Posted 10/26/2023 9:08 AM (GMT -5)
Great question, Bhava. I find it helpful to think about what a herx and MCAS actually are, which you may know already, but I want to define them just in case others don't know.
A herx is the body's reaction to the build-up of endotoxins in your body caused by dead pathogens during antimicrobial treatment. It's not just any flare in symptoms, even though the term is often used that way. You can kill a bunch of pathogens but not herx if your body processes out the endotoxins quickly and effectively. Some people never herx, rarely herx, or herx mildly and briefly. You could herx relatively quickly if your detox pathways (bowels, liver, kidneys, skin, lungs) are impaired in some way. Pathogens die and release endotoxins at different speeds and to different degrees, but it's unlikely that someone would herx immediately after taking something. First the antimicrobial treatment needs to be processed and absorbed by the body, then pathogens need to be killed, then they need to release endotoxins, then the endotoxins need to build up to such high numbers that your detox pathways can't keep up anymore. It could take hours, but it's more likely to take days or even weeks to build up. It's also worth considering that some antibiotics are bacteriostatic (keep bacteria from reproducing) rather than bactericidal (kill bacteria) so technically cannot cause a herx. A herx should go away as the detox pathways have a chance to catch up and as the pathogen load in your body decreases.
Mast cell activation syndrome (MCAS) is the over-reaction of mast cells, which causes the release of histamine, cytokines, and other inflammatory chemicals. Mast cells are part of the immune system and can be found in every organ and tissue in the body, so MCAS symptoms can occur literally in any body part or system. When considering MCAS, people usually think about rashes, respiratory symptoms, and food allergies, but symptoms can include headaches, brain fog, agitation, IBS, anaphylaxis, joint pain, peripheral neuropathy, interstitial cystitis, flushing, dizziness, fatigue, tingling, swelling, sore throat, reflux, and many other symptoms. MCAS can be triggered immediately, and by a wide range of possible triggers: herxing, fragrances, stress, friction, exertion, sweating, showers, heat, mold, shivering, overstimulation, loud noise, electromagnetic radiation, allergens, air pollution, cleaning products, body products, high-histamine foods or supplements, new drugs or herbs, etc. Testing exists but is unreliable, so it's a clinical diagnosis (based on signs, symptoms, and response to trigger avoidance and treatment with mast cell stabilizers and antihistamines). People who feel like they are "unusually sensitive" or "react poorly" to lots of Lyme treatments that others tolerate fine often are actually experiencing MCAS. They can tolerate antimicrobial treatment much better (and have far fewer symptoms overall) once they treat and stabilize MCAS. You can have MCAS reactions for an unlimited amount of time as long as triggers continue to be present, and it's dangerous to push through an MCAS flare - best to remove triggers and let the system recover.
People can end up having dangerously severe herxes (I certainly have), but with anaphylaxis as an MCAS symptom, I would be more worried about MCAS. Severity of symptoms is not a useful way to tell them apart.
If you are someone who feels like you herx right away to a new antimicrobial treatment - like within hours - and/or that you herx for a very long time relative to other people, then it's probably a side effect or MCAS, and not actually a herx. As far as I have read, timing is the best way to tell whether a symptom flare is MCAS versus a herx. It's not always easy to tell!
Another condition which can be confused with both of these is porphyria.
I found that fully 85% of the symptoms I had been attributing to tick-borne infections vanished when I treated MCAS.