Disease Risk & Landscape Attributes of Tick-Borne Borrelia Pathogens in the San Francisco Bay Area, California

Habitat heterogeneity influences pathogen ecology by affecting vector abundance and the reservoir host communities. We investigated spatial patterns of disease risk for two human pathogens in the Borrelia genus–B. burgdorferi and B. miyamotoi–that are transmitted by the western black-legged tick, Ixodes pacificus. We collected ticks (349 nymphs, 273 adults) at 20 sites in the San Francisco Bay Area, California, USA. Tick abundance, pathogen prevalence and density of infected nymphs varied widely across sites and habitat type, though nymphal western black-legged ticks were more frequently found, and were more abundant in coast live oak forest and desert/semi-desert scrub (dominated by California sagebrush) habitats. We observed Borrelia infections in ticks at all sites where we able to collect >10 ticks. The recently recognized human pathogen, B. miyamotoi, was observed at a higher prevalence (13/349 nymphs = 3.7%, 95% CI = 2.0–6.3; 5/273 adults = 1.8%, 95% CI = 0.6–4.2) than recent studies from nearby locations (Alameda County, east of the San Francisco Bay), demonstrating that tick-borne disease risk and ecology can vary substantially at small geographic scales, with consequences for public health and disease diagnosis

Recreational areas in the San Francisco Bay area (left) sampled for western black-legged ticks, with a close-up (right) of recreational areas sampled in San Mateo and Santa Clara Counties.

<p>Recreational areas in the San Francisco Bay area (left) sampled for western black-legged ticks, with a close-up (right) of recreational areas sampled in San Mateo and Santa Clara Counties.</p

Study area showing independent variables: soil type (top left), elevation (top right), vegetation type (bottom left) and distance to roads (bottom right).

<p>Purple dots represent tick- drag transects in Santa Clara and San Mateo counties.</p

Prevalence data for <i>Borrelia</i> in <i>I</i>. <i>pacificus</i> ticks from previous studies in California.

<p>¹Number positive/number tested (percentage positive, 95% CI).</p><p>Prevalence data for <i>Borrelia</i> in <i>I</i>. <i>pacificus</i> ticks from previous studies in California.</p

Summary statistics of entomologic risk for <i>Ixodes pacificus</i> infected with <i>Borrelia</i> spp. in recreational areas of the San Francisco Bay Area.

<p>¹Betsy Crowder Trail is comprised of coast live oak woodland.</p><p>²Anniversary Trail is comprised of coastal scrub and moderate grassland.</p><p>³Meadow Trail is comprised of coast live oak woodland and Douglas fir forest.</p><p>Data are presented on density of nymphal (DON) <i>I</i>. <i>pacificus</i> (i.e., number of nymphs/100m² (calculated by (number of ticks collected/meters covered by transects) x100); density of infected nymphs (DIN) for <i>Borrelia</i> spp. (calculated by <i>Borrelia</i> prevalence x DON); and for <i>B</i>. <i>miyamotoi</i>, <i>B</i>. <i>burgdorferi</i> ss, and for <i>B</i>. <i>burgdorferi</i> sl.</p

High Infection Rates for Adult Macaques after Intravaginal or Intrarectal Inoculation with Zika Virus

Unprotected sexual intercourse between persons residing in or traveling from regions with Zika virus transmission is a risk factor for infection. To model risk for infection after sexual intercourse, we inoculated rhesus and cynomolgus macaques with Zika virus by intravaginal or intrarectal routes. In macaques inoculated intravaginally, we detected viremia and virus RNA in 50% of macaques, followed by seroconversion. In macaques inoculated intrarectally, we detected viremia, virus RNA, or both, in 100% of both species, followed by seroconversion. The magnitude and duration of infectious virus in the blood of macaques suggest humans infected with Zika virus through sexual transmission will likely generate viremias sufficient to infect competent mosquito vectors. Our results indicate that transmission of Zika virus by sexual intercourse might serve as a virus maintenance mechanism in the absence of mosquito-to-human transmission and could increase the probability of establishment and spread of Zika virus in regions where this virus is not present