedemann), Ochlerotatus sollicitans (Walker), Culex quinquefasciatus Say, Culex interrogator (Dyar & Knab), and Aedes aegypti (L.). Oc. taeniorhynchus was the species most frequently collected, and it had the highest rate of feeding success and the highest attack rates. First (L1) and third (L3) instars of Dirofilaria were observed in all mosquito species collected except for Ae. aegypti: 23 of 223 Oc. taeniorhynchus, three of 40 Cx. quinquefasciatus, and one of six Oc. sollicitans and one of one Cx. interrogator were infected with Dirofilaria (10.3% of total examined mosquitoes). This is the first report of Dirofilaria (presumably D. immitis) isolation from wild-caught mosquitoes in Mexico. Results imply that D. immitis can develop from microfilaria to infective L3s in Oc. taeniorhynchus, Cx. quinquefasciatus, and Oc. sollicitans, but L3s (infectious) were only recovered in heads of Oc. taeniorhynchus. Thus, Oc. taeniorhynchus can be considered a potential vector of dirofilariasis in Celestun, because infective L3 Dirofilaria were found in the vicinity of the mouthparts, and this mosquito has a strong association with microfilaremic dogs in this study area.

Spatiotemporal oviposition and habitat preferences of Ochlerotatus triseriatus and Aedes albopictus in an emerging focus of La Crosse virus.

The number of cases of encephalitis caused by La Crosse virus recently has increased in southwestern Virginia counties. This article presents results of a study conducted from May to September 2000 in Wise County, VA, that examined the area-wide oviposition activity and habitat preferences of Ochlerotatus triseriatus and Aedes albopictus, potential vectors of La Crosse virus in the region. Data from 490 ovitrap collections made throughout the county showed that mean oviposition activity throughout the study was higher for Oc. triseriatus (20.4 eggs/trap-day) than for Ae. albopictus (3.7 eggs/trap-day). The 2 species also had distinct habitat preferences for oviposition, with Oc. triseriatus favoring forested habitats and Ae. albopictus favoring urban/residential habitats. A landcover map of 6 habitat types derived from Landsat satellite imagery of the county showed that 63% of the county was forested and 18% was urban/residential. A Bayesian decision-rule model that incorporated the ovitrap data and landcover map was moderately successful at predicting the occurrence of high oviposition activity and abundance of the 2 species. The predictions reflected seasonal and spatial fluctuations in oviposition activity, with accuracies between 55 and 79% for Oc. triseriatus and 70 and 94% for Ae. albopictus. Kappa (K), a measure of the predictive power of the model, varied from poor (K < 0.4) to good (0.4 < K < 0.75) for both species, and was highest during periods when actual egg abundance was high. This suggests that the predictions were most accurate during periods when the risk for La Crosse virus transmission is greatest. Limitations and suggestions for improving the model are discussed.