If you live in Minnesota, you already know about ticks. We're one of the highest-incidence states in the country for Lyme disease, and every spring and summer the cases start rolling in. Most of the time, when Lyme is caught early, it gets treated and that's the end of the story.
But not always.
Some patients come to rheumatology months after a tick bite they don't even remember, with a single, hugely swollen knee. Interestingly, I'll often diagnose these cases in the winter, when Lyme is the last thing on anyone's mind for obvious reasons. That's the version of Lyme I want to talk about today.
A quick primer on early Lyme
Lyme disease is caused by a bacterium called Borrelia burgdorferi, transmitted through the bite of an infected deer tick. The classic early presentation is a combination of:
- Erythema migrans, the bullseye rash, present in roughly 70 to 80% of cases
- Flu-like symptoms: fevers, chills, fatigue, headache
- Muscle aches and migratory joint aches
Caught at this stage, Lyme is very treatable. A standard course of antibiotics (usually doxycycline) clears the infection and most people are done with it.
The problem is that not everyone notices the bite. Not everyone gets the rash, or recognizes it when they do. And if the early symptoms are mild or get attributed to a virus, the infection can go untreated and quietly progress.
Late-stage Lyme arthritis
This is where I see Lyme in clinic. Not the early flu-like illness, but the late manifestation that shows up months later as arthritis.
Lyme arthritis has a pretty distinctive presentation:
- Usually a single large joint, most often the knee
- Massive joint swelling
- Surprisingly, it's often less painful than you'd expect from how swollen it looks
That last point is part of what makes it tricky. Patients sometimes assume it's a sports injury, an overuse problem, or just a weird flare of something they can manage. By the time they get evaluated, the original tick bite is long forgotten.
If a patient walks into my office in Minnesota with a giant, relatively painless knee and no clear cause, Lyme is high on my list.
Diagnosis
Lyme is diagnosed through a two-step blood test: an initial screening test (ELISA), followed by a confirmatory test (Western blot) if the screen is positive.
A few important points:
- The screening test (ELISA) can stay positive for years after a treated infection. A positive screen alone doesn't tell you whether you have active disease.
- The confirmatory test (Western blot) is what really matters. The pattern of bands on this test is what tells us whether Lyme is active and how to interpret the result.
- We can also aspirate the joint and send the fluid for Lyme PCR, which is often positive in active Lyme arthritis. That said, a negative PCR doesn't rule it out.
The clinical picture and the labs need to fit together. Neither alone tells the whole story.
Treatment
The good news: Lyme arthritis usually responds well to antibiotics. A four-week course of oral doxycycline is the typical first-line treatment. Most patients improve significantly.
A small subset of patients have what's called post-antibiotic Lyme arthritis, where the joint inflammation persists even after the infection has been adequately treated. In those cases, the immune system seems to keep reacting even though the bacteria are gone. This subset sometimes needs anti-inflammatory medications or DMARDs (like methotrexate) to settle things down. That's typically when I get more involved long-term.
When rheumatology gets involved
Most uncomplicated Lyme is managed by your primary care doctor, and that's appropriate. Rheumatology usually gets involved when:
- There's a large, swollen joint of unclear cause
- The arthritis isn't responding to standard antibiotic treatment
- The diagnostic picture is mixed or confusing, and we need to sort out whether it's Lyme or something else
- There's persistent inflammation suggesting post-antibiotic Lyme arthritis
The bottom line
In Minnesota, Lyme is part of the differential for a lot of joint problems we wouldn't think twice about elsewhere. If you've got a swollen knee that came on without a clear injury, especially after a summer outside, it's worth asking the question.
The earlier we identify and treat Lyme, the better the outcomes. And if it's already showing up as arthritis, we still have very good treatments. The key is recognizing it for what it is.