Bacterial myocarditis

Article written and reviewed by Dr Luca Moretti

Bacterial myocarditis is a rare but probably underdiagnosed form of myocardial involvement during infection. The finding may consist of disseminated microabscesses during bacteremia, an abscess contiguous with a valve annulus, a focal lesion, or inflammation associated with gastroenteritis or another infection without demonstration of bacteria in the heart. These configurations do not have the same degree of causal certainty or the same treatment.

Before the antibiotic era, diphtheria, scarlet fever and other infections were recognized causes; today, Staphylococcus aureus plays an important role in pyogenic forms, especially in sepsis, endocarditis, device-associated infection and injection drug use. Pneumococcus, streptococci, meningococcus, enteric gram-negative bacteria, Salmonella, Campylobacter, Coxiella and many other pathogens have been described, but for some of them the evidence comes from only a few observations and does not permit reliable estimates.

The term must be distinguished from septic cardiodepression, an often global and reversible dysfunction produced by cytokines, vasoplegia, metabolic and microcirculatory abnormalities without necessarily implying focal infection of the myocardium. The two processes may coexist: a bacteremic patient may simultaneously have systemic cardiodepression, microabscesses and ischemia; diagnosis therefore becomes an anatomical and microbiological reconstruction rather than an inference from troponin.

Therapeutic urgency often precedes certainty; in septic or mixed shock, blood cultures and other specimens should be collected rapidly, but antibiotics are not delayed to obtain ideal imaging or biopsy. At the same time, source investigation must begin at admission, because an infected valve, a colonized catheter or an uncontrolled abscess continues to seed the heart despite use of an appropriate drug.

The clinical definition should specify whether there is demonstrated myocardial invasion, a perivalvular abscess, myopericarditis associated with an extracardiac infection, or septic cardiodepression. These conditions are often grouped together in reports, but they have different levels of evidence and different surgical requirements; precise anatomical and microbiological description allows experience to be transferred appropriately from one patient to another.

Empirical therapy and diagnostics are not activities arranged in a rigid sequence. In a stable patient, multiple blood culture sets are obtained and imaging is performed before antibiotics; in shock, rapid sampling and treatment proceed almost simultaneously, with the awareness that subsequent diagnostic yield will decline. Documenting the sampling time, first dose and previous exposures allows the microbiologist to interpret a negative result without assigning it excessive weight.

The distinction between primary myocardial infection and spread from endocarditis also changes the choice of imaging and treatment duration, because an isolated intramural focus does not carry the same embolic risk as a vegetation or the same surgical indication as an annular abscess. The report and discharge letter should preserve this anatomy instead of reducing it to generic infectious myocarditis.

Etiology, Pathogenesis and Pathophysiology

Hematogenous dissemination allows bacteria to reach the coronary microcirculation and form interstitial foci; neutrophils, necrosis and colonies produce microabscesses, sometimes numerous but invisible on gross examination. Staphylococcus aureus adheres to tissues and forms metastatic foci particularly efficiently, so persistent bacteremia requires systematic investigation for endocarditis and deep sites of infection.

Contiguous extension occurs from endocarditis, the valve annulus, prostheses, pericardium or mediastinum. A perivalvular abscess may invade the septum and interrupt the AV node and His bundle, making new conduction block a sign of an anatomical complication. Fistulas, pseudoaneurysms and prosthetic dehiscence form a continuum that transesophageal echocardiography and CT can define better than standard echocardiography.

Toxins and superantigens may depress function even when bacterial density in the myocardium is low. Diphtheria toxin is the clearest model and is addressed on the page on diphtheritic myocarditis; staphylococcal and streptococcal toxins instead contribute to vasoplegia, increased permeability and multiorgan dysfunction. In these cases, eradication of the bacterium must be accompanied by source and shock control.

Campylobacter jejuni may be associated with myopericarditis in young adults during or shortly after enterocolitis. The short interval suggests a direct or toxin-mediated effect rather than delayed molecular mimicry in some cases, but the mechanism has not been demonstrated and detection in stool does not equal myocardial invasion. The published course is often favorable, with the inevitable bias toward cases that were recognized and reported.

Coxiella burnetii, Borrelia and other intracellular or difficult-to-culture bacteria require specific methods. Lyme carditis has a predominantly conduction-system phenotype and its own serologic diagnostic pathway, while Q fever is supported by epidemiology, phase-specific serology and investigation for endocarditis; an undifferentiated bacterial panel does not replace these pathways.

Inflammation and edema reduce contractility, while necrosis and abscesses disrupt architecture and create arrhythmic substrates. Microvascular ischemia results from emboli, thrombosis, hypotension and compression; in shock, acidosis and catecholamines amplify oxygen demand and make it difficult to separate the infectious component from the hemodynamic cascade.

An immunocompromised host may have a high organism burden with little fever and an attenuated neutrophilic infiltrate; neutropenia, transplantation, malignancy, diabetes and devices increase the risk of dissemination and alter the spectrum of pathogens. In the immunocompetent patient, a skin source or injection drug use may be the only initial clues.

Pyogenic foci may evolve from invisible microcolonies to cavities with a necrotic center, inflammatory wall and risk of rupture; antibiotic penetration varies across the different components, and prosthetic material provides surfaces for biofilm. This explains why favorable in vitro susceptibility does not guarantee sterilization of an abscess and why source control has equal standing with drug selection.

Sepsis alters cardiac function even without myocardial colonization through vasoplegia, cytokines, mitochondrial abnormalities and beta-adrenergic uncoupling. If focal necrosis is superimposed on this state, function may not recover as rapidly as in ordinary septic cardiodepression; the echocardiographic trajectory after infection control therefore becomes an important retrospective clue, although it does not replace anatomical proof.

Persistent bacteremia may repeatedly seed new foci during the time required to identify the source, and culture clearance does not guarantee that previously formed foci have been sterilized. Control is therefore demonstrated with serial cultures, clinical improvement and imaging of deep sites when indicated; a fall in C-reactive protein alone is not sufficiently specific to close the diagnostic pathway.

Clinical Manifestations

Fever, chills, hypotension and signs of an infectious source often dominate the presentation, while dyspnea and chest pain are attributed to sepsis. Troponin elevation is common in critically ill patients and does not demonstrate myocarditis, but a marked or persistent rise associated with regional abnormalities, conduction block or arrhythmia warrants specific investigation; new heart failure may be left-sided, right-sided or biventricular.

A focal myocardial abscess may cause pain, persistent fever, arrhythmias or emboli without major global dysfunction; invasion of the septum or perivalvular tissue produces PR prolongation and progressive conduction block. A new conduction abnormality during endocarditis is therefore a sign of extension and may change both the indication for and urgency of surgery.

Myopericarditis associated with gastroenteritis generally presents with chest pain and troponin elevation while intestinal symptoms are still present. ECG may show diffuse ST elevation and echocardiography may remain normal; stool microbiological evidence increases plausibility, but other causes must be considered and antibiotic therapy follows the severity of enteritis and relevant guidelines, not the biomarker alone.

Shock may be distributive, cardiogenic or mixed; warm extremities and a wide pulse pressure do not exclude ventricular dysfunction, while vasoconstriction and low output may emerge after initial resuscitation. Repeated echocardiography and perfusion measurements guide therapy that must adapt to changing physiology.

Cutaneous, splenic, renal or cerebral emboli, murmurs and immunologic phenomena suggest endocarditis. Pneumonia, an osteoarticular infection or a soft-tissue abscess may represent the primary source or a parallel metastatic focus; daily examination should therefore search for new sites even after antibiotics have been started.

In neutropenia, fever and suppuration may be minimal and sudden deterioration may be the first manifestation; in a child, irritability, tachycardia and reduced feeding require a high index of suspicion. The threshold for imaging and intensive care depends on physiological reserve and the rate of deterioration, not on the apparent intensity of the inflammatory response.

Recovery may leave premature beats, conduction block or regional dysfunction; persistence of fever or bacteremia despite appropriate antibiotics signals an undrained source, resistance, inadequate drug concentration or a new complication. Treating fever as a normal course of myocarditis delays necessary source control.

The fever pattern is interpreted together with cultures and interventions; transient defervescence after an antipyretic or antibiotic does not demonstrate control, while persistence may reflect an abscess, a new metastatic site, a drug reaction or thrombosis. Repeating the physical examination and searching for vertebral, joint, abdominal or device-related pain is part of the cardiac assessment because it identifies the source that continues to seed the myocardium.

In Campylobacter-associated disease, the short interval between diarrhea and chest pain, stool culture and absence of bacteremia establish a different causal framework from staphylococcal microabscesses. The reported prognosis is often favorable and antibiotics are chosen according to severity of enteritis and host risk; presenting these forms as identical would lead to inappropriate use of prolonged intravenous therapy.

Arrhythmias may result from fever and electrolyte abnormalities, but new conduction block or a localized tachycardia suggests a deeper anatomical lesion. Correcting potassium and temperature is necessary, but temporary rhythm normalization does not exclude an abscess; ECG therefore becomes a surveillance tool for extension as well as for immediate stability.

Investigations and Diagnosis

Multiple sets of blood cultures from distinct sites are obtained before antibiotics whenever this does not cause a dangerous delay. Complete blood count, lactate, renal and liver function, coagulation studies, troponin and natriuretic peptides characterize severity; cultures from catheters, urine, sputum, stool or focal sites are selected according to symptoms and do not replace blood cultures.

Serial ECGs and telemetry assess conduction and arrhythmias; transthoracic echocardiography evaluates function and vegetations, but transesophageal echocardiography is superior for endocarditis, prostheses and perivalvular abscesses. The absence of vegetations does not exclude primary myocardial infection, and imaging is repeated if bacteremia or anatomical signs persist.

CMR identifies edema, LGE and collections, distinguishing an inflammatory pattern from infarction when the patient is stable. Cardiac CT better defines calcification, gas, pseudoaneurysms and surgical relationships; PET may localize prosthetic material or selected metastatic foci; no imaging study alone identifies the bacterial species.

Endomyocardial biopsy is not required when blood cultures, source and imaging already provide an adequate diagnosis, but becomes valuable in unexplained fulminant dysfunction or in the differential diagnosis from immune myocarditis. Tissue should be sent sterile for culture as well as fixed for histology; prior antibiotics reduce yield, while stains and PCR may preserve diagnostic information.

Pyogenic histology with neutrophils, necrosis and colonies supports direct invasion; a lymphocytic infiltrate without organisms during enteritis suggests an associated but less certain mechanism. Microbiological concordance among blood, tissue and source strengthens causality; broad-range sequencing may help in selected cases, but it is vulnerable to contamination and does not replace culture and susceptibility testing.

The differential diagnosis includes septic cardiodepression, acute coronary syndrome, coronary emboli from endocarditis, Takotsubo syndrome, purulent pericarditis, and viral or immune myocarditis. Septic infarction and myocarditis may coexist; coronary anatomy, CMR and tissue findings separate these mechanisms when the distinction changes management.

Source evaluation includes skin, teeth, catheters, joints, spine, abdomen and devices, guided by the pathogen. S. aureus bacteremia requires particular attention to metastatic foci; clearance of blood cultures is documented with serial samples and becomes an independent therapeutic indicator rather than being inferred from troponin.

When imaging shows a mass or cavity, the differential diagnosis includes tumor, thrombus, hematoma and abscess; perfusion, mobility, uptake, gas and relationships with valves and coronary arteries contribute, but the surgical specimen may be definitive. Agreeing in advance on aerobic, anaerobic, fungal and molecular cultures prevents all material from being fixed and rendered microbiologically unusable.

Antimicrobial susceptibility is interpreted together with the site of infection, because breakpoints, biofilm and penetration may change the meaning of the minimum inhibitory concentration. Dose, interval and infusion are adjusted to renal function, weight, extracorporeal support and volume of distribution; therapeutic drug monitoring, when available, reduces both underdosing during shock and toxicity during organ recovery.

When culture-negative endocarditis is suspected, zoonotic exposures, prior antibiotics and intracellular pathogens guide dedicated serology and PCR, while broad-range tests are reserved for unresolved cases. The probability of contamination increases when the amount of DNA is minimal, and every unexpected sequence is verified by an independent method before major treatment changes are made.

Treatment and Prognosis

Early empirical antibiotics cover plausible pathogens based on the source, community- or hospital-acquired setting, local resistance patterns, immune status and severity. After identification, the regimen is narrowed to bactericidal therapy with appropriate dose and penetration. Duration and route depend on bacteremia, endocarditis, abscesses, prosthetic material and source control, and there is no single regimen for the word myocarditis.

Source control includes catheter removal, drainage of collections, debridement and valvular or perivalvular surgery when indicated. Antibiotics alone may fail in closed cavities and on prosthetic material; the cardiologist, infectious disease specialist, microbiologist and cardiac surgeon should agree on urgency before conduction block, fistula or shock makes intervention more hazardous.

Shock resuscitation uses reassessed fluid aliquots, vasopressors, inotropes and ventilation according to perfusion and congestion. Circulatory support may be necessary for the cardiogenic component, but uncontrolled sepsis increases complications and influences selection; doses are adjusted to renal function and extracorporeal support.

Arrhythmias and conduction block require correction of electrolytes, drugs and temporary pacing when unstable. A permanent device is deferred, when possible, until bacteremia is controlled, using safe temporary strategies; early implantation while blood cultures remain positive exposes the patient to device colonization and further extraction procedures.

Corticosteroids are not routine therapy for bacterial invasion; any post-infectious immune syndromes are diagnosed using rigorous criteria and treated after pathogen control, because attenuating the immune response during bacteremia may promote dissemination. Improvement with corticosteroids does not compensate for inadequate microbiological diagnosis.

Prognosis is poor in forms associated with sepsis, abscesses, delayed source control, immunosuppression and shock, whereas myopericarditis associated with enteritis may recover completely. The literature consists mainly of case reports and small series and overrepresents exceptional presentations; individual prediction should be based on the microbiological and hemodynamic response.

Follow-up verifies sterilization, anatomical resolution and cardiac recovery; cultures and imaging of the source follow infectious-disease timelines, whereas ECG, Holter monitoring and echocardiography document conduction and function. CMR is useful if uncertainty persists about scar or return to activity.

Treatment duration is anchored to the first day of negative cultures, source control and the presence of endocarditis, abscess or prosthetic material, following the relevant guidelines. Normalization of troponin does not shorten treatment of a deep infection. Conversely, a mildly persistent troponin elevation does not mandate indefinite antibiotics if microbiology and anatomy document healing.

Discharge requires a plan for venous access, adherence, blood tests, cultures and imaging in addition to cardiology follow-up. Fever, chills, new pain, syncope or neurologic signs must have a rapid return pathway; the hospital-to-community transition is particularly vulnerable when multiple specialists prescribe drugs with nephrotoxicity, interactions and different monitoring requirements.

Rehabilitation begins after microbiological and hemodynamic stability and takes into account scar, intensive-care-acquired weakness and any access lines that are still required. Return to exertion does not follow antibiotic completion alone, because residual myocarditis may retain electrical risk; function, rhythm and capacity are reassessed gradually.

Complications

Myocardial abscesses may rupture into the pericardium, create fistulas or extend to the valve annuli. A new murmur, conduction block, pain or effusion requires urgent imaging and surgical assessment; reduction in fever does not prove that an infected cavity has been sterilized.

Mixed shock combines vasoplegia, cardiac dysfunction and sometimes obstruction from mechanical complications. Multiorgan injury limits antibiotic and surgical options and increases mortality; repeated reassessment avoids giving fluids to a congested ventricle or attributing all hypotension to pump failure.

Complete heart block and ventricular arrhythmias may persist after infection control if tissue has become necrotic. A septal scar creates long-term pacing and defibrillation issues; device timing balances reversibility, immediate risk and sterility.

Septic emboli may affect the brain, spleen, kidney, limbs and coronary arteries; anticoagulation does not resolve a vegetation and may increase hemorrhagic risk in some neurologic complications. Antithrombotic decisions are based on the mechanism and imaging.

Valvular insufficiency, pseudoaneurysm and prosthetic dehiscence transform a myocardial infection into structural heart disease. Medical therapy temporizes or accompanies intervention but does not reconstruct destroyed tissue; surgical follow-up should continue after cultures have become negative.

Nephrotoxicity, cytopenias, Clostridioides difficile diarrhea, QT prolongation and drug interactions are complications of prolonged antibiotics. Stewardship means maintaining adequate efficacy while reducing unnecessary exposure, not arbitrarily shortening treatment of a deep infection; monitoring and dose adjustments are especially important during extracorporeal support.

Residual cardiomyopathy may require chronic heart failure therapy, temporary restriction of exertion and arrhythmic surveillance. Microbiological and myocardial recovery occur on different timelines; the patient is considered recovered only after both pathways have been reassessed.

Prevention of recurrence depends on the source: dental and valvular care, catheter asepsis, treatment of skin infections, harm reduction in injection drug use and management of prosthetic material carry more weight than generic antibiotic prophylaxis. In patients with recognized indications, endocarditis prophylaxis is applied to relevant procedures, whereas indiscriminate use increases resistance and adverse reactions without demonstrated protection. The plan is individualized according to the anatomy that generated the first episode.