This is a key piece of the chronic Lyme puzzle. The terms goes back to the early treatment of syphilis. Drs Jarish and Herxheimer noted that when patients were first treated for syphilis they frequently experienced a toxic, dramatic deterioration in their symptoms, after which the patients would show signs of improvement. This response was named the Jarish-Herxheimer reaction. Over time it was shortened to Herxheimer and then Herx reaction. The term has even become a verb. Patients frequently speak of "herxing." It turns out that only a small number of infectious diseases are know to be associated with a Herxheimer reaction. In addition to syphilis and Lyme, it is limited to several obscure spirochetal diseases such as relapsing fever, rat bite fever and to anthrax. It is a short list. Having a Herxheimer reaction when treated for Lyme is considered good evidence that the diagnosis was on the mark. A Lyme Herxcheimer may not be as severe as that seen with other diseases such as syphilis, but is usually not subtle. It typically starts several days or a week after antibiotic treatment is started in earnest. The primary symptom is profound fatigue, which can at times be disabling. Patients may experience low grade fevers, chills, flu like symptoms and joint and muscle pains. A brain Herx occurs when cognitive and or neurological symptoms temporarily get worse. They are many variations of the Herx response. The bottom line is that there is a worsening of pre-treatment symptoms. The typical Herx reaction lasts for 3 t0 4 weeks, but it can persist for months is some cases. Usually after the Herx abates the patient starts to suddenly feel better and have more energy. The primary Lyme symptoms may not start to imrove for weeks or months yet to come. There are different theories about how to manage a Herx reaction. Some physicians start patients off with very low doses of antibiotics and gradually ramp them up over many months in an effort to stave off the Herx response. I have not found this response effective. I have seen patients treated by other physicians who have still not reached a therapeutic dose of antibiotics based on this method. In my experience a Herx is something the patient is going to have to deal with, so I prefer to jump right into regular doses of medicines. If a Herx is excessive, then the antibiotic(s) can be held for 48 hours and re-introduced at a lower dose. I still try to ramp up quickly. Other methods have been used to mitigate the Herx. A low dose of prednisone can be used. Many physicians would avoid this. Vitamin D can be used on a short term basis because of its anti-inflammatory properties, although I do not recommend long term use of this supplement. Other strategies are used by other clinicians.
A Herxheimer reaction probably occurs for a couple of reasons. First, there is probably a huge number of spirochete organisms present in the body if killing them is causing a systemic reaction, experienced throughout your entire body. Second, there may be something unique about the immue reaction to the organism. The intensely red rash which can been seen with early infection indicates a marked inflammatory response. Apparently, the killing of the organisms is also associated with a dramatic inflammatory response. This is medicated by special proteins called cytokines, which mediate the functions of various aspects of the immune response. Lyme may be associated with a greater amount of cytokine release than is typically seen with other infectious processes. These cytokines are categorized into various classes, such as interferons, interleukins, tumor necrosis factor and others. Specifically, it may be the germ fighting molecule interferon which causes much of the Herxheimer reaction. When you have the flu, it is not the germ which makes you feel so sick. It actually the release of cytokines, like interferon which causes the so called "flu like symptoms." This is pretty much what happens with a Herxheimer reaction. It just lasts longer than the flu does. It also is not associated with a high fever like the flu. With the flu it is probably the particles themselves which become pyrogens and trigger the germ killing response of elevating your body temperature.
Herxheimer reactions are seen by some as a good thing. It means the buggers are being killed. It is also associated with an increased immune response to Lyme. Frequently while it the throws of such a reaction, a repeat Lyme serological test, like the Western Blot test will become positive when it was previously negative. This is called sero-conversion. This is excellent news which confirms the accuracy of the diagnosis and the correctness of the treatment.