Infectious myocarditis

Article written and reviewed by Dr Luca Moretti

Infectious myocarditis encompasses conditions that differ profoundly in causative agent, mechanism, geographic distribution and treatment. A virus may infect cardiomyocytes and trigger a persistent immune response; a bacterium may produce microabscesses during bacteremia; a toxin may disrupt protein synthesis without massive invasion of the heart; a protozoan may remain in tissues for decades. Grouping them within a single category is useful only if this heterogeneity is not lost.

Infectious causality is more difficult to demonstrate than the coincidence between a febrile syndrome and cardiac injury might suggest; many infections are common, clinical myocarditis is rare, and antibodies or genomes may reflect previous exposure or incidental presence. A reliable diagnosis therefore requires a coherent chain linking epidemiologic probability, temporal phase, phenotype, the appropriate microbiological test and, when necessary, tissue analysis.

Viral myocarditis is the most common paradigm in high-income countries, but the term should not be assigned simply because a respiratory episode preceded chest pain. Bacterial myocarditis is less common and often accompanies sepsis, endocarditis or dissemination, whereas some entities have a recognizable pattern: Lyme carditis preferentially involves the conduction system and diphtheritic myocarditis is toxin mediated.

Chagas heart disease shows how artificial the distinction between an infectious phase and chronic cardiomyopathy can be: Trypanosoma cruzi persists at low density, while the immune response, microcirculation, denervation and fibrosis progressively build an arrhythmogenic and thromboembolic disease. For each pathogen, therefore, the question is not only whether it is present, but also what role it is playing in the phase being observed.

The safest method is to formulate a specific microbiological question before ordering a test: demonstrating current bacteremia, documenting recent Borrelia infection, identifying a genome in tissue or monitoring reactivation are not equivalent objectives. Sample type, technique and time window differ for each; an accurate result used to answer the wrong question may produce a clinical conclusion just as erroneous as a technically poor test.

Classification by pathogen must be complemented by classification by mechanism, separating cytolysis, endothelial infection, microabscesses, toxin-mediated injury, persistent parasitism and immune response. Two pathogens in the same class may require opposite pathways, whereas different viruses and bacteria may converge on the same shock phenotype; organizing the reasoning along both axes prevents a microbiological label from replacing the pathophysiology needed for intensive care.

Etiology, Pathogenesis and Pathophysiology

Enteroviruses, adenoviruses, parvovirus B19, human herpesvirus 6, influenza, HIV, SARS-CoV-2 and other viruses have been associated with myocardial injury with varying strength of evidence. Tropism and mechanism differ: some replicate in cardiomyocytes, others involve endothelium or immune cells, and the presence of a genome does not prove pathogenic replication. Age, immune status, viral variant and host genetics modify the probability that a systemic infection will progress to cardiac disease.

The innate antiviral response limits replication through interferons and cytotoxic cells, but also causes edema and contractile depression. Subsequently, T lymphocytes and antibodies eliminate infected cells; molecular mimicry, exposure of cardiac antigens and failure of resolution may sustain inflammation after the viral burden declines. This sequence is not identical in every patient and does not permit the disease phase to be inferred from symptom duration alone.

Bacteria reach the myocardium through the bloodstream, by extension from the endocardium or pericardium, or through toxins; Staphylococcus aureus and other pyogenic pathogens may form microabscesses, whereas Borrelia burgdorferi is associated mainly with an infiltrate that disrupts conduction. Corynebacterium diphtheriae produces a circulating toxin capable of damaging cardiomyocytes after pharyngeal infection, and treatment must neutralize toxin that has not yet bound to cells in addition to eradicating the bacterium.

Protozoa and helminths cause injury through invasion, cyst formation, eosinophilia and immune responses. Trypanosoma cruzi has an acute phase with parasitemia and a chronic phase in which focal persistence and inflammation sustain fibrosis, aneurysms and denervation. Toxoplasma gondii becomes particularly relevant in immunocompromised patients, whereas Trichinella may involve skeletal muscle and the heart after ingestion of contaminated meat.

Fungal myocardial infections occur mainly in patients with neutropenia, transplantation, immunosuppressive therapy, endocarditis or disseminated infection. Candida and Aspergillus may produce microabscesses, infarctions and emboli; dimorphic fungi cause granulomas according to the endemic area. Empiric therapy must take organs involved, cultures, antigens, imaging and susceptibility into account because tissue penetration and source control affect the outcome.

Final injury results from a combination of cytolysis, toxicity, microvascular ischemia, edema, arrhythmias and fibrosis; the same ejection fraction may therefore represent recoverable stunned myocardium or extensive cell loss. Cardiac magnetic resonance and temporal evolution help estimate this distinction, whereas biopsy can identify the histologic type and pathogen when sampling and methodology are appropriate.

The host determines a substantial part of the disease; immunosuppression increases pathogen burden and dissemination but may reduce the histologic response; genetic predispositions to cardiomyopathy may be unmasked by infection; pregnancy, age and comorbidities alter reserve. The pathogen-host relationship is therefore more informative than an abstract list of microorganisms.

The distinction between direct invasion and a post-infectious response is not always binary. During the transition, declining pathogen burden, residual antigens and activated immunity may contribute simultaneously, while hypoxia and shock add noninfectious injury. For this reason, the phase is reconstructed from chronology, sample type and histology rather than inferred solely from the number of days since the prodrome.

Persistence of an agent has different meanings: a viral genome may be silent, a bacterium within an abscess remains an active source, and T. cruzi retains a biological role in the chronic phase even when blood tests are negative. The term persistence should therefore specify which material was detected, in which compartment and with what activity, avoiding automatic transfer of therapeutic implications from one microorganism to another.

Clinical Manifestations

The infarct-like phenotype combines chest pain, ST-T abnormalities and troponin elevation, often after respiratory or gastrointestinal symptoms; the coronary arteries are nonobstructive and cardiac magnetic resonance shows nonischemic injury, but these findings do not identify a virus. Persistent fever, positive blood cultures, immunosuppression or an extracardiac focus immediately broaden the investigation toward bacteria, fungi and parasites.

The heart-failure presentation ranges from moderate dyspnea to fulminant shock. Sepsis and myocarditis may coexist, and global septic dysfunction does not exclude abscesses or focal inflammation; rapid improvement with hemodynamic control favors reversible functional depression, whereas persistent troponin elevation, late gadolinium enhancement and arrhythmias support structural injury; in critically ill patients the distinction evolves during treatment.

Palpitations, syncope and cardiac arrest reflect ventricular instability or conduction disturbances; rapidly fluctuating atrioventricular block after tick exposure is typical of Lyme disease, whereas new blocks after diphtheria have serious prognostic significance. Apical aneurysms and inferolateral scars in Chagas disease sustain tachycardia and thromboembolism even many years after the initial infection.

Systemic manifestations guide the investigation: erythema migrans, facial palsy or arthritis in Lyme disease; a pharyngeal pseudomembrane and cervical lymphadenopathy in diphtheria; Romaña sign or a chagoma during acute Chagas disease; sepsis, emboli and murmurs in endocarditis. Their absence does not exclude cardiac involvement, but their presence greatly changes pre-test probability.

In immunocompromised patients, fever and leukocytosis may be absent while progression is rapid; opportunistic infections may simultaneously involve the lungs, brain and heart, making an extracardiac sample more accessible. Starting a new immunosuppressive treatment for presumed autoimmune myocarditis should be avoided until this possibility has been assessed.

In children, viruses and vaccine-preventable diseases retain particular relevance, with lower physiologic reserve and sometimes nonspecific signs such as poor feeding, tachypnea or irritability. Local epidemiology and vaccination status carry more weight than the subjective description of pain; management requires pediatric centers when dysfunction or arrhythmias are significant.

The chronic course includes fatigue, exercise intolerance, ectopy and progressive dilatation, but demonstration of a remote infection does not prove that ongoing replication continues to sustain cardiomyopathy. Chagas disease is a well-defined exception, whereas in many presumed post-viral forms autoimmunity and predisposition have a greater role; this distinction determines whether antimicrobial therapy has a plausible target.

The severity of the extracardiac syndrome does not predict cardiac involvement linearly: an apparently mild infection may precede a severe arrhythmia, whereas profound sepsis may raise troponin without histologic myocarditis. Surveillance is therefore guided by cardiac symptoms, ECG, function and hemodynamic dynamics; this principle avoids both indiscriminate advanced testing and minimization of discordant warning signals.

Shared warning signs include syncope, sustained arrhythmia, advanced block, hypotension, rising lactate, biventricular dysfunction and failure to respond, but their meaning changes with context. A block in Lyme disease is often reversible, the same finding in diphtheria indicates severe toxic injury, and in a perivalvular abscess it signals anatomic extension; common monitoring therefore leads to different etiologic interventions.

Investigations and Diagnosis

Diagnosis begins with a structured epidemiologic history: symptoms and date of infection, contacts, travel, geographic origin, vectors, food, animals, procedures, injection drug use, immune status and vaccinations. Antimicrobial drugs already taken may sterilize cultures and alter the presentation; an explicit timeline prevents every positive antibody result from being converted into a cardiac cause.

ECG, telemetry, troponin, natriuretic peptides, complete blood count, inflammatory markers, renal and liver function and blood gas analysis describe severity; blood cultures are obtained before antibiotics when the patient is stable enough to allow this, whereas treatment is not delayed in shock. Additional tests are selected according to probability: two-tier serology for Lyme disease, smear or PCR for acute parasitemia, and culture and toxigenicity testing for diphtheria.

Echocardiography assesses function, effusion, valves, vegetations, abscesses and thrombi; transesophageal echocardiography is strongly recommended when transthoracic echocardiography is inconclusive, when it is negative but suspicion of endocarditis remains high and, in general, even when it is positive to assess complications, except in isolated native right-sided endocarditis with high-quality conclusive transthoracic imaging. Cardiac magnetic resonance characterizes edema and nonischemic injury and may guide biopsy, but it does not identify the pathogen; positron emission tomography and computed tomography search for disseminated foci or infected material in selected settings.

Microbiology must be interpreted in context; isolated IgG often demonstrates remote contact; IgM may be nonspecific or persist; a negative blood PCR does not exclude an agent confined to tissue. Even myocardial PCR requires quality controls, quantification, localization and histologic correlation to distinguish causality, latency and contamination.

Endomyocardial biopsy is indicated in high-risk phenotypes, when treatable forms are suspected and when the result changes immunosuppression or antimicrobial therapy. Samples are correctly separated among formalin, sterile medium and molecular preservation before collection; stains, immunohistochemistry, microscopy and selected sequencing are agreed upon with the pathologist and microbiologist rather than added retrospectively to exhausted tissue.

The differential diagnosis includes acute coronary syndrome, Takotsubo syndrome, sepsis with myocardial depression, endocarditis without myocardial invasion, genetic cardiomyopathy and immune-mediated myocarditis; a response to antibiotics does not prove causality if shock and hypoxia were corrected during the same period. Similarly, spontaneous improvement does not exclude an agent already eliminated by the host.

The required isolation level and public-health notification depend on the infection; suspected diphtheria requires precautions and immediate coordination with public-health authorities; Chagas disease may affect blood and organ donation; vector-borne infections require targeted prevention. The cardiac diagnosis therefore has consequences extending beyond the individual patient.

The preanalytical phase of biopsy must be planned before entering the procedure room: some fragments are fixed for histology, others are kept sterile or frozen for culture and molecular analysis. A sample entirely fixed in formalin cannot later be recovered for every method, while contamination from blood or the environment makes low-burden results difficult to interpret. The pathologist and microbiologist therefore participate in the strategy, not only in final interpretation.

Microbiological negativity after therapy must be distinguished from negativity obtained before treatment because antibiotics, antivirals and the immune response rapidly alter pathogen burden and culturability. The laboratory needs the timing of doses and the nature of the specimen to estimate the weight of the result; a strong clinical diagnosis is not erased by a late sample, but the remaining uncertainty is declared and followed.

Treatment and Prognosis

Treatment combines therapy directed at the causative agent, source control and cardiac support. Antibiotics, antitoxin, antiparasitic agents or antifungals are used when a plausible target exists, with drug, dose and duration selected according to guidelines for the specific infection. A generic broad-spectrum antibiotic is not complete etiologic therapy and is narrowed as soon as microbiology and susceptibility allow.

For most viral myocarditis in immunocompetent patients, no antiviral drug with proven cardiac efficacy is available; treatment is supportive, while antivirals and immunoglobulins are reserved for selected infections and conditions with specific evidence. The presence of a genome in biopsy does not automatically mean that an available antiviral drug reaches or eliminates that virus in the heart.

Congestion, low output and shock are treated according to physiology, with early mechanical circulatory support when hypoperfusion progresses. Arrhythmias are managed by correcting precipitants, using drugs and devices; reversible Lyme-related block favors temporary pacing, whereas a chronic scar may require permanent protection. Intense physical activity is suspended during the inflammatory phase and resumed only after reassessment.

Immunosuppression requires caution; it is appropriate in specific immune-mediated histotypes or selected protocols after tissue characterization, but may impair control of replicating viruses, fungi or parasites. If shock and a suspected hyperinflammatory response require a decision before certainty is available, adequate samples and rational antimicrobial coverage should precede treatment whenever possible.

Acute prognosis depends on shock, arrhythmias, biventricular function, the pathogen and timeliness of therapy. Treated Lyme disease often shows recovery of conduction, whereas diphtheritic myocarditis with advanced block retains high mortality; bacteremia with abscesses requires source control, and chronic Chagas disease retains progressive risk. Speaking generically of the prognosis of infectious myocarditis would therefore be misleading.

Follow-up documents eradication or control of the infection with appropriate tests and separates this objective from cardiac recovery. ECG, echocardiography, Holter monitoring and selected cardiac magnetic resonance assess residual injury, whereas organ function and drug interactions monitor antimicrobial toxicity; a scar may require arrhythmic surveillance even after microbiological cure.

Prevention includes vaccination against diphtheria and other infections, control of ticks and triatomine vectors, food and transfusion safety, procedural hygiene and prompt treatment of sepsis and endocarditis. Secondary prevention includes completing therapy, avoiding unnecessary immunosuppression and informing donors or relatives when the specific disease requires it.

When etiology and disease phase remain uncertain, decisions are built around the cost of error. Delaying antitoxin in diphtheria or antibiotics in shock has immediate consequences, whereas starting an immunosuppressant in the presence of a plausible fungal infection may promote dissemination. Rapid sampling, targeted coverage and predefined reassessments allow action under uncertainty without turning empiric therapy into an irreversible diagnosis.

The discharge plan brings together two trajectories that often have different time courses: control or eradication of the infection and recovery of the heart. Cultures, serology or PCR are repeated only when validated for the first objective, whereas ECG, function, rhythm and scar address the second; normalization of one trajectory does not justify stopping surveillance of the other.

Complications

Cardiogenic shock may overlap with septic or distributive shock, making a static classification insufficient. Serial echocardiography, perfusion assessment and selected catheterization distinguish the ventricular contribution and guide drugs and support; escalating catecholamines without reassessment increases myocardial oxygen demand and arrhythmias.

Abscesses, infected aneurysms, endocarditis and purulent pericarditis require surgical source control or drainage in addition to antimicrobials. A focus protected from blood flow may persist despite negative blood cultures; serial imaging and cardiac surgical collaboration should be anticipated when prosthetic material, vegetations or collections are present.

Conduction blocks and arrhythmias may resolve with control of the infection or leave a permanent scar; the choice among monitoring, a temporary device and definitive implantation considers the pathogen and reversibility without underestimating risk during the waiting period. In Lyme disease, avoiding an unnecessary pacemaker is important, but unstable bradycardia still requires immediate protection.

Emboli arise from vegetations, ventricular thrombi, aneurysms or the inflammatory state; these mechanisms require different treatments, and anticoagulation is not automatically safe in endocarditis with cerebral emboli. Echocardiography, brain imaging and microbiology define the decision in an interdisciplinary manner.

Antimicrobial toxicity and interactions may worsen renal function, liver function, cytopenias and QT prolongation, particularly during intensive care. Antifungals, macrolides and antiarrhythmic drugs share metabolic pathways and electrical risks; daily review of dose, organ function and drug levels when available prevents some complications.

Immunosuppression favors relapse or reactivation of latent infections and may attenuate their signs; new deterioration during corticosteroid therapy requires cultures and imaging before the dose is automatically increased. Distinguishing an immune inflammatory syndrome from pathogen proliferation remains one of the most delicate problems in the entire group.

Transition to chronic cardiomyopathy combines fibrosis, dilatation, failure and arrhythmic risk. Persistence of a genome does not always coincide with replication and does not guarantee a therapeutic target, whereas in Chagas disease parasite persistence has a recognized biological role. Follow-up must respect these differences and not apply the same model to every infection.

The final report separates the level of certainty for myocarditis from that for its etiology, because convincing cardiac magnetic resonance findings may coexist with an agent that is only possible, and definite microbiology may accompany multifactorial cardiac injury. This dual classification improves follow-up and antimicrobial use: the patient does not remain indefinitely labeled as infectious after the pathogen has been eradicated, while the scar continues to receive the surveillance it warrants.

Collaboration with public health is part of treatment for diphtheria, transfusion-related or congenital Chagas disease and other notifiable infections, whereas it is unnecessary for every presumed viral form. Isolation, contact management, blood screening and vector prevention are applied only when the route of transmission requires them; precise communication avoids unnecessary restrictions while protecting people who were truly exposed.

Final medication review removes prescriptions started empirically that no longer have a target, while maintaining those required for source control or residual dysfunction. This reconciliation reduces interactions, resistance and false attribution of adverse effects to the disease. The discharge document also specifies which pending results could still change the regimen and who is responsible for communicating them, preventing a late culture result from being left without follow-up.