Origin of the Dengue Fever Mosquito, <i>Aedes aegypti</i>, in California

<div><p>Dengue fever is among the most widespread vector-borne infectious diseases. The primary vector of dengue is the <i>Aedes aegypti</i> mosquito. <i>Ae. aegypti</i> is prevalent in the tropics and sub-tropics and is closely associated with human habitats outside its native range of Africa. While long established in the southeastern United States of America where dengue is re-emerging, breeding populations have never been reported from California until the summer of 2013. Using 12 highly variable microsatellite loci and a database of reference populations, we have determined that the likely source of the California introduction is the southeastern United States, ruling out introductions from abroad, from the geographically closer Arizona or northern Mexico populations, or an accidental release from a research laboratory. The power to identify the origin of new introductions of invasive vectors of human disease relies heavily on the availability of a panel of reference populations. Our work demonstrates the importance of generating extensive reference databases of genetically fingerprinted human-disease vector populations to aid public health efforts to prevent the introduction and spread of vector-borne diseases.</p></div

Genetic structure within pantropical populations of <i>Aedes aegypti</i>.

<p>STRUCTURE bar plots indicating relatedness of <i>Aedes aegypti</i> populations based on 12 microsatellite loci. Each vertical bar represents an individual. The height of each bar represents the probability of assignment to each of K optimal clusters (different colors) determined using the Delta K method. (<b>A</b>) North America and Asian populations (K = 2), and (<b>B</b>) North American populations (K = 3). (<b>C</b>) Map indicating the North American geographic locations sampled in this study. (<b>Δ</b>) California, (○) other locations in North America.</p

Individual and group mosquito genetic assignments.

<p>Percentage of individuals from Madera (<b>A</b>), Fresno (<b>B</b>), and San Mateo (<b>C</b>) counties assigned with the highest probability to each of the reference populations. (<b>D</b>) Scores calculated for each of the reference populations after group assignment of each of three California populations. Assignments were performed using Bayesian criteria for likelihood estimation with GENECLASS 2.0.</p

Genetic diversity of <i>Aedes aegypti</i> populations.

<p>H_o = observed heterozygosity; H_e = expected heterozygosity; AR = Allelic richness estimated by rarefaction (N = 30 genes).</p><p>*Pantropical = mean across populations from Asia and the Americas.</p><p>**Lab strains = mean across Hamburg, Rockefeller, and Liverpool laboratory strains provided by David Severson (University of Notre Dame, Indiana).</p

Partial 16S–23S rRNA intragenic spacer region of <i>Borrelia miyamotoi</i> (450 bp) and <i>B. burgdorferi</i> sl (970 bp).

<p>Partial 16S–23S rRNA intragenic spacer region of <i>Borrelia miyamotoi</i> (450 bp) and <i>B. burgdorferi</i> sl (970 bp).</p

Large Scale Spatial Risk and Comparative Prevalence of <i>Borrelia miyamotoi</i> and <i>Borrelia burgdorferi</i> Sensu Lato in <i>Ixodes pacificus</i>

<div><p><i>Borrelia miyamotoi</i> is a newly described emerging pathogen transmitted to people by <i>Ixodes</i> species ticks and found in temperate regions of North America, Europe, and Asia. There is limited understanding of large scale entomological risk patterns of <i>B. miyamotoi</i> and of <i>Borreila burgdorferi</i> sensu stricto (ss), the agent of Lyme disease, in western North America. In this study, <i>B. miyamotoi</i>, a relapsing fever spirochete, was detected in adult (n = 70) and nymphal (n = 36) <i>Ixodes pacificus</i> ticks collected from 24 of 48 California counties that were surveyed over a 13 year period. Statewide prevalence of <i>B. burgdorferi</i> sensu lato (sl), which includes <i>B. burgdorferi</i> ss, and <i>B. miyamotoi</i> were similar in adult <i>I. pacificus</i> (0.6% and 0.8%, respectively). In contrast, the prevalence of <i>B. burgdorferi</i> sl was almost 2.5 times higher than <i>B. miyamotoi</i> in nymphal <i>I. pacificus</i> (3.2% versus 1.4%). These results suggest similar risk of exposure to <i>B. burgdorferi</i> sl and <i>B. miyamotoi</i> from adult <i>I. pacificus</i> tick bites in California, but a higher risk of contracting <i>B. burgdorferi</i> sl than <i>B. miyamotoi</i> from nymphal tick bites. While regional risk of exposure to these two spirochetes varies, the highest risk for both species is found in north and central coastal California and the Sierra Nevada foothill region, and the lowest risk is in southern California; nevertheless, tick-bite avoidance measures should be implemented in all regions of California. This is the first study to comprehensively evaluate entomologic risk for <i>B. miyamotoi</i> and <i>B. burgdorferi</i> for both adult and nymphal <i>I. pacificus</i>, an important human biting tick in western North America.</p></div

Adult and nymphal <i>B. burgdorferi</i> sl pooled results (minimum infection prevalence) and individual tick results (prevalence), 2000–2012.

<p>Adult and nymphal <i>B. burgdorferi</i> sl pooled results (minimum infection prevalence) and individual tick results (prevalence), 2000–2012.</p

Adult and nymphal <i>B. miyamotoi</i> pooled results (minimum infection prevalence) and individual tick results (prevalence), 2000–2012.

<p>Adult and nymphal <i>B. miyamotoi</i> pooled results (minimum infection prevalence) and individual tick results (prevalence), 2000–2012.</p

Proportion of individual adult and nymphal <i>Ixodes pacificus</i> ticks with <i>Borrelia burgdorferi</i> sensu lato and <i>B. miyamotoi</i> detected in California, 2009–2012.

<p>Proportion of individual adult and nymphal <i>Ixodes pacificus</i> ticks with <i>Borrelia burgdorferi</i> sensu lato and <i>B. miyamotoi</i> detected in California, 2009–2012.</p

California <i>Ixodes pacificu</i>s adult and nymphal ticks, tested in pools or tested individually, 2000–2012.

<p>California <i>Ixodes pacificu</i>s adult and nymphal ticks, tested in pools or tested individually, 2000–2012.</p