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.

An insight into the sialome, mialome and virome of the horn fly, Haematobia irritans

BACKGROUND

The horn fly (Haematobia irritans) is an obligate blood feeder that causes considerable economic losses in livestock industries worldwide. The control of this cattle pest is mainly based on insecticides; unfortunately, in many regions, horn flies have developed resistance. Vaccines or biological control have been proposed as alternative control methods, but the available information about the biology or physiology of this parasite is rather scarce.

RESULTS

We present a comprehensive description of the salivary and midgut transcriptomes of the horn fly (Haematobia irritans), using deep sequencing achieved by the Illumina protocol, as well as exploring the virome of this fly. Comparison of the two transcriptomes allow for identification of uniquely salivary or uniquely midgut transcripts, as identified by statistically differential transcript expression at a level of 16 x or more. In addition, we provide genomic highlights and phylogenetic insights of Haematobia irritans Nora virus and present evidence of a novel densovirus, both associated to midgut libraries of H. irritans.

CONCLUSIONS

We provide a catalog of protein sequences associated with the salivary glands and midgut of the horn fly that will be useful for vaccine design. Additionally, we discover two midgut-associated viruses that infect these flies in nature. Future studies should address the prevalence, biological effects and life cycles of these viruses, which could eventually lead to translational work oriented to the control of this economically important cattle pest.

Risk of introduction of lumpy skin disease in France by the import of vectors in animal trucks

BACKGROUND

The lumpy skin disease virus (LSDV) is a dsDNA virus belonging to the Poxviridae family and the Capripoxvirus genus. Lumpy skin diseases (LSD) is a highly contagious transboundary disease in cattle producing major economic losses. In 2014, the disease was first reported in the European Union (in Cyprus); it was then reported in 2015 (in Greece) and has spread through different Balkan countries in 2016. Indirect vector transmission is predominant at small distances, but transmission between distant herds and between countries usually occurs through movements of infected cattle or through vectors found mainly in animal trucks.

METHODS AND PRINCIPAL FINDINGS

In order to estimate the threat for France due to the introduction of vectors found in animal trucks (cattle or horses) from at-risk countries (Balkans and neighbours), a quantitative import risk analysis (QIRA) model was developed according to the international standard. Using stochastic QIRA modelling and combining experimental/field data and expert opinion, the yearly risk of LSDV being introduced by stable flies (Stomoxys calcitrans), that travel in trucks transporting animals was between 6 x 10-5 and 5.93 x 10-3 with a median value of 89.9 x 10-5; it was mainly due to the risk related to insects entering farms in France from vehicles transporting cattle from the at-risk area. The risk related to the transport of cattle going to slaughterhouses or the transport of horses was much lower (between 2 x 10-7 and 3.73 x 10-5 and between 5 x 10-10 and 3.95 x 10-8 for cattle and horses, respectively). The disinsectisation of trucks transporting live animals was important to reduce this risk.

CONCLUSION AND SIGNIFICANCE

The development of a stochastic QIRA made it possible to quantify the risk of LSD being introduced in France through the import of vectors that travel in trucks transporting animals. This tool is of prime importance because the LSD situation in the Balkans is continuously changing. Indeed, this model can be updated to process new information on vectors and the changing health situation, in addition to new data from the TRAde Control and Expert System (TRACES, EU database). This model is easy to adapt to different countries and to other vectors and diseases.

Effect of Diet on Adult House Fly (Diptera: Muscidae) Injected With the Salivary Gland Hypertrophy Virus (MdSGHV)

Research to date on the salivary gland hypertrophy virus (SGHV) in three species of flies has focused on adult flies having access to and taking a proteinaceous diet. Since many studies have shown that diet affects viral infection in numerous organisms, this study examined the effect of a protein-free diet on the effect of the SGHV virus in adult house flies, Musca domestica. L. Adults infected with the virus, and maintained on a sugar diet only, showed salivary glands with a blue rather than a grayish color and mild hypertrophy compared with protein-fed flies. It was possible to retrieve the virus from these glands and successfully infect noninfected flies. When injected at various ages, female flies fed only sugar showed that regardless of age, sugar-fed flies still became infected and showed the pathology of the glands. In addition, electron microscope studies revealed at the ultrastructural level that there was no difference between viral replication in cells from salivary glands of adults fed a proteinaceous-free diet and those feeding on protein.

High relative abundance of the stable fly Stomoxys calcitrans is associated with lumpy skin disease outbreaks in Israeli dairy farms

The vector of lumpy skin disease (LSD), a viral disease affecting Bovidae, is currently unknown. To evaluate the possible vector of LSD virus (LSDV) under field conditions, a yearlong trapping of dipterans was conducted in dairy farms that had been affected by LSD, 1-2 years previously. This was done in order to calculate monthly relative abundances of each dipteran in each farm throughout the year. The relative abundances of Stomoxys calcitrans (Diptera: Muscidae) in the months parallel to the outbreaks (December and April) were significantly higher than those of other dipterans. A stable fly population model based on weather parameters for the affected area was used to validate these findings. Its results were significantly correlated with S. calcitrans abundance. This model, based on weather parameters during the epidemic years showed that S. calcitrans populations peaked in the months of LSD onset in the studied farms. These observations and model predictions revealed a lower abundance of stable flies during October and November, when LSD affected adjacent grazing beef herds. These findings therefore suggest that S. calcitrans is a potential vector of LSD in dairy farms and that another vector is probably involved in LSDV transmission in grazing herds. These findings should be followed up with vector competence studies.

Persistence and Retention of Porcine Reproductive and Respiratory Syndrome Virus in Stable Flies (Diptera: Muscidae)

We investigated the acquisition of porcine reproductive and respiratory syndrome (PRRS) virus by the stable fly (Diptera: Muscidae; Stomoxys calcitrans (L.)) through a bloodmeal, and virus persistence in the digestive organs of the fly using virus isolation and quantitative reverse-transcription PCR (qRT-PCR). Stable flies were fed blood containing live virus, modified live vaccine virus, chemically inactivated virus, or no virus. Stable flies acquired PRRSV from the bloodmeal and the amount of virus in the flies declined with time, indicating virus did not replicate in fly digestive tissues. Virus RNA was recovered from the flies fed live virus up to 24 h postfeeding using virus isolation techniques and 96 h using qRT-PCR. We further examined the fate of PRRSV in the hemolymph of the flies following intrathoracic injection to bypass the midgut barrier. PRRSV was detected in intrathoracically inoculated adult stable flies for 10 d using qRT-PCR. In contrast to what we observed in the digestive tract, detectable virus quantities in the intrathoracically inoculated stable flies followed an exponential decay curve. The amount of virus decreased fourfold in the first 3 d and remained stable thereafter, up to 10 d.

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.

Assessment of Stomoxys calcitrans (Diptera: Muscidae) as a vector of porcine reproductive and respiratory syndrome virus

Porcine Reproductive and respiratory syndrome (PRRS) is a globally significant swine disease caused by an arterivirus. The virus replicates in alveolar macrophages of infected pigs, resulting in pneumonia in growing pigs and late-term abortions in sows. Outbreaks occur on disparate farms within an area despite biosecurity measures, suggesting mechanical transport by arthropods. We investigated the vector potential of stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), in the transmission of porcine reproductive and respiratory syndrome virus (family Arteriviridae, genus Arterivirus, PRRSV) under laboratory conditions. Stable flies were collected around PRRS-negative boar stud barns in North Carolina and tested for presence of the virus. Stable flies were collected on alsynite traps placed near the exhaust fan of the close-sided tunnel-ventilated buildings, suggesting blood seeking flies are attracted by olfactory cues. No flies were positive for PRRSV. We assessed transmission of the virus through an infective bite by feeding laboratory reared stable flies on blood containing virus and transferring them to naive pigs for subsequent bloodmeals. Transmission of the virus to naive pigs by infective bites failed in all attempts. The volume of blood contained within the closed mouthparts of the stable fly seems to be insufficient to deliver an infective dose of the virus. Stable flies are unlikely to transmit PRRSV from one pig to another while blood feeding. The fate of the virus after a bloodmeal remains to be determined.

Vector competence of the stable fly (Diptera: Muscidae) for West Nile virus

In 2006-2007, stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), were suspected of being enzootic vectors of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) during a die-off of American white pelicans (Pelecanus erythrorhynchos Gmelin) (Pelecanidae) in Montana, USA. WNV-positive stable flies were observed feeding en masse on incapacitated, WNV-positive pelicans, arousing suspicions that the flies could have been involved in WNV transmission among pelicans, and perhaps to livestock and humans. We assessed biological transmission by infecting stable flies intrathoracically with WNV and testing them at 2-d intervals over 20 d. Infectious WNV was detected in fly bodies in decreasing amounts over time for only the first 6 d postinfection, an indication that WNV did not replicate within fly tissues and that stable flies cannot biologically transmit WNV. We assessed mechanical transmission using a novel technique. Specifically, we fed WNV-infected blood to individual flies by using a cotton swab (i.e., artificial donor), and at intervals of 1 min-24 h, we allowed flies to refeed on a different swab saturated with WNV-negative blood (i.e., artificial recipient). Flies mechanically transmitted viable WNV from donor to recipient swabs for up to 6 h postinfection, with the majority of the transmission events occurring within the first hour. Flies mechanically transmitted WNV RNA to recipient swabs for up to 24 h, mostly within the first 6 h. Given its predilection to feed multiple times when disturbed, these findings support the possibility that the stable fly could mechanically transmit WNV.

Potential for stable flies and house flies (Diptera: Muscidae) to transmit Rift Valley fever virus

Rift Valley fever (RVF), a disease of ruminants and humans, has been responsible for large outbreaks in Africa that have resulted in hundreds of thousands of human infections and major economic disruption due to loss of livestock and to trade restrictions. As indicated by the rapid spread of West Nile viral activity across North America since its discovery in 1999 and the rapid and widespread movement of chikungunya virus from Africa throughout the Indian Ocean Islands to Asia and Europe, an introduced exotic arbovirus can be rapidly and widely established across wide geographical regions. Although RVF virus (RVFV) is normally transmitted by mosquitoes, we wanted to determine the potential for this virus to replicate in 2 of the most globally distributed and common higher flies: house flies, Musca domestica, and stable flies, Stomoxys calcitrans. Neither species supported the replication of RVFV, even after intrathoracic inoculation. However, S. calcitrans was able to mechanically transmit RVFV to susceptible hamsters (Mesocricetus auratus) after probing on infected hamsters with high viral titers. Therefore, S. calcitrans, because of its close association with domestic animals that serve as amplifying hosts of RVFV, should be considered a possible mechanical vector of RVFV, and it may contribute to the rapid spread of a RVF outbreak. Other Stomoxys species present in Africa and elsewhere may also play similar roles.

Detection of West Nile virus in stable flies (Diptera: Muscidae) parasitizing juvenile American white pelicans

Stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), an economically important pest of livestock and humans, were observed parasitizing prefledged American white pelicans, Pelecanus erythrorhynchos (Pelecaniformes: Pelecanidae), in a pelican breeding colony in northeastern Montana where die-offs attributed to West Nile virus (family Flaviviridae, genus Flavivirus, WNV) have occurred since 2002. Engorged and unengorged flies were collected off nine moribund chicks. Of 29 blood-engorged flies testing positive for vertebrate DNA, all 29 contained pelican DNA. Virus isolation was performed on 60 pools (1,176 flies) of unengorged flies using Vero cell plaque assay. Eighteen pools were positive for WNV for an estimated infection rate of 18.0 per 1,000 flies. Fifty-four percent (36/67) of abdomens from blood-engorged flies tested positive for WNV. Pelican viremia levels from the blood-engorged fly abdomens revealed that at least one of the ill pelicans circulated a viremia capable of infecting Culex mosquito vectors. Stable flies may be involved in WNV transmission within the pelican breeding colony by serving as either a mechanical vector or as a source for oral infection if ingested by predators.

Detection and isolation of exotic Newcastle disease virus from field-collected flies

Flies were collected by sweep net from the vicinity of two small groups of "backyard" poultry (10-20 chickens per group) that had been identified as infected with exotic Newcastle disease virus (family Paramyxoviridae, genus avulavirus, ENDV) in Los Angeles County, CA, during the 2002-2003 END outbreak. Collected flies were subdivided into pools and homogenized in brain-heart infusion broth with antibiotics. The separated supernatant was tested for the presence of ENDV by inoculation into embryonated chicken eggs. Exotic Newcastle disease virus was isolated from pools of Phaenicia cuprina (Wiedemann), Fannia canicularis (L.), and Musca domestica L., and it was identified by hemagglutination inhibition with Newcastle disease virus antiserum. Viral concentration in positive pools was low (<1 egg infectious dose50 per fly). Isolated virus demonstrated identical monoclonal antibody binding profiles as well as 99% sequence homology in the 635-bp fusion gene sequence compared with ENDV recovered from infected commercial egg layer poultry during the 2002 outbreak.

Detection and isolation of exotic Newcastle disease virus from field-collected flies

Flies were collected by sweep net from the vicinity of two small groups of "backyard" poultry (10-20 chickens per group) that had been identified as infected with exotic Newcastle disease virus (family Paramyxoviridae, genus avulavirus, ENDV) in Los Angeles County, CA, during the 2002-2003 END outbreak. Collected flies were subdivided into pools and homogenized in brain-heart infusion broth with antibiotics. The separated supernatant was tested for the presence of ENDV by inoculation into embryonated chicken eggs. Exotic Newcastle disease virus was isolated from pools of Phaenicia cuprina (Wiedemann), Fannia canicularis (L.), and Musca domestica L., and it was identified by hemagglutination inhibition with Newcastle disease virus antiserum. Viral concentration in positive pools was low (<1 egg infectious dose50 per fly). Isolated virus demonstrated identical monoclonal antibody binding profiles as well as 99% sequence homology in the 635-bp fusion gene sequence compared with ENDV recovered from infected commercial egg layer poultry during the 2002 outbreak.

Stability of equine infectious anemia virus in Aedes aegypti (Diptera: Culicidae), Stomoxys calcitrans (Diptera:Muscidae), and Tabanus fuscicostatus (Diptera:Tabanidae) stored at -70 degrees C

Equine infectious anemia virus (EIAV) was injected intrathoracically into Aedes aegypti, Stomoxys calcitrans, and Tabanus fuscicostatus, and fed to Ae. aegypti in suspensions of either artificial blood of Eagle's Minimum Essential Medium. Insects were stored at -70 degrees C for up to 9 months before testing for the presence of EIAV. The viral tissue culture titers detected from stored insects were similar to those from insects tested at time 0.