VectorInfo: A web resource for medically important Indian arthropod disease vectors

VectorInfo is a freely accessible web resource, emphasised on medically important Indian arthropods funded by Indian Council of Medical Research (ICMR) and maintained by one of its premier institute, Vector Control Research Centre (VCRC). VectorInfo elucidates and curates medically important Indian arthropod's biological, omics technologies to adopt a holistic view of the molecules that make up an organism, aimed at the detection of genomics, transcriptomics, proteomics, enzymes & pathways and immune specific genes. The nitty-gritty of VectorInfo is aimed at scrutinizing all the possible information on Indian disease vectors in a single window for the scientific community. The database affords 53 medically important Indian arthropod's biological and omics information well-structured and provided with downloadable facilities. In addition to this, huge number of research articles were mined in the quest for gathering the recommended insecticide targets and their mechanisms, that pave ways to design and develop novel lead molecules through computational means. This current up-to-date database contains 2,498 omics entries beneficial for the molecular studies and analysis. In order to maintain regular updates, user forms were provided for the scientific community to submit research data to the database administrator. The VectorInfo furthermore conveys various resources for vector control and diagnostics and the links to the crucial software tools used for the Bioinformatics analysis.

Identification of African swine fever virus-like elements in the soft tick genome provides insights into the virus' evolution

BACKGROUND

African swine fever virus (ASFV) is a most devastating pathogen affecting swine. In 2007, ASFV was introduced into Eastern Europe where it continuously circulates and recently reached Western Europe and Asia, leading to a socio-economic crisis of global proportion. In Africa, where ASFV was first described in 1921, it is transmitted between warthogs and soft ticks of the genus Ornithodoros in a so-called sylvatic cycle. However, analyses into this virus' evolution are aggravated by the absence of any closely related viruses. Even ancient endogenous viral elements, viral sequences integrated into a host's genome many thousand years ago that have proven extremely valuable to analyse virus evolution, remain to be identified. Therefore, the evolution of ASFV, the only known DNA virus transmitted by arthropods, remains a mystery.

RESULTS

For the identification of ASFV-like sequences, we sequenced DNA from different recent Ornithodoros tick species, e.g. O. moubata and O. porcinus, O. moubata tick cells and also 100-year-old O. moubata and O. porcinus ticks using high-throughput sequencing. We used BLAST analyses for the identification of ASFV-like sequences and further analysed the data through phylogenetic reconstruction and molecular clock analyses. In addition, we performed tick infection experiments as well as additional small RNA sequencing of O. moubata and O. porcinus soft ticks.

CONCLUSION

Here, we show that soft ticks of the Ornithodoros moubata group, the natural arthropod vector of ASFV, harbour African swine fever virus-like integrated (ASFLI) elements corresponding to up to 10% (over 20 kb) of the ASFV genome. Through orthologous dating and molecular clock analyses, we provide data suggesting that integration could have occurred over 1.47 million years ago. Furthermore, we provide data showing ASFLI-element specific siRNA and piRNA in ticks and tick cells allowing for speculations on a possible role of ASFLI-elements in RNA interference-based protection against ASFV in ticks. We suggest that these elements, shaped through many years of co-evolution, could be part of an evolutionary virus-vector 'arms race', a finding that has not only high impact on our understanding of the co-evolution of viruses with their hosts but also provides a glimpse into the evolution of ASFV.

Highlights in Medical Entomology, 2019: Familiar Foes and New Frontiers

The 2019 Entomological Society of America annual meeting was held in St. Louis, Missouri, just blocks away from the iconic Gateway Arch. Representing a 'gateway to the West', this inspired the theme of the Highlights in Medical Entomology to reflect on the accomplishments of the past year as we move into a 'new frontier' of vector biology research. Papers were selected broadly across arthropods that influence public health, focusing on topics ranging from West Nile virus transmission, ticks and tick-borne disease, to advances in genetics and 'big data' studies. This included current perspectives on West Nile virus ecology and epidemiology, which has now been endemic in the United States for 20 yr. Additional topics such as the advantages of citizen science and the importance of scientific communication were also discussed. Together, these papers demonstrate the achievements of the vector community while emphasizing the challenges that we collectively face to reduce the burden of vector-borne disease.

Arthropod Containment Guidelines, Version 3.2

The Arthropod Containment Guidelines are a product of the work of the American Committee of Medical Entomology, a subcommittee of the American Society of Tropical Medicine and Hygiene. The guidelines provide a reference for research laboratories to assess risk and establish protocols for the safe handling of arthropod vectors of human and animal disease agents. The guidelines were originally published in 2004 and have been updated here to reflect the spectrum of vector taxa under investigation, and the demands of working with vector arthropods in the context of the Select Agent Rule.

G protein-coupled receptors in arthropod vectors: omics and pharmacological approaches to elucidate ligand-receptor interactions and novel organismal functions

Regulation of many physiological processes in animals, certainly those controlled by neuropeptide hormones, involves G protein-coupled receptors (GPCRs). Our work focusing on endocrine regulation of diuresis and water balance in mosquitoes and ticks started in 1997 with the kinin receptor, at the dawn of the omics era. After the genomic revolution, we began work on the endocrinology of reproduction in the red imported fire ant. We will use the template of this comparative work to summarize key points about GPCRs and signaling, and emphasize the most recent developments in the pharmacology of arthropod neuropeptide GPCRs. We will discuss omics' contributions to the advancement of this field, and its influence on peptidomimetic design while emphasizing work on blood feeding arthropods.

Fitness estimates from experimental infections predict the long-term strain structure of a vector-borne pathogen in the field

The populations of many pathogen species consist of a collection of common and rare strains but the factors underlying this strain-specific variation in frequency are often unknown. Understanding frequency variation among strains is particularly challenging for vector-borne pathogens where the strain-specific fitness depends on the performance in both the vertebrate host and the arthropod vector. Two sympatric multiple-strain tick-borne pathogens, Borrelia afzelii and B. garinii, that use the same tick vector, Ixodes ricinus, but different vertebrate hosts were studied. 454-sequencing of the polymorphic ospC gene was used to characterize the community of Borrelia strains in a local population of I. ricinus ticks over a period of 11 years. Estimates of the reproduction number (R0), a measure of fitness, were obtained for six strains of B. afzelii from a previous laboratory study. There was substantial variation in prevalence among strains and some strains were consistently common whereas other strains were consistently rare. In B. afzelii, the strain-specific estimates of R0 in laboratory mice explained over 70% of the variation in the prevalences of the strains in our local population of ticks. Our study shows that laboratory estimates of fitness can predict the community structure of multiple-strain pathogens in the field.

Genetic make-up of arthropod vectors

A better understanding of the molecular aspects of arthropod vector biology and the processes that determine pathogen transmission can lead to the development of novel or improved control methods for vectors and vector-borne diseases. The 'omics' era provides unprecedented opportunities to explore these aspects of vectors and the diseases which they transmit. This review aims to summarise recent developments in the field of vector genomics and to provide basic insight into the application of functional genetic tools such as RNA interference, RNA sequencing and genetic transformation in vector control development.

Functional genomics of tick vectors challenged with the cattle parasite Babesia bigemina

Ticks are obligate hematophagous ectoparasites considered as vectors of animal diseases, having a huge economic impact in cattle industry. Babesia spp. are tick-borne pathogens that cause a disease called babesiosis in a wide range of animals and in humans. Control of tick infestations is mainly based on the use of acaricides, which have limited efficacy reducing tick infestations, mostly due to wrong usage, and is often accompanied by the selection of acaricide-resistant ticks, environmental contamination, and contamination of milk and meat products. Vaccines affecting both vector and pathogens constitute new control strategies for tick and tick-borne diseases and are, therefore, a good alternative to chemical control. In this chapter we describe the identification of Rhipicephalus (Boophilus) annulatus genes differentially expressed in response to infection with B. bigemina by using suppression-subtractive hybridization (SSH), which allows the identification of differentially expressed genes. The results of the SSH studies are validated by real-time reverse transcription (RT)-PCR. Functional analyses are conducted by RNAi on selected R. annulatus genes to determine their putative role in B. bigemina-tick interactions. Gathered data may be useful for the future development of improved vaccines and vaccination strategies to control babesiosis.

Characterization of the bacterial communities of life stages of free living lone star ticks (Amblyomma americanum)

The lone star tick (Amblyomma americanum) is an abundant and aggressive biter of humans, domestic animals, and wildlife in the southeastern-central USA and an important vector of several known and suspected zoonotic bacterial pathogens. However, the biological drivers of bacterial community variation in this tick are still poorly defined. Knowing the community context in which tick-borne bacterial pathogens exist and evolve is required to fully understand the ecology and immunobiology of the ticks and to design effective public health and veterinary interventions. We performed a metagenomic survey of the bacterial communities of questing A. americanum and tested 131 individuals (66 nymphs, 24 males, and 41 females) from five sites in three states. Pyrosequencing was performed with barcoded eubacterial primers targeting variable 16S rRNA gene regions 5–3. The bacterial communities were dominated by Rickettsia (likely R. amblyommii) and an obligate Coxiella symbiont, together accounting for 6.7–100% of sequences per tick. DNAs from Midichloria, Borrelia, Wolbachia, Ehrlichia, Pseudomonas, or unidentified Bacillales, Enterobacteriaceae, or Rhizobiales groups were also detected frequently. Wolbachia and Midichloria significantly co-occurred in Georgia (p<0.00001), but not in other states. The significance of the Midichloria-Wolbachia co-occurrence is unknown. Among ticks collected in Georgia, nymphs differed from adults in both the composition (p = 0.002) and structure (p = 0.002) of their bacterial communities. Adults differed only in their community structure (p = 0.002) with males containing more Rickettsia and females containing more Coxiella. Comparisons among adult ticks collected in New York and North Carolina supported the findings from the Georgia collection despite differences in geography, collection date, and sample handling, implying that the differences detected are consistent attributes. The data also suggest that some members of the bacterial community change during the tick life cycle and that some sex-specific attributes may be detectable in nymphs.

Decoding the ubiquitin-mediated pathway of arthropod disease vectors

Protein regulation by ubiquitin has been extensively described in model organisms. However, characterization of the ubiquitin machinery in disease vectors remains mostly unknown. This fundamental gap in knowledge presents a concern because new therapeutics are needed to control vector-borne diseases, and targeting the ubiquitin machinery as a means for disease intervention has been already adopted in the clinic. In this study, we employed a bioinformatics approach to uncover the ubiquitin-mediated pathway in the genomes of Anopheles gambiae, Aedes aegypti, Culex quinquefasciatus, Ixodes scapularis, Pediculus humanus and Rhodnius prolixus. We observed that (1) disease vectors encode a lower percentage of ubiquitin-related genes when compared to Drosophila melanogaster, Mus musculus and Homo sapiens but not Saccharomyces cerevisiae; (2) overall, there are more proteins categorized as E3 ubiquitin ligases when compared to E2-conjugating or E1-activating enzymes; (3) the ubiquitin machinery within the three mosquito genomes is highly similar; (4) ubiquitin genes are more than doubled in the Chagas disease vector (R. prolixus) when compared to other arthropod vectors; (5) the deer tick I. scapularis and the body louse (P. humanus) genomes carry low numbers of E1-activating enzymes and HECT-type E3 ubiquitin ligases; (6) R. prolixus have low numbers of RING-type E3 ubiquitin ligases; and (7) C. quinquefasciatus present elevated numbers of predicted F-box E3 ubiquitin ligases, JAB and UCH deubiquitinases. Taken together, these findings provide novel opportunities to study the interaction between a pathogen and an arthropod vector.

Functional insights into recombinant TROSPA protein from Ixodes ricinus

Lyme disease (also called borreliosis) is a prevalent chronic disease transmitted by ticks and caused by Borrelia burgdorferi s. l. spirochete. At least one tick protein, namely TROSPA from I. scapularis, commonly occurring in the USA, was shown to be required for colonization of the vector by bacteria. Located in the tick gut, TROSPA interacts with the spirochete outer surface protein A (OspA) and initiates the tick colonization. Ixodes ricinus is a primary vector involved in B. burgdorferi s. l. transmission in most European countries. In this study, we characterized the capacities of recombinant TROSPA protein from I. ricinus to interact with OspA from different Borrelia species and to induce an immune response in animals. We also showed that the N-terminal part of TROSPA (a putative transmembrane domain) is not involved in the interaction with OspA and that reduction of the total negative charge on the TROSPA protein impaired TROSPA-OspA binding. In general, the data presented in this paper indicate that recombinant TROSPA protein retains the capacity to form a complex with OspA and induces a significant level of IgG in orally immunized rats. Thus, I. ricinus TROSPA may be considered a good candidate component for an animal vaccine against Borrelia.

Identification of novel arthropod vector G protein-coupled receptors

BACKGROUND

The control of vector-borne diseases, such as malaria, dengue fever, and typhus fever is often achieved with the use of insecticides. Unfortunately, insecticide resistance is becoming common among different vector species. There are currently no chemical alternatives to these insecticides because new human-safe classes of molecules have yet to be brought to the vector-control market. The identification of novel targets offer opportunities for rational design of new chemistries to control vector populations. One target family, G protein-coupled receptors (GPCRs), has remained relatively under explored in terms of insecticide development.

METHODS

A novel classifier, Ensemble*, for vector GPCRs was developed. Ensemble* was validated and compared to existing classifiers using a set of all known GPCRs from Aedes aegypti, Anopheles gambiae, Apis Mellifera, Drosophila melanogaster, Homo sapiens, and Pediculus humanus. Predictions for unidentified sequences from Ae. aegypti, An. gambiae, and Pe. humanus were validated. Quantitative RT-PCR expression analysis was performed on previously-known and newly discovered Ae. aegypti GPCR genes.

RESULTS

We present a new analysis of GPCRs in the genomes of Ae, aegypti, a vector of dengue fever, An. gambiae, a primary vector of Plasmodium falciparum that causes malaria, and Pe. humanus, a vector of epidemic typhus fever, using a novel GPCR classifier, Ensemble*, designed for insect vector species. We identified 30 additional putative GPCRs, 19 of which we validated. Expression of the newly discovered Ae. aegypti GPCR genes was confirmed via quantitative RT-PCR.

CONCLUSION

A novel GPCR classifier for insect vectors, Ensemble*, was developed and GPCR predictions were validated. Ensemble* and the validation pipeline were applied to the genomes of three insect vectors (Ae. aegypti, An. gambiae, and Pe. humanus), resulting in the identification of 52 GPCRs not previously identified, of which 11 are predicted GPCRs, and 19 are predicted and confirmed GPCRs.

Tubular structure induced by a plant virus facilitates viral spread in its vector insect

Rice dwarf virus (RDV) replicates in and is transmitted by a leafhopper vector in a persistent-propagative manner. Previous cytopathologic and genetic data revealed that tubular structures, constructed by the nonstructural viral protein Pns10, contain viral particles and are directly involved in the intercellular spread of RDV among cultured leafhopper cells. Here, we demonstrated that RDV exploited these virus-containing tubules to move along actin-based microvilli of the epithelial cells and muscle fibers of visceral muscle tissues in the alimentary canal, facilitating the spread of virus in the body of its insect vector leafhoppers. In cultured leafhopper cells, the knockdown of Pns10 expression due to RNA interference (RNAi) induced by synthesized dsRNA from Pns10 gene strongly inhibited tubule formation and prevented the spread of virus among insect vector cells. RNAi induced after ingestion of dsRNA from Pns10 gene strongly inhibited formation of tubules, preventing intercellular spread and transmission of the virus by the leafhopper. All these results, for the first time, show that a persistent-propagative virus exploits virus-containing tubules composed of a nonstructural viral protein to traffic along actin-based cellular protrusions, facilitating the intercellular spread of the virus in the vector insect. The RNAi strategy and the insect vector cell culture provide useful tools to investigate the molecular mechanisms enabling efficient transmission of persistent-propagative plant viruses by vector insects.

Molecular characterization and tissue-specific gene expression of Dermacentor variabilis α-catenin in response to rickettsial infection

Alpha catenin is a cytoskeleton protein that acts as a regulator of actin rearrangement by forming an E-cadherin adhesion complex. In Dermacentor variabilis, a putative α-catenin (Dvα-catenin) was previously identified as differentially regulated in ovaries of ticks chronically infected with Rickettsia montanensis. To begin characterizing the role(s) of Dvα-catenin during rickettsial infection, the full-length Dvα-catenin cDNA was cloned and analysed. Comparative sequence analysis demonstrates a 3069-bp cDNA with a 2718-bp open reading frame with a sequence similar to Ixodes scapularisα-catenin. A portion of Dvα-catenin is homologous to the vinculin-conserved domain containing a putative actin-binding region and β-catenin-binding and -dimerization regions. Quantitative reverse-transcription PCR analysis demonstrated that Dvα-catenin is predominantly expressed in tick ovaries and is responsive to tick feeding. The tissue-specific gene expression analysis of ticks exposed to Rickettsia demonstrates that Dvα-catenin expression was significantly downregulated 12 h after exposure to R. montanensis, but not in Rickettsia amblyommii-exposed ovaries, compared with Rickettsia-unexposed ticks. Studying tick-derived molecules associated with rickettsial infection will provide a better understanding of the transmission dynamics of tick-borne rickettsial diseases.

Molecular identification of Salp15, a key salivary gland protein in the transmission of lyme disease spirochetes, from Ixodes persulcatus and Ixodes pacificus (Acari: Ixodidae)

Salp15 is a multifunctional protein, vital to the tick in its need to obtain vertebrate host blood without stimulating a host inflammatory and immune response. The Salpl5 protein from both Ixodes scapularis Say and Ixodes ricinus (L.), the principal vectors of the Lyme disease spirochete in eastern North America and Europe, respectively, have been well characterized and found to bind the murine CD4 receptor, DC-SIGN, and the OspC protein of Borrelia burgdorferi. In the current study, we characterized the full salp15 gene in Ixodes pacificus Cooley & Kohls and Ixodes persulcatus Schulze, the principal vectors of Lyme disease spirochetes in western North America and Asia, respectively. In comparing the Salp15 protein of all four principal vector ticks of public health importance for the transmission of Lyme disease spirochetes, we find the 53 C-terminal amino acids to have a high degree of similarity. There are at least three clades in the tree of Salp15 and its homologues, probably representing a multigene family.

Disentangling vector-borne transmission networks: a universal DNA barcoding method to identify vertebrate hosts from arthropod bloodmeals

Emerging infectious diseases represent a challenge for global economies and public health. About one fourth of the last pandemics have been originated by the spread of vector-borne pathogens. In this sense, the advent of modern molecular techniques has enhanced our capabilities to understand vector-host interactions and disease ecology. However, host identification protocols have poorly profited of international DNA barcoding initiatives and/or have focused exclusively on a limited array of vector species. Therefore, ascertaining the potential afforded by DNA barcoding tools in other vector-host systems of human and veterinary importance would represent a major advance in tracking pathogen life cycles and hosts. Here, we show the applicability of a novel and efficient molecular method for the identification of the vertebrate host's DNA contained in the midgut of blood-feeding arthropods. To this end, we designed a eukaryote-universal forward primer and a vertebrate-specific reverse primer to selectively amplify 758 base pairs (bp) of the vertebrate mitochondrial Cytochrome c Oxidase Subunit I (COI) gene. Our method was validated using both extensive sequence surveys from the public domain and Polymerase Chain Reaction (PCR) experiments carried out over specimens from different Classes of vertebrates (Mammalia, Aves, Reptilia and Amphibia) and invertebrate ectoparasites (Arachnida and Insecta). The analysis of mosquito, culicoid, phlebotomie, sucking bugs, and tick bloodmeals revealed up to 40 vertebrate hosts, including 23 avian, 16 mammalian and one reptilian species. Importantly, the inspection and analysis of direct sequencing electropherograms also assisted the resolving of mixed bloodmeals. We therefore provide a universal and high-throughput diagnostic tool for the study of the ecology of haematophagous invertebrates in relation to their vertebrate hosts. Such information is crucial to support the efficient management of initiatives aimed at reducing epidemiologic risks of arthropod vector-borne pathogens, a priority for public health.

Molecular analysis of Ixodes granulatus, a possible vector tick for Borrelia burgdorferi sensu lato in Taiwan

The genetic identity of Ixodes granulatus ticks was determined for the first time in Taiwan. The phylogenetic relationships were analyzed by comparing the sequences of mitochondrial 16S ribosomal DNA gene obtained from 19 strains of ticks representing seven species of Ixodes and two outgroup species (Rhipicephalus sanguineus and Haemaphysalis inermis). Four major clades could be easily distinguished by neighbour-joining analysis and were congruent by maximum-parsimony method. All these I. granulatus ticks of Taiwan were genetically affiliated to a monophyletic group with highly homogeneous sequences (92.2-99.3% similarity), and can be discriminated from other Ixodes species and other genera of ticks with a sequence divergence ranging from 11.7 to 30.8%. Moreover, intraspecific analysis revealed that two distinct lineages are evident between the same species of I. granulatus ticks collected from Taiwan and Malaysia. Our results demonstrate that all these I. granulatus ticks of Taiwan represent a unique lineage distinct from the common vector ticks (I. ricinus complex) for Borrelia burgdorferi spirochetes.

Genetic variation in the 16S mitochondrial DNA gene of two Canadian populations of Dermacentor andersoni (Acari: Ixodidae)

The Rocky Mountain wood tick, Dermacentor andersoni Stiles, 1908, is of medical and veterinary importance because it can transmit pathogenic agents to humans, domestic livestock, and wildlife. The preferred attachment sites of D. andersoni adults and their ability to induce paralysis in hosts vary among populations, which may have a genetic basis. In this study, polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) analyses and DNA sequencing were used to determine the genetic variation in the 16S mitochondrial DNA gene of two D. andersoni populations from the Canadian prairies: Saskatchewan Landing Provincial Park, Saskatchewan, and Lethbridge, Alberta. Five haplotypes were detected in each population, but this was considerably lower than the 14 haplotypes reported in a previous study of a laboratory colony of D. andersoni originating from the Rocky Mountains in Montana. In addition, the Canadian populations did not share any haplotypes with the population from Montana. Differences in the genetic composition of the two Canadian prairie populations of D. andersoni compared with the montane population in the United States may have arisen through geographical isolation. These genetic differences between tick populations may also have important implications with respect to their ability to transmit pathogens to hosts. Further studies are needed to determine the extent of genetic variation and the vector potential of ticks from different populations throughout the range of D. andersoni in the United States and Canada.

Genetic differences in internal transcribed spacer 1 between Dermanyssus gallinae from wild birds and domestic chickens

We investigated the presence of the poultry red mite or the chicken mite, Dermanyssus gallinae De Geer, Acari: Dermanyssidae, in wild bird populations in four different geographical regions of Sweden. The mites identified as D. gallinae were compared genetically with D. gallinae from egg-producing poultry farms in the same regions. The small subunit (SSU) gene, the 5.8S ribosomal RNA (rRNA) gene and the two internal transcribed spacers (ITS) of the rRNA genes were used in the genetic analysis. All D. gallinae mites had identical SSU rRNA, 5.8S rRNA and ITS2 sequences independent of their origin. By contrast, we identified significant differences in the ITS1 sequences. Based on the differences in the ITS1 sequences, the mites could be divided into two genotypes, of wild and domesticated origin, with no variation within the groups. These results imply that wild bird populations are of low importance, if any, as natural reservoirs of D. gallinae in these four geographical regions of Sweden.

Genetic diversity and gene flow of humans, Plasmodium falciparum, and Anopheles farauti s.s. of Vanuatu: inferred malaria dispersal and implications for malaria control

A comparison of the patterns of gene flow within and between islands and the genetic diversities of the three species required for malaria transmission (humans, Plasmodium falciparum, and Anopheles farauti s.s.) within the model island system of Vanuatu, shows that the active dispersal of An. farauti s.s. is responsible for within island movement of parasites. In contrast, since both P. falciparum and An. farauti s.s. populations are largely restricted to islands, movement of parasites between islands is likely due to human transport. Thus, control of vectors is crucial for controlling malaria within islands, while control of human movement is essential to control malaria transmission across the archipelago.

The biogeography and population genetics of neotropical vector species

Phylogenetic and population genetic data support the Pliocene or Pleistocene divergences of the co-distributed hematophagous insect vectors, the sand fly Lutzomyia longipalpis s.l., the mosquitoes Anopheles darlingi and A. albitarsis s.l., and the triatomines Rhodnius prolixus and R. robustus. We examined patterns of divergence and distribution in relation to three hypotheses of neotropical diversification: Miocene/Pliocene marine incursion, Pliocene/Pleistocene riverine barriers and Pleistocene refugia. Only R. prolixus has a pattern concordant with the refugia hypothesis, and R. robustus conforms to the marine incursion predictions. A. darlingi partially fits the refugia hypothesis. For L. longipalpis s.l. and A. albitarsis s.l., elements of both incursion and refugia hypotheses seem to fit, suggesting perhaps an interaction of factors determining their distribution patterns.

Arthropod-borne diseases: vector control in the genomics era

Diseases that are transmitted by arthropods cause severe morbidity and mortality throughout the world. The burden of many of these diseases is borne largely by developing countries. Advances in vector genomics offer new promise for the control of arthropod vectors of disease. Radical changes in vector-biology research are required if scientists are to exploit genomic data and implement changes in public health.

ECTOPARASITES OF GRAY SQUIRRELS IN TWO DIFFERENT HABITATS AND SCREENING OF SELECTED ECTOPARASITES FOR BARTONELLAE

Gray squirrels, Sciurus carolinensis, were livetrapped in 2 different habitat types, woodland (67 squirrels) and parkland (53 squirrels), in southeastern Georgia. Ectoparasites were recovered from anesthetized squirrels and compared between hosts from the 2 habitats. Because of the absence of low vegetation in parkland habitats, it was hypothesized that the ectoparasite fauna, especially ticks and chiggers, would be more diverse on woodland squirrels. The results were generally in agreement with this hypothesis. Seventeen species of ectoparasites were recovered from woodland squirrels, compared with 6 species from parkland squirrels. Five species of ticks and 3 species of chiggers parasitized the woodland squirrels compared with no ticks or chiggers on the parkland squirrels. Significantly higher infestation prevalences were recorded on woodland compared with parkland squirrels for the flea Orchopeas howardi, the tick Amblyomma americanum, and the mesostigmatid mite Androlaelaps fahrenholzi. The mean intensity for O. howardi also was significantly higher on woodland than on parkland squirrels. Because a new strain of Bartonella sp. was isolated recently from S. carolinensis in Georgia, selected ectoparasites from this study were screened for bartonellae by polymerase chain reaction (PCR). Some of the fleas and lice, but none of the mites tested, were PCR positive, suggesting that fleas, or lice, or both, might be vectors of bartonellae between squirrels. Six distinct strains of Bartonella sp. were detected, 2 in fleas and 4 in lice.

High-throughput approaches to study salivary proteins and genes from vectors of disease

Blood-feeding arthropods have in their saliva a variety of molecules that affect the vertebrate host's hemostatic, inflammatory, and immune systems. The saliva of blood feeders also helps to facilitate infection of the pathogens they carry to the host, making vector saliva an attractive target to control pathogen transmission. The isolation and identification of salivary molecules from vectors of disease has been slow and difficult. Emerging technologies and new approaches in the fields of molecular biology and protein chemistry are facilitating this work. Massive sequencing of high quality, full-length cDNA libraries, coupled with proteomics and functional genomic approaches has led to the discovery of novel proteins, transcription products (genes), and biologic activities from the salivary glands of blood-feeding arthropods. This review focuses on the biologic activities identified in the saliva of various vectors of disease using classic biochemical and molecular biology approaches and new types of molecules and activities identified with high-throughput strategies.

[Chromosomal polymorphism of populations of Anopheles arabiensis (Diptera: Culicidae) from Reunion island and cross-fertility among continental African populations]

Cytological examination of a sample of Anopheles gambiae complex mosquitoes from Reunion island revealed the presence of An. arabiensis only. Chromosomal polymorphisms were observed only for inversion 3Ra, the standard homozygote form being predominant. Cross-mating experiments with laboratory specimens originating from continental Africa produced viable and fertile offspring with no chromosomal asynapsis observed in the F1 female progeny. There was no evidence for speciation of the Reunion island populations. The results are discussed with regard to the behaviour of the vector and its influence on the vectorial capacity of this species, and the history of malaria and malaria control in the South-West islands of the Indian Ocean and on Reunion island in particular.

Genetics of mosquito vector competence

Mosquito-borne diseases are responsible for significant human morbidity and mortality throughout the world. Efforts to control mosquito-borne diseases have been impeded, in part, by the development of drug-resistant parasites, insecticide-resistant mosquitoes, and environmental concerns over the application of insecticides. Therefore, there is a need to develop novel disease control strategies that can complement or replace existing control methods. One such strategy is to generate pathogen-resistant mosquitoes from those that are susceptible. To this end, efforts have focused on isolating and characterizing genes that influence mosquito vector competence. It has been known for over 70 years that there is a genetic basis for the susceptibility of mosquitoes to parasites, but until the advent of powerful molecular biological tools and protocols, it was difficult to assess the interactions of pathogens with their host tissues within the mosquito at a molecular level. Moreover, it has been only recently that the molecular mechanisms responsible for pathogen destruction, such as melanotic encapsulation and immune peptide production, have been investigated. The molecular characterization of genes that influence vector competence is becoming routine, and with the development of the Sindbis virus transducing system, potential antipathogen genes now can be introduced into the mosquito and their effect on parasite development can be assessed in vivo. With the recent successes in the field of mosquito germ line transformation, it seems likely that the generation of a pathogen-resistant mosquito population from a susceptible population soon will become a reality.

Reports from the cutting edge of parasitic genome analysis

This new feature in Parasitology Today will host reports from the laboratories involved in genomics of parasites, be that sequencing, mapping or 'functional genomics' - the mining and analysis of the sequence datasets, and the development of postgenomics tools to examine gene expression, response to drugs and population variability. It will publicize new technology to wider audiences, let communities of researchers know about novel resources (particularly those available through the World Wide Web) and highlight significant advances in the understanding of parasitic genomes through functional genomics.

Speciation and phylogeography of Hawaiian terrestrial arthropods

The Hawaiian archipelago is arguably the world's finest natural laboratory for the study of evolution and patterns of speciation. Arthropods comprise over 75% of the endemic biota of the Hawaiian Islands and a large proportion belongs to species radiations. We classify patterns of speciation within Hawaiian arthropod lineages into three categories: (i) single representatives of a lineage throughout the islands; (ii) species radiations with either (a) single endemic species on different volcanoes or islands, or (b) multiple species on each volcano or island; and (iii) single widespread species within a radiation of species that exhibits local endemism. A common pattern of phylogeography is that of repeated colonization of new island groups, such that lineages progress down the island chain, with the most ancestral groups (populations or species) on the oldest islands. While great dispersal ability and its subsequent loss are features of many of these taxa, there are a number of mechanisms that underlie diversification. These mechanisms may be genetic, including repeated founder events, hybridization, and sexual selection, or ecological, including shifts in habitat and/or host affiliation. The majority of studies reviewed suggest that natural selection is a primary force of change during the initial diversification of taxa.

Genetic variation in arthropod vectors of disease-causing organisms: obstacles and opportunities

An overview of the genetic variation in arthropods that transmit pathogens to vertebrates is presented, emphasizing the genetics of vector-pathogen relationships and the biochemical genetics of vectors. Vector-pathogen interactions are reviewed briefly as a prelude to a discussion of the genetics of susceptibility and refractoriness in vectors. Susceptibility to pathogens is controlled by maternally inherited factors, sex-linked dominant alleles, and dominant and recessive autosomal genes. There is widespread interpopulation (including intercolony) and temporal variation in susceptibility to pathogens. The amount of biochemical genetic variation in vectors is similar to that found in other invertebrates. However, the amount varies widely among species, among populations within species, and temporally within populations. Biochemical genetic studies show that there is considerable genetic structuring of many vectors at the local, regional, and global levels. It is argued that genetic variation in vectors is critical in understanding vector-pathogen interactions and that genetic variation in vectors creates both obstacles to and opportunities for application of genetic techniques to the control of vectors.

Generic approaches to obtaining efficacious antigens from vector arthropods

The development of vaccines to control ectoparasites is dependent upon the identification of key parasite antigens. While a rational, pragmatic approach to antigen identification has yielded a successful vaccine candidate from ticks, there may be problems with such an approach when dealing with other ectoparasites. As an alternative approach, the search for vaccine candidates may be facilitated by cloning and expressing parasite genes encoding proteins involved in key physiological roles. A number of criteria may be applied to short-list candidate vaccines, these being; (a) host antibodies should be able to gain access to the parasite antigen; (b) sufficient antibody must gain access to the antigen target; (c) the formation of antibody-antigen complex should disrupt the normal function of the parasite antigen (d) the antigen should share conserved structural/sequence motifs with related, characterised, proteins, thus allowing the use of recombinant DNA methods to clone and express the candidate antigen. We propose three major groups of parasite antigens which may fulfill these criteria; serine proteases, chemoreceptors/ion channels and neuropeptides.

Bunyavirus-vector interactions

Recent advances in the genetics and molecular biology of bunyaviruses have been applied to understanding bunyavirus-vector interactions. Such approaches have revealed which virus gene and gene products are important in establishing infections in vectors and in transmission of viruses. However, much more information is required to understand the molecular mechanisms of persistent infections of vectors which are lifelong but apparently exert no untoward effect. In fact, it seems remarkable that LAC viral antigen can be detected in almost every cell in an ovarian follicle, yet no untoward effect on fecundity and no teratology is seen. Similarly the lifelong infection of the vector would seem to provide ample opportunity for bunyavirus evolution by genetic drift and, under the appropriate circumstances, by segment reassortment. The potential for bunyavirus evolution by segment reassortment in vectors certainly exists. For example the Group C viruses in a small forest in Brazil seem to constitute a gene pool, with the 6 viruses related alternately by HI/NT and CF reactions, which assay respectively M RNA and S RNA gene products (Casals and Whitman, 1960; Shope and Causey, 1962). Direct evidence for naturally occurring reassortant bunyaviruses has also been obtained. Oligonucleotide fingerprint analyses of field isolates of LAC virus and members of the Patois serogroup of bunyaviruses have demonstrated that reassortment does occur in nature (El Said et al., 1979; Klimas et al., 1981; Ushijima et al., 1981). Determination of the genotypic frequencies of viruses selected by the biological interactions of viruses and vectors after dual infection and segment reassortment is an important issue. Should a virus result that efficiently interacts with alternate vector species, the virus could be expressed in different circumstances with serious epidemiologic consequences. Dual infection of vectors with different viruses is not unlikely, because many bunyaviruses are sympatric in nature. For example, the Ae. trivittatus-cottontail rabbit and the Ae. triseriatus-squirrel arbovirus cycles are sympatric in the ecotone between their respective grassland and forest ecosystems (LeDuc, 1979). Should a LaCrosse virus variant or reassortant evolve that was efficiently vectored by Ae. trivittatus mosquitoes, significantly more human infections with La Crosse virus would likely occur. Unlike Ae. triseriatus, Ae. trivittatus mosquitoes are not restricted to forested areas and consequently are more likely to encounter and to feed upon humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation of biotechnology methods on the study of arthropods

Biotechnological advances are rapidly being applied to the study of arthropods and arthropod-born diseases. Improved tools are available for species identification, study of species complexes, detection and identification of pathogens in vectors and detection and characterization of insecticide resistance. These techniques have their basis in hybridoma, DNA probe and biochemical assay technologies. Problem areas in assay development are discussed.