Bracing for the Worst — Range Expansion of the Lone Star Tick in the Northeastern United States
List of authors.
- Goudarz Molaei, Ph.D.,
- Eliza A.H. Little, Ph.D.,
- Scott C. Williams, Ph.D.,
- and Kirby C. Stafford, Ph.D.
Ticks and tickborne diseases are increasingly becoming a major health concern for humans, domesticated animals, and livestock. Reported cases of bacterial and protozoan tickborne disease doubled in the United States between 2004 and 2016. More than 90% of the nearly 60,000 cases of nationally notifiable vectorborne diseases reported in 2017 were linked to ticks. As the geographic ranges of multiple tick species continue to expand, invasive tick species are being discovered, new tickborne pathogens are emerging, and coinfections in ticks are surging. Rising global temperatures, ecologic changes, reforestation, and increases in commerce and travel are all important underlying factors influencing the rate and extent of range expansion for ticks and tickborne pathogens.
Both blacklegged (Ixodes scapularis) and lone star (Amblyomma americanum) ticks may be recolonizing areas where they thrived historically, before rampant deforestation and substantial local reduction of key hosts. Linked, in part, to a warming climate, there has been an increase in the number of ticks and associated diseases recorded in the United States and Canada,1 as well as in Europe.
Persistently warming temperatures may not only lead to the continued geographic range expansion of some ticks but may also extend their active season, thereby altering host availability and abundance; interactions among vectors, pathogens, and hosts; and the prevalence of infection in ticks. A warming climate and other environmental changes will affect abundance, distribution, seasonal activity patterns, and interactions among species differently for various ticks.
Lone star ticks of all life stages (larva, nymph, and adult) feed predominantly on large mammals, especially white-tailed deer. Larvae and nymphs also feed on birds. The resurgence of lone star ticks is linked to increased populations of deer, eastern coyotes, and wild turkeys. In addition to occupying its established range, the lone star tick has expanded into the upper midwestern and northeastern United States and eastern Canada.2,3 Since lone star ticks can lay several thousand eggs, even the dispersal of a small number of gravid females may be sufficient to establish populations in areas with abundant reproductive hosts, suitable habitats, and conducive temperatures.
Current Populations of Lone Star Ticks (Amblyomma americanum) in the Northeastern United States.
Lone star ticks have been established in the southeastern United States for well over a century; southern New Jersey was historically recognized as their northern range limit.4 Some reports of lone star ticks in the northeastern United States and more recently in eastern Canada may not necessarily reflect established breeding populations.2,3 In the past few decades, however, documented breeding populations have expanded into some parts of the Northeast. Such populations were reported in Suffolk County, New York, as early as 1971; Newport County, Rhode Island, in 1986; Somerset and Middlesex Counties, New Jersey, in 2017; Fairfield and New Haven Counties, Connecticut, in 2018 and 2019, respectively; and Barnstable, Nantucket, and Dukes Counties, Massachusetts, in 2019 (see map).2,4,5
Current environmental and climatic conditions favor the establishment and expansion of lone star ticks along the southern New England coast. Moderate maritime climates may be more conducive to the establishment of lone star tick populations than the climates inland, areas where immature ticks may not survive cold winters. Investigations of lone star ticks that can be traced back to an established southern population show that adults can successfully survive the winter in mainland Connecticut, however, and population simulations using current climate conditions suggest that southern Canada is already suitable for their establishment. Although the northward range expansion of the lone star tick is consistent with climate change, a recent study revealed that tick populations in New York are genetically distinct from those occupying the species’ historical range. This finding suggests the possibility of adaptive evolution causing or coinciding with this range expansion and probably favoring pathogen transmission.
It’s unclear how the lone star tick will compete and interact directly or indirectly with other tick species in the Northeast, and the nature of these interactions may vary depending on evolutionary context and changing environmental and climatic conditions. In the southeastern United States, range expansion of the lone star tick coincided with diminished populations of the American dog tick (Dermacentor variabilis). In New Jersey, populations of the lone star tick have increased in areas where it is endemic while blacklegged tick populations have remained static.4 Field studies indicate that the lone star tick establishes populations in habitats with specific humidity ranges and that tick abundance is associated with the presence of invasive plants. Areas colonized by invasive plants are frequented by white-tailed deer, a prominent tick host and pathogen reservoir. Lone star ticks will traverse long distances when searching for a mammalian host, thereby accelerating their establishment in new areas.2
Previously considered aggressive nuisance pests, lone star ticks have now been associated with several human diseases and medical conditions, including tularemia (Francisella tularensis), ehrlichiosis (Ehrlichia chaffeensis, E. ewingii, and Panola Mountain Ehrlichia), Heartland virus disease (Heartland virus), southern tick–associated rash illness, or STARI (pathogen unknown), and red meat allergy (alpha-gal syndrome) and are probably also associated with Bourbon virus disease (Bourbon virus).2 Lone star ticks have also been commonly found to be infected with Rickettsia amblyommatis; however, serologic evidence suggests that humans develop a robust immune response to this bacterium, though it may cause symptoms in some people. Local abundance of lone star ticks and the likelihood of getting multiple bites can be highly irritating, even in the absence of disease transmission.
Although lone star ticks don’t transmit Borrelia burgdorferi — the principal bacterium that causes Lyme disease in North America — symptoms of STARI and early Lyme disease are similar, and STARI may be misdiagnosed as Lyme disease in areas with both lone star ticks and blacklegged ticks. Reported cases of human ehrlichiosis have increased, but this disease is largely underrecognized and underreported. Infections of E. chaffeensis in lone star ticks have been identified in Connecticut, Rhode Island, and Massachusetts, but few cases of human disease have been attributed to this pathogen.
Most reports of lone star ticks in the northeastern United States come from tick submissions by the public to passive surveillance programs, which serve as an early warning system. Active surveillance is important for accurate determination of the extent of the northern range expansion of this vector, however. To be effective, active surveillance should be designed specifically for lone star ticks and should include targeting of areas with emerging populations identified by passive surveillance.3 In many areas of the mid-Atlantic region and emerging areas of the Northeast, coexistence of lone star and blacklegged ticks complicates management strategies. Rodent-targeted approaches used for the control of blacklegged ticks aren’t necessarily effective for lone star ticks, since small mammals aren’t major hosts for immature members of this species. By contrast, application of acaricides to deer by means of four-poster feeding stations has reduced the abundance of host-seeking lone star and blacklegged ticks.
Abundant reproductive hosts, an increasingly hospitable climate, and genetic plasticity of the lone star tick support the continued invasion and establishment of this tick in the Northeast. Increasing population densities and subsequent range expansion, in conjunction with nondiscriminating biting habits and the capacity to transmit diverse pathogens, position the lone star tick as an important emerging health threat to humans, domesticated animals, and wildlife. It’s also plausible that the lone star tick will displace local tick species, transmit different pathogens than those species, and alter the tickborne disease landscape. We believe it’s essential for practitioners and the public to develop a heightened awareness of the health risks associated with emergent tick vectors such as the lone star tick and their potential for changing the dynamics of tickborne diseases in the northeastern United States and elsewhere.
Disclosure forms provided by the authors are available at NEJM.org.