The vector: Black-legged ticks.
Left to right: The black-legged tick (Ixodes scapularis) adult female, adult male, nymph, and larva on a centimeter scale.
VECTOR [n] any agent (person or animal or microorganism) that carries and transmits a disease. "Ticks are vectors of Lyme disease."
Black-legged ticks (Ixodes scapularis) are responsible for transmitting Lyme disease bacteria to humans in the northeastern and north-central United States. On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick (Ixodes pacificus).
Ticks are related to spiders and mites in the Arachnid Class. The black-legged tick falls within the family Ixodidae, or hard ticks. Most ixodid ticks 'quest' (ambush) for hosts in low lying brush and leaf litter on the forest floor. This is accomplished by climbing to the edge of leaves or twigs, and holding their front legs out. When a host passes by, they attach themselves and look for a suitable feeding site. They are widely dispersed, but often accumulate along pathways, game trails, or bedding locations used by their hosts. Ixodes ticks are much smaller than common dog and cattle ticks.
IMMATURE STAGES (LARVA & NYMPH)
In their larval and nymphal stages, they are no bigger than a pinhead and prefer to feed on small rodents. Larval and nymphal black-legged ticks can usually be found in the lower vegetation of forests in areas where they are endemic. They quest for small mammal hosts in the early spring through late summer. Occasionally, however, people enjoying the outdoors may come into contact with one of these small ticks as an 'accidental host'. Larval ticks rarely infect humans, as few to none hatch with the infection. Larval ticks can acquire infection from their small mammal hosts. Nymphal ticks, however, are the most dangerous of the life stages. The nymphs are very small (appx. 1 1/2 mm) and may be infectious from a previous blood meal. They are most prevalent in the spring and early summer (many people enjoy the outdoors at this time), and begin questing as early as the spring thaw.
Adult female ticks (most prevalent in the spring and autumn seasons) prefer the white-tailed deer for a blood meal, but may bite humans. The adult female may also be infected with Lyme disease bacteria, but because of the size difference, are easier to notice. Adult male ticks rarely feed, and use the white-tailed deer to find their mates.
Ticks feed by inserting their mouthparts painlessly into the skin of a host, and slowly taking in blood. This feeding process can take from 3-7 days depending on the stage of the tick. Sites favored by the ticks are the waistline, thighs, armpit, hairline and head, but may be found in any location. Prompt removal of attached ticks, however, can prevent transmission of the disease. Studies have shown that removal of attached, infected ticks prior to 48 hours significantly reduces risk of acquiring the disease.
Below: A 'questing', female black-legged tick from Southwest Michigan.