Brugia malayi microfilariae (Nematoda: Filaridae) enhance the infectivity of Venezuelan equine encephalitis virus to Aedes mosquitoes (Diptera: Culicidae).

Immune interactions between mosquitoes and their hosts

Abstract: The intimate contact between mosquitoes and the immune system of their hosts is generally not considered important because of the transient nature of mosquito feeding. However, when hosts are exposed to many feeding mosquitoes, they develop immune responses against a range of salivary antigens. Understanding the importance of these responses will provide new tools for monitoring vector populations and identifying individuals at risk of mosquito-borne diseases, and allow the development of novel methods for monitoring control and mosquito-release programmes. Antibodies targeting the mosquito midgut are also important in the development of mosquito vaccines. The feasibility of this approach has been demonstrated and future research opportunities are considered in this review. The potential impact of mosquito vaccines is also discussed. Our understanding of the interplay between mosquitoes and the immune system of their hosts is still in its infancy, but it is clear that there is great potential for exploiting this interplay in the control of mosquito-borne diseases.

Mosquito appetite for blood is stimulated by Plasmodium chabaudi infections in themselves and their vertebrate hosts.

Abstract: Arthropod vectors of disease may encounter more than one infected host during the course of their lifetime. The consequences of super-infection to parasite development are rarely investigated, but may have substantial epidemiological and evolutionary consequences. Using a rodent malaria model system, behavioural avoidance of super-infection was tested by examining whether already-infected Anopheles stephensi mosquitoes were less responsive to new vertebrate hosts if they were infected. Additionally, a second dose of parasites was given to malaria-infected mosquitoes on a biologically realistic time scale to test whether it impeded the development of a first infection. No effect of a second infected blood meal on either the prevalence or parasite burden arising from a first was found. Furthermore, it was found that not only were infected mosquitoes more likely to take a second blood meal than their uninfected counterparts, they were disproportionately drawn to infected hosts. The alterations in mosquito feeding propensity reported here would occur if parasites have been selected to make infected vertebrate hosts more attractive to mosquitoes, and infected mosquitoes are more likely to seek out new blood meals. Although such a strategy might increase the risk of super-infection, this study suggests the cost to parasite development is not high and as such would be unlikely to outweigh the potential benefits of increasing the contact rate between the parasite's two obligate hosts.

Infection patterns of o'nyong nyong virus in the malaria-transmitting mosquito, Anopheles gambiae.

Abstract: Arthropod-borne alphaviruses transmitted by mosquitoes almost exclusively use culicines; however, the alphavirus o'nyong-nyong (ONNV) has the unusual characteristic of being transmitted primarily by anopheline mosquitoes This unusual attribute makes ONNV a valuable tool in the characterization of mosquito determinants of infection as well as a useful expression system in Anopheles species We developed a series of recombinant alphaviruses, based upon the genome of ONNV, designed for the expression of heterologous genes The backbone genome is a full-length infectious cDNA clone of ONNV from which wild-type virus can be rescued Additional constructs are variants of the primary clone and contain the complete genome plus a duplicated subgenomic promoter element with a multiple cloning site for insertion of heterologous genes We inserted a green fluorescent protein (GFP) gene downstream of this promoter and used it to characterize infection and dissemination patterns of ONNV within An gambiae mosquitoes These experiments allowed us to identify atypical sites of initial infection and dissemination patterns in this mosquito species not frequently observed in comparable culicine infections The utility of these ONNVs for studies in anopheline mosquitoes includes the potential for identification of vector infection determinants and to serve as tools for antimalaria studies Viruses that can express a heterologous gene in a vector and rapidly and efficiently infect numerous tissues in An gambiae mosquitoes will be a valuable asset in parasite-mosquito interaction and interference research

Potential for Mosquitoes (Diptera: Culicidae) From Florida to Transmit Rift Valley Fever Virus

Abstract: We evaluated Aedes atlanticus Dyar and Knab, Aedes infirmatus Dyar and Knab, Aedes vexans (Meigen), Anopheles crucians Wiedemann, Coquillettidia perturbans (Walker), Culex nigripalpus Theobald, Mansonia dyari Belkin, Heinemann, and Page, and Psorophora ferox (Von Humboldt) from Florida to determine which of these species should be targeted for control should Rift Valley fever virus (RVFV) be detected in North America. Female mosquitoes that had fed on adult hamsters inoculated with RVFV were incubated for 7-21 d at 26 degrees C, then allowed to refeed on susceptible hamsters, and tested to determine infection, dissemination, and transmission rates. We also inoculated mosquitoes intrathoracically, held them for 7 d, and then allowed them to feed on a susceptible hamster to check for a salivary gland barrier. When exposed to hamsters with viremias > or = 10(7.6) plaque-forming units per milliliter of blood, at least some individuals in each of the species tested became infected; however, Cx. nigripalpus, An. crucians, and Ae. infirmatus were essentially incompetent vectors in the laboratory because of either a midgut escape or salivary gland barrier. Each of the other species should be considered as potential vectors and would need to be controlled if RVFV were introduced into an area where they were found. Additional studies need to be conducted with other geographic populations of these species and to determine how environmental factors affect transmission.

Pathogenesis of Rift Valley fever virus in mosquitoes--tracheal conduits & the basal lamina as an extra-cellular barrier.

Abstract: Knowledge of the fate of an arbovirus in a mosquito is fundamental to understanding the mosquito’s competence to transmit the virus When a competent mosquito ingests viremic vertebrate blood, virus infects midgut epithelial cells and replicates, then disseminates to other tissues, including salivary glands and/or ovaries The virus is then transmitted to the next vertebrate host horizontally via bite and/or vertically to the mosquito’s offspring Not all mosquitoes that ingest virus become infected or, if infected, transmit virus Several “barriers” to arbovirus passage, and ultimately transmission, have been identified in incompetent or partially competent mosquitoes, including, among others, gut escape barriers and salivary gland infection barriers The extra-cellular basal lamina around the midgut epithelium and the basal lamina that surrounds the salivary glands may act as such barriers Midgut basal lamina pore sizes are significantly smaller than arboviruses and ultrastructural evidence suggests that midgut tracheae and tracheoles may provide a means for viruses to circumvent this barrier Further, immunocytochemical evidence indicates the existence of a salivary gland infection barrier in Anopheles stephensi The basal lamina may prevent access to mosquito cell surface virus receptors and help explain why anopheline mosquitoes are relatively incompetent arbovirus transmitters when compared to culicines

Multiparasite communities in animals and humans: frequency, structure and pathogenic significance.

Abstract: Individual humans and animals are subject to infection by a variety of parasites (broadly defined to include viruses, bacteria and other non-protozoan microparasites) at any one time Multiple parasite infections occur frequently in populations of wild animals as well as in humans from developing countries In some species and regions, hosts with multiple infections are more common than hosts with either no infection or a single infection Studies, predominantly on animals, show that a wide variety of environmental and host-dependent factors can influence the structure and dynamics of the communities of parasites that make up these multiple infections In addition, synergistic and competitive interactions can occur between parasite species, which can influence the likelihood of their successful transmission to other hosts and increase or decrease their overall pathogenic impact This review summarises aspects of our current knowledge on the frequency of multiparasite infections, the factors which influence them, and their pathogenic significance

In vitro synthesis of infectious venezuelan equine encephalitis virus RNA from a cDNA clone: analysis of a viable deletion mutant.

Abstract: A molecular clone of Venezuelan equine encephalitis virus (VEE) was constructed from four cDNAs that were synthesized using the viral RNA genome as template. Together, these cDNAs are believed to represent all but the nine 5'-terminal nucleotides of the VEE genome sequence. A T7 promoter, followed by a single intervening G residue, and the exact 5'-terminus of VEE were added to the 5'-most clone using in vitro mutagenesis. Appropriate restriction fragments isolated from the cloned cDNAs were joined to form a candidate full-length VEE cDNA clone. RNA transcripts synthesized in vitro from the cDNA clone were able to initiate a productive infection in DEAE-dextran-treated chicken embryo fibroblasts (CEF). VEE antigens were demonstrated in RNA-transfected cells, and supernatants from transfected cultures contained infectious virus particles. The candidate full-length cDNA clone lacked 102 nucleotides of the VEE genome sequence. The deletion, which also was present in the genomes of progeny virions derived from the clone, did not appear to affect growth in cultured CEF, baby hamster kidney cells, or Vero cells. The site of the deletion was mapped to the 3'-end of the nsP3 gene by comparison to other alphavirus sequences. In this region, the VEE genome sequence includes two tandem 102-nucleotide repeats which can be arranged in a stable stem and loop structure. The sequence remaining in the deleted clone retains one copy of the duplicated sequence and, in addition, faithfully preserves a portion of the predicted stem.

Replication and Dissemination of Rift Valley Fever Virus in Culex Pipiens

Abstract: Following ingestion of 10(4.2) to 10(7.2) plaque-forming units (PFU) of Rift Valley fever (RVF) virus, 662 of 850 female Culex pipiens (78%) became infected. Those mosquitoes that became infected separated into two distinct groups: 1) those with a nondisseminated infection limited to the gut, and 2) those with a disseminated infection. The former group contained a median of 10(3.2) PFU, while those females with a disseminated infection contained a median of 10(5.5) PFU. Only those females with a disseminated infection were capable of transmitting virus by bite to a susceptible hamster. This is consistent with a mesenteronal escape barrier to viral dissemination. Following intrathoracic inoculation of RVF virus, all females developed a disseminated infection (26/26) and successfully transmitted virus by bite (49/49) if allowed to feed on a susceptible hamster or suckling mouse. Examination of legs and bodies separately provided a rapid and efficient method of determining dissemination status.

The Effect of Laboratory Colonization on the Vector-Pathogen Interactions of Egyptian Culex pipiens and Rift Valley Fever Virus

Abstract: : Field and laboratory findings implicated Culex pipiens as a vector of Rift Valley fever (RVF) virus during the 1977-1978 epizoodtics/epidemics in Egypt. This study evaluated changes in infection and transmission rates, and viral titters in F sub 1 through F sub 16 generation Cx. pipiens mosquitoes orally infected with RVF virus. Infection and transmission rates of RVF virus by this species changed significantly during the colonization process. However, the ultimate viral titers of either the transmitting or the infected nontransmitting mosquitoes were not affected by the colonization process. Following ingestion of virus, Cx. pipiens could be separated into three distinct subpopulations, an uninfected group and two types of infected mosquitoes - transmitters and nontransmitters. Transmitters contained significantly more virus (approximately 100-fold) than nontransmitters. These results demonstrated that not every infected female mosquito should be considered a competent vector, even if the species (population) is known to be a primary vector. Transmission was also accomplished by probing mosquitoes which were unsuccessful in obtaining a blood meal. These data document the long-held suspicion that vector competence studies based upon laboratory-colonized specimens may not represent the field situation.