Antigenic
variation of B. burgdorferi sensu lato
In the United States, a number of genospecies of B. burgdorferi
sensu lato have been isolated from animals and ticks, but
only OspA expressing B. burgdorferi sensu stricto has
been isolated from humans (23), and existing evidence suggests
that rOspA vaccines will be protective against most if not all
human infections in the United States (24). B. burgdorferi
sensu stricto also occurs in Europe, but the dominant European
and Asian genospecies are B. garinii and B. afzelii,
both of which antigenically distinct from B. burgdorferi sensu
stricto (25), and vary in their expression of OspA. Vaccines
using combinations of immunogenic proteins may be necessary to
provide protection against multiple genospecies (26).
Routes of
Transmission
Borrelia burgdorferi infection is acquired from infected
ticks at the time they take a blood meal (27), and not by person-to-person
spread or direct contact with infected animals. Transplacental
transmission of B. burgdorferi has been reported (28,
19), but the effects of such transmission on the fetus remain
unclear. The results of several epidemiologic studies suggest
that congenital Lyme disease must be extremely rare if it occurs
at all (30, 31). Transmission in breast milk has not been described.
Although B. burgdorferi can be cultured from the blood
in some patients with early acute infection, and is able to survive
for several weeks in stored blood, the risk of transfusion acquired
infection is thought to be minimal (32).
Tick Vectors
of Lyme Disease
Borrelia burgdorferi is transmitted to humans by ticks
of the Ixodes ricinus complex (33). Ixodes scapularis
(the black-legged, or deer tick) is the vector in the eastern
United States; I. pacificus (the western black-legged
tick) transmits B. burgdorferi in the western United States
(34, 35). Ixodes scapularis is a vector for human granulocytic
ehrlichiosis and babesiosis (33, 36). In their nymphal stage,
these ticks feed predominantly in the late spring and early summer.
The majority of Lyme disease cases result from bites by infected
nymphs. In highly enzootic areas of the United States, approximately
15-30% of questing I. scapularis and up to 14% of I.
pacificus nymphs are infected with B. burgdorferi (37-40).
However, in the southern United States, the prevalence of infection
in I. scapularis ticks is generally 0-3% (35). The risk
of acquiring Lyme disease in the United States varies with the
distribution, density and infection prevalence of vector ticks
(Figure 1).
Over the past
several decades, the distribution of I. scapularis has
spread slowly in the northeastern and upper north-central regions
of the United States (41). Although deer are incompetent reservoirs
of B. burgdorferi, they are the principal maintenance
hosts for adult black-legged ticks, and the presence of deer
appears to be a pre-requisite for the establishment of I.
scapularis in any area (42). The explosive repopulation of
the eastern United States by white-tailed deer in recent decades
has been linked to the spread of I. scapularis ticks and
of Lyme disease in this region. The future limits of this spread
are not known (41).
Distribution
of Human Cases of Lyme Disease
Lyme disease is endemic in several regions in the United States,
Canada and temperate Eurasia (1, 43). Lyme disease accounts for
more than 95% of all reported cases of vector-borne illness in
the United States. Using a national surveillance case definition
(44), more than 62,000 cases were reported by states to the CDC
from 1993 - 1997, and the national mean annual rate in this 5-year
period was 5.5 cases per 100,000 population (CDC, unpublished).
Persons of all ages are thought to be equally susceptible to
infection, although the highest reported rates of Lyme disease
occur in children aged greater than 15 years of age, and in adults
aged 30-59 years (1). Both under-reporting and overdiagnosis
are common (45-47). Although cases of Lyme disease have been
reported by 48 states, only 13 states are considered by CDC to
be either highly or moderately endemic, and risk is thought to
be low or non-existent in the remaining 37 states and the District
of Columbia (Table 1). More than 90% of cases are reported by
about 150 countries in these 13 high and moderate risk states,
which are located along the northeastern and mid-Atlantic seaboard
and in the upper north-central region of the United States (Figure
1).
A rash similar
to erythema migrans of Lyme disease, but not caused by B.
burgdorferi infection, has been described in patients who
have been bitten by ticks in the southern United States (48,
49). This rash is suspected to be associated with the bite of
Amblyomma americanum ticks (50), and there is no evidence
that patients with the rash develop disseminated disease.
Populations
at Risk of Lyme Disease
Most B. burgdorferi infections are thought to result from
periresidential exposure to ticks (37, 51-54) during property
maintenance, recreation, and leisure activities. Thus individuals
who live or work in residential areas surrounded by woods or
overgrown brush infested by vector ticks are at risk of getting
Lyme disease. In addition, persons who participate in recreational
activities away from home such as hiking, camping, fishing and
hunting in tick habitat, and persons who engage in outdoor occupations,
such as landscaping, brush clearing, forestry, and wildlife and
parks management in endemic areas may also be at risk of getting
Lyme disease (55-57).
