Multiplication and distribution of type 2 dengue and Japanese encephalitis viruses in Toxorhynchites splendens after intrathoracic inoculation

Replication of Dengue Virus 4 in Plodia interpunctella (Lepidoptera: Pyralidae) Larvae Under Laboratory Conditions

Background: Developing methods for the isolation and replication of dengue virus (DENV), based on nonhematophagous insect models to assess virus-host interaction, would contribute, for instance, to the creation of drugs or vaccines and eventually to the control of the disease. In this regard, nonhematophagous mosquitoes have been used as biological hosts for the isolation of DENV because they are specific and sensitive to a low viral load and viral particles with low infectivity. However, implementation of these models is mainly affected by the complexity of the establishment of the entomological colonies. Materials and Methods: In this study, the susceptibility of DENV-4 infection in Plodia interpunctella larvae was evaluated. Ten larvae, previously inoculated with supernatant from DENV-4-infected C6/36 cells, were processed to determine viral replication by the optical density and 2-ΔΔCt methods at different time intervals (1 and 7 days postinoculation). Results: A prospective increase in viral replication was observed, which did not influence the survival and development of P. interpunctella. Conclusion: These results demonstrate the infectivity of DENV-4 in P. interpunctella, thus becoming an option as a biological model for the study of this etiological agent.

Biology and Transmission Dynamics of Aedes flavivirus

Aedes albopictus (Skuse) and Aedes aegypti (Linnaeus) (Diptera: Culicidae) mosquitoes transmit pathogenic arthropod-borne viruses, including dengue, chikungunya, and Zika viruses, with significant global health consequences. Both Ae. albopictus and Ae. aegypti also are susceptible to Aedes flavivirus (AEFV), an insect-specific flavivirus (ISF) first isolated in Japan from Ae. albopictus and Ae. flavopictus. ISFs infect only insect hosts and evidence suggests that they are maintained by vertical transmission. In some cases, ISFs interfere with pathogenic flavivirus infection, and may have potential use in disease control. We explored the host range of AEFV in 4 genera of mosquitoes after intrathoracic injection and observed greater than 95% prevalence in the species of Aedes and Toxorhynchites tested. Anopheles and Culex species were less permissive to infection. Vertical transmission studies revealed 100% transovarial transmission and a filial infection rate of 100% for AEFV in a persistently-infected colony of Ae. albopictus. Horizontal transmission potential was assessed for adult and larval mosquitoes following per os exposures and in venereal transmission experiments. No mosquitoes tested positive for AEFV infection after blood feeding, and infection with AEFV after sucrose feeding was rare. Similarly, 2% of adult mosquitoes tested positive for AEFV after feeding on infected cells in culture as larvae. Venereal transmission of AEFV was most frequently observed from infected males to uninfected females as compared with transmission from infected females to uninfected males. These results reveal new information on the infection potential of AEFV in mosquitoes and expand our understanding of both vertical and horizontal transmission of ISFs.

Toxorhynchites Species: A Review of Current Knowledge

Simple Summary

Mosquitoes are well known to spread diseases when they take a blood meal. However, not all species feed on blood but instead get their nourishment from other sources. One such species is Toxorhynchites, which are a paradox among mosquitoes. These mosquitoes are entirely non-blood feeding and, as a result, are not considered to be harmful to human health. Indeed, since their larvae feed on the larvae of pest species and other aquatic insects, they are a potential counter measure against the spread of mosquito-transmitted diseases. Their effective application has been hampered due to a lack of understanding and inconsistencies in their descriptions. This review aims to build upon previously published information and summarize recent findings to support their use in combating mosquito-transmitted infections.

Abstract

The increasing global incidence of mosquito-borne infections is driving a need for effective control methods. Vector populations have expanded their geographical ranges, while increasing resistance to chemical insecticides and a lack of effective treatments or vaccines has meant that the development of vector control methods is essential in the fight against mosquito-transmitted diseases. This review will focus on Toxorhynchites, a non-hematophagous mosquito genus which is a natural predator of vector species and may be exploited as a biological control agent. Their effectiveness in this role has been strongly debated for many years and early trials have been marred by misinformation and incomplete descriptions. Here, we draw together current knowledge of the general biology of Toxorhynchites and discuss how this updated information will benefit their role in an integrated vector management program.

Wolbachia-Based Dengue Virus Inhibition Is Not Tissue-Specific in Aedes aegypti

Background

Dengue fever, caused by the dengue virus (DENV), is now the most common arbovirus transmitted disease globally. One novel approach to control DENV is to use the endosymbiotic bacterium, Wolbachia pipientis, to limit DENV replication inside the primary mosquito vector, Aedes aegypti. Wolbachia that is naturally present in a range of insects reduces the capacity for viruses, bacteria, parasites and fungi to replicate inside insects. Wolbachia’s mode of action is not well understood but may involve components of immune activation or competition with pathogens for limited host resources. The strength of Wolbachia-based anti DENV effects appear to correlate with bacterial density in the whole insect and in cell culture. Here we aimed to determine whether particular tissues, especially those with high Wolbachia densities or immune activity, play a greater role in mediating the anti DENV effect.

Methodology/findings

Ae. aegypti mosquito lines with and without Wolbachia (Wildtype) were orally fed DENV 3 and their viral loads subsequently measured over two time points post infection in the midgut, head, salivary glands, Malpighian tubules, fat body and carcass. We did not find correlations between Wolbachia densities and DENV loads in any tissue, nor with DENV loads in salivary glands, the endpoint of infection. This is in contrast with strong positive correlations between DENV loads in a range of tissues and salivary gland loads for Wildtype mosquitoes. Lastly, there was no evidence of a heightened role for tissues with known immune function including the fat body and the Malpighian tubules in Wolbachia’s limitation of DENV.

Conclusion/significance

We conclude that the efficacy of DENV blocking in Wolbachia infected mosquitoes is not reliant on any particular tissue. This work therefore suggests that the mechanism of Wolbachia-based antiviral effects is either systemic or acts locally via processes that are fundamental to diverse cell types. We further conclude that the relationship between DENV blocking and Wolbachia density is not linear in mosquito tissues

Dengue fever caused by the dengue virus (DENV) is transmitted by the mosquito, Aedes aegypti. To control the disease, an intracellular bacterium called Wolbachia has been introduced into Ae. aegypti where it blocks/limits success of infection of DENV. The mechanistic basis of blocking is not well understood but may involve Wolbachia activating the host immune system or competing with DENV for host resources. The strength of blocking appears to correlate with Wolbachia density. Here, we aimed to determine if any particular tissues inside the mosquito play a greater role in blocking. Tissues were chosen based on their Wolbachia density and their roles in infection and immunity. Wolbachia infected and uninfected mosquitoes were orally infected with DENV and Wolbachia density and DENV load were assessed in midgut, salivary gland, head, Malpighian tubules, fat body and carcass. Wolbachia density did not correlate with DENV loads in the same tissues nor with DENV loads in the salivary glands. We also showed that no one tissue appeared to play a greater role in blocking. In summary, these finding suggest that in the mosquito a threshold Wolbachia density may be required for DENV blocking. Our findings also suggest that blocking may involve mechanisms that are fundamental to all cells.

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.

Laboratory evaluation of the response of Aedes aegypti and Aedes albopictus uninfected and infected with dengue virus to deet

Laboratory studies were conducted to compare the response of Aedes aegypti (L.) and Aedes albopictus (Skuse) adults, uninfected and infected with four serotypes of dengue virus to a repellent containing 5% N, N-diethyl-3-methylbenzamide (deet). The results showed that mosquitoes infected with the four serotypes of dengue responded similarly to uninfected mosquitoes.

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.