Lyme Disease

Lyme disease (LD) was named in 1977 when arthritis was observed in a cluster of children in and around Lyme, Connecticut. LD is a multisystem and multistage infection caused by a tick-borne spirochete. It is the most common arthropod-borne infection in the United States. There has been a steady increase in the incidence of the disease over the years and the distribution of the disease in the United States matches the distribution of ticks of the genus Ixodes. The tick Ixodes scapularis is responsible for the transmission of the LD bacteria in the Northeastern and Northcentral United States. On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick (Ixodes pacificus). Ixodes ticks are much smaller than common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Ticks feed by inserting their mouths into the skin of a host and slowly taking in blood. Ixodes ticks are most likely to transmit infection after feeding for two or more days. The disease is caused by Borrelia burgdorferi, a spriochete sharing sequence homology with Treponema and Leptospira. Borrelia burgdorferi is the longest and narrowest of the Borreliae. It contains several antigens that are important in pathogenesis and diagnosis including outer surface proteins, OspA through OspG, that are located on plasmids and a 41 kDa flagellar protein. Although there are three geno-species recognized within the Borrelia burgdorferi (B. burgdorferi sensu lato): B. burgdorferi sensu stricto, B. garinii, and B. afzelii, strains found in the United States are relatively homogeneous and conform to the definition of B. burgdorferi sensu stricto. The two other species are present in Europe and Asia and produce mixed infections in humans and mice. B. garinii is mainly associated with neuroborreliosis whereas B. afzelii is associated with arthritis and skin lesions. The risk of developing LD following a tick bite is less than 0.01 and it has been shown that it is not cost-effective to recommend prophylactic treatment for everyone that has been bitten by a tick. Like other spirochetal infections, the signs and symptoms of LD occur in stages and involve a variety of tissues and organs including the skin, joints, heart and nervous system. Early infection (stage 1) involves erythema migrans (EM), an annular skin rash that is seen days to weeks after a tick bite. Hematogenous dissemination of the bacteria days to weeks later (stage 2) can result in multiple skin lesions (secondary EM) as well as meningitis, rediculoneuritis, arterioventricular blockage, myocarditis and oligoarticular arthritis. Persistent infections (stage 3) occurs months to years following the initial exposure and can be associated with acrodermatitis chronica atrophicans, various encephalopathies and persistent arthritis. Clinical signs of LD among patients in North America tend to differ from those in Europe and Asia due to differences in Borrelia species in different parts of the world. The CDC has developed a case definition of LD for surveillance purposes that includes either physician-diagnosed EM along with solitary lesions of at least 5 cm or at least one joint, cardiac or neurological manifestation along with laboratory diagnosis. Culture isolation of B. burgdorferi sensu lato remains the gold standard for diagnosis although the recovery rate decreases as the disease stages advance with the most likelihood of isolating the bacteria in Barbour-Stoenner-Kelly medium (BSK or modified BSK) is in stage one EM. Detection of the bacteria in culture is accomplished using dark field microscopy, or by fluorescent microscopy using acridine orange stain or a specific antibody to the bacteria labeled with fluorescein. Serologic testing using antibodies to outer surface proteins (OsP-A to G), the 41 KDa flagellin protein and other heat shock proteins can be used although there have been reports about down regulation of OsPs A-G in the bacteria after a blood meal. Molecular testing is being widely used for the detection of the spirochete in lesions even before the appearance of antibodies in the patient’s serum. It was shown that PCR has close to 99% specificity and an average of 73% sensitivity and that molecular testing produces positive results in cases where the patients had already received prophylactic treatment and no antibodies or viable bacteria have been detected. The bacterial DNA tends to be detectable by PCR in joints and tissues for weeks following antimicrobial therapy. The PCR results from cerebrospinal fluid (CSF) vary and the overall sensitivity in CSF does not exceed 20%, therefore, a negative result in the CSF does not rule out LD. Urine has been shown not to be a good sample choice for diagnosis as the results showed large variations. In conclusion, the most important element in LD diagnosis is the clinical picture and patient history supported by laboratory testing using several methods to improve sensitivity. It is highly recommended that PCR testing be performed as early as possible following a possible exposure to ticks. If the results are positive, prophylactic treatment can be recommended by a clinician and other testing is performed to monitor the treatment efficacy.