Variation In Vector Competence Research Articles

Molecular identification and genetic analysis of Rhipicephalus sanguineus sensu lato of dogs in Nigeria, West Africa.

The brown dog tick, Rhipicephalus sanguineus sensu lato, is a ubiquitous and taxonomically controversial pest of dogs with immense veterinary and public health significance. Genetic analyses of specimens from various geographical origins reveal intraspecific diversity within the taxon. Little information is available on the genetic characteristics of R. sanguineus s.l. in Nigeria, West Africa. In this study, 460bp of the mitochondrial 16S rDNA gene of R. sanguineus s.l. collected from dogs in different ecological zones of Nigeria was amplified, sequenced and characterized. Phylogenetic and pairwise analyses were used to compare the sequences generated in this study to each other and to sequences in GenBank. The sequences in this study were highly similar (>98%) to each other and clustered with sequences of the R. sanguineus s.l. tropical lineage in GenBank. None of the sequences in this study clustered with the 'southeastern Europe' or temperate lineage. The mean intraspecific divergence among R. sanguineus s.l. in this study was 1.7% (range: 0-8.0%). Furthermore, the sequences in this study showed mean divergence of 1.5% (0-10%), 5.0% (3.8-13.9%) and 9.7% (6.9-19.8%) from sequences of the tropical, southeastern Europe and temperate lineages, respectively. Interestingly, sequences in this study showed a mean divergence of 9.3% (1.0-17.8%) from the Rhipicephalus sp. morphotype 4 (GenBank acc. nr. KC243850) earlier identified from cattle in Nigeria, suggesting diversity in this taxon in Nigeria. Further studies are needed to elucidate the veterinary and public health significance of R. sanguineus s.l. in Nigeria taking into cognizance the existence of intraspecific variation in vector competence.

Host blood-meal source has a strong impact on gut microbiota of Aedes aegypti.

Gut microbial communities of mosquitoes can influence vector susceptibility to pathogens, yet the factors that govern their composition remain poorly understood. We investigated the impact of host blood-meal source on gut microbiota of Aedes aegypti L. Adult mosquitoes were fed on human, rabbit or chicken blood and their gut microbiota compared to those of sugar-fed and newly emerged adults. Microbial diversity was significantly reduced in blood-fed and sugar-fed mosquitoes but was restored to the levels of newly emerged adults at 7-days post-blood meal. Microbial composition was strongly influenced by host blood-meal source. Leucobacter spp., Chryseobacterium spp., Elizabethkingia spp. and Serratia spp. were characteristic of newly emerged adults and adults fed on chicken, rabbit and human blood, respectively. Sugar-fed mosquitoes had higher abundance of Pseudomonas spp. and unclassified Acetobacteraceae. Shifts in gut microbial communities in response to host blood-meal source may fundamentally impact pathogen transmission given the well-documented link between specific bacterial taxa and vector susceptibility to a variety of mosquito-borne pathogens and may be a key determinant of individual and population variation in vector competence.

Genetic Variation in Insect Vectors: Death of Typology?

The issue of typological versus population thinking in biology is briefly introduced and defined. It is then emphasized how population thinking is most relevant and useful in vector biology. Three points are made: (1) Vectors, as they exist in nature, are genetically very heterogeneous. (2) Four examples of how this is relevant in vector biology research are presented: Understanding variation in vector competence, GWAS, identifying the origin of new introductions of invasive species, and resistance to inbreeding. (3) The existence of high levels of vector genetic heterogeneity can lead to failure of some approaches to vector control, e.g., use of insecticides and release of sterile males (SIT). On the other hand, vector genetic heterogeneity can be harnessed in a vector control program based on selection for refractoriness.

Transmission risk of two chikungunya lineages by invasive mosquito vectors from Florida and the Dominican Republic.

Between 2014 and 2016 more than 3,800 imported human cases of chikungunya fever in Florida highlight the high risk for local transmission. To examine the potential for sustained local transmission of chikungunya virus (CHIKV) in Florida we tested whether local populations of Aedes aegypti and Aedes albopictus show differences in susceptibility to infection and transmission to two emergent lineages of CHIKV, Indian Ocean (IOC) and Asian genotypes (AC) in laboratory experiments. All examined populations of Ae. aegypti and Ae. albopictus mosquitoes displayed susceptibility to infection, rapid viral dissemination into the hemocoel, and transmission for both emergent lineages of CHIKV. Aedes albopictus had higher disseminated infection and transmission of IOC sooner after ingesting CHIKV infected blood than Ae. aegypti. Aedes aegypti had higher disseminated infection and transmission later during infection with AC than Ae. albopictus. Viral dissemination and transmission of AC declined during the extrinsic incubation period, suggesting that transmission risk declines with length of infection. Interestingly, the reduction in transmission of AC was less in Ae. aegypti than Ae. albopictus, suggesting that older Ae. aegypti females are relatively more competent vectors than similar aged Ae. albopictus females. Aedes aegypti originating from the Dominican Republic had viral dissemination and transmission rates for IOC and AC strains that were lower than for Florida vectors. We identified small-scale geographic variation in vector competence among Ae. aegypti and Ae. albopictus that may contribute to regional differences in risk of CHIKV transmission in Florida.

Mosquito Microbiome Dynamics, a Background for Prevalence and Seasonality of West Nile Virus.

Symbiotic microbial communities augment host phenotype, including defense against pathogen carriage and infection. We sampled the microbial communities in 11 adult mosquito host species from six regions in southern Ontario, Canada over 3 years. Of the factors examined, we found that mosquito species was the largest driver of the microbiota, with remarkable phylosymbiosis between host and microbiota. Seasonal shifts of the microbiome were consistently repeated over the 3-year period, while region had little impact. Both host species and seasonal shifts in microbiota were associated with patterns of West Nile virus (WNV) in these mosquitoes. The highest prevalence of WNV, with a seasonal spike each year in August, was in the Culex pipiens/restuans complex, and high WNV prevalence followed a decrease in relative abundance of Wolbachia in this species. Indeed, mean temperature, but not precipitation, was significantly correlated with Wolbachia abundance. This suggests that at higher temperatures Wolbachia abundance is reduced leading to greater susceptibility to WNV in the subsequent generation of C. pipiens/restuans hosts. Different mosquito genera harbored significantly different bacterial communities, and presence or abundance of Wolbachia was primarily associated with these differences. We identified several operational taxonomic units (OTUs) of Wolbachia that drive overall microbial community differentiation among mosquito taxa, locations and timepoints. Distinct Wolbachia OTUs were consistently found to dominate microbiomes of Cx. pipiens/restuans, and of Coquilletidia perturbans. Seasonal fluctuations of several other microbial taxa included Bacillus cereus, Enterococcus, Methylobacterium, Asaia, Pantoea, Acinetobacter johnsonii, Pseudomonas, and Mycoplasma. This suggests that microbiota may explain some of the variation in vector competence previously attributed to local environmental processes, especially because Wolbachia is known to affect carriage of viral pathogens.

Distinct variation in vector competence among nine field populations of Aedes aegypti from a Brazilian dengue-endemic risk city.

BackgroundIn Brazil, dengue epidemics erupt sporadically throughout the country and it is unclear if outbreaks may initiate a sustainable transmission cycle. There are few studies evaluating the ability of Brazilian Aedes aegypti populations to transmit dengue virus (DENV). The aim of this study was to compare DENV susceptibility of field-captured Ae. aegypti populations from nine distinct geographic areas of the city of Belo Horizonte in 2009 and 2011. Infection Rate (IR), Vector Competence (VC) and Disseminated Infection Rate (DIR) were determined.MethodsAedes aegypti eggs from each region were collected and reared separately in an insectary. Adult females were experimentally infected with DENV-2 and the virus was detected by qPCR in body and head samples. Data were analyzed with the Statistical Package for the Social Sciences version 17.ResultsIR varied from 40.0% to 82.5% in 2009 and 60.0% to 100.0% in 2011. VC ranged from 25.0% to 77.5% in 2009 and 25.0% to 80.0% in 2011. DIR oscillated from 68.7% to 100.0% in 2009 and 38.4% to 86.8 in 2011. When the results were evaluated by a logistic model using IR as covariate, North, Barreiro, South-Central and Venda Nova showed the strongest association in 2009. In 2011, a similar association was observed for South-Central, Venda Nova, West and Northeast regions. Using VC as covariate, South-Central and Venda Nova showed the most relevant association in 2009. In 2011, South-Central, Venda Nova and Barreiro presented the greatest revelation associations. When DIR data were analyzed by logistic regression models, Pampulha, South-Central, Venda Nova, West, Northeast and East (2009) as well as South-Central, Venda Nova and West (2011) were the districts showing the strongest associations.ConclusionsWe conclude that Ae. aegypti populations from Belo Horizonte exhibit wide variation in vector competence to transmit dengue. Therefore, vector control strategies should be adapted to the available data for each region. Further analysis should be conducted to better understand the reasons for this large variability in vector competence and how these parameters correlate with epidemiological findings in subsequent years.

Ambient temperature and dietary supplementation interact to shape mosquito vector competence for malaria

The extent to which environmental factors influence the ability of Anopheles mosquitoes to transmit malaria parasites remains poorly explored. Environmental variation, such as change in ambient temperature, will not necessarily influence the rates of host and parasite processes equivalently, potentially resulting in complex effects on infection outcomes. As proof of principle, we used Anopheles stephensi and the rodent malaria parasite, Plasmodium yoelii, to examine the effects of a range of constant temperatures on one aspect of host defense (detected as alterations in expression of nitric oxide synthase gene – NOS) to parasite infection. We experimentally boosted mosquito midgut immunity to infection through dietary supplementation with the essential amino acid l-Arginine (l-Arg), which increases midgut nitric oxide (NO) levels by infection-induced NOS catalysis in A. stephensi. At intermediate temperatures, supplementation reduced oocyst prevalence, oocyst intensity, and sporozoite prevalence suggesting that the outcome of parasite infection was potentially dependent upon the rate of NOS-mediated midgut immunity. At low and high temperature extremes, however, infection was severely constrained irrespective of supplementation. The effects of l-Arg appeared to be mediated by NO-dependent negative feedback on NOS expression, as evidenced by depressed NOS expression in l-Arg treated groups at temperatures where supplementation decreased parasite infection. These results suggest the need to consider the direct (e.g. effects of mosquito body temperature on parasite physiology) and indirect effects (e.g. mediated through changes in mosquito physiology/immunity) of environmental factors on mosquito-malaria interactions in order to understand natural variation in vector competence.

Survival of West Nile virus-challenged Southern house mosquitoes, Culex pipiens quinquefasciatus, in relation to environmental temperatures.

We investigated the effect of West Nile virus (WNV) infection on survival in two colonies of Culex pipiens quinquefasciatus Say (Diptera: Culicidae) originating from Vero Beach and Gainesville, FL. Mosquitoes were fed West Nile virus-infected blood and checked daily for survival. Exposure to WNV decreased survival among Cx. p. quinquefasciatus from Gainesville relative to unexposed individuals at 31° C. In contrast, exposure to WNV enhanced survival among Cx. p. quinquefasciatus from Vero Beach relative to unexposed individuals at 27° C. These results may suggest that exposure to WNV and associated infection could increase or decrease components of fitness, dependent on environmental temperature and intraspecific variation in Cx. p. quinquefasciatus. The relationship between lifespan (time of death in days) and WNV titer differed in the colonies at 31° C and 27° C, suggesting that intraspecific species variation in response to temperature impacts interactions with WNV. While further work is needed to determine if similar effects occur under field conditions, this suggests intraspecific variation in vector competence for WNV and adult survival of Cx. p. quinquefasciatus, both aspects of vectorial capacity.

Vector competence of Culex pipiens quinquefasciatus (Diptera: Culicidae) for West Nile virus isolates from Florida.

To assess vector competence (infection, dissemination and transmission) of Culex pipiens quinquefasciatus for Florida (FL) West Nile virus (WNV) isolates. West Nile virus isolates (WN-FL-03: NY99 genotype; WN-FL-05-558, WN-FL-05-2186, WN-FL-05-510: WN02 genotype) collected from different regions of FL were used for vector competence experiments in Cx. p. quinquefasciatus from Alachua County and Indian River County in FL. Mosquitoes from both colonies were fed blood containing 7.9 ± 0.2 log10 plaque-forming units WNV/ml ± SE and incubated at 28 °C for 14 days. Vector competence, including rates of infection, dissemination, and transmission, was compared between colonies for WN-FL-03 using chi-squared. Virus titres in bodies, legs and saliva were compared using anova. Daily measurements of in vitro replication of WNV isolates were evaluated in Vero cells so that a standardised virus dose for each isolate could be delivered to mosquitoes. Infection and dissemination rates were high (≥ 95%) and not affected by isolate or colony (infection, P = 0.679; dissemination, P = 0.799). Transmission rates were low (≤ 20%), detected in one colony and affected by isolate (P = 0.008). Body and leg titres differed between isolates (body titre, P = 0.031; leg titre, P = 0.044) and colonies (body titre, P = 0.001; leg titre, P = 0.013) while saliva titre did not differ between isolates (P = 0.462). Variation in vector competence of mosquito populations may be attributed, in part, to exposures to WNV with genetic differences leading to different rates of replication in mosquitoes. Evaluation of vector competence for different WNV isolates may help us understand vector-virus interactions and, hence, the role of vectors in complex virus transmission cycles in nature.

No recent adaptive selection on the apyrase of Mediterranean Phlebotomus: implications for using salivary peptides to vaccinate against canine leishmaniasis.

Vaccine development is informed by a knowledge of genetic variation among antigen alleles, especially the distribution of positive and balancing selection in populations and species. A combined approach using population genetic and phylogenetic methods to detect selective signatures can therefore be informative for identifying vaccine candidates. Parasitic Leishmania species cause the disease leishmaniasis in humans and mammalian reservoir hosts after inoculation by female phlebotomine sandflies. Like other arthropod vectors of disease agents, sandflies use salivary peptides to counteract host haemostatic and immunomodulatory responses during bloodfeeding, and these peptides are vaccine candidates because they can protect against Leishmania infection. We detected no contemporary adaptive selection on one salivary peptide, apyrase, in 20 populations of Phlebotomus ariasi, a European vector of Leishmania infantum. Maximum likelihood branch models on a gene phylogeny showed apyrase to be a single copy in P. ariasi but an ancient duplication event associated with temporary positive selection was observed in its sister group, which contains most Mediterranean vectors of L. infantum. The absence of contemporary adaptive selection on the apyrase of P. ariasi may result from this sandfly's opportunistic feeding behaviour. Our study illustrates how the molecular population genetics of arthropods can help investigate the potential of salivary peptides for disease control and for understanding geographical variation in vector competence.

Variation in Vector Competence for Dengue Viruses Does Not Depend on Mosquito Midgut Binding Affinity

BackgroundDengue virus genotypes of Southeast Asian origin have been associated with higher virulence and transmission compared to other genotypes of serotype 2 (DEN-2). We tested the hypothesis that genetic differences in dengue viruses may result in differential binding to the midgut of the primary vector, Aedes aegypti, resulting in increased transmission or vectorial capacity.Methodology/Principal FindingTwo strains of each of the four DEN-2 genotypes (Southeast Asian, American, Indian, and West African) were tested to determine their binding affinity for mosquito midguts from two distinct populations (Tapachula, Chiapas, Mexico and McAllen, Texas, USA). Our previous studies demonstrated that Southeast Asian viruses disseminated up to 65-fold more rapidly in Ae. aegypti from Texas and were therefore more likely to be transmitted to humans. Results shown here demonstrate that viruses from all four genotypes bind to midguts at the same rate, in a titer-dependent manner. In addition, we show population differences when comparing binding affinity for DEN-2 between the Tapachula and McAllen mosquito colonies.ConclusionsIf midgut binding potential is the same for all DEN-2 viruses, then viral replication differences in these tissues and throughout the mosquito can thus probably explain the significant differences in dissemination and vector competence. These conclusions differ from the established paradigms to explain mosquito barriers to infection, dissemination, and transmission.

Spatial and Temporal Variation in Vector Competence of Culex pipiens and Cx. restuans Mosquitoes for West Nile Virus

Vector competence, the probability that a vector will transmit a pathogen after feeding on an infected host, is known to vary among vector species, populations, days since feeding, and temperature during the extrinsic incubation period. However, the extent of spatio-temporal variability and consistency in vector competence of populations is not known. We examined vector competence of Culex pipiens Linnaeus and Cx. restuans Theobald mosquitoes for West Nile virus collected over 3 years from 17 sites to measure spatial and temporal scales of variation in vector competence. We found extreme variation with 0-52% of mosquitoes transmitting West Nile virus at a single site between different sampling periods, and similar variation across populations. However, we also found that within a smaller geographic range, vector competence varied somewhat synchronously, suggesting that environmental and population genetic factors might influence vector competence. These results highlight the spatio-temporal variability in vector competence and the role of local processes.

Geographic Variation in Vector Competence for West Nile Virus in theCulex pipiens(Diptera: Culicidae) Complex in California

We evaluated the susceptibility to infection and transmission of West Nile virus (WNV) in seven populations of Culex pipiens pipiens (L.), Cx. p. quinquefasciatus Say, and from populations containing Cx. pipiens/quinquefasciatus hybrids in a north-south transect of California. Samples were identified to species or as hybrid forms based on morphology of male terminalia. After 7 and 14 days of extrinsic incubation, few females were infected and none transmitted WNV from samples of Cx. p. pipiens from northern Shasta County and of Cx. p. quinquefasciatus from southern Los Angeles County. Seven days after infective feeding, 13%-36% of mosquitoes from the counties of Merced, Fresno, Kern, and San Bernardino were infected, and 12%-40% of infected mosquitoes expressed WNV in salivary expectorate. Fourteen days after infective feeding, 18%-43% of mosquitoes from these counties were infected, and 50%-69% of infected mosquitoes transmitted WNV in salivary expectorate. A sample of Cx. p. quinquefasciatus from Riverside County did not transmit WNV after 7 days, but 71% transmitted 14 days after infective feeding. These results reveal extensive geographic variation in vector competence for WNV in the Culex pipiens complex in California.

Variation in vector competence for dengue 2 virus among 24 collections of Aedes aegypti from Mexico and the United States.

Aedes aegypti from 24 collections in Mexico and the United States were challenged orally with dengue 2 virus JAM1409 (DEN-2 JAM1409). The vector competence (VC) of the populations ranged from 24% to 83%. Mosquito populations from the Yucatan exhibited greater VC than those from other areas of Mexico. The presence or absence of a midgut infection barrier (MIB) and a midgut escape barrier (MEB) was determined for mosquitoes in each population. The percentage of mosquitoes exhibiting an MIB ranged from 14% to 59%, and those exhibiting an MEB ranged from 4% to 43% in the collections. The MIB and MEB were not completely independent as determined by regression analysis. Midgut infection rates were dose dependent.