by Brian Schwartz, M.D., M.S.
Professor and Director,
Division of Occupational and Environmental Health,
Department of Environmental Health Sciences,
Johns Hopkins School of Hygiene and Public Health
- What is Lyme disease?
- How do people get Lyme disease?
- What are the high-risk areas in the United States?
- Are there other risk factors, other than geography?
- What is the clinical presentation of early Lyme disease?
- Are all target-shaped rashes from Lyme disease?
- What is the clinical presentation of the later manifestations of Lyme disease?
- How is Lyme disease diagnosed?
- How is Lyme disease treated?
- How can individuals protect themselves?
- Is there a vaccination available to prevent Lyme disease?
What is Lyme disease?
Lyme disease is the most common vector-borne disease in the United States. It is caused by the spirochetal bacterium Borrelia burgdorferi, and it is transmitted to humans by the “bites” of certain kinds of ticks. Lyme disease cannot be transmitted from an infected person to another person. The infection can involve multiple organ systems and has myriad manifestations, but is rarely, if ever, fatal.
How do people get Lyme disease?
Lyme disease is transmitted to people by ticks of the Ixodes ricinus complex. The ticks painlessly insert a mouth part, not their whole bodies, into the skin of a person or animal to slowly feed on the blood. These include I. scapularis in the eastern United States, I. pacificus in the western United States, I. ricinus in Europe, and I. persulcatus in Asia. There is some controversy regarding the tick species in the northeastern United States. In the 1980s, some investigators published that the vector of Lyme disease in the northeast was a new tick species, which they called I. dammini. Recent data suggest that this tick is probably the same species as one that had been known in the southeast United States for around a hundred years, I. scapularis. Most scientists accept that these two species are now the same.
B. burgdorferi has been identified in other ticks, including Dermacentor variabilis and Amblyomma americanum, but there is little convincing epidemiologic data to suggest that these ticks are important vectors of Lyme disease. Ticks generally also have common names, which will be most often used by your patients (Table 1). There have also been case reports of Lyme disease after bites of biting flies.
Table 1. Tick species and common names.
|Amblyomma americanum||lone star tick||Probably uncommon vector|
|Dermacentor variabilis||dog tick||Probably uncommon vector|
|Ixodes scapularis||black legged tick||Vector species|
|Ixodes pacificus||western black legged tick||Vector species|
|Ixodes dammini||northern deer tick||This common name is applied to ticks in the northeast U.S., but many scientists do not believe this species is distinct from I. scapularis|
These ticks seek blood meals from a variety of mammal, reptile, and bird species. The animal that provides the blood meal is termed the host. An animal in which B. burgdorferi can live and from which a feeding tick can acquire the bacteria for subsequent transmission to the next host is termed a reservoir of the infection. There are data to suggest that I. scapularis must feed for 24-48 hours before B. burgdorferi is transmitted to the host. This is an important point in the prevention of Lyme disease, which will be discussed further, below. On animals, which cannot remove the ticks once attached, these ticks can feed for 4 to 7 days, and during that time they increase dramatically in size and weight. Humans probably detect attached ticks after several days, after they become engorged with blood.
The first stage, larvae, appear in the late summer and fall of any given calendar year. The larva feeds once, then molts to the next stage, nymph, which appears in the spring of year two of the two-year life cycle. The nymph feeds once, then molts to the adult form, which appears in the fall of year two. Male and female adults then mate, and the female over-winters, then lays eggs which will produce larvae within a few months. Sub-adult forms (larvae, nymphs) feed on a wide variety of small mammals, birds, and reptiles, but prefer to feed on white-footed mice (Peromyscus leucopus), which are the important reservoir of infection in nature. Adults prefer to feed and mate on white-tailed deer (Odocoileus virginianus).
What are the high-risk areas in the United States?
Over 95% of cases of Lyme disease come from three distinct geographic regions of the U.S. These are the northeastern U.S. from Maryland to Maine, parts of the upper mid-west in Wisconsin and Minnesota, and parts of northern California and southern Oregon. Several very important points must be emphasized. First, adequate tick habitats are very localized. Even within endemic areas, vector ticks are not homogeneously dispersed throughout the geographic area. Second, cases of Lyme disease have been reported from almost every state in the U.S., including from some states where vector ticks, infected reservoirs, or infected human tissues have never been identified. This illustrates difficulties in the diagnosis of the disease. Since 1989, approximately 75% of cases have been reported from just four states – New York, Connecticut, New Jersey, and Pennsylvania – and from just several specific counties within each state.
Approximately 12,500 cases of Lyme disease have been reported in the U.S. annually from 1993-1997. Since national surveillance began in 1982, the number of annual reported cases has increased 25-fold, and the cumulative number of reported cases now exceeds 128,000 individuals. The overall national incidence of reported cases of the disease is 5 per 100,000. In 1998, 16,801 cases were reported, with approximately 27% of cases from New York, 20% from Connecticut, 17% from Pennsylvania, 11% from New Jersey, 5% from Rhode Island, and 4% from each of Maryland, Massachusetts, and Wisconsin. The highest 1998 incidence rates were found in Connecticut (105 per 100,000), followed by Rhode Island (79.6), New York (25.5), New Jersey (24.0), Pennsylvania (22.9), Maryland (13.1), and Wisconsin (12.8). It is thought that there is likely to be under-reporting, so these numbers may be underestimates of the true burden of the disease. The highest attack rates are in children from 0 to 14 years of age and in persons over the age of 30 years.
Are there other risk factors, other than geography?
Most cases of Lyme disease are thought to result from peri-residential exposure to infected ticks. Persons who reside, work, or recreate in wooded areas or areas of overgrown brush are at risk of acquiring the infection. Several environmental factors have been identified that favor increased tick abundance. Some studies have shown that certain soil types, elevations, watersheds, and ground cover are associated with tick abundance, but these are of little clinical relevance. Patients often know whether they live or work in tick-infested areas. A useful clue to the possible presence of ticks is whether deer have been commonly observed in the area.
Self-reported tick exposure, age, outdoor recreational activities, pet ownership, residential area, and other factors have not been consistently identified as conferring risk from study to study. One cross-sectional study of outdoor workers in New Jersey identified occupational tick exposure, hunting, and male sex to be risk factors and antibiotic use and insect repellent use to be protective factors for anti-B. burgdorferi antibody seropositivity.
Several epidemiologic studies, including those that were based in the general population or confined to groups of workers, have documented that outdoor workers have an increased risk of Lyme disease. Seroprevalences of Lyme disease antibody measured by enzyme-linked immunosorbent assay (ELISA) or indirect fluorescent antibody have ranged from 5.6%-35% in populations with varying degrees of risk in several areas of the U.S. and Europe. In these cross-sectional studies, outdoor workers have been reported to have a 4-6 fold elevation in risk of clinical Lyme disease or seropositivity for antibodies to B. burgdorferi. It should be noted that seroprevalences in populations of normal, healthy volunteers from low- to moderate-risk areas have been as high as 5-10%.
Annual or seasonal risk of seroconversion in several longitudinal studies of B. burgdorferi infection have ranged from 0.4%-10% in populations at high-risk by virtue of area of residence or outdoor employment. Seroconversion in these studies was associated with an asymptomatic infection in 26-98% of cases. The average annual risk of clinical Lyme disease in several high-risk populations has ranged from 0-3.3%.
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