What is the Jarisch-Herxheimer reaction?

This acute febrile reaction, associated with shaking chills, malaise, sore throat, myalgia, headache, and localized inflammation of infected mucocutaneous sites, usually occurs 6 to 8 hours following penicillin treatment. Tetracycline, doxycycline, and ceftriaxone are less commonly associated with this reaction. It develops in 50% of patients with primary syphilis, 75% with secondary syphilis, and 30% with neurosyphilis. There is indirect evidence that this reaction is due to the release of a treponemal lipopolysaccharide that acts like a bacterial endotoxin. Similar reactions have been reported with other infectious diseases, including leptospirosis and louse-borne relapsing fever.