Microbiology

Lyme disease is a multisystem disease caused by infection with the bacterial spirochete, B. burgdorferi, which is transmitted by Ixodes ticks. The enzootic life cycle of B. burgdorferi is dependent upon its transmission between an insect vector, the Ixodes tick, and a reservoir host, most commonly the white-footed mouse. Tick larvae usually feed in the late summer and acquire B. burgdorferi from an infected animal host. Nymphal ticks feed in the late spring and summer, and serve as the most common source of human infection. Adult ticks feed in the fall, winter and early spring, with the white-tailed deer being the preferred host. Adult ticks can also transmit B. burgdorferi to humans.¹ Both deer and rodent hosts are necessary to maintain the enzootic cycle of B. burgdorferi.

Epidemiology

Lyme disease is the most commonly diagnosed vector-borne disease in the United States, with over 99,000 cases reported to the Centers for Disease Control and Prevention (CDC) from 1982 to 1996. During that time, the incidence of reported cases increased by at least 32-fold. Although most cases have been reported in the Northeast, upper Midwest and Pacific coastal areas of the United States, infections have been reported in almost all states.² The incidence rates vary considerably from state to state and even within states at the county level.²

The trend of an increasing incidence in some established endemic areas continues, along with the geographic spread of the causative organism to new areas.^1,2,4,5

Lyme disease has a bimodal age distribution, with the highest number of cases occurring in children 2 to 15 years of age and adults 30 to 55 years of age.⁴

The primary risk factor for Lyme disease is exposure to wooded or grassy areas inhabited by B. burgdorferi-infected ticks. Such areas may include woodlands, meadows, or residential yards in endemic areas.⁵ Cases have been reported in people whose only exposure to B. burgdorferi has been while on vacation in an endemic area.¹

Lyme disease has been reported to occur throughout the year.^{7, 8} Peak incidence of Lyme disease varies by region and may vary annually based on fluctuations in local climatic conditions.^1,5,7,8 For example, the peak occurs in the late spring and summer in the Northeast United States, coincident with the feeding of nymphal ticks, the most common source of human infection. Transmission can occur also in the fall, winter, and early spring when adult ticks are feeding.¹

Clinical Manifestations: Lyme disease has a variable incubation period.⁵ Lyme disease is a multisystem disease, which has been described as having early and late stages. The early stage is usually characterized by a rash (erythema migrans) and may be accompanied by fever, fatigue, myalgias and/or arthralgias. Erythema migrans represents a localized cutaneous infection and is the presenting symptom in 60% to 80% of cases. Early disseminated manifestations include secondary skin lesions, neurologic involvement (meningitis, facial palsy, other cranial neuritides, radiculoneuritis), cardiac involvement (atrioventricular block, myocarditis), and musculoskeletal symptoms usually consisting of migratory pain in joints and the surrounding soft tissue structures.⁹

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