Lyme Disease and Lyme Arthritis

What Lyme disease is

Lyme disease is an infection caused by Borrelia bacteria, spread by the bite of certain small Ixodes ticks (the deer or black-legged tick). It tends to unfold in stages. Early on, there may be an expanding skin rash (erythema migrans, sometimes — but not always — a target shape) with flu-like symptoms. If untreated, it can spread over weeks to months and, later, settle in a joint as Lyme arthritis.

Lyme arthritis — the part most relevant to an orthopaedic surgeon — is typically a late manifestation. It usually affects one or a few large joints, most often the knee, with marked swelling that is sometimes out of proportion to the pain. It is very treatable when recognized.

How Lyme disease is actually diagnosed

The diagnosis depends on the stage, and using the right test at the right time is what avoids confusion. The classic expanding rash in someone with likely tick exposure is a clinical diagnosis — it is treated on sight, without waiting for blood tests, because antibodies take a few weeks to appear.

For later manifestations, including arthritis, the standard is two-tier antibody (serology) testing: a sensitive screening test followed by a confirmatory test. These antibody tests are first-line and are highly reliable for a late manifestation like arthritis. A key point that surprises many people: by the time Lyme arthritis is present, the antibody test is essentially always positive — so a negative IgG result in someone who has been ill for months effectively rules Lyme out.

Two cautions guard against misdiagnosis. First, testing is not recommended for nonspecific symptoms alone — fatigue, aches, or “brain fog” without an exposure or objective findings — because that mostly turns up misleading results. Second, the antibody test should not be repeated to judge whether treatment worked: antibodies can stay positive for years after a cure, so a lingering positive test does not mean lingering infection.

Lyme arthritis — the orthopaedic part

Lyme arthritis is treated with a course of oral antibiotics, usually for 28 days. If the joint only partly settles, a second course — oral or, less often, intravenous — is used. A minority of patients have what is called antibiotic-refractory Lyme arthritis: the infection is gone, but an immune-driven inflammation persists in the joint. That situation is managed with anti-inflammatory and disease-modifying medicines through rheumatology, not with still more antibiotics.

Common misconceptions

• “You always get a bull’s-eye rash, and you need the rash to have Lyme.” — Many people never see a rash, and when it appears it is not always a target shape. Its absence does not rule Lyme out.
• “A positive Lyme test means I have an active infection right now.” — Antibodies reflect exposure and can persist for years; a positive test has to be read in the context of symptoms and timing, not on its own.
• “‘Chronic Lyme’ should be treated with long-term antibiotics.” — Persistent, nonspecific symptoms after proper treatment are real, and are known as post-treatment Lyme disease syndrome; their cause is not yet fully understood. But long courses or repeated rounds of antibiotics are not supported by the evidence and carry real harms, such as bloodstream infections and serious intestinal infections. The symptoms deserve to be taken seriously and worked up — the disagreement is about cause and treatment, not about whether people feel unwell.
• “Lyme explains all of my symptoms.” — Sometimes it does, and sometimes something else treatable is going on. A careful evaluation protects against missing another diagnosis.

How we help and what to expect

For a swollen joint — especially a knee — in someone with possible tick exposure, the evaluation includes Lyme serology and, when helpful, an analysis of the joint fluid to sort Lyme arthritis from other causes of a swollen joint. Treatment follows the guidelines above, with infectious-disease and rheumatology colleagues involved for partial responders or refractory cases.

Prevention is worth the effort in tick country: tick checks after time outdoors, repellents and protective clothing, and prompt, proper tick removal. A high-risk tick bite is sometimes treated with a single preventive dose of antibiotic.

Learn more

CDC — Lyme Disease: cdc.gov

IDSA / AAN / ACR — 2020 Lyme Disease Clinical Practice Guideline: idsociety.org

This information is for educational purposes only and does not replace evaluation by a qualified clinician. A formal diagnosis of these conditions means that established, published criteria have been met — not simply that someone has been told they have the condition — and the workup often involves genetics, cardiology, or infectious-disease and rheumatology specialists alongside orthopaedic care.