Differential Diagnosis for Tick-Borne Illnesses Causing Knee Swelling
Single Most Likely Diagnosis
- Lyme disease: This is the most common tick-borne illness that can cause knee swelling, particularly in the United States. Lyme disease is caused by the bacterium Borrelia burgdorferi, transmitted to humans through the bite of infected black-legged ticks. Knee swelling in Lyme disease is often associated with Lyme arthritis, which typically occurs weeks to months after the initial infection.
Other Likely Diagnoses
- Southern Tick-Associated Rash Illness (STARI): Although less common than Lyme disease, STARI can cause a rash and, in some cases, joint pain or swelling, including in the knees. It is transmitted by the lone star tick.
- Rocky Mountain Spotted Fever (RMSF): While RMSF primarily causes fever, headache, and rash, it can also lead to joint pain and swelling. It is transmitted by the American dog tick, Rocky Mountain wood tick, and the brown dog tick.
- Ehrlichiosis: This bacterial infection, transmitted by the lone star tick, can cause fever, headache, and fatigue, and in some cases, joint pain or swelling.
Do Not Miss Diagnoses
- Tularemia: Caused by Francisella tularensis, tularemia can be transmitted by the dog tick, wood tick, and lone star tick. It can cause severe illness, including fever, skin ulcers, and swollen lymph nodes. Joint pain or swelling can occur, and it's crucial not to miss this diagnosis due to its potential severity and need for specific antibiotic treatment.
- Anaplasmosis: Similar to ehrlichiosis, anaplasmosis is a bacterial infection that can cause fever, headache, and muscle pain, and occasionally joint pain or swelling. It is transmitted by the black-legged tick and the western black-legged tick.
Rare Diagnoses
- Babesiosis: This parasitic infection, transmitted by the black-legged tick, primarily affects red blood cells and can cause anemia, fever, and fatigue. Joint pain is less common but can occur.
- Tick-Borne Relapsing Fever (TBRF): Caused by several species of Borrelia, TBRF is transmitted by soft ticks and can cause recurring episodes of fever, headache, and muscle or joint pain.
- Powassan Virus Disease: A rare but potentially severe tick-borne illness that can cause fever, headache, and encephalitis or meningitis. Joint pain or swelling is less commonly reported but could be part of the initial presentation.