The Overlooked Drivers That Keep the Alarm Firing
By Krystle Alves | Naturopath, Nutritionist & Yoga Therapist | B.HSc, Adv. Dip. Naturopathy
Parts One and Two of this series covered what mast cells are, how mast cell activation syndrome produces its wide-ranging symptom picture, the role of the HPA axis and trauma in priming mast cell reactivity, and the genetic architecture that determines individual susceptibility. If you are coming to this article without that foundation, it is worth reading them first.
This article addresses a specific question that many MCAS patients are never asked, and that conventional medicine is not well-equipped to investigate: what was the initiating event?
For a significant proportion of people with MCAS, there is an identifiable point at which the mast cell system shifted from background normal to chronically dysregulated. Not a gradual onset, not a lifelong pattern, but a before and after: before that illness, before that mould exposure, before that holiday to Southeast Asia. The mast cells were on a hair trigger before the event (the genetic susceptibility was always there), but something flipped the switch from manageable to symptomatic. That something is almost never investigated. The patient receives their MCAS picture without anyone connecting it to its origin, and the origin continues to drive reactivity from underneath.
This article covers the infectious and environmental triggers most commonly implicated in MCAS onset and perpetuation, why they are so consistently missed, and what investigation looks like when this dimension is taken seriously.
Why Infectious and Environmental Triggers Are Overlooked
The oversight is structural. Infectious disease medicine is organised around the acute infection model: identify the pathogen, treat it, clear it, move on. The concept that a pathogen cleared months or years ago might have left a permanently altered mast cell landscape, or that a chronic low-grade infection might be the ongoing signal keeping mast cells perpetually activated, falls outside both the training and the investigative framework of most clinicians.
Environmental medicine, the study of how toxins, moulds, and chemical exposures drive chronic illness, occupies an even more marginal position in mainstream practice. The mechanisms linking mycotoxin exposure to immune dysregulation and mast cell activation are not taught in standard medical training. The tests that assess these exposures are not funded by Medicare in Australia and are not part of any standard screening panel. A patient who has been living in a water-damaged building for three years and whose MCAS developed eighteen months into that exposure will almost never be asked about their home environment by anyone they see in the conventional medical system.
The time lag compounds the problem. Infectious and environmental triggers often do not produce MCAS symptoms at the point of exposure. They alter the mast cell threshold and the immune environment over weeks to months, so that by the time the clinical picture is established, the causative event feels temporally remote. Patients may not connect their current symptom state to an illness they had two years ago, or to a mouldy bathroom they remediated last year. Clinicians asking about recent exposures are already asking the wrong timeframe.
The result is that many people with MCAS are managed for their reactivity without the driver of that reactivity ever being identified. They are given tools to reduce mast cell mediator burden, antihistamines, mast cell stabilisers, low-histamine diets, and they improve partially. Then they plateau. Then they relapse. Because the source signal has not been removed, and the mast cells continue to receive it.
Viral Infections
Viruses are among the most potent mast cell activators known. They interact with mast cells through multiple mechanisms: direct binding to surface receptors, innate immune pattern recognition through toll-like receptors (TLRs), and the release of danger signals (extracellular ATP, heat shock proteins) that activate mast cells as part of the cell danger response framework described in Part One.
Epstein-Barr Virus (EBV) is the pathogen most consistently associated with MCAS onset in clinical practice. EBV, the virus responsible for infectious mononucleosis, infects mast cells directly via the CD21 receptor on their surface. This is not an incidental finding: mast cells are a genuine reservoir of latent EBV, and viral reactivation triggers mast cell activation as part of the same event. The clinical picture of EBV reactivation (fatigue, lymphadenopathy, sore throat, rash, neurological symptoms) overlaps so heavily with a MCAS flare that distinguishing the two without serology is impossible. More significantly, primary EBV infection in a genetically susceptible individual appears capable of resetting the mast cell activation threshold permanently, so that the clinical MCAS picture persists long after the acute infection is resolved. Many patients date the onset of their MCAS to a severe case of glandular fever in their teens or twenties.
The Th2 immune skewing driven by chronic mast cell activation, discussed in Part One, also creates a permissive environment for EBV reactivation. Th2 dominance suppresses the Th1 cellular immunity responsible for controlling latent herpesvirus infections. This creates a reinforcing loop in which MCAS drives the immune state that allows EBV to reactivate, and EBV reactivation drives further mast cell activation. Breaking this loop requires addressing both the viral and the mast cell dimensions simultaneously.
Human Herpesvirus 6 (HHV-6) follows a similar pattern. HHV-6 is a neurotropic herpesvirus that establishes latency in the central nervous system and can reactivate under conditions of immune stress. It has been documented in association with chronic fatigue syndrome, fibromyalgia, and multiple sclerosis, all of which carry significant mast cell activation components. HHV-6 reactivation drives neurological mast cell activation and contributes to the brain fog, cognitive impairment, and central sensitisation common in MCAS.
SARS-CoV-2 and long COVID have produced perhaps the most visible recent evidence for the mast cell-viral connection. The spike protein of SARS-CoV-2 directly activates mast cells through multiple pathways, including ACE2 receptor engagement on mast cell surfaces and innate immune TLR pattern recognition. Mast cells in the lungs, gut, heart, and brain are central to the pathogenesis of severe acute COVID-19, and the subsequent long COVID syndrome shows a pattern of symptoms, post-exertional malaise, cardiovascular dysregulation, gastrointestinal disturbance, cognitive impairment, and widespread hypersensitivity, that is clinically indistinguishable from MCAS in many patients. Research groups including Theoharides and colleagues have specifically proposed that mast cell activation is a central mechanism in long COVID pathophysiology. Patients who already had sub-threshold or undiagnosed MCAS before COVID infection frequently find that the acute infection pushes their reactivity above the symptomatic threshold, and they emerge from the illness with an overt MCAS clinical picture for the first time.
Enteroviruses and post-viral gut syndromes deserve mention in the context of gut-predominant MCAS. Enterovirus infections (including coxsackievirus and echovirus) can trigger sustained mast cell activation in the gut wall, altering enteric nervous system function and gut permeability in ways that are indistinguishable from post-infectious IBS. The distinction that the mast cell framework provides is mechanistic: the gut symptoms are not simply altered motility following infection, but reflect ongoing mast cell-mediated inflammation in the gut wall that was initiated by the infection and has not resolved.
Bacterial Infections
Borrelia burgdorferi (Lyme disease) has one of the most extensively documented relationships with mast cell activation of any bacterium. Borrelia directly activates mast cells through TLR2 binding, the toll-like receptor that recognises bacterial lipoproteins. TLR2 activation on mast cell surfaces triggers full degranulation. Substance P, the neuropeptide released in elevated quantities during the neurological inflammation of Lyme disease, is simultaneously a potent mast cell activator. In Lyme disease, mast cells and the neurological stress response are both being driven by the same infection, producing a compounding activation loop that is part of why neurological Lyme is so severely symptomatic.
The diagnostic landscape for Lyme disease in Australia is contested in ways that directly harm patients. The official position of Australian health authorities is that locally-acquired Lyme disease does not exist in Australia, and tick-borne illness presentations are attributed to other causes. The lived experience of many Australian patients, practitioners, and the growing body of research on tick-associated illness in Australia tells a different story. What matters clinically is that tick exposure, regardless of the specific pathogen involved, is capable of triggering a mast cell activation pattern through the same TLR2-mediated mechanism. Patients with a history of tick bites followed by illness and the subsequent development of MCAS-like symptoms warrant investigation for tick-borne pathogens regardless of the jurisdictional controversy.
Bartonella is a tick and flea-borne bacterial genus that coinfects with Borrelia in a significant proportion of tick-borne illness cases. Bartonella has its own direct mast cell activation pathway and produces a characteristic pattern of vasoproliferative effects, stretch marks (striae) in atypical locations, neuropsychiatric symptoms, and lymphadenopathy that overlaps significantly with mast cell activation presentations. The skin findings of Bartonella (linear, stretch-mark-like striae on the flanks, thighs, and abdomen that are not explained by weight change) are a pattern worth noting in MCAS patients with tick exposure histories. Bartonella is often not included in standard tick-borne disease panels and requires specific serology to detect.
Helicobacter pylori is the most clinically common bacterial driver of gastrointestinal MCAS in my clinical experience, and it is one of the most straightforward to identify and address. H. pylori colonises the gastric mucosa and produces histamine directly as part of its metabolic activity. It also drives chronic mast cell activation in the gastric and duodenal mucosa. The clinical picture is often one of gastric MCAS: nausea, early satiety, bloating, upper abdominal pain, and food intolerances that are difficult to map consistently. H. pylori is not rare. Australian prevalence estimates suggest it affects between 15 and 30 percent of the population, with higher rates in older adults and specific demographic groups. It is testable, treatable, and its successful eradication frequently produces significant improvement in MCAS symptoms in patients where it is a contributing driver.
Small intestinal bacterial overgrowth (SIBO) as a mast cell driver operates through two overlapping mechanisms. First, the bacterial species overpopulating the small intestine include histamine-producing strains (Morganella morganii, Klebsiella pneumoniae, Hafnia alvei, and others) that continuously produce histamine from dietary histidine, directly adding to the histamine load the body must clear. Second, the bacterial endotoxin (lipopolysaccharide, LPS) released by gram-negative bacteria in the small intestinal overgrowth is a potent direct activator of mast cells through TLR4. A patient with significant SIBO is simultaneously increasing their histamine burden and providing continuous TLR4-mediated mast cell activation signals from their own gut microbiome. SIBO is discussed in detail in a separate article on this site, and its connection to MCAS is one of the most clinically actionable relationships in this space: SIBO is testable, and its treatment reliably reduces mast cell symptom burden in patients where it is a contributor.
Mycoplasma pneumoniae and other atypical bacteria capable of producing chronic low-grade infection have documented mast cell activation effects and are worth considering in MCAS patients with a history of persistent respiratory illness, recurrent sinus infections, or respiratory symptoms preceding the onset of wider MCAS reactivity.
Parasitic Infections
Mast cells are the primary immune defence against parasites. The IgE-mast cell-eosinophil axis, the same immune pathway dysregulated in allergy, evolved specifically to expel parasites from mucosal surfaces. This biological reality means that parasitic infection is a uniquely potent mast cell activator, and that the Th2 immune skewing driving parasitic expulsion is indistinguishable from the Th2 skewing seen in allergic disease and MCAS.
Giardia lamblia produces one of the most clearly documented post-infectious mast cell syndromes. Giardia infection in the small intestine triggers strong mast cell activation in the gut mucosa as part of the immune expulsion response. In a significant proportion of patients, this mast cell activation persists after the parasite is cleared, producing a post-Giardia IBS pattern with continuing food intolerances, gut permeability, and mast cell reactivity that can persist for years. The mechanism is believed to involve persistent mast cell sensitisation during the acute infection that does not fully de-escalate after resolution. For MCAS patients with gut-predominant symptoms and a history of travel to regions where Giardia is endemic, or a history of drinking from natural water sources, Giardia as an initiating event is a clinically important consideration.
Blastocystis hominis occupies a more complex position. Its pathogenicity is debated, with some researchers regarding it as a commensal and others as a significant pathogen. What is clinically consistent is that Blastocystis is substantially overrepresented in patients with urticaria, chronic skin conditions, and MCAS-like presentations, and that its treatment (where appropriate) sometimes produces marked improvement in these symptoms. Blastocystis appears to activate mast cells in the gut mucosa and to alter intestinal permeability, providing a continuous antigenic load that maintains systemic mast cell reactivity. It is detected on comprehensive stool analysis and is frequently present without causing obvious gastrointestinal symptoms, which is part of why it goes uninvestigated.
Hookworm, Strongyloides, and intestinal roundworms are classical Th2 activators and mast cell triggers. In the Australian context, Strongyloides stercoralis deserves specific mention because it is endemic in parts of Queensland and Northern Territory, can establish chronic autoinfection persisting for decades, and is massively underdiagnosed in people who have lived in or travelled to endemic regions. Strongyloides hyperinfection in immunosuppressed individuals can be life-threatening, but chronic low-level Strongyloides autoinfection in immunocompetent individuals produces a persistent mast cell activation pattern with skin manifestations (larva currens, a migrating urticarial rash), gastrointestinal symptoms, and eosinophilia. It is an entirely treatable condition that will not be identified unless it is specifically tested for.
Mould and Mycotoxins
Mould exposure is the environmental trigger most consistently underrecognised in MCAS, and the one whose investigation requires the most deliberate clinical intent, because it is genuinely invisible in a standard workup.
Moulds growing in water-damaged buildings produce mycotoxins: secondary metabolites including trichothecenes, aflatoxins, ochratoxin A, zearalenone, and gliotoxin that are released into the indoor environment along with mould spores, volatile organic compounds, and actinobacteria from building materials. The combination of these agents in a water-damaged building is not a single exposure but a continuous, low-level, multi-agent toxic assault that the body must process and clear on an ongoing basis.
Mycotoxins directly activate mast cells through innate immune pattern recognition and through their direct cytotoxic effects on cell membranes. They activate the NF-kB inflammatory pathway, a master switch for cytokine production, and drive the production of IL-6, IL-1 beta, and TNF-alpha, the same pro-inflammatory cytokines that are elevated in MCAS and in the autoimmune conditions that overlap with it. They also impair mitochondrial function, contributing to the cell danger response state that keeps mast cells in a perpetual activation mode.
The overlap with Chronic Inflammatory Response Syndrome (CIRS), the framework developed by Dr Ritchie Shoemaker for understanding biotoxin illness, is substantial. In CIRS, a subset of genetically susceptible individuals (those carrying certain HLA-DR variants that impair biotoxin recognition and clearance) are unable to mount an adequate immune response to mycotoxins and other biotoxins, leading to their continuous recirculation and accumulating immune damage. The immune activation pattern of CIRS, characterised by elevated transforming growth factor-beta 1 (TGF-beta1), elevated C4a complement split product, elevated matrix metallopeptidase 9 (MMP-9), and suppressed melanocyte-stimulating hormone (MSH) and vascular endothelial growth factor (VEGF), overlaps significantly with MCAS. Many patients with confirmed CIRS also meet clinical criteria for MCAS, and addressing the mould exposure produces improvement in both the CIRS markers and the mast cell reactivity simultaneously.
What makes mould particularly insidious as a trigger is the re-exposure problem. A person who has been sensitised to mycotoxins by prolonged exposure in a water-damaged building will continue to react to subsequent mould exposures, even low-level ones, with a mast cell response disproportionate to the exposure. Their threshold for mast cell activation has been permanently lowered by the sensitisation period. The mould no longer needs to be present in large quantities to drive reactivity; trace exposures in new environments, damp bathrooms, food moulds, outdoor fungal spore loads, can be sufficient to trigger significant mast cell episodes in highly sensitised individuals.
Clinically, mould-associated MCAS should be suspected when: symptoms began or significantly worsened after moving to a new building or after water damage (a leak, flooding, or persistent condensation) in an existing building; symptoms are consistently worse at home or at a particular workplace than elsewhere; there is a history of visible mould in the living or working environment; symptoms include a pattern of cognitive symptoms (brain fog, word retrieval difficulty, difficulty with directions and spatial memory) alongside the typical MCAS picture; and when standard MCAS interventions produce only partial and unstable improvement despite apparent adherence.
Heavy Metals and Chemical Exposures
Environmental toxins beyond mycotoxins are an increasingly recognised dimension of MCAS perpetuation, though they receive less research attention than infectious drivers.
Heavy metals, particularly lead, mercury, cadmium, and arsenic, are documented mast cell activators. They activate mast cells through oxidative stress mechanisms and through direct interaction with cell membrane receptors, and they impair the enzymatic clearance pathways for histamine and other mast cell mediators. Mercury in particular impairs the methylation cycle (reducing SAMe availability and therefore HNMT-mediated histamine clearance), compound the genetic vulnerabilities described in Part Two. Sources include amalgam dental fillings (mercury), industrial or occupational exposure, contaminated water, and high fish consumption (methylmercury).
Pesticides and herbicides affect mast cell reactivity through their disruption of the gut microbiome. Glyphosate, the active ingredient in the most widely used herbicide worldwide, has documented inhibitory effects on the gut bacterial species responsible for DAO enzyme activity and histamine degradation. A gut microbiome chronically exposed to glyphosate residues through diet has reduced histamine-clearing capacity, meaning that dietary histamine and mast cell-released histamine persists at higher concentrations. Organophosphate pesticides have direct mast cell activation effects through their inhibition of acetylcholinesterase, altering the cholinergic-mast cell signalling relationship.
Multiple chemical sensitivity (MCS) frequently coexists with MCAS and may represent, at least in part, the same underlying mast cell hyperreactivity expressed as sensitivity to chemical exposures rather than biological ones. Perfumes, cleaning products, paint, exhaust fumes, and synthetic fragrances trigger symptoms in MCS that closely mirror MCAS flares, and the neurogenic mast cell activation pathway (through substance P and neuropeptides in sensory nerve endings) provides the mechanistic link. In patients with both MCAS and MCS, the chemical sensitivity tends to resolve alongside the mast cell reactivity when the underlying drivers are adequately addressed.
What Investigation for Infectious and Environmental Drivers Looks Like
Investigating this dimension of MCAS requires deliberate clinical intent, because the tests involved are not part of any standard panel and require specific ordering and interpretation.
For viral assessment, comprehensive herpesvirus serology provides the most useful picture. EBV serology interpreted correctly requires distinguishing between past resolved infection (VCA IgG positive, EBNA IgG positive, EA-D IgG negative) and chronic active or reactivated infection (VCA IgG positive with high titres, EA-D IgG positive, EBNA IgG variable). High VCA IgG titres alongside positive EA-D IgG in a patient with MCAS and fatigue is a clinically significant finding. HHV-6 IgG, CMV IgG and IgM, and where relevant, SARS-CoV-2 antibody profiles and spike protein persistence markers, complete the viral picture.
For bacterial and parasitic assessment, a comprehensive stool analysis (not a standard culture, which misses the majority of relevant organisms) tests for Giardia antigen, Blastocystis, Cryptosporidium, Dientamoeba fragilis, and ova and parasite microscopy for helminths. Strongyloides serology (IgG) is essential in any patient with tropical travel history or time spent in endemic Australian regions. H. pylori is most reliably confirmed by faecal antigen test or carbon-13 urea breath test. SIBO is diagnosed by glucose or lactulose hydrogen and methane breath testing, with interpretation including the methane component (elevated methane points to intestinal methanogen overgrowth, now formally classified as IMO). Lyme and Bartonella serology requires a laboratory with experience in tick-borne illness interpretation; standard hospital serology misses a significant proportion of positive cases.
For mould and mycotoxin assessment, urinary mycotoxin testing measures actual mycotoxin excretion, providing direct evidence of ongoing or recent exposure. The Visual Contrast Sensitivity (VCS) test is a validated, inexpensive screening tool for biotoxin illness that can be performed online and has documented sensitivity for identifying mould-exposed individuals. CIRS markers including TGF-beta1, C4a, MMP-9, MSH, and VEGF can be ordered through functional pathology laboratories and provide an objective picture of the biotoxin immune activation pattern. Where results are positive, a detailed environmental assessment of the patient's living and working spaces is an essential next step.
For heavy metal assessment, a red blood cell (RBC) elements panel measures intracellular mineral and metal status more accurately than serum testing for most elements. A provoked urine heavy metal test (collected after a chelating agent is administered under medical supervision) provides the most complete picture of total body metal burden, though this requires appropriate clinical supervision and is not appropriate for all patients.
The Pattern That Matters Clinically
In practice, infectious and environmental triggers in MCAS rarely present as a single clean cause. What is more commonly seen is an accumulation: a person with genetic susceptibility (slow AOC1, MTHFR, HAT) who experienced a significant EBV illness in their twenties that shifted their mast cell threshold, subsequently spent years in a water-damaged workplace, had a course of antibiotics that disturbed their microbiome and allowed Blastocystis to establish, and gradually accumulated mercury from dietary sources and amalgam fillings. No single factor in this picture explains the MCAS. Together, they explain the total burden that pushed the system past its capacity to self-regulate.
This is why the clinical history matters as much as the test results. The sequence of events, what happened when, what changed the patient's trajectory, is often where the most important information lives. And it is why the question that is almost never asked in conventional medicine is the one that functional medicine starts with: what happened before this began?
The MCAS patient who has been told their condition is unexplained, their triggers are mysterious, and their best management option is lifelong antihistamines deserves a better framework. Not because antihistamines have no place, but because the conversation about why the mast cells are firing, and what has been keeping them firing for years, has the potential to produce something antihistamines never will: resolution.
This post is for educational purposes only and does not constitute medical advice. If you are concerned about infectious exposures or environmental illness, please consult a qualified health practitioner with experience in this area for personalised assessment.
Krystle Alves is a naturopath, nutritionist and yoga therapist with over 17 years of clinical experience. She works with clients presenting with MCAS, tick-borne illness, post-viral syndromes, biotoxin illness, and complex multi-system chronic conditions where the initiating and perpetuating drivers have not yet been identified. Consultations are available via telehealth Australia-wide.