Ixodes scapularis

Say, 1821

deer tick, black-legged tick, bear tick, Eastern Black-legged Tick

Ixodes scapularis is a hard-bodied to the eastern and upper midwestern United States, with expanding in Canada. It is the primary of (Borrelia burgdorferi) in North America and transmits multiple other including microti, Anaplasma phagocytophilum, and Powassan virus. The tick has a two-year with three blood-feeding stages (, , ), each requiring a different individual. Climate change is driving northward range expansion and increased human encounters.

Ixodes scapularis by (c) Senna Bryce Robeson, some rights reserved (CC BY), uploaded by Senna Bryce Robeson. Used under a CC-BY license.

I-scapularis nymph by G. Hickling, University of Tennessee.. Used under a Public domain license.

Ixodes scapularis by Content Provider(s) : CDC/ Michael L. Levin, Ph. D. Photo Credit : Jim Gathany. Used under a Public domain license.

Pronunciation

How to pronounce Ixodes scapularis: //ɪkˈsoʊdiːz skəˈpjʊlərɪs//

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

females are 3–4 mm long when unfed, with a dark, covering the portion of the body; the exposed idiosoma is orange to red. Adult males are smaller (2–3 mm) with a rigid scutum covering the entire body, preventing engorgement. and adults have eight legs; have six. Unlike in other , Ixodes scapularis lacks . The groove forms a horseshoe shape anterior to the anal pore on the surface. Male are shorter than female palps. Engorged females become greatly enlarged and grayish. Distinguished from Ixodes pacificus (western black-legged tick) by geographic range; from () by smaller size, lack of , and dark scutum; from Amblyomma americanum (lone star tick) by absence of a spot.

Images

Habitat

Wooded and forested with dense leaf litter, particularly areas with high tree stem count, midstory cover, and cover. Found in between forest and meadow, grassy meadows, and vegetation along stream banks. Spends off- periods in leaf litter or soil. abundance correlates with oak mast production with a two-year lag due to small mammal .

Distribution

Eastern and upper midwestern United States, from Florida to Maine and west to Minnesota and Texas; expanding northward into Canada (Ontario, Quebec, Maritime provinces) and westward into central U.S. states. Range expansion attributed to climate warming, increasing deer , and land use changes.

Seasonality

Activity varies by and latitude. active primarily in summer. most active in spring and early summer (peak May–July), presenting highest risk for human transmission. active in fall and early spring; can remain active after hard frosts when daytime temperatures warm sufficiently. In warmer regions, may be active on mild winter days. Two-year typical in northern temperate regions.

Diet

Obligate blood feeder; requires a blood meal from a vertebrate to develop at each . , , and each feed on different host individuals, typically of different .

Host Associations

White-tailed deer - primary for feeding and Odocoileus virginianus; preferred of stage, though deer do not transmit Borrelia to
White-footed mouse - for stages and acquisitionPeromyscus leucopus; for Borrelia burgdorferi and other
Eastern chipmunk - bloodmeal Tamias striatus; bloodmeal source in some (36% of analyzed meals)
Eastern gray squirrel - bloodmeal Sciurus carolinensis; significant bloodmeal source (26% of analyzed meals)
Humans - incidental Homo sapiens; most commonly implicated in human transmission due to small size and summer activity
Various birds and lizards - for stagesParticularly important for larval and nymphal via migratory birds

Life Cycle

Three- with four stages: , , , and . Eggs hatch into six-legged larvae, which feed once and to eight-legged nymphs. Nymphs feed once and molt to adults. Adults feed, mate on host, and females drop off to lay eggs. Each feeding stage takes a blood meal from a different host individual. Two-year cycle: eggs laid spring of year one, larvae feed summer year one, nymphs feed spring-summer year two, adults feed fall-spring and lay eggs. Females lay 1,500–3,000 eggs. Off-host survival periods in leaf litter can extend months.

Behavior

Exhibits : climbs vegetation and extends forelegs to detect and grasp passing . Responds to CO₂, heat, movement, and host odors; CO₂ occurs even without the Haller's organ, indicating additional sensory structures. speed increases and foreleg waving (questing posture) initiates upon CO₂ exposure above 1% concentration. Host-seeking height varies geographically: northern quest above leaf litter, southern populations remain beneath leaf litter. Can be active after hard frosts when daytime temperatures permit.

Ecological Role

Principal of and other -borne in eastern North America. Serves as bridge between wildlife pathogen (small mammals) and human . Blood feeding influences small mammal . Endosymbiotic relationship with buchneri, which may influence tick and pathogen interactions. in transmission cycles.

Human Relevance

Major public health threat as of (30,000+ reported U.S. cases annually, estimated 300,000 actual ), , , and Powassan virus . Nymphal bites most commonly associated with human disease due to small size and summer activity when human exposure is highest. Risk of transmission increases with attachment duration; removal within 24 hours reduces disease risk. Prevention relies on avoidance, protective clothing, , and prompt removal. Photo-based identification services (e.g., TickSpotters) achieve >98% for this .

Similar Taxa

Ixodes pacificusWestern black-legged ; morphologically similar but restricted to Pacific Coast states; historically distinguished by geography though some overlap may occur
Dermacentor variabilis; larger size ( 5–15 mm), has (rectangular plates along margin), and more patterning
Amblyomma americanumLone star ; females have distinctive spot on ; more aggressive human biter; different complement including Ehrlichia chaffeensis

More Details

Genomic features

sequenced at approximately 2.1 gigabases, among the largest genomes; - assembly resolved into 15 pseudochromosomes (13 plus X and Y ). Possesses horizontally acquired dae2 genes encoding amidase effectors that limit Borrelia burgdorferi proliferation in the , influencing transmission .

Epigenetic variation

Northern and southern exhibit distinct methylation profiles; northern from Minnesota show reduced global methylation compared to southern Texas populations, potentially contributing to phenotypic plasticity and regional .

Alpha-gal syndrome

Contains alpha-gal ; bites have been associated with of red meat allergy (Alpha-Gal ) in sensitive individuals, though this association is more strongly documented for Amblyomma americanum.