chronic lymphocytic meningitis and sometimes cranial or peripheral neuritis. In a few cases, the neurologic syndrome was preceded by an erythema.

These various syndromes were brought together conclusively in 1982 when a previously unrecognized spirochete, now called Borrelia burgdorferi, was recovered from Ixodes dammini ticks (also called Ixodes scapularis) (9). The spirochete was then isolated from patients with Lyme disease in the United States (10, 11) and from those with erythema migrans, Bannwarth's syndrome, or acrodermatitis in Europe (12–14). In addition, the immune responses of patients were linked conclusively with infection with this organism.

Although the basic outlines of the illness are similar worldwide, regional variations have been noted (15). Lymphocytoma, acrodermatitis, and encephalomyelitis have been seen primarily in Europe (16, 17), whereas widely disseminated early infection, secondary annular skin lesions, and arthritis have been found more commonly in America (18, 19). Recent work in the classification of B. burgdorferi has begun to clarify the issue of geographic differences in Lyme disease. Using a variety of methods, three genomic groups of B. burgdorferi have now been identified (20, 21). To date, all North American strains have belonged to the first group, B. burgdorferi sensu stricto. Although all three groups have been found in Europe, most isolates have been group 2 or 3 strains. Group 2 strains have been renamed Borrelia garinii (21), and group 3 strains have been renamed Borrelia afzelii (81).

Lyme disease is transmitted in all of these locations by several closely related ixodid ticks that are part of the Ixodes ricinus complex. These include I. dammini (also named I. scapularis) in the northeastern and midwestern United States (3, 22), Ixodes pacificus in the western United States (82), I. ricinus in Europe (23), and Ixodes persulcatus in Asia (24). The endemic cycle of B. burgdorferi varies among geographic locations. In the northeastern and midwestern United States, the preferred host for both the larval and nymphal stages of I. dammini is the white-footed mouse Peromyscus leucopus (22). It is critical that this rodent host is tolerant to infection with B. burgdorferi and that both of the tick's immature stages feed upon this host, since the life cycle of the spirochete depends upon horizontal transmission between immature ticks and mice (25). The preferred host for the adult stage of I. dammini is the white-tailed deer Odocoileus virginianus (26). Although deer are not involved in the life cycle of the spirochete, they seem to be critical for