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

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.

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.

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.

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.

Transgenerational transmission of the Glossina pallidipes hytrosavirus depends on the presence of a functional symbiome

The vertically transmitted endosymbionts (Sodalis glossinidius and Wigglesworthia glossinidia) of the tsetse fly (Diptera: Glossinidae) are known to supplement dietary deficiencies and modulate the reproductive fitness and the defense system of the fly. Some tsetse fly species are also infected with the bacterium, Wolbachia and with the Glossina hytrosavirus (GpSGHV). Laboratory-bred G. pallidipes exhibit chronic asymptomatic and acute symptomatic GpSGHV infection, with the former being the most common in these colonies. However, under as yet undefined conditions, the asymptomatic state can convert to the symptomatic state, leading to detectable salivary gland hypertrophy (SGH(+)) syndrome. In this study, we investigated the interplay between the bacterial symbiome and GpSGHV during development of G. pallidipes by knocking down the symbionts with antibiotic. Intrahaemocoelic injection of GpSGHV led to high virus titre (10(9) virus copies), but was not accompanied by either the onset of detectable SGH(+), or release of detectable virus particles into the blood meals during feeding events. When the F1 generations of GpSGHV-challenged mothers were dissected within 24 h post-eclosion, SGH(+) was observed to increase from 4.5% in the first larviposition cycle to >95% in the fourth cycle. Despite being sterile, these F1 SGH(+) progeny mated readily. Removal of the tsetse symbiome, however, suppressed transgenerational transfer of the virus via milk secretions and blocked the ability of GpSGHV to infect salivary glands of the F1 progeny. Whereas GpSGHV infects and replicates in salivary glands of developing pupa, the virus is unable to induce SGH(+) within fully differentiated adult salivary glands. The F1 SGH(+) adults are responsible for the GpSGHV-induced colony collapse in tsetse factories. Our data suggest that GpSGHV has co-evolved with the tsetse symbiome and that the symbionts play key roles in the virus transmission from mother to progeny.

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.

Alimentary Canal of the Adult Blow Fly, Chrysomya megacephala (F.) (Diptera: Calliphoridae)-Part I: Ultrastructure of Salivary Glands

The salivary gland ultrastructure of the adult male blow fly, Chrysomya megacephala (F.) (Diptera: Calliphoridae), was investigated at the ultrastructural level using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The salivary glands are paired structures composed of a single median deferent duct bifurcated into two long, narrow efferent ducts connected to the coiled tubular glands. The SEM image of the gland surface revealed that the basal lamina is relatively smooth in general, but the whole surface appeared as a trace of rough swollen insertion by intense tracheal ramification. Ultrastructurally, the salivary gland is enclosed within the basal lamina, and interdigitation cytoplasmic extensions were apparent between the adjacent gland cells. The basement membrane appeared infoldings that is similar to the complex of the labyrinth channel. The cytoplasm characteristic of the gland revealed high activity, based on the abundance of noticeable secretory granules, either singly or in an aggregated reservoir. In addition, mitochondria were found to intersperse among rich parallel of arrays rough endoplasmic reticulum. Thick cuticle, which was well-delineated and electron dense, apically lined the gland compartments, with discontinuity of the double-layer cuticle revealing a trace of secretion discharged into the lumen. Gross anatomy of the adult salivary gland was markedly different from that of the third instar of the same species, and structural dissimilarity is discussed briefly.

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.