Any long term effect from septic arthritis?

Several.: If the diagnosis is delayed by two weeks, structural damage will occur to the joint. To get septic arthritis, one must have another site infected, spreading to the joint with blood flow. Only other ways are from a contiguous infection (bone, skin, etc.) or direct penetration of the joint. Early DX and effective antibiotic rx is most important!

Untreated ..yes: Untreated septic arthritis can permanently damage the joint, causing permanent disability. That's why we consider such a condition an orthopedic emergency, proper and prompt diagnosis and treatment can lead to a full uncomplicated recovery.