Mapping Where Lyme-Infected Ticks Lurk Across the Western United States

In the western United States, the landscape of Lyme disease risk has long been shadowed by uncertainty. Unlike the Northeast, where dense surveillance networks track the blacklegged tick with fine resolution, the Pacific states have faced a persistent data gap: simply put, public health officials often did not know which counties could realistically harbor the infected, host-seeking nymphal ticks that transmit Borrelia burgdorferi sensu stricto, the bacterium responsible for nearly all Lyme disease cases in North America. A new modeling study led by researchers at the Centers for Disease Control and Prevention now offers the most detailed county-level picture to date of where the western blacklegged tick, Ixodes pacificus, is likely to carry and transmit the pathogen in its most dangerous life stage.

The research, published in the journal Parasites & Vectors, was conducted by James C. Burtis, Erik Foster, and Rebecca J. Eisen of the CDC’s Division of Vector-Borne Diseases in Fort Collins, Colorado. Their central question was deceptively simple: across seven western states, which counties have environmental conditions suitable both for the bacterium to maintain itself in local wildlife cycles and for infected nymphs to climb up onto vegetation and wait for a host? That second condition matters enormously for human health. Nymphal ticks are tiny, roughly the size of a poppy seed, and their host-seeking behavior above the ground is what brings them into contact with people hiking, gardening, or working outdoors. A tick that never quests above vegetation poses far less risk of a bite.

To build their analysis, the team confronted the fundamental scarcity of western tick surveillance data. Systematic records documenting the densities of host-seeking, infected nymphs are limited in the region, in stark contrast to the eastern United States where such data underpin county-level risk maps. The researchers turned to the ArboNET Tick Module, a national database maintained by the CDC that aggregates tick surveillance submissions from public health partners. Because those records alone could not cover the full geographic extent of potential tick habitat, they supplemented the database with an exhaustive review of the published literature and public health records, collating county-level observations of host-seeking nymphs and separately compiling reports of B. burgdorferi sensu stricto infections detected in both nymphal and adult I. pacificus.

The compilation revealed a striking imbalance. Host-seeking I. pacificus nymphs had been recorded in 54 counties across the seven western states, while evidence of Bbss-infected I. pacificus existed in only 36 counties, all of them in California, Oregon, and Washington. That distinction is critical. A tick population can thrive in a county without the bacterium circulating among the wildlife that feed the ticks. Enzootic transmission, the sustained cycling of the spirochete between ticks and reservoir hosts such as small mammals and birds, requires a specific confluence of vector abundance, competent reservoir species, and favorable climate. Some counties may host abundant ticks but, for reasons of host community composition or microclimate, never sustain the pathogen.

With these records in hand, the researchers built separate habitat suitability models for the two phenomena: one predicting where host-seeking nymphs are likely to occur, and another predicting where Bbss-infected ticks of either stage have been or are likely to be found. The modeling approach drew on multiple algorithms rather than a single statistical technique, a strategy that guards against the idiosyncrasies of any one method. Crucially, the team set the suitability thresholds of all algorithms to achieve 90 percent sensitivity, meaning the models were deliberately tuned to flag nearly every county with a known record, even at the cost of including some counties where the tick or pathogen may ultimately be absent. In public health terms, this conservative choice favors caution: it is better to overestimate the map of potential risk than to tell residents of an affected county they are safe.

The final step was an overlay. By intersecting the suitable habitat for host-seeking nymphs with the suitable habitat for infected I. pacificus, the researchers identified counties where the environment plausibly supports the full combination of conditions that generate acarological risk: infected nymphs actively questing above vegetation. The resulting map told a geographically focused story. Suitable habitat for encountering Bbss-infected host-seeking nymphs clustered mostly in central and northern California and along the coastal corridors of Washington and Oregon. The pattern was markedly narrower than the overall footprint of the tick itself, confirming that I. pacificus occupies a far broader range than the Lyme disease transmission zones within it.

The numbers quantify that contrast vividly. An earlier modeling effort had classified 113 counties across the western states as suitable for I. pacificus. Yet the new analysis found only 76 counties characterized as suitable for detecting Bbss-infected host-seeking nymphs by at least one of the team’s models. In other words, more than a third of the counties that could host the tick lack evidence of conditions supporting the infected, questing nymphs that pose the greatest hazard to people. For residents and clinicians, this reframes the geography of concern: living in a county where western blacklegged ticks exist is not equivalent to living in a county where Lyme transmission cycles are plausible.

The study’s authors are careful about what their models can and cannot say. The suitability maps identify where infected, host-seeking nymphs are environmentally plausible, but they do not differentiate the magnitude of risk within or among counties. A county flagged as suitable might support low densities of infected nymphs while another supports densities orders of magnitude higher, and the binary classification cannot capture that gradient. Nor can the models replace direct measurement; density of infected nymphs, the metric most tightly linked to human case risk, requires labor-intensive field dragging and pathogen testing that surveillance programs in the West have struggled to sustain. What the models provide instead is a prioritization tool, a way to direct scarce surveillance resources toward the counties where field data would be most informative.

That prioritization is precisely how the authors frame the study’s practical payoff. As a next step toward improving acarological risk assessment in the western United States, the research highlights specific counties where enhanced tick surveillance is warranted. Field confirmation in modeled-suitable counties without existing records could verify the presence of infected questing nymphs, while surveillance in borderline counties could sharpen the thresholds of future models. The approach mirrors strategies used in the East, where layered surveillance and modeling have produced the county maps that clinicians and the public routinely consult. Bringing the West closer to that standard has been a long-standing goal of CDC vector-borne disease programs, and the intramurally funded study represents a substantial advance toward it.

For the public, the message embedded in the maps is one of informed vigilance rather than alarm. The core risk zones in central and northern coastal California and the Pacific Northwest coasts are places where outdoor recreation and residential development interleave with tick habitat, and where standard precautions remain essential: using repellents, performing tick checks after time outdoors, and promptly removing attached ticks. At the same time, the finding that infected questing nymphs are plausible in fewer counties than the tick’s total range offers reassurance to residents of inland and southern areas where the tick occurs but the pathogen apparently does not. As climate and land use continue to shift, the interplay between tick distributions and pathogen transmission will evolve, and models like these, anchored to surveillance and repeatedly updated, will be central to keeping western Lyme disease risk maps honest, current, and actionable.

Subject of Research: Habitat suitability modeling for Borrelia burgdorferi-infected western blacklegged tick nymphs in the western United States

Article Title: Identifying counties with suitable habitat for host-seeking Borrelia burgdorferi sensu stricto-infected Ixodes pacificus nymphs across seven states in the western United States

Article References: Burtis, J. C., Foster, E., & Eisen, R. J. (2026). Identifying counties with suitable habitat for host-seeking Borrelia burgdorferi sensu stricto-infected Ixodes pacificus nymphs across seven states in the western United States. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07730-5

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07730-5

Keywords: Lyme disease, Ixodes pacificus, Borrelia burgdorferi, tick surveillance, habitat suitability modeling, nymphal ticks, western United States, CDC, acarological risk, vector-borne disease, county-level mapping, Parasites & Vectors