The Nurse of Parasites: Gender Concepts in Patrick Manson's Parasitological Research

Patrick Manson (1844-1922), the so-called father of tropical medicine, played a pivotal role in making that discipline into a specialty. During his early career in China he discovered that the mosquito was the intermediate host of the filarial parasite and he somewhat peculiarly called the mosquito the "nurse" of the filarial worm. The discovery contributed greatly to the intellectual foundation of modern parasitology. In this paper I situate Manson's nomenclature in the context of nineteenth-century biological research on reproductive mech-anisms and argue that Manson's concept of the "nurse" was derived from nineteenth-century theories of sexual division of labor in nature's economy. The way he framed the relation between the mosquito and the parasite, moreover, can be understood in the terms of the domestic arrangement of the colonial European household. Manson's research demonstrates the significant exchange between medical concerns over European women's procreative role in the tropics and biological studies of parasitic reproduction.     

Prior exposure to uninfected mosquitoes enhances mortality in naturally-transmitted West Nile virus infection

Background

The global emergence of West Nile virus (WNV) has highlighted the importance of mosquito-borne viruses. These are inoculated in vector saliva into the vertebrate skin and circulatory system. Arthropod-borne (arbo)viruses such as WNV are transmitted to vertebrates as an infectious mosquito probes the skin for blood, depositing the virus and saliva into the skin and circulation. Growing evidence has demonstrated that arthropod, and recently mosquito, saliva can have a profound effect on pathogen transmission efficiency, pathogenesis, and disease course. A potentially important aspect of natural infections that has been ignored is that in nature vertebrates are typically exposed to the feeding of uninfected mosquitoes prior to the mosquito that transmits WNV. The possibility that pre-exposure to mosquito saliva might modulate WNV infection was explored.

Principal Findings

Here we report that sensitization to mosquito saliva exacerbates viral infection. Prior exposure of mice to mosquito feeding resulted in increased mortality following WNV infection. This aggravated disease course was associated with enhanced early viral replication, increased interleukin-10 expression, and elevated influx of WNV-susceptible cell types to the inoculation site. This exacerbated disease course was mimicked by passive transfer of mosquito-sensitized serum.

Significance

This is the first report that sensitization to arthropod saliva can exacerbate arthropod-borne infection, contrary to previous studies with parasite and bacteria infections. This research suggests that in addition to the seroreactivity of the host to virus, it is important to take into account the immune response to vector feeding.     

The multifaceted mosquito anti-Plasmodium response

Plasmodium development within its mosquito vector is an essential step in malaria transmission, as illustrated in world regions where malaria was successfully eradicated via vector control. The innate immune system of most mosquitoes is able to completely clear a Plasmodium infection, preventing parasite transmission to humans. Understanding the biological basis of this phenomenon is expected to inspire new strategies to curb malaria incidence in countries where vector control via insecticides is unpractical, or inefficient because insecticide resistance genes have spread across mosquito populations. Several aspects of mosquito biology that condition the success of the parasite in colonizing its vector begin to be understood at the molecular level, and a wealth of recently published data highlights the multifaceted nature of the mosquito response against parasite invasion. In this brief review, we attempt to provide an integrated view of the challenges faced by the parasite to successfully invade its mosquito host, and discuss the possible intervention strategies that could exploit this knowledge for the fight against human malaria.     

Interspecific transfer of Wolbachia into the mosquito disease vector Aedes albopictus

Intracellular Wolbachia bacteria are obligate, maternally inherited endosymbionts found frequently in insects and other invertebrates. The evolutionary success of Wolbachia is due in part to an ability to manipulate reproduction. In mosquitoes and many other insects, Wolbachia causes a form of sterility known as cytoplasmic incompatibility (CI). Wolbachia-induced CI has attracted interest as a potential agent for affecting medically important disease vectors. However, application of the approach has been restricted by an absence of appropriate, naturally occurring Wolbachia infections. Here, we report the interspecific transfer of Wolbachia infection into a medically important mosquito. Using embryonic microinjection, Wolbachia is transferred from Drosophila simulans into the invasive pest and disease vector: Aedes albopictus (Asian tiger mosquito). The resulting infection is stably maintained and displays a unique pattern of bidirectional CI in crosses with naturally infected mosquitoes. Laboratory population cage experiments examine a strategy in which releases of Wolbachia-infected males are used to suppress mosquito egg hatch. We discuss the results in relation to developing appropriate Wolbachia-infected mosquito strains for population replacement and population suppression strategies.     

Parasitoid-induced behavioral alterations of Aedes aegypti mosquito larvae infected with mermithid nematodes (Nematoda: Mermithidae).

A wide range of parasites are known to cause behavioral changes in their hosts and parasitized insects are especially amenable to the study of such changes. The majority of studies addressing parasite-induced behavioral alterations have focused on parasites with complex life cycles and the adaptive nature of such changes. Behavioral changes caused by parasitoids, single-host parasites that kill their host upon emergence, have been studied less and the adaptive nature of these changes is likely to be different than those in complex life cycles. I investigated behavioral alterations in Aedes aegypti mosquito larvae infected with parasitoid nematodes (family Mermithidae). I conducted several experiments in which I tested the following hypotheses: 1) Mermithid nematodes induce behavioral changes in mosquito larvae and the changes are density dependent. 2) Different species of mermithid nematodes induce similar changes in mosquito larvae behavior. 3) Behavioral alterations vary with mermithid developmental stage. 4) Mosquito larvae infected with mermithid nematodes behave similarly to uninfected food-deprived mosquito larvae. I found that 4th instar Ae. aegypti infected with Romanomermis culicivorax or Strelkovimermis spiculatus exhibited resting behaviors significantly more often than uninfected controls but that intensity of infection did not affect activity levels. In earlier instars, infected mosquito larvae were more active than uninfected control larvae in some behaviors associated with feeding. There was no significant difference between infected and uninfected food-deprived mosquitoes in nine of the ten behaviors observed. The decrease in activity of late instar Ae. aegypti larvae infected with mermithids may be a parasitoid adaptation that reduces the risk of predation and thus increases host and parasitoid survival. The increase in feeding activity in earlier instars as well as the similarity between uninfected food-deprived and infected Ae. aegypti behavior may indicate that these behaviors are adaptive for the parasitoid, increasing nutritional acquisition for successful parasitoid development.     

Mosquito (Diptera: Culicidae) fauna in inland areas of south-west Western Australia

Abstract  Previously existing sources of data regarding mosquitoes in inland areas of south-west Western Australia are few and qualitative in nature. This is the first attempt to quantitatively investigate mosquito fauna in this region. The existing data are reviewed and compared with the results of the quantitative surveys presented in this paper. Temporal comparisons appear to indicate that mosquito community structure in the region may have changed since the initial surveys in the 1950s from a combination of freshwater-breeding species towards a strong dominance of Aedes camptorhynchus (Thomson), a major vector of Ross River virus in southern Australia. It is speculated that this shift may have been brought about by the increasing area and severity of dryland salinity in the region over the last century, and also may increase the potential for Ross River virus disease transmission.     

A spatial model of mosquito host-seeking behavior

Mosquito host-seeking behavior and heterogeneity in host distribution are important factors in predicting the transmission dynamics of mosquito-borne infections such as dengue fever, malaria, chikungunya, and West Nile virus. We develop and analyze a new mathematical model to describe the effect of spatial heterogeneity on the contact rate between mosquito vectors and hosts. The model includes odor plumes generated by spatially distributed hosts, wind velocity, and mosquito behavior based on both the prevailing wind and the odor plume. On a spatial scale of meters and a time scale of minutes, we compare the effectiveness of different plume-finding and plume-tracking strategies that mosquitoes could use to locate a host. The results show that two different models of chemotaxis are capable of producing comparable results given appropriate parameter choices and that host finding is optimized by a strategy of flying across the wind until the odor plume is intercepted. We also assess the impact of changing the level of host aggregation on mosquito host-finding success near the end of the host-seeking flight. When clusters of hosts are more tightly associated on smaller patches, the odor plume is narrower and the biting rate per host is decreased. For two host groups of unequal number but equal spatial density, the biting rate per host is lower in the group with more individuals, indicative of an attack abatement effect of host aggregation. We discuss how this approach could assist parameter choices in compartmental models that do not explicitly model the spatial arrangement of individuals and how the model could address larger spatial scales and other probability models for mosquito behavior, such as Lévy distributions.     

Fitness advantage and cytoplasmic incompatibility in Wolbachia single- and superinfected Aedes albopictus

Wolbachia are obligate, maternally inherited, intracellular bacteria that infect numerous insects and other invertebrates. Wolbachia infections have evolved multiple mechanisms to manipulate host reproduction and facilitate invasion of naive host populations. One such mechanism is cytoplasmic incompatibility (CI) that occurs in many insect species, including Aedes albopictus (Asian tiger mosquito). The multiple Wolbachia infections that occur naturally in A. albopictus make this mosquito a useful system in which to study CI. Here, experiments employ mosquito strains that have been introgressed to provide genetically similar strains that harbor differing Wolbachia infection types. Cytoplasmic incompatibility levels, host longevity, egg hatch rates, and fecundity are examined. Crossing results demonstrate a pattern of additive unidirectional cytoplasmic incompatibility. Furthermore, relative to uninfected females, infected females are at a reproductive advantage due to both cytoplasmic incompatibility and a fitness increase associated with Wolbachia infection. In contrast, no fitness difference was observed in comparisons of single- and superinfected females. We discuss the observed results in regard to the evolution of the Wolbachia/A. albopictus symbiosis and the observed pattern of Wolbachia infection in natural populations.     

Dissemination barriers to Ross River virus in Aedes vigilax and the effects of larval nutrition on their expression

Effects of larval nutrition on vector competence of the mosquito Aedes vigilax (Skuse) (Diptera: Culicidae) from Townsville, north Queensland, for Ross River virus (RR) were examined. Larvae were reared on three different diets to create three significantly different size classes of adult mosquito. These were fed on serial dilutions of RR and then sampled on alternate days so the progression of the virus through the mosquito could be examined. No differences of vector competence could be attributed to larval nutrition. Barriers to infection and the correlation between infection rate, viral titre and transmission of RR by Ae. vigilax were also examined. The mesenteronal barrier was the only infection barrier expressed. No correlation between viral titre and transmission was detected, but a strong correlation was found between salivary gland infection and transmission rate. From this it was possible to estimate that 98.7+/-1.3% of Ae. vigilax with infected salivary glands transmit RR.     

昆虫内共生菌沃尔巴克氏体抗病毒研究进展

植物病毒病是危害我国蔬菜生产的第一大病害,而烟粉虱Bemisia tabaci Gennadius、蓟马和蚜虫等小型昆虫是蔬菜病毒病的主要传播媒介.虫传病毒病害的防控策略复杂且难度大,目前生产上主要依赖化学农药防治介体昆虫,预防与控制蔬菜病毒病.种植户化学杀虫药剂的不合理使用、甚至滥用,导致媒介昆虫抗药性、杀虫剂污染与...     

Punch in the gut: parasite tolerance of phytochemicals reflects host diet

Gut parasites of plant-eating insects are exposed to antimicrobial phytochemicals that can reduce infection. Trypanosomatid gut parasites infect insects of diverse nutritional ecologies as well as mammals and plants, raising the question of how host diet-associated phytochemicals shape parasite evolution and host specificity. To test the hypothesis that phytochemical tolerance of trypanosomatids reflects the chemical ecology of their hosts, we compared related parasites from honey bees and mosquitoes – hosts that differ in phytochemical consumption – and contrasted our results with previous studies on phylogenetically related, human-parasitic Leishmania. We identified one bacterial and 10 plant-derived substances with known antileishmanial activity that also inhibited honey bee parasites associated with colony collapse. Bee parasites exhibited greater tolerance of chrysin – a flavonoid found in nectar, pollen and plant resin-derived propolis. In contrast, mosquito parasites were more tolerant of cinnamic acid – a product of lignin decomposition present in woody debris-rich larval habitats. Parasites from both hosts tolerated many compounds that inhibit Leishmania, hinting at possible trade-offs between phytochemical tolerance and mammalian infection. Our results implicate the phytochemistry of host diets as a potential driver of insect–trypanosomatid associations and identify compounds that could be incorporated into colony diets or floral landscapes to ameliorate infection in bees.     

Attraction of mosquitoes to primate odours and implications for zoonotic Plasmodium transmission

Vector-borne diseases often originate from wildlife and can spill over into the human population. One of the most important determinants of vector-borne disease transmission is the host preference of mosquitoes. Mosquitoes with a specialised host preference are guided by body odours to find their hosts in addition to carbon dioxide. Little is known about the role of mosquito host preference in the spillover of pathogenic agents from humans towards animals and vice versa. In the Republic of Congo, the attraction of mosquitoes to primate host odours was determined, as well as their possible role as malaria vectors, using odour-baited traps mimicking the potential hosts of mosquitoes. Most of the mosquito species caught showed a generalistic host preference. Anopheles obscurus was the most abundant Anopheles mosquito, with a generalistic host preference observed from the olfactory response and the detection of various Plasmodium parasites. Interestingly, Culex decens showed a much higher attraction towards chimpanzee odours than to human or cow odours. Human Plasmodium parasites were observed in both human and chimpanzee blood, although not in the Anopheles mosquitoes that were collected. Understanding the role of mosquito host preference for cross-species parasite transmission provides information that will help to determine the risk of spillover of vector-borne diseases.     

Discovery of Mosquito Saliva MicroRNAs during CHIKV Infection

Mosquito borne pathogens are transmitted to humans via saliva during blood feeding. Mosquito saliva is a complex concoction of many secretory factors that modulate the feeding foci to enhance pathogen infection and establishment. Multiple salivary proteins/factors have been identified/characterized that enhance pathogen infection. Here, we describe, for the first time, the identification of exogenous microRNAs from mosquito saliva. MicroRNAs are short, 18–24 nucleotide, non-coding RNAs that regulate gene expression, and are generally intracellular. However, circulating miRNAs have been described from serum and saliva of humans. Exogenous miRNAs have not been reported from hematophagous arthropod saliva. We sought to identify miRNAs in the mosquito saliva and their role in Chikungunya virus (CHIKV) infection. Next generation sequencing was utilized to identify 103 exogenous miRNAs in mosquito saliva of which 31 miRNAs were previously unidentified and were designated novel. Several miRNAs that we have identified are expressed only in the CHIKV infected mosquitoes. Five of the saliva miRNAs were tested for their potential to regulated CHIKV infection, and our results demonstrate their functional role in the transmission and establishment of infection during blood feeding on the host.     

Togavirus-associated pathologic changes in the midgut of a natural mosquito vector.

Arthropod-borne viruses were not previously believed to cause discernible pathologic changes in their natural mosquito vectors. We report cytopathologic lesions in the midgut of the mosquito, Culiseta melanura, 2 to 5 days after oral infection with eastern equine encephalomyelitis virus. Sloughing of densely staining, heavily infected epithelial cells into the midgut lumen was observed by light and transmission electron microscopy, along with degeneration of cells within the epithelium. Pathological changes in midgut epithelial cells sometimes included loss of brush border and basal lamina integrity. Disruption of the midgut basal lamina could result in bypassing of barriers to virus dissemination within the mosquito and allow rapid transmission to occur. Alternatively, luminal sloughing of heavily infected midgut epithelial cells may serve to modulate mosquito infections. These findings challenge previous beliefs regarding the benign nature of arbovirus-invertebrate host relationships.     

Parasite immune escape: new views into host-parasite interactions

For parasites of humans and animals that rely on vectors or on sexual contact for transmission, it is particularly important that infection does not to terminate before the occurrence of the crucial event that completes its lifecycle (e.g. another mosquito bite). For chronic infection to occur, it is essential that the parasite avoids clearance by the host immune system. Much progress has been made in elucidating the immunological interactions and the molecular mechanisms involved in the process of immune evasion. Mathematical models have also been invaluable in understanding these processes, particularly in the generation of new ideas about a complex form of immune evasion known as antigenic variation whereby a major target of the host immune response is varied during the course of a single infection to avoid recognition.     

Exploration of mosquito immunity using cells in culture

The propagation of immune-responsive cells in vitro has provided the basis for substantial contributions to our understanding of many aspects of the mammalian immune response. In contrast, the potential for exploring the innate immune response of insects using cultured cells is only beginning to be developed, particularly with various mosquito cell lines from the genera Aedes and Anopheles. Immune-reactive mosquito cell lines express various defensive factors, including transferrin, lysozyme, cecropin, defensin, and prophenoloxidase activities. In this review, we discuss insect immunity in the context of key concepts that have emerged in the study of the mammalian immune system, with emphasis on the properties of the cells that participate in the immune response. The nature of established cell lines and their contributions to our understanding of immune functions in humans and insects is described, with emphasis on our own work with the C7-10 and Aag-2 mosquito cell lines from Aedes albopictus and Aedes aegypti, respectively. Finally, we offer some speculation on further advances in insect immunology that may be facilitated by work with cells in culture.     

Different mosquito species host Wickerhamomyces anomalus (Pichia anomala): perspectives on vector-borne diseases symbiotic control

The genetic manipulation of the microbial community associated with hematophagus insects is particularly relevant for public health applications. Within mosquito populations, this relationship has been overlooked until recently. New advances in molecular biotechnology propose the genetic manipulation of mosquito symbionts to prevent the transmission of pathogens to humans by interfering with the obligatory life cycle stages within the insect through the use of effector molecules. This approach, defined as 'paratransgenesis', has opened the way for the investigation and characterization of microbes residing in the mosquito body, particularly those localised within the gut. Some interesting bacteria have been identified as candidates for genetic modification, however, endosymbiotic yeasts remain largely unexplored with little information on the symbiotic relationships to date. Here we review the recent report of symbiotic relationship between Wickerhamomyces anomalus (Pichia anomala) and several mosquito vector species as promising methods to implement control of mosquito-borne diseases.     

Dual miRNA Targeting Restricts Host Range and Attenuates Neurovirulence of Flaviviruses

Mosquito-borne flaviviruses are among the most significant arboviral pathogens worldwide. Vaccinations and mosquito population control programs remain the most reliable means for flavivirus disease prevention, and live attenuated viruses remain one of the most attractive flavivirus vaccine platforms. Some live attenuated viruses are capable of infecting principle mosquito vectors, as demonstrated in the laboratory, which in combination with their intrinsic genetic instability could potentially lead to a vaccine virus reversion back to wild-type in nature, followed by introduction and dissemination of potentially dangerous viral strains into new geographic locations. To mitigate this risk we developed a microRNA-targeting approach that selectively restricts replication of flavivirus in the mosquito host. Introduction of sequences complementary to a mosquito-specific mir-184 and mir-275 miRNAs individually or in combination into the 3’NCR and/or ORF region resulted in selective restriction of dengue type 4 virus (DEN4) replication in mosquito cell lines and adult Aedes mosquitos. Moreover a combined targeting of DEN4 genome with mosquito-specific and vertebrate CNS-specific mir-124 miRNA can silence viral replication in two evolutionally distant biological systems: mosquitoes and mouse brains. Thus, this approach can reinforce the safety of newly developed or existing vaccines for use in humans and could provide an additional level of biosafety for laboratories using viruses with altered pathogenic or transmissibility characteristics.