Musca domestica salivary gland hypertrophy virus, a globally distributed insect virus that infects and sterilizes female houseflies

Rescuing the Inhibitory Effect of the Salivary Gland Hypertrophy Virus of Musca domestica on Mating Behavior

Infection with salivary gland hypertrophy virus (MdSGHV) of Musca domestica prevents female flies from accepting copulation attempts by healthy or virus-infected males. This study focused on supplemental hormonal rescue therapy for mating behavior in virus-infected female house flies. The inhibitory effect of the virus on mating behavior in females injected with MdSGHV was reversed by hormonal therapy in the form of octopamine injections, topical application of methoprene, or both therapies combined along with 20-hydroxyecdysone. Infected females whose mating responsiveness had been restored continued to have other viral pathologies associated with infection such as hypertrophy of the salivary glands and a lack of ovarian development.

A Systematic Review on Viruses in Mass-Reared Edible Insect Species

Edible insects are expected to become an important nutrient source for animals and humans in the Western world in the near future. Only a few studies on viruses in edible insects with potential for industrial rearing have been published and concern only some edible insect species. Viral pathogens that can infect insects could be non-pathogenic, or pathogenic to the insects themselves, or to humans and animals. The objective of this systematic review is to provide an overview of the viruses detected in edible insects currently considered for use in food and/or feed in the European Union or appropriate for mass rearing, and to collect information on clinical symptoms in insects and on the vector role of insects themselves. Many different virus species have been detected in edible insect species showing promise for mass production systems. These viruses could be a risk for mass insect rearing systems causing acute high mortality, a drastic decline in growth in juvenile stages and in the reproductive performance of adults. Furthermore, some viruses could pose a risk to human and animal health where insects are used for food and feed.

The Effect of the Hypertrophy Virus (MdSGHV) on the Ultrastructure of the Salivary Glands of Musca domestica (Diptera: Muscidae)

The salivary glands of insects play a key role in the replication cycle and vectoring of viral pathogens. Consequently, Musca domestica (L.) (Diptera: Muscidae) and the Salivary Gland Hypertrophy Virus (MdSGHV) serve as a model to study insect vectoring of viruses. A better understanding of the structural changes of the salivary glands by the virus will help obtain a better picture of the pathological impact the virus has on adult flies. The salivary glands are a primary route for viruses to enter a new host. As such, studying the viral effect on the salivary glands is particularly important and can provide insights for the development of strategies to control the transmission of vector-borne diseases, such as dengue, malaria, Zika, and chikungunya virus. Using scanning and transmission electron microscopic techniques, researchers have shown the effects of infection by MdSGHV on the salivary glands; however, the exact location where the infection was found is unclear. For this reason, this study did a close examination of the effects of the hypertrophy virus on the salivary glands to locate the specific sites of infection. Here, we report that hypertrophy is present mainly in the secretory region, while other regions appeared unaffected. Moreover, there is a disruption of the cuticular, chitinous lining that separates the secretory cells from the lumen of the internal duct, and the disturbance of this lining makes it possible for the virus to enter the lumen. Thus, we report that the chitinous lining acts as an exit barrier of the salivary gland.

The discovery, distribution, and diversity of DNA viruses associated with Drosophila melanogaster in Europe

Drosophila melanogaster is an important model for antiviral immunity in arthropods, but very few DNA viruses have been described from the family Drosophilidae. This deficiency limits our opportunity to use natural host-pathogen combinations in experimental studies, and may bias our understanding of the Drosophila virome. Here, we report fourteen DNA viruses detected in a metagenomic analysis of 6668 pool-sequenced Drosophila, sampled from forty-seven European locations between 2014 and 2016. These include three new nudiviruses, a new and divergent entomopoxvirus, a virus related to Leptopilina boulardi filamentous virus, and a virus related to Musca domestica salivary gland hypertrophy virus. We also find an endogenous genomic copy of galbut virus, a double-stranded RNA partitivirus, segregating at very low frequency. Remarkably, we find that Drosophila Vesanto virus, a small DNA virus previously described as a bidnavirus, may be composed of up to twelve segments and thus represent a new lineage of segmented DNA viruses. Two of the DNA viruses, Drosophila Kallithea nudivirus and Drosophila Vesanto virus are relatively common, found in 2 per cent or more of wild flies. The others are rare, with many likely to be represented by a single infected fly. We find that virus prevalence in Europe reflects the prevalence seen in publicly available datasets, with Drosophila Kallithea nudivirus and Drosophila Vesanto virus the only ones commonly detectable in public data from wild-caught flies and large population cages, and the other viruses being rare or absent. These analyses suggest that DNA viruses are at lower prevalence than RNA viruses in D.melanogaster, and may be less likely to persist in laboratory cultures. Our findings go some way to redressing an earlier bias toward RNA virus studies in Drosophila, and lay the foundation needed to harness the power of Drosophila as a model system for the study of DNA viruses.

Quantification of dose-mortality responses in adult Diptera: Validation using Ceratitis capitata and Drosophila suzukii responses to spinosad

Quantitative laboratory bioassay methods are required to evaluate the toxicity of novel insecticidal compounds for pest control and to determine the presence of resistance traits. We used a radioactive tracer based on 32P-ATP to estimate the volume of a droplet ingested by two dipteran pests: Ceratitis capitata (Tephritidae) and Drosophila suzukii (Drosophilidae). Using blue food dye it was possible to distinguish between individuals that ingested the solution from those that did not. The average volume ingested by C. capitata adults was 1.968 μl. Females ingested a ~20% greater volume of solution than males. Adults of D. suzukii ingested an average of 0.879 μl and females ingested ~30% greater volume than males. The droplet feeding method was validated using the naturally-derived insecticide spinosad as the active ingredient (a.i.). For C. capitata, the concentration-mortality response did not differ between the sexes or among three different batches of insects. Lethal dose values were calculated based on mean ingested volumes. For C. capitata LD50 values were 1.462 and 1.502 ng a.i./insect for males and females, respectively, equivalent to 0.274 and 0.271 ng a.i./mg for males and females, respectively, when sex-specific variation in body weight was considered. Using the same process for D. suzukii, the LD50 value was estimated at 2.927 ng a.i./insect, or 1.994 ng a.i./mg based on a mean body weight of 1.67 mg for both sexes together. We conclude that this technique could be readily employed for determination of the resistance status and dose-mortality responses of insecticidal compounds in many species of pestiferous Diptera.

Coevolution of hytrosaviruses and host immune responses

BACKGROUND

Hytrosaviruses (SGHVs; Hytrosaviridae family) are double-stranded DNA (dsDNA) viruses that cause salivary gland hypertrophy (SGH) syndrome in flies. Two structurally and functionally distinct SGHVs are recognized; Glossina pallidipes SGHV (GpSGHV) and Musca domestica SGHV (MdSGHV), that infect the hematophagous tsetse fly and the filth-feeding housefly, respectively. Genome sizes and gene contents of GpSGHV (~ 190 kb; 160-174 genes) and MdSGHV (~ 124 kb; 108 genes) may reflect an evolution with the SGHV-hosts resulting in differences in pathobiology. Whereas GpSGHV can switch from asymptomatic to symptomatic infections in response to certain unknown cues, MdSGHV solely infects symptomatically. Overt SGH characterizes the symptomatic infections of SGHVs, but whereas MdSGHV induces both nuclear and cellular hypertrophy (enlarged non-replicative cells), GpSGHV induces cellular hyperplasia (enlarged replicative cells). Compared to GpSGHV's specificity to Glossina species, MdSGHV infects other sympatric muscids. The MdSGHV-induced total shutdown of oogenesis inhibits its vertical transmission, while the GpSGHV's asymptomatic and symptomatic infections promote vertical and horizontal transmission, respectively. This paper reviews the coevolution of the SGHVs and their hosts (housefly and tsetse fly) based on phylogenetic relatedness of immune gene orthologs/paralogs and compares this with other virus-insect models.

RESULTS

Whereas MdSGHV is not vertically transmitted, GpSGHV is both vertically and horizontally transmitted, and the balance between the two transmission modes may significantly influence the pathogenesis of tsetse virus. The presence and absence of bacterial symbionts (Wigglesworthia and Sodalis) in tsetse and Wolbachia in the housefly, respectively, potentially contributes to the development of SGH symptoms. Unlike MdSGHV, GpSGHV contains not only host-derived proteins, but also appears to have evolutionarily recruited cellular genes from ancestral host(s) into its genome, which, although may be nonessential for viral replication, potentially contribute to the evasion of host's immune responses. Whereas MdSGHV has evolved strategies to counteract both the housefly's RNAi and apoptotic responses, the housefly has expanded its repertoire of immune effector, modulator and melanization genes compared to the tsetse fly.

CONCLUSIONS

The ecologies and life-histories of the housefly and tsetse fly may significantly influence coevolution of MdSGHV and GpSGHV with their hosts. Although there are still many unanswered questions regarding the pathogenesis of SGHVs, and the extent to which microbiota influence expression of overt SGH symptoms, SGHVs are attractive 'explorers' to elucidate the immune responses of their hosts, and the transmission modes of other large DNA viruses.

The microbiomes of blowflies and houseflies as bacterial transmission reservoirs

Blowflies and houseflies are mechanical vectors inhabiting synanthropic environments around the world. They feed and breed in fecal and decaying organic matter, but the microbiome they harbour and transport is largely uncharacterized. We sampled 116 individual houseflies and blowflies from varying habitats on three continents and subjected them to high-coverage, whole-genome shotgun sequencing. This allowed for genomic and metagenomic analyses of the host-associated microbiome at the species level. Both fly host species segregate based on principal coordinate analysis of their microbial communities, but they also show an overlapping core microbiome. Legs and wings displayed the largest microbial diversity and were shown to be an important route for microbial dispersion. The environmental sequencing approach presented here detected a stochastic distribution of human pathogens, such as Helicobacter pylori, thereby demonstrating the potential of flies as proxies for environmental and public health surveillance.

Responses of the Housefly, Musca domestica, to the Hytrosavirus Replication: Impacts on Host's Vitellogenesis and Immunity

Hytrosaviridae family members replicate in the salivary glands (SGs) of their adult dipteran hosts and are transmitted to uninfected hosts via saliva during feeding. Despite inducing similar gross symptoms (SG hypertrophy; SGH), hytrosaviruses (SGHVs) have distinct pathobiologies, including sex-ratio distortions in tsetse flies and refusal of infected housefly females to copulate. Via unknown mechanism(s), SGHV replication in other tissues results in reduced fecundity in tsetse flies and total shutdown of vitellogenesis and sterility in housefly females. We hypothesized that vitellogenesis shutdown was caused by virus-induced modulation of hormonal titers. Here, we used RNA-Seq to investigate virus-induced modulation of host genes/pathways in healthy and virus-infected houseflies, and we validated expression of modulated genes (n = 23) by RT-qPCR. We also evaluated the levels and activities of hemolymph AMPs, levels of endogenous sesquiterpenoids, and impacts of exogenous hormones on ovarian development in viremic females. Of the 973 housefly unigenes that were significantly modulated (padj ≤ 0.01, log2FC ≤ -2.0 or ≥ 2.0), 446 and 527 genes were downregulated and upregulated, respectively. While the most downregulated genes were related to reproduction (embryogenesis/oogenesis), the repertoire of upregulated genes was overrepresented by genes related to non-self recognition, ubiquitin-protease system, cytoskeletal traffic, cellular proliferation, development and movement, and snRNA processing. Overall, the virus, Musca domestica salivary gland hytrosavirus (MdSGHV), induced the upregulation of various components of the siRNA, innate antimicrobial immune, and autophagy pathways. We show that MdSGHV undergo limited morphogenesis in the corpora allata/corpora cardiaca (CA/CC) complex of M. domestica. MdSGHV replication in CA/CC potentially explains the significant reduction of hemolymph sesquiterpenoids levels, the refusal to mate, and the complete shutdown of egg development in viremic females. Notably, hormonal rescue of vitellogenesis did not result in egg production. The mechanism underlying MdSGHV-induced sterility has yet to be resolved.

Viruses of insects reared for food and feed

The use of insects as food for humans or as feed for animals is an alternative for the increasing high demand for meat and has various environmental and social advantages over the traditional intensive production of livestock. Mass rearing of insects, under insect farming conditions or even in industrial settings, can be the key for a change in the way natural resources are utilized in order to produce meat, animal protein and a list of other valuable animal products. However, because insect mass rearing technology is relatively new, little is known about the different factors that determine the quality and yield of the production process. Obtaining such knowledge is crucial for the success of insect-based product development. One of the issues that is likely to compromise the success of insect rearing is the outbreak of insect diseases. In particular, viral diseases can be devastating for the productivity and the quality of mass rearing systems. Prevention and management of viral diseases imply the understanding of the different factors that interact in insect mass rearing. This publication provides an overview of the known viruses in insects most commonly reared for food and feed. Nowadays with large-scale sequencing techniques, new viruses are rapidly being discovered. We discuss factors affecting the emergence of viruses in mass rearing systems, along with virus transmission routes. Finally we provide an overview of the wide range of measures available to prevent and manage virus outbreaks in mass rearing systems, ranging from simple sanitation methods to highly sophisticated methods including RNAi and transgenics.

Metagenomic characterization of the virome associated with bovine respiratory disease in feedlot cattle identified novel viruses and suggests an etiologic role for influenza D virus

Bovine respiratory disease (BRD) is the most costly disease affecting the cattle industry. The pathogenesis of BRD is complex and includes contributions from microbial pathogens as well as host, environmental and animal management factors. In this study, we utilized viral metagenomic sequencing to explore the virome of nasal swab samples obtained from feedlot cattle with acute BRD and asymptomatic pen-mates at six and four feedlots in Mexico and the USA, respectively, in April-October 2015. Twenty-one viruses were detected, with bovine rhinitis A (52.7 %) and B (23.7 %) virus, and bovine coronavirus (24.7 %) being the most commonly identified. The emerging influenza D virus (IDV) tended to be significantly associated (P=0.134; odds ratio=2.94) with disease, whereas viruses commonly associated with BRD such as bovine viral diarrhea virus, bovine herpesvirus 1, bovine respiratory syncytial virus and bovine parainfluenza 3 virus were detected less frequently. The detection of IDV was further confirmed using a real-time PCR assay. Nasal swabs from symptomatic animals had significantly more IDV RNA than those collected from healthy animals (P=0.04). In addition to known viruses, new genotypes of bovine rhinitis B virus and enterovirus E were identified and a newly proposed species of bocaparvovirus, Ungulate bocaparvovirus 6, was characterized. Ungulate tetraparvovirus 1 was also detected for the first time in North America to our knowledge. These results illustrate the complexity of the virome associated with BRD and highlight the need for further research into the contribution of other viruses to BRD pathogenesis.

Enhancement of the Musca domestica hytrosavirus infection with orally delivered reducing agents

House flies (Musca domestica L.) throughout the world are infected with the salivary gland hypertrophy virus MdSGHV (Hytrosaviridae). Although the primary route of infection is thought to be via ingestion, flies that are old enough to feed normally are resistant to infection per os, suggesting that the peritrophic matrix (PM) is a barrier to virus transmission. Histological examination of the peritrophic matrix of healthy flies revealed a multilaminate structure produced by midgut cells located near the proventriculus. SEM revealed the PM to form a confluent sheet surrounding the food bolus with pores/openings less than 10nm in diameter. TEM revealed the PM to be multilayered, varying in width from 350 to 900 nm, and generally thinner in male than in female flies. When flies were fed on the reducing agents dithiothetriol (DTT) or tris (2-caboxyethyl)phosphine hydrochloride (TCEP) for 48 h before per os exposure to the virus, infection rates increased 10- to 20-fold compared with flies that did not receive the reducing agent treatments. PM's from flies treated with DTT and TCEP displayed varying degrees of disruption, particularly in the outer layer, and lacked the electron-dense inner layer facing the ectoperitrophic space. Both drugs were somewhat toxic to the flies, resulting in>40% mortality at doses greater than 10mM (DTT) or 5 mM (TCEP). DTT increased male fly susceptibility (55.1% infected) more than that of females (7.8%), whereas TCEP increased susceptibility of females (42.9%) more than that of males (26.2%). The cause for the sex differences in response to oral exposure the reducing agents is unclear. Exposing flies to food treated with virus and the reducing agents at the same time, rather than pretreating flies with the drugs, had no effect on susceptibility to the virus. Presumably, the reducing agent disrupted the enveloped virus and acted as a viricidal agent. In summary, it is proposed that the reducing agents influence integrity of the PM barrier and increase the susceptibility of flies to infection by MdSGHV.

Selecting optimal partitioning schemes for phylogenomic datasets

BACKGROUND

Partitioning involves estimating independent models of molecular evolution for different subsets of sites in a sequence alignment, and has been shown to improve phylogenetic inference. Current methods for estimating best-fit partitioning schemes, however, are only computationally feasible with datasets of fewer than 100 loci. This is a problem because datasets with thousands of loci are increasingly common in phylogenetics.

METHODS

We develop two novel methods for estimating best-fit partitioning schemes on large phylogenomic datasets: strict and relaxed hierarchical clustering. These methods use information from the underlying data to cluster together similar subsets of sites in an alignment, and build on clustering approaches that have been proposed elsewhere.

RESULTS

We compare the performance of our methods to each other, and to existing methods for selecting partitioning schemes. We demonstrate that while strict hierarchical clustering has the best computational efficiency on very large datasets, relaxed hierarchical clustering provides scalable efficiency and returns dramatically better partitioning schemes as assessed by common criteria such as AICc and BIC scores.

CONCLUSIONS

These two methods provide the best current approaches to inferring partitioning schemes for very large datasets. We provide free open-source implementations of the methods in the PartitionFinder software. We hope that the use of these methods will help to improve the inferences made from large phylogenomic datasets.

Selecting optimal partitioning schemes for phylogenomic datasets

BACKGROUND

Partitioning involves estimating independent models of molecular evolution for different subsets of sites in a sequence alignment, and has been shown to improve phylogenetic inference. Current methods for estimating best-fit partitioning schemes, however, are only computationally feasible with datasets of fewer than 100 loci. This is a problem because datasets with thousands of loci are increasingly common in phylogenetics.

METHODS

We develop two novel methods for estimating best-fit partitioning schemes on large phylogenomic datasets: strict and relaxed hierarchical clustering. These methods use information from the underlying data to cluster together similar subsets of sites in an alignment, and build on clustering approaches that have been proposed elsewhere.

RESULTS

We compare the performance of our methods to each other, and to existing methods for selecting partitioning schemes. We demonstrate that while strict hierarchical clustering has the best computational efficiency on very large datasets, relaxed hierarchical clustering provides scalable efficiency and returns dramatically better partitioning schemes as assessed by common criteria such as AICc and BIC scores.

CONCLUSIONS

These two methods provide the best current approaches to inferring partitioning schemes for very large datasets. We provide free open-source implementations of the methods in the PartitionFinder software. We hope that the use of these methods will help to improve the inferences made from large phylogenomic datasets.

Selecting optimal partitioning schemes for phylogenomic datasets

BACKGROUND

Partitioning involves estimating independent models of molecular evolution for different subsets of sites in a sequence alignment, and has been shown to improve phylogenetic inference. Current methods for estimating best-fit partitioning schemes, however, are only computationally feasible with datasets of fewer than 100 loci. This is a problem because datasets with thousands of loci are increasingly common in phylogenetics.

METHODS

We develop two novel methods for estimating best-fit partitioning schemes on large phylogenomic datasets: strict and relaxed hierarchical clustering. These methods use information from the underlying data to cluster together similar subsets of sites in an alignment, and build on clustering approaches that have been proposed elsewhere.

RESULTS

We compare the performance of our methods to each other, and to existing methods for selecting partitioning schemes. We demonstrate that while strict hierarchical clustering has the best computational efficiency on very large datasets, relaxed hierarchical clustering provides scalable efficiency and returns dramatically better partitioning schemes as assessed by common criteria such as AICc and BIC scores.

CONCLUSIONS

These two methods provide the best current approaches to inferring partitioning schemes for very large datasets. We provide free open-source implementations of the methods in the PartitionFinder software. We hope that the use of these methods will help to improve the inferences made from large phylogenomic datasets.

A Mathematic Model That Describes Modes of MdSGHV Transmission within House Fly Populations

In this paper it is proposed that one potential component by which the Musca domestica salivary gland hypertrophy virus (MdSGHV) infects individual flies is through cuticular damage. Breaks in the cuticle allow entry of the virus into the hemocoel causing the infection. Male flies typically have a higher rate of infection and a higher rate of cuticular damage than females. A model for the transmission of MdSGHV was formulated assuming several potential and recognized means of transmission. The model yields results that are in agreement with field data that measured the infection rate in house flies on dairy farms in Florida. The results from this model indicate that MdSGHV will be maintained at a stable rate within house fly populations and support the future use of MdSGHV as a birth control agent in house fly management.

Virology, Epidemiology and Pathology of Glossina Hytrosavirus, and Its Control Prospects in Laboratory Colonies of the Tsetse Fly, Glossina pallidipes (Diptera; Glossinidae)

The Glossina hytrosavirus (family Hytrosaviridae) is a double-stranded DNA virus with rod-shaped, enveloped virions. Its 190 kbp genome encodes 160 putative open reading frames. The virus replicates in the nucleus, and acquires a fragile envelope in the cell cytoplasm. Glossina hytrosavirus was first isolated from hypertrophied salivary glands of the tsetse fly, Glossina pallidipes Austen (Diptera; Glossinidae) collected in Kenya in 1986. A certain proportion of laboratory G. pallidipes flies infected by Glossina hytrosavirus develop hypertrophied salivary glands and midgut epithelial cells, gonadal anomalies and distorted sex-ratios associated with reduced insemination rates, fecundity and lifespan. These symptoms are rare in wild tsetse populations. In East Africa, G. pallidipes is one of the most important vectors of African trypanosomosis, a debilitating zoonotic disease that afflicts 37 sub-Saharan African countries. There is a large arsenal of control tactics available to manage tsetse flies and the disease they transmit. The sterile insect technique (SIT) is a robust control tactic that has shown to be effective in eradicating tsetse populations when integrated with other control tactics in an area-wide integrated approach. The SIT requires production of sterile male flies in large production facilities. To supply sufficient numbers of sterile males for the SIT component against G. pallidipes, strategies have to be developed that enable the management of the Glossina hytrosavirus in the colonies. This review provides a historic chronology of the emergence and biogeography of Glossina hytrosavirus, and includes researches on the infectomics (defined here as the functional and structural genomics and proteomics) and pathobiology of the virus. Standard operation procedures for viral management in tsetse mass-rearing facilities are proposed and a future outlook is sketched.

Improving Sterile Insect Technique (SIT) for tsetse flies through research on their symbionts and pathogens

Tsetse flies (Diptera: Glossinidae) are the cyclical vectors of the trypanosomes, which cause human African trypanosomosis (HAT) or sleeping sickness in humans and African animal trypanosomosis (AAT) or nagana in animals. Due to the lack of effective vaccines and inexpensive drugs for HAT, and the development of resistance of the trypanosomes against the available trypanocidal drugs, vector control remains the most efficient strategy for sustainable management of these diseases. Among the control methods used for tsetse flies, Sterile Insect Technique (SIT), in the frame of area-wide integrated pest management (AW-IPM), represents an effective tactic to suppress and/or eradicate tsetse flies. One constraint in implementing SIT is the mass production of target species. Tsetse flies harbor obligate bacterial symbionts and salivary gland hypertrophy virus which modulate the fecundity of the infected flies. In support of the future expansion of the SIT for tsetse fly control, the Joint FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture implemented a six year Coordinated Research Project (CRP) entitled "Improving SIT for Tsetse Flies through Research on their Symbionts and Pathogens". The consortium focused on the prevalence and the interaction between the bacterial symbionts and the virus, the development of strategies to manage virus infections in tsetse colonies, the use of entomopathogenic fungi to control tsetse flies in combination with SIT, and the development of symbiont-based strategies to control tsetse flies and trypanosomosis. The results of the CRP and the solutions envisaged to alleviate the constraints of the mass rearing of tsetse flies for SIT are presented in this special issue.

Muscavirus (MdSGHV) disease dynamics in house fly populations--how is this virus transmitted and has it potential as a biological control agent?

The newly classified family Hytrosaviridae comprises several double-stranded DNA viruses that have been isolated from various dipteran species. These viruses cause characteristic salivary gland hypertrophy and suppress gonad development in their hosts. One member, Muscavirus or MdSGHV, exclusively infects adult house flies (Musca domestica) and, owing to its massive reproduction in and release from the salivary glands, is believed to be transmitted orally among feeding flies. However, results from recent experiments suggest that additional transmission routes likely are involved in the maintenance of MdSGHV in field populations of its host. Firstly, several hours before newly emerged feral flies begin feeding activities, the fully formed peritrophic matrix (PM) constitutes an effective barrier against oral infection. Secondly, flies are highly susceptible to topical virus treatments and intrahemocoelic injections. Thirdly, disease transmission is higher when flies are maintained in groups with infected conspecifics than when flies have access to virus-contaminated food. We hypothesize that interactions between flies may lead to cuticular damage, thereby providing an avenue to viral particles for direct access to the hemocoel. Based on our current knowledge, two options seem plausible for developing Muscavirus as a sterilizing agent to control house fly populations: The virus may either be formulated with PM-disrupting materials to facilitate oral infection from a feeding bait system, or amended with abrasive materials to enhance infection through a damaged cuticle after topical aerosol applications.

Enhancing tsetse fly refractoriness to trypanosome infection--a new IAEA coordinated research project

To date, IAEA-supported Sterile Insect Technique (SIT) projects for tsetse and trypanosomiasis control have been in areas without human sleeping sickness, but future projects could include areas of actual or potential human disease transmission. In this context it would be imperative that released sterile tsetse flies are incompetent to transmit the disease-causing trypanosome parasite. Therefore, development of tsetse fly strains refractory to trypanosome infection is highly desirable as a simple and effective method of ensuring vector incompetence of the released flies. This new IAEA Coordinated Research Project (CRP) focuses on gaining a deeper knowledge of the tripartite interactions between the tsetse fly vectors, their symbionts and trypanosome parasites. The objective of this CRP is to acquire a better understanding of mechanisms that limit the development of trypanosome infections in tsetse and how these may be enhanced.

Prevalence and genetic variation of salivary gland hypertrophy virus in wild populations of the tsetse fly Glossina pallidipes from southern and eastern Africa

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is a rod-shaped, non-occluded double-stranded DNA virus that causes salivary gland hypertrophy (SGH) and reduced fecundity in the tsetse fly G. pallidipes. High GpSGHV prevalence (up to 80%) makes it impossible to mass-rear G. pallidipes colonies for the sterile insect technique (SIT). To evaluate the feasibility of molecular-based GpSGHV management strategies, we investigated the prevalence and genetic diversity of GpSGHV in wild populations of G. pallidipes collected from ten geographical locations in eastern and southern Africa. Virus diversity was examined using a total sequence of 1497 nucleotides (≈ 1% of the GpSGHV genome) from five putative conserved ORFs, p74, pif1, pif2, pif3 and dnapol. Overall, 34.08% of the analyzed flies (n=1972) tested positive by nested PCR. GpSGHV prevalence varied from 2% to 100% from one location to another but phylogenetic and gene genealogy analyses using concatenated sequences of the five putative ORFs revealed low virus diversity. Although no correlation of the virus diversity to geographical locations was detected, the GpSGHV haplotypes could be assigned to one of two distinct clades. The reference (Tororo) haplotype was the most widely distributed, and was shared by 47 individuals in seven of the 11 locations. The Ethiopian haplotypes were restricted to one clade, and showed the highest divergence (with 14-16 single nucleotide mutation steps) from the reference haplotype. The current study suggests that the proposed molecular-based virus management strategies have a good prospect of working throughout eastern and southern Africa due to the low diversity of the GpSGHV strains.

Analysis of the structural proteins from the Musca domestica hytrosavirus with an emphasis on the major envelope protein

The Musca domestica hytrosavirus (MdHV), a member of the family Hyrosaviridae, is a large, dsDNA, enveloped virus that infects adult house flies and causes a diagnostic hypertrophy of the salivary gland. Herein, studies were directed at identifying key structural components of the viral envelope and nucleocapsid. SDS-PAGE of detergent-treated virus fractions identified protein bands unique to the envelope and nucleocapsid components. Using prior LC-MSMS data we identified the viral ORF associated with the major envelope band, cloned and expressed recombinant viral antigens, and prepared a series of polyclonal sera. Western blots confirmed that antibodies recognized the target viral antigen and provided evidence that the viral protein MdHV96 underwent post-translational processing; antibodies bound to the target high molecular weight parent molecule as well as distinct sets of smaller bands. Immuno gold electron microscopy demonstrated that the anti-MdHV96 sera recognized target antigens associated with the envelope. The nucleocapsids migrated from the virogenic stroma in the nucleus through the nuclear membrane into the cytoplasm, where they acquired an initial envelope that contained MdHV96. This major envelope protein, appeared to incorporate into intracellular membranes of both the caniculi and rough endoplasmic reticulum membranes and mediate binding to the nucleocapsids. Oral infection bioassays demonstrated that the anti-HV96 polyclonal sera acted as neutralizing agents in suppressing the levels of orally acquired infections.

Disease dynamics and persistence of Musca domestica salivary gland hypertrophy virus infections in laboratory house fly (Musca domestica) populations

Past surveys of feral house fly populations have shown that Musca domestica salivary gland hypertrophy virus (MdSGHV) has a worldwide distribution, with an average prevalence varying between 0.5% and 10%. How this adult-specific virus persists in nature is unknown. In the present study, experiments were conducted to examine short-term transmission efficiency and long-term persistence of symptomatic MdSGHV infections in confined house fly populations. Average rates of disease transmission from virus-infected to healthy flies in small populations of 50 or 100 flies ranged from 3% to 24% and did not vary between three tested geographical strains that originated from different continents. Introduction of an initial proportion of 40% infected flies into fly populations did not result in epizootics. Instead, long-term observations demonstrated that MdSGHV infection levels declined over time, resulting in a 10% infection rate after passing through 10 filial generations. In all experiments, induced disease rates were significantly higher in male flies than in female flies and might be explained by male-specific behaviors that increased contact with viremic flies and/or virus-contaminated surfaces.

Salivary gland hypertrophy virus of house flies in Denmark: prevalence, host range, and comparison with a Florida isolate

House flies (Musca domestica) infected with Musca domestica salivary gland hypertrophy virus (MdSGHV) were found in fly populations collected from 12 out of 18 Danish livestock farms that were surveyed in 2007 and 2008. Infection rates ranged from 0.5% to 5% and averaged 1.2%. None of the stable flies (Stomoxys calcitrans), rat-tail maggot flies (Eristalis tenax) or yellow dung flies (Scathophaga stercoraria) collected from MdSGHV-positive farms displayed characteristic salivary gland hypertrophy (SGH). In laboratory transmission tests, SGH symptoms were not observed in stable flies, flesh flies (Sarcophaga bullata), black dump flies (Hydrotaea aenescens), or face flies (Musca autumnalis) that were injected with MdSGHV from Danish house flies. However, in two species (stable fly and black dump fly), virus injection resulted in suppression of ovarian development similar to that observed in infected house flies, and injection of house flies with homogenates prepared from the salivary glands or ovaries of these species resulted in MdSGHV infection of the challenged house flies. Mortality of virus-injected stable flies was the highest among the five species tested. Virulence of Danish and Florida isolates of MdSGHV was similar with three virus delivery protocols, as a liquid food bait (in sucrose, milk, or blood), sprayed onto the flies in a Potter spray tower, or by immersiion in a crude homogenate of infected house flies. The most effective delivery system was immersion in a homogenate of ten infected flies/ml of water, resulting in 56.2% and 49.6% infection of the house flies challenged with the Danish and Florida strains, respectively.

Impact of house fly salivary gland hypertrophy virus (MdSGHV) on a heterologous host, Stomoxys calcitrans

The effect of Musca domestica salivary gland hypertrophy virus (MdSGHV) on selected fitness parameters of stable flies, Stomoxys calcitrans (L.), was examined in the laboratory. Virus-injected stable flies of both genders suffered substantially higher mortality than control flies. By day 9, female mortality was 59.3 +/- 10.1% in the virus group compared with 23.7 +/- 3.7% in the controls; mortality in virus-injected males was 78.1 +/- 3.1% compared with 33.3 +/- 9.3% for controls. Fecundity of control flies on days 6-9 was 49-54 eggs deposited per live female per day (total, 8,996 eggs deposited), whereas virus-injected flies produced four to five eggs per female on days 6-7 and less then one egg per female per day thereafter (total, 251 eggs). Fecal spot deposition by virus-injected flies was comparable to controls initially but decreased to approximately 50% of control levels by day 4 after injection; infected flies produced only 26% as many fecal spots as healthy flies on days 6 and 7. None of the virus-injected stable flies developed symptoms of salivary gland hypertrophy. Quantitative real-time polymerase chain reaction demonstrated virus replication in injected stable flies, with increasing titers of virus genome copies from one to four days after injection. MdSGHV in stable flies displayed tissue tropism similar to that observed in house fly hosts, with higher viral copy numbers in fat body and salivary glands compared with ovaries. Virus titers were approximately 2 orders of magnitude higher in house fly than in stable fly hosts, and this difference was probably due to the absence of salivary gland hypertrophy in the latter species.

Two hytrosaviruses, MdSGHV and GpSGHV, induce distinct cytopathologies in their respective host insects

Recently, a new virus family (Hytrosaviridae) was proposed for double-stranded DNA viruses that cause salivary gland hypertrophy in their dipteran hosts. The two type species, MdSGHV and GpSGHV, induce similar gross pathology and share several morphological, biological, and molecular characteristics. This histological study revealed profound differences in the cytopathology of these viruses supporting their previously proposed placement in different genera.

Salivary gland hypertrophy viruses: a novel group of insect pathogenic viruses

Salivary gland hypertrophy viruses (SGHVs) are a unique, unclassified group of entomopathogenic, double-stranded DNA viruses that have been reported from three genera of Diptera. These viruses replicate in nuclei of salivary gland cells in adult flies, inducing gland enlargement with little obvious external disease symptoms. Viral infection inhibits reproduction by suppressing vitellogenesis, causing testicular aberrations, and/or disrupting mating behavior. Historical and present research findings support a recent proposal of a new virus family, the Hytrosaviridae. This review describes the discovery and prevalence of different SGHVs, summarizes their biochemical characterization and taxonomy, compares morphological and histopathological properties, and details transmission routes and the influence of infection on host biology and reproduction. In addition, the potential use of SGHVs as sterilizing agents for house fly control and the deleterious impact of SGHVs on colonized tsetse flies reared for sterile insect technique are discussed.

Universal primers for rapid detection of hytrosaviruses

Hytrosaviridae is a proposed virus family encompassing viruses that cause salivary gland hypertrophy (SGH) syndrome in infected insects and reduce the fertility in their dipteran insect hosts. They contain a large, double stranded DNA genome of 120-190 kbp. To date, these viruses have been detected only in adult Diptera. These include hytrosaviruses detected in various tsetse fly species (Glossina spp.), the narcissus bulb fly Merodon equestris and the house fly Musca domestica. The limited number of hytrosaviruses reported to date may be a reflection of the frequent absence of external symptoms in infected adult flies and the fact that the virus does not cause rapid mortality. Based on the complete genome sequence of Glossinia pallidipes (GpSGHV) and Musca domestica (MdSGHV) salivary gland hypertrophy viruses, a PCR based methodology was developed to detect the viruses in these species. To be able to detect hytrosaviruses in other Diptera, five degenerate primer pairs were designed and tested on GpSGHV and MdSGHV DNA using gradient PCR with annealing temperatures from 37 to 61°C. Two pairs of primers were selected from p74, two pairs from PIF-1 and one pair from ODV-e66 homologous proteins. Four primer pairs generated a virus specific PCR product on both MdSGHV and GpSGHV at all tested annealing temperatures, while the ODV-e66 based primers did not generate a virus specific product with annealing temperatures higher that 47°C. No non-specific PCR product was found when using genomic DNA of infected flies as template DNA. These results offer new sets of primers that could be used to detect hytrosaviruses in other insects.

Tissue tropism of the Musca domestica salivary gland hypertrophy virus

The tissue tropism of Musca domestica salivary gland hypertrophy virus (MdSGHV) infecting adult house flies was examined by transmission electron microscopy (TEM) and quantitative real-time PCR. TEM demonstrated that characteristic MdSGHV-induced nuclear and cellular hypertrophy was restricted to the salivary glands. Both nucleocapsids and enveloped virions were present in salivary gland cells. In contrast, thin sections of midguts, ovaries, abdominal fat body, crops, air sacs and brains showed the presence of enveloped virions in vacuoles of tracheal cells associated with these tissues. However, no sites of viral morphogenesis were detected in the tracheal cells. Quantitative analysis of MdSGHV DNA and transcript titers revealed that viral DNA was present in all hemolymph and tissue samples collected from MdSGHV-infected flies. Average numbers of MdSGHV genome copies per 50 ng of DNA varied significantly between examined tissues and ranged from 3.83 × 10(8) (±3.75 × 10(7)) in salivary gland samples to 7.98 × 10(5) (±2.91 × 10(5)) in hemolymph samples. High levels of viral genome copies were detected in midgut, fat body and brain samples. Viral transcripts were present in all examined samples, and transcript abundance was also at the highest level in salivary glands and at the lowest level in hemolymph. However, over the range of different tissues that were analyzed, there was no correlation between estimated quantities of genome copies and viral transcripts. The function of viral transcripts in host tissues that do not show sites of viral morphogenesis remains to be elucidated.