Japanese encephalitis virus in mosquito salivary glands

Mosquito Defensins Enhance Japanese Encephalitis Virus Infection by Facilitating Virus Adsorption and Entry within the Mosquito

Japanese encephalitis virus (JEV) is a viral zoonosis that can cause viral encephalitis, death, and disability. Although the Culex mosquito is the primary vector of JEV, little is known about JEV transmission by this kind of mosquito. Here, we found that mosquito defensin facilitated the adsorption of JEV on target cells via the defensin/lipoprotein receptor-related protein 2 (LRP2) axis. Mosquito defensin bound the ED III domain of the viral envelope (E) protein and directly mediated efficient virus adsorption on the target cell surface; the receptor LRP2, which is expressed on the cell surface, affected defensin-dependent adsorption. As a result, mosquito defensin enhanced JEV infection in the salivary gland, increasing the possibility of viral transmission by mosquitoes. These findings demonstrate the novel role of mosquito defensin in JEV infection and the mechanisms through which the virus exploits mosquito defensin for infection and transmission.IMPORTANCE In this study, we observed the complex roles of mosquito defensin in JEV infection; mosquito defensin exhibited a weak antiviral effect but strongly enhanced binding. In the latter, defensin directly binds the ED III domain of the viral E protein and promotes the adsorption of JEV to target cells by interacting with lipoprotein receptor-related protein 2 (LRP2), thus accelerating virus entry. Together, our results indicate that mosquito defensin plays an important role in facilitating JEV infection and potential transmission.

Viral Determinants and Vector Competence of Zika Virus Transmission

Zika virus (ZIKV) has emerged as a new global health threat. Since its first discovery in Zika forest in Uganda, this virus has been isolated from several mosquito species, including Aedes aegypti and Aedes albopictus. The geographical distribution of these mosquito species across tropical and subtropical regions has led to several outbreaks, including the recent pandemic in Brazil, followed by the Pacific islands and other areas of North and South America. This has gained attention of the scientific community to elucidate the epidemiology and transmission of ZIKV. Despite its strong attention on clinical aspects for healthcare professionals, the relationships between ZIKV and its principal vectors, A. aegypti and A. albopictus, have not gained substantial interest in the scientific research community. As such, this review aims to summarize the current knowledge on ZIKV tropism and some important mechanisms which may be employed by the virus for effective strategies on viral survival in mosquitoes. In addition, this review identifies the areas of research that should be placed attention to, for which to be exploited for novel mosquito control strategies.

A novel sheet-like virus particle array is a hallmark of Zika virus infection

Zika virus (ZIKV) is an emerging flavivirus that caused thousands of human infections in recent years. Compared to other human flaviviruses, ZIKV replication is not well understood. Using fluorescent, transmission electron, and focused ion beam-scanning electron microscopy, we examined ZIKV replication dynamics in Vero 76 cells and in the brains of infected laboratory mice. We observed the progressive development of a perinuclear flaviviral replication factory both in vitro and in vivo. In vitro, we illustrated the ZIKV lifecycle from particle cell entry to egress. ZIKV particles assembled and aggregated in an induced convoluted membrane structure and ZIKV strain-specific membranous vesicles. While most mature virus particles egressed via membrane budding, some particles also likely trafficked through late endosomes and egressed through membrane abscission. Interestingly, we consistently observed a novel sheet-like virus particle array consisting of a single layer of ZIKV particles. Our study further defines ZIKV replication and identifies a novel hallmark of ZIKV infection.

Mosquitoes Transmit Unique West Nile Virus Populations during Each Feeding Episode

Arthropod-borne viruses (arboviruses), such as Zika virus, chikungunya virus, and West Nile virus (WNV), pose continuous threats to emerge and cause large epidemics. Often, these events are associated with novel virus variants optimized for local transmission that first arise as minorities within a host. Thus, the conditions that regulate the frequency of intrahost variants are important determinants of emergence. Here, we describe the dynamics of WNV genetic diversity during its transmission cycle. By temporally sampling saliva from individual mosquitoes, we demonstrate that virus populations expectorated by mosquitoes are highly diverse and unique to each feeding episode. After transmission to birds, however, most genetic diversity is removed by strong purifying selection. Further, transmission of potentially mosquito-adaptive WNV variants is strongly influenced by genetic drift in mosquitoes. These results highlight the complex evolutionary forces a novel virus variant must overcome to alter infection phenotypes at the population level.

Flavivirus-mosquito interactions

The Flavivirus genus is in the family Flaviviridae and is comprised of more than 70 viruses. These viruses have a broad geographic range, circulating on every continent except Antarctica. Mosquito-borne flaviviruses, such as yellow fever virus, dengue virus serotypes 1-4, Japanese encephalitis virus, and West Nile virus are responsible for significant human morbidity and mortality in affected regions. This review focuses on what is known about flavivirus-mosquito interactions and presents key data collected from the field and laboratory-based molecular and ultrastructural evaluations.

Role of antibodies in controlling dengue virus infection

The incidence and disease burden of arthropod-borne flavivirus infections have dramatically increased during the last decades due to major societal and economic changes, including massive urbanization, lack of vector control, travel, and international trade. Specifically, in the case of dengue virus (DENV), the geographical spread of all four serotypes throughout the subtropical regions of the world has led to larger and more severe outbreaks. Many studies have established that recovery from infection by one DENV serotype provides immunity against that serotype, whereas reinfection with another serotype may result in severe disease. Pre-existing antibodies thus play a critical role in controlling viral infection. Both neutralization and enhancement of DENV infection by antibodies are thought to be related to the natural route of viral entry into cells. In this review, we will describe the current knowlegde on the mechanisms involved in flavivirus cell entry and discuss how antibodies may influence the course of infection towards neutralization or enhancement of viral disease.

Ultrastructural study of West Nile virus pathogenesis in Culex pipiens quinquefasciatus (Diptera: Culicidae)

The ultrastructural features of West Nile virus (WNV) replication and dissemination in orally infected Culex pipiens quinquefasciatus Say were analyzed over a 25-d infection period. To investigate the effects of virus replication on membrane induction, cellular organization, and cell viability in midgut and salivary gland tissues, midguts were dissected on days 3, 7, 14, and 21, and salivary glands were collected on days 7, 14, 21, and 25 postinfection (d.p.i.) for examination by transmission electron microscopy (TEM). Whole mosquito heads were embedded for TEM analysis 14 d.p.i. to localize WNV particles and to investigate the effects of replication on nervous tissues of the brain. Membrane proliferation was induced by WNV in the midgut epithelium, midgut muscles, and salivary glands, although extensive endoplasmic reticulum swelling was a unique feature of salivary gland infection. TEM revealed WNV-induced pathology in salivary glands at 14, 21, and 25 d.p.i., and we hypothesize that long-term virus infection of this tissue results in severe cellular degeneration and apoptotic-like cell death. This finding indicates that the efficiency of WNV transmission may decrease with mosquito age postinfection.

West Nile Virus Dissemination and Tissue Tropisms in Orally InfectedCulex pipiens quinquefasciatus

We investigated the spatial and temporal distribution of West Nile virus (WNV) in organs and tissues of Culex pipiens quinquefasciatus mosquitoes for up to 27 days following oral infection. WNV antigen was detected in paraffin-embedded mosquitoes by immunohistochemistry. Immunofluorescence assays were performed on dissected salivary glands and midguts and analyzed by confocal microscopy. We evaluated the route of virus dissemination following midgut escape and the relative importance of amplifying tissues in mosquito susceptibility to infection. WNV infection was persistent in all tissues analyzed including the midgut, salivary glands, nervous system, and fat body and only declined in the cytoplasm of posterior midgut epithelial cells over time. The phenomenon of cell-to-cell spread was observed in the midgut epithelium and WNV intensely infected both circular and longitudinal muscles of the same organ. It is possible that muscle tissue serves as a conduit for virus dissemination and contributes to WNV amplification, particularly late in infection. These findings provide insight into WNV infection dynamics in a highly susceptible, epidemiologically important, North American vector. Our results pave the way for future studies to analyze physical and biological barriers to WNV dissemination in less competent mosquitoes.

Flavivirus susceptibility in Aedes aegypti

Aedes aegypti is the primary vector of yellow fever (YF) and dengue fever (DF) flaviviruses worldwide. In this review we focus on past and present research on genetic components and environmental factors in Aedes aegypti that appear to control flavivirus transmission. We review genetic relationships among Ae. aegypti populations throughout the world and discuss how variation in vector competence is correlated with overall genetic differences among populations. We describe current research into how genetic and environmental factors jointly affect distribution of vector competence in natural populations. Based on this information, we propose a population genetic model for vector competence and discuss our recent progress in testing this model. We end with a discussion of approaches being taken to identify the genes that may control flavivirus susceptibility in Ae. aegypti.

Potential role of Armigeres subalbatus (Diptera: Culicidae) in the transmission of Japanese encephalitis virus in the absence of rice culture on Liu-chiu islet, Taiwan

Mosquitoes known to be involved in the transmission of Japanese encephalitis virus (JE) on Taiwan typically develop in rice fields. However, recent serological evidence indicated that JE virus was being transmitted on Liu-Chiu, a rice-free islet. To identify the mosquito vector in this unusual epidemiological situation, 4 mosquito species commonly found in Liu-Chiu were evaluated for their vector competence for a strain of JE (CH1392) virus isolated from central Taiwan. Armigeres subalbatus (Coquillett) was the most susceptible species, indicating its status as a potential vector. In addition, an isolate of JE (T1P1) virus from Ar. subalbatus collected on Liu-Chiu readily infected the salivary glands of orally infected Ar. subalbatus originating from Liu-Chiu. The infection rate reached 79% (11/14) after a 20-d period of extrinsic incubation at 28 degrees C. We conclude that JE likely was transmitted between vertebrate hosts by Ar. subalbatus in this rice-free islet and that this species should be considered as a potential vector in similar ecological conditions.

Dengue 3 virus distribution in the mosquito Aedes aegypti: an immunocytochemical study

The dissemination of dengue (DEN) 3 virus in parenterally infected female Aedes aegypti mosquitoes was studied immunocytochemically. Antigen was first detected in fat body cells near the thoracic site of virus inoculation. The intussuscepted foregut, salivary glands and nervous tissue were the first major tissues infected. Nervous tissue appeared to be the primary site of amplification. Muscles, tracheae, Malphigian tubules and the posterior midgut did not become infected. The only part of the reproductive system to be infected was the calyx (71% of specimens 16-22 days post-infection) consistent with low rates of vertical transmission. After 7 days post-inoculation the salivary glands of 100% of the specimens examined were infected. Virus dissemination was slow and the most common sequence of infection following intrathoracic inoculation was as follows: thoracic fat body, intussuscepted foregut, salivary glands, cardial epithelium, thoracic ganglion, brain, compound eye, anterior midgut, intermediate midgut/anterior abdominal ganglia, and calyx/hindgut/posterior abdominal ganglia. Fat body and intussuscepted foregut tissues lost infections after 16 days post-inoculation.

Reduced survival of adult Culex pipiens infected with Rift Valley fever virus

The effect of Rift Valley fever (RVF) viral infection on the survival of female Culex pipiens was examined. In 3 experiments in which mosquitoes ingested RVF virus, there was a 44% decrease in survival to days 14-16 for transmitting vs. nontransmitting mosquitoes, and a 48% decrease in survival for individuals with disseminated vs. nondisseminated infections. These results were corroborated by other experiments in which survival of mosquitoes intrathoracically inoculated with RVF virus was compared with that of those inoculated with diluent. In both the per os and inoculation tests, uninfected mosquitoes survived significantly longer than infected mosquitoes. Even though mosquitoes with disseminated infections had a lower survival rate than did uninfected mosquitoes, dissemination and transmission rates were similar at days 7 and 14-18 after the infectious bloodmeal. This suggests that nondisseminated individuals were developing disseminated infections and becoming capable of transmitting virus between days 7 and 14-18 at approximately the same rate older transmitters were dying. The decreased survival associated with RVF viral infection should be considered in predictive models of this disease.

The pathogenesis of Japanese encephalitis virus in Culex tritaeniorhynchus mosquitoes

Culex tritaeniorhynchus were inoculated intrathoracically with mosquito and human strains of Japanese encephalitis virus and maintained at 26 degrees C or 32 degrees C. Virus titration and localization of viral antigen by avidin-biotin immunoperoxidase staining were done at intervals up to 21 days. Marked differences were noted in the tempo of organ involvement at the 2 temperatures; at both there was initial infection of fat body cells followed by selective infection of the central nervous system (CNS), with consistent infection of cells of the compound eye, patchy involvement of cephalic, thoracic and abdominal ganglia and no infection of Johnston's organ. CNS infection was always present 4 days after infection, when salivary gland involvement was first seen at 32 degrees C; at 26 degrees C CNS infection preceded salivary gland infection by 2 weeks. Late involvement of gut cells, pericardial cells and oviducts was also found, with no involvement of muscle.

Western equine encephalomyelitis virus: in vivo infection and morphogenesis in mosquito mesenteronal epithelial cells

The infection and morphogenetic events associated with the replication of Western equine encephalomyelitis (WEE) virus within the mesenterons of Aedes dorsalis and three strains of Culex tarsalis are compared and contrasted. WEE virus apparently penetrates mesenteronal epithelial cells in vivo through membrane fusion. Profiles of apparent membrane fusion events were observed between virus particles and the microvillar surface of the mesenteron and naked nucleocapsids are observed intracellularly along the apical margin of the mesenteronal epithelial cell within 3 h of ingestion of the bloodmeal. Further, no viral particles were found in association with endocytotic nor lysosomal vacuoles during the initial phases of infection. In those strains of Cx. tarsalis that supported viral replication and in Ae. dorsalis, accumulations of nucleocapsids and maturation of WEE virus were evident along basolateral membranes of the mesenteron by 22-24 h after ingestion of the blood-meal. Maximal extracellular nascent virus occurred between 30-36 hrs. The Knights Landing strain of Cx. tarsalis revealed no subcellular morphological alteration in response to infection throughout the period of study. However, distinct morphological structures associated with the infection were observed in strains or species with enhanced susceptibility compared to Knights Landing (i.e., Cx. tarsalis WS-3 and Ae. dorsalis). In both, apical accumulations of nucleocapsids were apparent by 29 h post infection. These nucleocapsids were most often embedded in a rather amorphous matrix and occasionally in association with membrane profiles; presumably endoplasmic reticulum. Ae. dorsalis also demonstrated some alterations in response to WEE viral infection that were unique relative to Cx. tarsalis and some of these may be considered cytopathological. First, progeny virions were observed repeatedly within lysosomal figures. Second, extensive cytoplasmic vacuolization was noted and occasionally it appeared that these vacuolated cells were being sloughed off into the lumen of the mesenteron.