INTRODUCTION — Lyme disease is a spirochetal infection transmitted by the bite of infected ticks of the Ixodes ricinus complex. It is caused primarily by Borrelia burgdorferi in the United States, and primarily B. afzelii, B. burgdorferi, and B garinii in Europe and Asia. In the northeastern United States, rodents such as the white-footed mouse are the primary reservoir of Borrelia species.
Lyme disease is the most common tick-borne infection in the United States and Europe. Because the ticks that transmit Lyme disease are frequently encountered in backyards and outdoor recreational areas, a high degree of public health awareness of Lyme disease must be maintained wherever the disease is known to occur.
The epidemiology of Lyme disease and the ecology of Ixodes species ticks will be reviewed here. The microbiology of Lyme disease, the evaluation of a tick bite for possible Lyme disease, and the clinical manifestations, diagnosis, treatment, and prevention of Lyme disease are discussed separately. (See "Microbiology of Lyme disease" and "Evaluation of a tick bite for possible Lyme disease" and "Clinical manifestations of Lyme disease in adults" and "Lyme disease: Clinical manifestations in children" and "Diagnosis of Lyme disease" and "Treatment of Lyme disease" and "Prevention of Lyme disease".)
The epidemiology of other tick-borne diseases is reviewed elsewhere. (See "Clinical manifestations and diagnosis of Rocky Mountain spotted fever" and "Human ehrlichiosis and anaplasmosis" and "Southern tick-associated rash illness (STARI)" and "Babesiosis: Microbiology, epidemiology, and pathogenesis" and "Tick paralysis" and "Microbiology, pathogenesis, and epidemiology of relapsing fever".)
EPIDEMIOLOGY — Overall, Lyme disease is the most commonly reported tick-borne disease in the United States and Europe [1,2]. Lyme disease was first recognized clinically in 1977 as "Lyme arthritis" during studies of a cluster of children in Connecticut who were thought to have juvenile rheumatoid arthritis [3]. The etiologic agent was discovered to be a spirochete in the early 1980s [4,5]. In 1991, the disease was designated a nationally notifiable condition in the United States, and since then, the incidence and geographic distribution of cases in the United States have increased substantially [6-9]. The incidence in some regions of Europe may be increasing as well [10].
The ongoing emergence of Lyme disease appears to be driven in part by environmental factors. As an example, in Europe and Canada, climate change is expanding the geographic areas where Ixodes ticks can survive [11,12]. In the United States, changes in land use practices and a marked increase in the deer population have increased the risk of exposure to infected ticks in certain regions [13,14].
There are also factors related to the organism itself that may influence the incidence of Lyme disease. A particularly virulent strain of B. burgdorferi called OspC type A is prevalent in the northeastern United States [15,16]. This genotype has a high transmission frequency among ticks and may be increasing in frequency in nature. This may have been important in the emergence of Lyme disease in the northeastern United States during the late 20th century [17].
United States — A total of 252,681 confirmed cases of Lyme disease were reported in the United States from 2010 to 2019 (figure 1). In 2019, the disease was the sixth most common Nationally Notifiable disease [18]. In an analysis of national surveillance data for 16 reportable vector-borne diseases, Lyme disease accounted for 63 percent of all cases reported from 2004 to 2016 and 82 percent of reported tick-borne infections [1].
As with other diseases, not all cases of Lyme disease are captured through routine surveillance. Reporting practices vary by location, and underreporting of cases is common. One study suggests that approximately 476,000 people are diagnosed and treated for Lyme disease in the United States each year [19].
In 2019, 14 states accounted for over 93 percent of confirmed cases; these states included Connecticut, Delaware, Maine, Maryland, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia, West Virginia, and Wisconsin (figure 2) [18]. Historically, Massachusetts has ranked among the top 14 states with the highest Lyme disease incidence; however, changes in local surveillance practices resulted in fewer cases being reported in recent years. The reported number of cases by state or locality can be found on the CDC website.
Lyme disease occurs most commonly in forested regions, including some suburban areas, such as those near Boston, New York, and Philadelphia [20]. Within highly endemic regions, the abundance of ticks and the percentage of ticks that are infected can vary greatly depending upon local ecological features. As an example, among over 11,000 Ixodes scapularis collected from 27 sites in New York from 2003 to 2006, rates of B. burgdorferi infection ranged from 2.1 to 30.7 percent among nymphs and from 17.1 to 51.5 percent among adults [21]. Additionally, up to 20 percent of adult ticks were infected with the agent responsible for human granulocytic anaplasmosis (caused by Anaplasma phagocytophilum, formerly called human granulocytic ehrlichiosis). (See "Human ehrlichiosis and anaplasmosis".)
Although all age groups are affected by Lyme disease, the distribution of cases is distinctly bimodal, with peaks among children 5 to 14 years old and adults 45 to 55 years old (figure 3) [7]. Males account for 56 percent of reported cases in the United States. This age and sex pattern may reflect the amount of time certain groups spend outdoors in contact with forested habitats.
Europe and Asia — Although the etiology of Lyme disease was unknown until the early 1980s [4,5], cases of acrodermatitis chronica atrophicans (a skin manifestation of Lyme disease in Europe) have been described in Europe as far back as 1883 [22,23]. (See "Clinical manifestations of Lyme disease in adults", section on 'Acrodermatitis chronica atrophicans'.)
In Europe, as in the United States, there are regions of high prevalence of Lyme disease in forested areas [24]. Although the risk of Lyme disease is widespread (figure 4), rates of reported disease vary widely among countries. In general, transmission appears to be greatest in northeastern and central Europe and then decreases moving south and westward [25]. Rates exceeding 100 per 100,000 population have been reported in areas of Austria, Germany, Slovenia, Switzerland, Sweden, Norway, and the Netherlands [25,26]. To facilitate comparisons across countries, the European Union has designated neuroborreliosis a notifiable condition and has developed a standard case definition [27,28].
Lyme disease also occurs in Russia, Japan, and China (figure 4) [29-31].
BORRELIA SPECIES — Several species of Borrelia account for most cases of Lyme disease in the world:
●B. burgdorferi is the primary cause of Lyme disease in North America. A second genospecies, B. mayonii, has also been described as a cause of human illness in the upper midwestern United States [32].
●In Europe and Asia, B. afzelii, B. garinii, and B. burgdorferi, are the predominant species causing human illness.
The diversity of Borrelia species in different geographic regions is discussed in greater detail separately. (See "Microbiology of Lyme disease", section on 'Diversity of Lyme Disease species'.)
TICK VECTORS — The three tick species that commonly bite people in the eastern United States include the blacklegged tick (I. scapularis) (picture 1), the Lone star tick (Amblyomma americanum), and the American dog tick (Dermacentor variabilis) (figure 5) [33]. Of these three, field and laboratory studies show that the blacklegged tick (I. scapularis) is the sole competent vector of B. burgdorferi in the hyperendemic regions of the eastern United States [34-37].
Regional distribution — Lyme disease is transmitted by various Ixodes ticks, depending on geography [38]. The regional distribution of the Ixodes ticks that transmit Lyme disease follows (figure 4):
●I. scapularis in eastern and north central regions of North America
●I. pacificus in western North America (eg, northern California and Oregon)
●I. persulcatus in Asia and Eastern Europe
●I. ricinus in Europe and Western Asia
The distribution of these ticks is Holarctic, spanning both the Old World (Palearctic Region) and the New World (Nearctic Region) in the northern hemisphere [38]. These vector ticks are forest dwellers, spending most of their time hiding in the leaf litter of the forest floor, where humidity is high and the risk of desiccation is low. Thus, Lyme disease is associated with heavily forested areas of the northern hemisphere.
Life cycle — There are three free-living stages of I. scapularis ticks: the six-legged larva, the eight-legged nymph, and the sexually dimorphic eight-legged adult tick. The larvae, nymphs, and adult females require a blood meal from a vertebrate host, whereas the adult male does not. Larvae are rarely, if ever, infected with B. burgdorferi, whereas approximately 25 percent of nymphal I. scapularis and up to 50 percent of adult female I. scapularis are infected in some areas [34].
I. scapularis ticks in the northeastern and north-central United States go through a two-year cycle from larva to nymph to adult [39]:
●Ixodes larvae hatch in the early summer and begin questing in the late summer in the leaf litter of the forest floor. Emerging larvae search for a blood meal, usually from mice, other small mammals, or birds. Tick larvae are uninfected upon hatching, even in hyperendemic areas. The infection is acquired by the larva during its first blood meal from an infected mouse. Once infected, mice remain healthy, but they are persistently infectious. After its meal, the tick larva falls off the mouse, molts, and re-emerges the next spring as a nymph.
●The infected nymph will spread the organism to a second host (eg, mouse, vole, chipmunk) during its blood meal in the late spring or early summer. This host then spreads the organism to future generations of larvae to complete the horizontal cycle of transmission. Once fed, the tick drops off the host and molts to an adult in the fall.
●Adult ticks seek a blood meal during warm days in the fall, winter, and early spring. Their preferred host is the white-tailed deer.
●In the fall and winter, after feeding, the adult female tick falls to the forest floor but does not lay eggs in the leaf litter until the next spring; thus, the two-year life cycle repeats itself.
Ticks seeking a meal reside on the underside of low-lying shrubs or grass. Ticks sense warmth and carbon dioxide; they will latch on to a mammal with these attributes that brushes against them. The tick must usually be attached for 36 to 48 hours before transmission of B. burgdorferi occurs.
Dissemination within ticks — Using live imaging of B. burgdorferi expressing green fluorescent protein, it has been observed that B. burgdorferi uses a biphasic mode of dissemination within ticks [40]. In the first phase, replicating spirochetes in the gut form networks of nonmotile organisms that advance toward the basolateral surface of epithelial cells. In the second phase, the spirochetes become motile and penetrate the basement membrane and enter the hemocoel before reaching and entering the salivary glands.
Transmission to humans — The nymphal stage of I. scapularis is the primary vector of B. burgdorferi. One reason that nymphal I. scapularis are much more effective at transmitting B. burgdorferi to people than are adult female I. scapularis is the relative size difference. Nymphal I. scapularis may be quite small when they first attach to people (<2 mm), whereas adult female I. scapularis are much larger (figure 5).
Nymphal ticks commonly feed on people for the minimal period required to transmit an infectious dose of spirochetes (two days), whereas adult females rarely remain attached and escape detection for >2 days [41,42] (see "Evaluation of a tick bite for possible Lyme disease", section on 'Tick attachment'). Thus, the risk of Lyme disease is closely associated with areas where nymphal I. scapularis infected with B. burgdorferi commonly bite people. An ecological risk map study demonstrated that infected host-seeking I. scapularis nymphs are concentrated in the northeastern and upper midwestern United States and are much less common in the southern United States [43].
SEASONALITY — In the Lyme disease-endemic regions of the northeastern United States, nymphal I. scapularis become active in mid-May, peak in activity in June, and become less active during late July. Lyme disease onset occurs mainly during the summer months of June, July, and August, since the incubation period between the tick bite and the onset of Lyme disease is approximately two to three weeks (figure 6). Although the life cycle of the Ixodes tick vectors of Lyme disease spirochetes may vary in different regions, the onset of Lyme disease generally occurs in the summer wherever it is found. Arthritis, a late disease manifestation, may present at any time of year.
In contrast to nymphs, adult I. scapularis generally seek hosts during the colder months from October to December and March to April, when few cases of Lyme disease are diagnosed [41,44].
RESERVOIR HOSTS — In the northeastern United States, the primary reservoir hosts for B. burgdorferi are rodents. In particular, the white-footed mouse, Peromyscus leucopus, was identified as the primary reservoir of B. burgdorferi soon after Lyme disease was described [45]. The majority of white-footed mice are infected with B. burgdorferi in areas of the northeastern United States where Lyme disease is hyperendemic. More recent work suggests that the small rodent hosts for I. scapularis are broader than P. leucopus and also commonly include chipmunks and voles [46], shrews and birds [47].
Larval I. scapularis hatch from egg batches in May and seek hosts in late summer (August to September). These larvae ingest spirochetes when feeding on an infected rodent [48]. Infected larvae molt to nymphs, and then overwinter, and are ready to feed the next year in early summer (May to July) when they may feed on another rodent, bird, or human (figure 7). Thus, the Lyme disease spirochete enzootic cycle is perpetually extended from year to year in an extremely efficient fashion.
Although small rodents, particularly white-footed mice, are central to the Lyme disease enzootic cycle, immature I. scapularis feed on a wide variety of hosts including a number of types of rodents, insectivores, lagomorphs, medium-sized mammals, birds, and reptiles. In suburban settings, where human dwellings exert a strong influence on the environment, other hosts such as chipmunks, squirrels, raccoons, skunks, and even shrews may come to play a larger role than white-footed mice as reservoirs of B. burgdorferi [49,50]. Indeed, in certain areas of the north central United States, chipmunks may be more important than white-footed mice as reservoirs of B. burgdorferi [51].
Birds may play a special role as reservoirs of B. burgdorferi, but not as enzootic reservoirs in highly endemic regions of the eastern United States. Rather, they may be phoretic hosts, carrying infected ticks to regions where I. scapularis and B. burgdorferi are not well established. In Canada, birds have been identified as a likely source of infected ticks in southern Quebec, where the Lyme disease spirochete enzootic cycle is not well established [52]. Lizards serve as hosts for immature I. scapularis in the southern United States. Since lizards may be relatively refractory hosts for B. burgdorferi, lizards may reduce tick infection rates, thus serving as "zooprophylactic hosts" and lowering transmission potential in the south [13].
In contrast to the pattern of transmission seen in the eastern United States, in northern California, two intersecting cycles occur [53,54]. One cycle involves western grey squirrels and I. pacificus ticks, which are infected with B. burgdorferi and bite humans [55-57]. Another cycle involves the dusky-footed wood rat and I. spinipalpis (formerly I. neotomae) ticks, which do not bite humans but maintain the cycle of B. bissettii in nature [58]. However, B. bissettii does not cause Lyme disease in North America. (See "Microbiology of Lyme disease", section on 'Diversity of Lyme Disease species'.)
In certain areas of Europe, where B. garinii is the predominant spirochete infecting humans, song birds may play a key role as reservoirs [59]. In Japan, rodents are the main reservoir host of B. garinii strains that cause disease in humans [30].
ROLE OF DEER — Adult I. scapularis ticks feed on larger animals, particularly deer, but will not feed on smaller hosts like rodents [60]. Indeed, although the accepted common name for I. scapularis is the blacklegged tick, it is known colloquially in many areas as the deer tick. Adult females feed on deer, acquire a blood meal, and fall off into the leaf litter to lay their eggs. The dramatic rise in white-tailed deer populations during the second half of the 20th century played a key role in supporting the expansion of I. scapularis and prompting, in part, the rapid emergence of Lyme disease that began in the 1980s [13,14]. While deer are required to support I. scapularis populations, they do not play a role as reservoirs of the Lyme disease spirochete; indeed, deer complement appears to be lytic to B. burgdorferi [61]. Thus, deer are important for the survival of the ticks that transmit Lyme disease but do not support the spirochetes themselves.
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Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topic (See "Patient education: Lyme disease (The Basics)".)
●Beyond the Basics topic (See "Patient education: Lyme disease symptoms and diagnosis (Beyond the Basics)".)
SUMMARY
●Lyme disease is a spirochetal infection caused by Borrelia species (B. burgdorferi and B. mayonii in the United States and primarily B. afzelii and B. garinii in Europe and Asia) and transmitted by the bite of infected Ixodes ricinus complex ticks. (See 'Introduction' above.)
●The incidence of Lyme disease in the United States has steadily increased since its discovery in the late 1970s. It is currently the most commonly reported tick-borne disease in both the United States and Europe. (See 'Epidemiology' above.)
●I. scapularis, also known as the blacklegged tick or deer tick, is the principal vector of B. burgdorferi in the hyperendemic regions of the eastern United States. Other Ixodes species are the vectors of Lyme disease in certain other regions: I. pacificus in western North America, I. persulcatus in Asia, and I. ricinus in Europe. (See 'Tick vectors' above.)
●The peak incidence for the onset of early Lyme disease in the eastern United States occurs during the summer months of June, July, and August (figure 6). Arthritis, a late disease manifestation, may present at any time of year. (See 'Seasonality' above.)
●In the eastern United States, the primary reservoir host for B. burgdorferi is the white-footed mouse, Peromyscus leucopus, or other small rodents, including chipmunks and voles. (See 'Reservoir hosts' above.)
●Deer are important for the survival of the ticks that transmit Lyme disease because they provide adult ticks with a blood meal. However, they do not support the spirochetes themselves. (See 'Role of deer' above.)
ACKNOWLEDGMENTS — The UpToDate editorial staff acknowledges Joseph Piesman, DSci, C Benjamin Beard, PhD, and Amy Schwartz, MPH who contributed to earlier versions of this topic review.
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