Vector Competence of American Mosquitoes for Three Strains of Zika Virus

In 2015, Zika virus (ZIKV; Flaviviridae; Flavivirus) emerged in the Americas, causing millions of infections in dozens of countries. The rapid spread of the virus and the association with disease outcomes such as Guillain-Barré syndrome and microcephaly make understanding transmission dynamics essential. Currently, there are no reports of vector competence (VC) of American mosquitoes for ZIKV isolates from the Americas. Further, it is not clear whether ZIKV strains from other genetic lineages can be transmitted by American Aedes aegypti populations, and whether the scope of the current epidemic is in part facilitated by viral factors such as enhanced replicative fitness or increased vector competence. Therefore, we characterized replication of three ZIKV strains, one from each of the three phylogenetic clades in several cell lines and assessed their abilities to be transmitted by Ae. aegypti mosquitoes. Additionally, laboratory colonies of different Culex spp. were infected with an American outbreak strain of ZIKV to assess VC. Replication rates were variable and depended on virus strain, cell line and MOI. African strains used in this study outcompeted the American strain in vitro in both mammalian and mosquito cell culture. West and East African strains of ZIKV tested here were more efficiently transmitted by Ae. aegypti from Mexico than was the currently circulating American strain of the Asian lineage. Long-established laboratory colonies of Culex mosquitoes were not efficient ZIKV vectors. These data demonstrate the capacity for additional ZIKV strains to infect and replicate in American Aedes mosquitoes and suggest that neither enhanced virus replicative fitness nor virus adaptation to local vector mosquitoes seems likely to explain the extent and intensity of ZIKV transmission in the Americas

Mapping geographical inequalities in access to drinking water and sanitation facilities in low-income and middle-income countries, 2000-17

Background: Universal access to safe drinking water and sanitation facilities is an essential human right, recognised in the Sustainable Development Goals as crucial for preventing disease and improving human wellbeing. Comprehensive, high-resolution estimates are important to inform progress towards achieving this goal. We aimed to produce high-resolution geospatial estimates of access to drinking water and sanitation facilities. Methods: We used a Bayesian geostatistical model and data from 600 sources across more than 88 low-income and middle-income countries (LMICs) to estimate access to drinking water and sanitation facilities on continuous continent-wide surfaces from 2000 to 2017, and aggregated results to policy-relevant administrative units. We estimated mutually exclusive and collectively exhaustive subcategories of facilities for drinking water (piped water on or off premises, other improved facilities, unimproved, and surface water) and sanitation facilities (septic or sewer sanitation, other improved, unimproved, and open defecation) with use of ordinal regression. We also estimated the number of diarrhoeal deaths in children younger than 5 years attributed to unsafe facilities and estimated deaths that were averted by increased access to safe facilities in 2017, and analysed geographical inequality in access within LMICs. Findings: Across LMICs, access to both piped water and improved water overall increased between 2000 and 2017, with progress varying spatially. For piped water, the safest water facility type, access increased from 40·0% (95% uncertainty interval [UI] 39·4–40·7) to 50·3% (50·0–50·5), but was lowest in sub-Saharan Africa, where access to piped water was mostly concentrated in urban centres. Access to both sewer or septic sanitation and improved sanitation overall also increased across all LMICs during the study period. For sewer or septic sanitation, access was 46·3% (95% UI 46·1–46·5) in 2017, compared with 28·7% (28·5–29·0) in 2000. Although some units improved access to the safest drinking water or sanitation facilities since 2000, a large absolute number of people continued to not have access in several units with high access to such facilities (>80%) in 2017. More than 253 000 people did not have access to sewer or septic sanitation facilities in the city of Harare, Zimbabwe, despite 88·6% (95% UI 87·2–89·7) access overall. Many units were able to transition from the least safe facilities in 2000 to safe facilities by 2017; for units in which populations primarily practised open defecation in 2000, 686 (95% UI 664–711) of the 1830 (1797–1863) units transitioned to the use of improved sanitation. Geographical disparities in access to improved water across units decreased in 76·1% (95% UI 71·6–80·7) of countries from 2000 to 2017, and in 53·9% (50·6–59·6) of countries for access to improved sanitation, but remained evident subnationally in most countries in 2017. Interpretation: Our estimates, combined with geospatial trends in diarrhoeal burden, identify where efforts to increase access to safe drinking water and sanitation facilities are most needed. By highlighting areas with successful approaches or in need of targeted interventions, our estimates can enable precision public health to effectively progress towards universal access to safe water and sanitation

Ivermectin-treated bird feed to control West Nile virus transmission

2018 Spring.Includes bibliographical references.West Nile virus is the leading cause of arboviral fever and encephalitis in the United States. The highest WNV disease incidence occurs along the Great Plains region of the United States, as the ecology and land use provide a supportive habitat for the main WNV enzootic and bridge vector of the region, Culex tarsalis. However, due to the lack of dense human population, this area often does not benefit current WNV control measures as applied by conventional mosquito control districts. Based on the ecology of WNV transmission in the Great Plains region, a strategy that targets Cx. tarsalis through its ornithophilic blood feeding behavior could disrupt WNV transmission. Given that the majority of Cx. tarsalis blood meals on the northern Colorado plains may come from doves and passerine species during the WNV transmission season, effective targeting of these or other local preferred hosts with endectocide-treated bird feed could result control of WNV transmission. This study develops and characterizes the effects of IVM-treated bird feed in birds and biting Cx. tarsalis mosquitoes in both a laboratory and field setting. In Chapter 2, the effects of IVM on Cx. tarsalis survival were examined using both in vitro membrane blood meals and direct blood feeding on IVM-treated birds. Chickens and wild Eurasian Collared Doves fed solely on IVM-treated bird feed concentrations up to 200 mg IVM/kg feed exhibited no signs of toxicity, and most Cx. tarsalis that blood fed on these birds died compared to controls. Mosquito survivorship following blood feeding correlated with IVM serum concentrations at the time of blood feeding, which dropped rapidly after the withdrawal of treated feed. These results suggested IVM-treated bird feed should be further explored as a hyper-localized control strategy for WNV transmission. Chapter 3 presents the development of a method to detect and quantify IVM in individual blood meals of Anopheles gambiae and Cx. tarsalis, which will be important in measuring the coverage of this intervention in the field, and accurately assessing IVM's mosquitocidal effects in field situations. This ability to detect IVM in mosquito blood meals was similar between blood fed Cx. tarsalis and An. gambiae, and between sampling times of 0 or 12 hours post blood feed. The quantity of IVM ingested in individual mosquitoes was also compared to the venous serum concentrations of live animals. Chapter 4 presents promising results from two separate pilot field trials of IVM-treated bird feed that were conducted during the summers of 2016 and 2017. Results from 2016 showed that wild birds frequently visit the IVM-treated feeders. In addition, there was an observable trend where "far" traps that are expected to be beyond the zone of control had more WNV-positive pools compared to "near" traps at both ELC and ARDEC South sites. Results from the 2017 study continued to be promising, where birds were again visiting IVM-treated feeders and IVM could be detected in the sera of birds sampled by IVM feeders. There was also a trend of higher VI for the control sites compared to IVM sites for the 2017 season. The efficacy of IVM-treated bird feed was evaluated in two pilot trials where natural WNV transmission cycles occurred in wild birds and Cx. tarsalis, but should be followed up with field seasons with many control and IVM sites to allow for a robust analysis of IVM effects. This study introduces the novel concept of using systemic endectocides for controlling WNV transmission, and this concept could be explored for other arboviruses

Predicted reduction in transmission from deployment of ivermectin-treated birdfeeders for local control of West Nile virus

Ivermectin (IVM)-treated birds provide the potential for targeted control of Culex mosquitoes to reduce West Nile virus (WNV) transmission. Ingestion of IVM increases mosquito mortality, which could reduce WNV transmission from birds to humans and in enzootic maintenance cycles affecting predominantly bird-feeding mosquitoes and from birds to humans. This strategy might also provide an alternative method for WNV control that is less hampered by insecticide resistance and the logistics of large-scale pesticide applications. Through a combination of field studies and modeling, we assessed the feasibility and impact of deploying IVM-treated birdfeed in residential neighborhoods to reduce WNV transmission. We first tracked 105 birds using radio telemetry and radio frequency identification to monitor their feeder usage and locations of nocturnal roosts in relation to five feeder sites in a neighborhood in Fort Collins, Colorado. Using these results, we then modified a compartmental model of WNV transmission to account for the impact of IVM on mosquito mortality and spatial movement of birds and mosquitoes on the neighborhood level. We found that, while the number of treated lots in a neighborhood strongly influenced the total transmission potential, the arrangement of treated lots in a neighborhood had little effect. Increasing the proportion of treated birds, regardless of the WNV competency status, resulted in a larger reduction in infection dynamics than only treating competent birds. Taken together, model results indicate that deployment of IVM-treated feeders could reduce local transmission throughout the WNV season, including reducing the enzootic transmission prior to the onset of human infections, with high spatial coverage and rates of IVM-induced mortality in mosquitoes. To improve predictions, more work is needed to refine estimates of daily mosquito movement in urban areas and rates of IVM-induced mortality. Our results can guide future field trials of this control strategy

Effects of ivermectin treatment of backyard chickens on mosquito dynamics and West Nile virus transmission.

BackgroundVector control strategies typically rely on pesticides to target mosquitoes involved in enzootic and zoonotic transmission of West Nile virus (WNV). Nevertheless, increasing insecticide resistance and a desire to reduce pesticide usage provide the impetus for developing alternative strategies. Ivermectin (IVM), an antiparasitic drug which is widely used in human and veterinary medicine, is a potential alternative for targeted control because Culex mosquitoes experience increased mortality following ingestion of IVM in bloodmeals.Methodology/principal findingsWe conducted a randomized field trial to investigate the impact of treating backyard chicken flocks with IVM in urban neighborhoods across Davis, California on mosquito populations and WNV transmission dynamics. We observed a significant reduction in WNV seroconversions in treated vs. untreated chickens, suggesting a reduction in WNV transmission intensity around treated flocks. We also detected a reduction in parity rates of Cx. tarsalis near treated vs. untreated flocks and increased mortality in wild mosquitoes following a bloodmeal on treated chickens (IVM serum concentration > 5ng/mL) vs. chickens with IVM serum concentrations < 5 ng/mL. However, we did not find a significant difference in abundance or infection prevalence in mosquitoes between treatment groups associated with the reductions in seroconversions. Mosquito immigration from surrounding larval habitat, relatively low WNV activity in the study area, and variable IVM serum concentrations likely contributed to uncertainty about the impact.Conclusions/significanceTaken together, our results point to a reduction in WNV transmission due to the impact of IVM on Culex mosquito populations and support the ongoing investigation of oral administration of IVM to wild birds for local control of WNV transmission, although further work is needed to optimize dosing and understand effects on entomological endpoints

Evaluation of a novel West Nile virus transmission control strategy that targets Culex tarsalis with endectocide-containing blood meals.

Control of arbovirus transmission remains focused on vector control through application of insecticides directly to the environment. However, these insecticide applications are often reactive interventions that can be poorly-targeted, inadequate for localized control during outbreaks, and opposed due to environmental and toxicity concerns. In this study, we developed endectocide-treated feed as a systemic endectocide for birds to target blood feeding Culex tarsalis, the primary West Nile virus (WNV) bridge vector in the western United States, and conducted preliminary tests on the effects of deploying this feed in the field. In lab tests, ivermectin (IVM) was the most effective endectocide tested against Cx. tarsalis and WNV-infection did not influence mosquito mortality from IVM. Chickens and wild Eurasian collared doves exhibited no signs of toxicity when fed solely on bird feed treated with concentrations up to 200 mg IVM/kg of diet, and significantly more Cx. tarsalis that blood fed on these birds died (greater than 80% mortality) compared to controls (less than 25% mortality). Mosquito mortality following blood feeding correlated with IVM serum concentrations at the time of blood feeding, which dropped rapidly after the withdrawal of treated feed. Preliminary field testing over one WNV season in Fort Collins, Colorado demonstrated that nearly all birds captured around treated bird feeders had detectable levels of IVM in their blood. However, entomological data showed that WNV transmission was non-significantly reduced around treated bird feeders. With further development, deployment of ivermectin-treated bird feed might be an effective, localized WNV transmission control tool

Select strains of East and West African ZIKV have higher fitness <i>in vitro</i> than the American strain PRVABC59.

<p>Equal amounts of virus (MOI 0.01) was used to infect the indicated cell line. Supernatant was harvested and proportion of each virus was determined by sequencing and analyzing chromatograms. Data are presented as proportion of the first virus listed present at the indicated time point in (A) Vero, (B) Huh7, C6/36 (C), and (D) Aag2 cells.</p

Phylogenetic tree of 29 Zika virus isolates.

<p>The tree was generated by Neighbor-Joining methods using the Tamura-Nei genetic distance model (bootstrapped 1,000 times). Similar results were obtained using maximum parsimony or maximum likelihood algorithms. Strains used in this study are shown in bold; distinct lineages are East African (MR766), West African (41525), and Asian (PRVABC59).</p

Long term freezing of virus lowers infection rates of <i>Aedes aegypti</i> mosquitoes.

<p><i>Ae</i>. <i>aegypti</i> (Poza Rica strain) mosquitoes were fed an infectious blood-meal containing 1.6x10⁷ PFU of ZIKV (PRVABC59 strain) that was harvested directly from Vero cells (and stored at 4°C for 4 hours) or frozen for 4 hours or more than a week at -80°C. Mosquitoes were held for 7 or 14 days (n = 48 each time point for each group) and then (A) Infection rate (% of mosquitoes with virus in their bodies), (B) Dissemination Rate (% of mosquitoes, regardless of infection status, with virus in their legs), (C) Transmission Rate (% of mosquitoes, regardless of infection status, with virus in their saliva) and (D) Transmission (D) Rate (% of mosquitoes with a disseminated infection, with virus in their saliva) were collected. * Indicates p<0.05, ** p<0.01, and *** p<0.001 by two-tailed Fisher’s exact test. n = 48–144 for each group at each time point. n.d. indicates that samples were not collected for this time-point.</p

Mexican <i>Aedes aegypti</i> have increased vector competence for African strains of ZIKV.

<p>Mosquitoes were fed a blood-meal containing 1.6x10⁷ PFU of different ZIKV. Mosquitoes were then held for 7 or 14 days (n = 48 each time point for each group) and then (A) Infection rate (% of mosquitoes with virus in their bodies), (B) Dissemination Rate (% of mosquitoes, regardless of infection status, with virus in their legs), (C) Transmission Rate (% of mosquitoes, regardless of infection status, with virus in their saliva) and (D) Transmission (D) Rate (% of mosquitoes with a disseminated infection, with virus in their saliva) were collected. * Indicates p<0.05, ** p<0.01, and *** p<0.001 by two-tailed Fisher’s exact test. n = 126–144 for each group at each time point.</p