Fig 1.
Location of ivermectin (IVM)-treated (red) and untreated (blue) chicken flocks and CO2-baited mosquito traps in Davis, California.
Numbered insets illustrate the arrangement of dry-ice baited CO2-traps around each flock, with three traps within 10m (triangle) and three traps approximately 150m from the flock (square). Bar in each inset indicates 100m. Top right inset illustrates the location of the study site in relation to the state of California. Geographic boundaries for the state of California and the city of Davis were obtained from the 2020 TIGER/Line shapefiles for States and Places, respectively, provided by the United States Census Bureau (https://www.census.gov/cgi-bin/geo/shapefiles/index.php).
Fig 2.
Categories used for ovarian grading in Cx. tarsalis.
Parity status graded as A) nulliparous with all tightly wound skeins, B) intermediate with a combination of tight and loose skeins, or C) parous with all unwound skeins. D) Dark mass observed in some ovaries, likely due to egg protein, which is E) removed with washing with deionized water. Ovarian mounts presented at 400x magnification (A-C) or 100x magnification (D-E).
Fig 3.
Reduced West Nile virus (WNV) seroconversions in ivermectin (IVM)-treated vs. untreated chicken flocks.
Four flocks per treatment group. Chickens seropositive at baseline were excluded from the analysis.
Fig 4.
Ivermectin (IVM) serum concentrations (ng/mL) in treated and untreated chickens.
Grey dashed line indicates the limit of quantification (LOQ, 5 ng/mL). Bold line indicates chicken used in mosquito bioassay at end of study.
Fig 5.
Blood feeding on ivermectin (IVM)-treated chickens increased wild Culex tarsalis mortality.
Wild-caught Cx. tarsalis survival following blood feeding on a randomly chosen chicken from each coop A) by treatment group and B) stratified by IVM serum concentration above or below the limit of quantification (LOQ, 5 ng/mL). Mantel-Haenszel chi-square P-value indicated for comparison of Kaplan-Meier survival curves for 1–3 and 1–5 days (duration indicated by grey segment). IVM-related effects were expected to occur within 3 days post-bloodmeal.
Fig 6.
Parity rates in Culex tarsalis around ivermectin (IVM)-treated and untreated chicken flocks.
Parity grading during the West Nile virus (WNV) season near (≤10m) and far (~150m) from treated and untreated flocks for (A) individual weeks and (B-C) collapsed by group for weeks with observations in each group (weeks 29–37). Number of ovaries in each category by week indicated in bars in A.
Table 1.
Odds ratio (95% CI) of a mosquito being parous grouped by distances from ivermectin (IVM)-treated and untreated flocks.
Fig 7.
Entomological indices of Culex tarsalis around ivermectin (IVM)-treated and untreated chicken flocks.
Weekly (A) abundance, (B) infection prevalence per 1,000, and (C) vector index (VI) near (≤ 10m) and far (~150m) from IVM-treated and untreated flocks. VI is a risk metric that approximates the number of infectious mosquitoes present as the product of abundance and infection prevalence. Vertical dashed lines indicate the first and last day IVM-treated feed was provided.
Table 2.
Pathologic and histological findings at necropsy of ivermectin-treated (n = 6) and untreated (n = 6) chickens.