Swollen Left Knee Caused by Borrelia burgdorferi

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A 64-year-old man from New Jersey presents to his primary care physician complaining of a swollen left knee. The patient stated that the swelling began approximately 1 week prior after playing 18 holes of golf.  He denies fever, traumatic injury to the knee or other systemic symptoms. The physical exam is significant for warm, tender swelling of the left knee. 

Lab Testing

Based on the physicians’ clinical differential, there was concern for gout or infection.  An aspirate of the knee fluid was sent for cell count, Gram stain, bacterial culture and fluid smear to look for the presence of crystals (gout).  The Gram stain showed many white blood cells (WBC’s) and no organisms seen (NOS).  Bacterial culture was negative and the cell count of the fluid showed a differential of 81% neutrophils, 8% lymphocytes and 11% monocytes.  The fluid was also negative for crystals.  Based on the negative lab results and outdoor exposure, the physician ordered additional testing for Lyme disease. The Lyme immunoblot run on this patient was positive for Lyme antibodies. A polymerase chain reaction (PCR) test was performed on the joint fluid of this patient to rule in the presence of organisms in the fluid and active infection.

Cause of Symptoms

The differential diagnosis in this patient consists of a variety of causes such as trauma, arthritis, disseminated gonococcus infection, Lyme disease, septic arthritis (commonly caused by Staphylococcus aureus), gout or pseudogout (uric acid or calcium pyrophosphate dihydrate crystals in the joint) or systemic autoimmune diseases such as psoriasis and rheumatoid arthritis.  Since the initial laboratory testing described above was negative, further testing for Lyme disease proved beneficial. The patients’ serum was positive for Borrelia burgdorferi (causative agent of Lyme disease) as was the patients joint fluid, which was positive by PCR. The final diagnosis in this patient was Late Disseminated Stage Lyme Disease.  Patients with untreated Lyme disease may experience joint pain and swelling months to years after the initial Tick bite.


Treatment for Lyme disease typically consists of doxycycline though other antibiotics are also effective. 

Contact Information

Nicole Jackson, clinmicro@asmusa.org