Lyme disease is an infection caused by the bacterium Borrelia burgdorferi, transmitted to humans through the bite of infected blacklegged ticks. The impact of this tick-borne illness is wide-ranging, affecting multiple body systems if left untreated. Lyme disease can cause arthritis, a late-stage manifestation known as Lyme arthritis. This condition is a well-documented consequence of the bacterial spread within the body.
Understanding Lyme Arthritis
Lyme arthritis (LA) is a late-stage manifestation, typically occurring weeks to months following the initial tick bite. The onset of joint symptoms averages approximately six months after the initial infection. This condition develops when B. burgdorferi bacteria disseminate through the bloodstream and colonize the joint tissues.
The presence of the bacteria in the joint space triggers an intense inflammatory response from the body’s immune system. Specific bacterial components bind to Toll-like receptors (TLRs) on host cells, initiating inflammation within the synovium, the lining of the joint capsule. This immune-mediated inflammation directly causes the swelling and discomfort characteristic of the condition. Historically, roughly 60% of untreated patients with Lyme disease developed this form of arthritis.
Recognizing Joint Symptoms
The clinical presentation of Lyme arthritis is characteristic, usually involving a specific pattern of joint inflammation. This condition most frequently presents as a monoarticular or oligoarticular arthritis, affecting only one or a few joints at a time. The knee is the most common site of involvement, affected in up to 90% of cases.
Other large joints, such as the shoulder, ankle, elbow, wrist, and hip, can also be involved, often in an asymmetric pattern. A distinguishing feature is the disproportionate swelling: patients experience large joint effusions with significant fluid accumulation, but relatively mild pain. The affected joint may feel warm, and the swelling can be massive, unlike the generalized stiffness seen in other forms of arthritis.
The course of Lyme arthritis is typically episodic, with attacks of swelling and pain beginning suddenly and lasting for several weeks or months. These inflammatory episodes can resolve spontaneously, only to return later if the underlying infection is not addressed. Without appropriate intervention, persistent inflammation risks causing permanent damage to the joint cartilage and bone structure.
Confirming the Diagnosis
The diagnosis of Lyme arthritis relies on clinical symptoms and specific laboratory evidence of the B. burgdorferi infection. Since Lyme arthritis is a late-stage disease, the patient’s immune system has produced a robust antibody response. Therefore, all patients with true Lyme arthritis are expected to have a positive result on serologic testing.
The standard procedure involves a two-tiered testing protocol. This starts with an initial screening test, such as an Enzyme Immunoassay (EIA) or Immunofluorescence Assay (IFA). If this first test is positive or equivocal, a more specific confirmatory test, the Western blot, is performed. For late-stage manifestations, the presence of IgG antibodies on the Western blot confirms exposure to the bacterium.
In addition to blood tests, arthrocentesis is often performed, which involves drawing fluid from the swollen joint. This synovial fluid is analyzed to rule out other causes of joint swelling, such as septic arthritis. The fluid can also be tested using Polymerase Chain Reaction (PCR) to detect the DNA of B. burgdorferi. PCR is positive in approximately 70% to 75% of untreated Lyme arthritis cases and supports the diagnosis.
Treating the Condition
The primary treatment for Lyme arthritis involves antibiotics to eradicate the B. burgdorferi bacteria from the body and joint tissue. For most adults, the initial protocol consists of a 28-day course of oral antibiotics, typically doxycycline or amoxicillin. This regimen is highly effective, leading to the resolution of joint symptoms in the vast majority of patients.
If joint swelling persists or recurs after the initial oral course, a second course of oral antibiotics or a switch to intravenous (IV) antibiotics, such as ceftriaxone, may be recommended. The prognosis is excellent, with arthritis resolution occurring in about 90% of patients following a single course of oral treatment.
In a small subset of individuals (approximately 10%), joint inflammation may persist despite completing two full courses of antibiotic therapy. This condition is known as antibiotic-refractory Lyme arthritis. In these cases, the inflammation is driven by an ongoing immune response rather than a continued active infection. Treatment for refractory arthritis shifts away from antibiotics and toward anti-inflammatory medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or disease-modifying anti-rheumatic drugs. Eventual resolution is expected for almost all patients.