Lyme disease was first described in the United States in the mid 1970s when Steere investigated an outbreak of arthritis in 51 patients living near Lyme, Connecticut. These patients developed recurrent attacks of arthritis and nearly all lived in heavily wooded areas. Steere noted that a significant number of these patients remembered having an expanding erythematous rash. Steere and others became aware that "Lyme disease" was a multisystem illness that involved not only joints and skin, but could also involve the heart and the nervous system. Several of Steere's patients remembered having a tick bite prior to developing the expanding erythematous rash; one of the patients saved the tick, and it was identified as Ixodes scapularis (formerly I. dammini). In 1982, Burgdorfer identified spirochetes in the tick midgut region. Soon thereafter this spirochete was isolated from patients, classified as a Borreli a species, and given the name Borrelia burgdorferi.

Vector

In the United States, Borrelia burgdorferi is known to be transmitted by certain Ixodes ticks. Ixodes scapularis or "deer tick" is found predominantly in the northeastern United States as well as in Minnesota and Wisconsin. The tick has a two-year life cycle and takes a blood meal during each of the three stages of the life cycle — larval, nymphal and adult (Fig 1). The preferred host for larvae and nymphs is the white-footed mouse, while adult ticks prefer to feed on white-tailed deer. Ticks can feed on humans at any stage, but most often the nymph transmits the spirochete.

In the western United States, limited information is available on the tick vector, its life cycle, and its preferred hosts. Ixodes pacificus (Fig. 2), the "western black-legged tick," is believed to be the main vector in Pacific states.

Ixodes nymphs are small (2-3 mm), which explains why over 50 percent of patients with Lyme disease have no recollection of a tick bite (Fig. 3). Animal studies have shown that Ixodes ticks must remain attached for at least 24 hours in order to transmit B. burgdorferi. During feeding, ticks gradually become engorged and more noticeable (Fig. 4).

Dermacentor ticks, which are common in eastern Washington, are not competent vectors of Lyme disease. They are readily differentiated from Ixodes ticks by their darker body and lighter head region (Fig. 5a and 5b).