# Seminar 12: Infectious Disease Emergencies

## Emergency Medicine Clerkship

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## Learning Objectives

By the end of this seminar, students will be able to:

1. Recognize sepsis using current definitions based on infection with organ dysfunction and apply the Sequential Organ Failure Assessment criteria to identify at-risk patients
2. Implement the hour-1 sepsis bundle including lactate measurement, blood cultures, broad-spectrum antibiotics, fluid resuscitation, and vasopressor initiation for refractory hypotension
3. Identify necrotizing soft tissue infections by recognizing pain out of proportion to examination, rapid progression, and systemic toxicity, and initiate emergent surgical consultation
4. Select appropriate empiric antimicrobial therapy based on suspected source, community versus healthcare acquisition, and special patient populations including neutropenic and immunocompromised hosts
5. Diagnose and manage bacterial meningitis with emphasis on the urgency of antibiotic administration and the role of adjunctive dexamethasone therapy
6. Evaluate the immunocompromised patient with fever by identifying the type of immune deficiency and selecting antimicrobial coverage targeting the most likely opportunistic pathogens

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## Seminar Outline

### Section 1: Sepsis Recognition

Sepsis is currently defined by the Third International Consensus Definitions (Sepsis-3) as a life-threatening organ dysfunction caused by a dysregulated host response to infection. This definition replaced the earlier systemic inflammatory response syndrome (SIRS) criteria, which lacked specificity for infection-related organ dysfunction. Under the current framework, organ dysfunction is quantified using the Sequential Organ Failure Assessment (SOFA) score, with an increase of 2 or more points from baseline indicating sepsis. Septic shock represents the most severe end of the spectrum and is defined by the need for vasopressor therapy to maintain a mean arterial pressure of at least 65 mmHg in conjunction with a serum lactate level greater than 2 mmol/L despite adequate fluid resuscitation. The quick SOFA (qSOFA) score, which incorporates a respiratory rate of 22 or greater, altered mental status, and systolic blood pressure of 100 mmHg or less, serves as a bedside screening tool but is not diagnostic and has limited sensitivity.

The SOFA criteria evaluate six organ systems, each scored from 0 to 4 based on the degree of dysfunction. The respiratory system is assessed by the PaO2/FiO2 ratio, with lower values indicating more severe respiratory failure. The coagulation system is evaluated by platelet count, with thrombocytopenia reflecting consumptive coagulopathy. Hepatic function is assessed by serum bilirubin elevation. Cardiovascular function incorporates mean arterial pressure and the need for vasopressor support. Neurologic function is assessed using the Glasgow Coma Scale. Renal function is evaluated by serum creatinine and urine output. A baseline SOFA score of zero is assumed for patients without known pre-existing organ dysfunction, and an acute increase of 2 or more points identifies the sepsis patient at significantly elevated risk of in-hospital mortality exceeding 10 percent.

Identifying the source of infection is a critical early step in sepsis management because it guides empiric antibiotic selection and determines the need for source control interventions. Pulmonary infections are the most common source of sepsis, suggested by cough, hypoxia, and chest radiographic infiltrates. Urinary tract infections are the second most common source, identified by dysuria, pyuria on urinalysis, and positive urine culture. Intra-abdominal infections present with abdominal pain, peritoneal signs, and may require imaging for diagnosis. Skin and soft tissue infections are identified by erythema, warmth, swelling, and fluctuance suggesting abscess. Central nervous system infections present with headache, nuchal rigidity, and altered mental status. Line-related and catheter-associated infections should be suspected in patients with indwelling devices or recent procedures, particularly when no other source is readily apparent.

Certain populations are at heightened risk for developing sepsis and may present with atypical or attenuated clinical features that delay recognition. Elderly patients frequently exhibit blunted febrile responses, minimal tachycardia, and subtle mental status changes as the primary manifestation of severe infection. Immunocompromised patients, including those receiving chemotherapy, transplant recipients, and patients with HIV/AIDS, may harbor atypical organisms and present without the classic inflammatory response. Diabetic patients have increased susceptibility to infection from both impaired immune function and microvascular disease. Patients with chronic medical conditions including chronic kidney disease, liver cirrhosis, and heart failure have reduced physiologic reserve and may decompensate rapidly. Patients with recent hospitalization or healthcare exposure are at risk for infection with antibiotic-resistant organisms including MRSA, ESBL-producing Enterobacteriaceae, and Pseudomonas aeruginosa.

<image>Panel A: Sepsis-3 definition framework showing the progression from infection through organ dysfunction (SOFA increase of 2 or more) to septic shock (vasopressors plus lactate greater than 2 despite fluids), with the qSOFA screening tool positioned as a bedside assessment. Panel B: SOFA scoring grid showing the six organ systems (respiratory, coagulation, hepatic, cardiovascular, neurologic, renal) with their respective parameters and scoring thresholds from 0 to 4. Panel C: Body diagram illustrating common infection sources with associated clinical clues, ordered by frequency: pulmonary, urinary, abdominal, skin and soft tissue, central nervous system, and line-related. Panel D: High-risk population icons showing elderly, immunocompromised, diabetic, chronically ill, and recently hospitalized patients with their respective atypical presentation patterns.</image>

### Section 2: Sepsis Management

The hour-1 sepsis bundle represents the current standard of care and consists of five interventions that should be initiated within the first hour of sepsis recognition. Serum lactate should be measured immediately, as it serves as both a prognostic marker and a guide for resuscitation adequacy; if the initial lactate exceeds 2 mmol/L, it should be remeasured within 2 to 4 hours to assess the response to treatment. Blood cultures from at least two separate sites should be obtained before antibiotic administration, provided that culture collection does not delay antimicrobial therapy. Broad-spectrum antibiotics should be administered within 1 hour of sepsis recognition, as each hour of delay in antibiotic administration is associated with a measurable increase in mortality. For patients who are hypotensive or have a lactate level of 4 mmol/L or greater, 30 mL/kg of crystalloid fluid should be initiated. Vasopressor therapy should be started during or after fluid resuscitation if hypotension persists.

Fluid resuscitation is a cornerstone of early sepsis management, addressing the relative and absolute hypovolemia that results from vasodilation, capillary leak, and decreased oral intake. The initial fluid bolus of 30 mL/kg of crystalloid (either normal saline or lactated Ringer solution) is administered rapidly, with reassessment after each aliquot to evaluate hemodynamic response. Dynamic measures of fluid responsiveness, including passive leg raise testing, pulse pressure variation during mechanical ventilation, and bedside echocardiographic assessment of inferior vena cava collapsibility, are superior to static measures such as central venous pressure for determining whether additional fluid will improve cardiac output. Over-resuscitation is increasingly recognized as harmful, contributing to pulmonary edema, abdominal compartment syndrome, and worsened organ function, and clinicians should reassess the need for further fluid administration at each decision point.

Vasopressor therapy is indicated when fluid resuscitation fails to restore adequate perfusion pressure. Norepinephrine is the first-line vasopressor for septic shock, providing potent alpha-1 adrenergic vasoconstriction with modest beta-1 inotropic effects. The target mean arterial pressure is 65 mmHg or greater. Vasopressin at a fixed dose of 0.03 units per minute is added as a second-line agent when norepinephrine alone is insufficient, acting through V1 receptors to cause vasoconstriction independent of the adrenergic system. Epinephrine may be added or substituted when the combination of norepinephrine and vasopressin fails to achieve target perfusion pressure. Vasopressor administration through a peripheral intravenous line is acceptable as a temporizing measure while central venous access is being established, as the risk of delayed vasopressor initiation outweighs the risk of peripheral extravasation in the short term.

Source control encompasses the physical interventions required to eradicate the focus of infection and is as important as antimicrobial therapy. Identification of a drainable collection, removable device, or necrotic tissue requiring debridement should be pursued through targeted physical examination and imaging. Abscesses require drainage, whether percutaneous or surgical. Empyema requires chest tube placement or surgical drainage. Infected intravascular catheters, prosthetic devices, and hardware should be removed when feasible. Necrotic tissue, as seen in necrotizing soft tissue infections, requires emergent surgical debridement. The timing of source control should be as soon as feasible after initial resuscitation and should not be unnecessarily delayed, as failure to achieve source control is a major contributor to refractory sepsis and mortality.

<image>Panel A: Hour-1 sepsis bundle infographic showing the five concurrent interventions (lactate measurement, blood cultures, antibiotics, crystalloid fluids, vasopressors) with timing targets and decision triggers for each step. Panel B: Fluid resuscitation algorithm showing the initial 30 mL/kg bolus, reassessment using dynamic measures (passive leg raise, pulse pressure variation, IVC ultrasound), and decision point for additional fluids versus vasopressors. Panel C: Vasopressor escalation ladder showing norepinephrine as first-line, vasopressin as add-on, and epinephrine as third-line, with the MAP target of 65 mmHg and peripheral versus central access considerations. Panel D: Source control examples showing abscess drainage, infected catheter removal, empyema chest tube placement, and necrotizing fasciitis debridement with corresponding imaging modalities for each.</image>

### Section 3: Empiric Antibiotic Selection

Empiric antibiotic selection for community-acquired sepsis is guided by the suspected source of infection and local resistance patterns. When the source is unknown, broad-spectrum coverage should include a third-generation cephalosporin such as ceftriaxone, combined with azithromycin for atypical pulmonary coverage, and vancomycin if methicillin-resistant Staphylococcus aureus is a concern. For pulmonary sources, the combination of ceftriaxone plus azithromycin provides standard community-acquired pneumonia coverage, with the addition of vancomycin for critically ill patients or those with risk factors for MRSA. Urinary sources are adequately covered by ceftriaxone or a fluoroquinolone. Intra-abdominal sources require anaerobic coverage, provided by piperacillin-tazobactam or the combination of ceftriaxone plus metronidazole. Skin and soft tissue infections with systemic sepsis require vancomycin plus piperacillin-tazobactam to cover both MRSA and gram-negative organisms.

Healthcare-associated infections require broader antimicrobial coverage due to the increased prevalence of resistant organisms. MRSA risk factors include prior MRSA colonization or infection, recent hospitalization, hemodialysis, and residence in long-term care facilities, and these patients should receive empiric vancomycin or linezolid. Pseudomonas aeruginosa risk is elevated in patients with recent broad-spectrum antibiotic exposure, structural lung disease, and prolonged hospitalization, necessitating antipseudomonal beta-lactam coverage with agents such as piperacillin-tazobactam, cefepime, or meropenem. Extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae are increasingly prevalent in healthcare settings and require carbapenem coverage. Candida should be considered in patients with prolonged intensive care stays, total parenteral nutrition, broad-spectrum antibiotic use, and immunosuppression, with the addition of an echinocandin antifungal. Prior culture data should always be reviewed to tailor empiric therapy to known colonization or infection patterns.

Special clinical situations require specific antimicrobial strategies that deviate from standard empiric regimens. Neutropenic fever, defined as an absolute neutrophil count below 500 cells per microliter with a temperature of 38.3 degrees Celsius or higher, is a medical emergency requiring immediate initiation of antipseudomonal monotherapy with cefepime, piperacillin-tazobactam, or a carbapenem. Functional or anatomic asplenia places patients at risk for overwhelming infection with encapsulated organisms, particularly Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae, and ceftriaxone is the empiric agent of choice. Bacterial meningitis requires the combination of ceftriaxone plus vancomycin plus adjunctive dexamethasone, with the addition of ampicillin for patients aged 50 years or older or those who are immunocompromised to cover Listeria monocytogenes. Suspected necrotizing fasciitis requires triple coverage with vancomycin plus piperacillin-tazobactam plus clindamycin, the latter added specifically for its ability to inhibit toxin production.

Dosing considerations in sepsis are important because the pharmacokinetic changes associated with critical illness can significantly alter drug levels and efficacy. Loading doses of antibiotics should generally be administered at full dose regardless of renal or hepatic function, as the first dose establishes the initial drug concentration necessary for bacterial killing. Subsequent doses should be adjusted for organ dysfunction, particularly renal impairment, to avoid accumulation and toxicity. In severe sepsis, the increased volume of distribution from capillary leak and aggressive fluid resuscitation may result in subtherapeutic drug levels, supporting the use of higher initial doses for concentration-dependent antibiotics. Pharmacokinetic and pharmacodynamic principles should guide dosing optimization: beta-lactams, which exhibit time-dependent killing, may benefit from extended or continuous infusions, while aminoglycosides and fluoroquinolones, which exhibit concentration-dependent killing, require adequate peak levels.

<image>Panel A: Community-acquired sepsis empiric antibiotic selection chart organized by suspected source (unknown, pulmonary, urinary, abdominal, skin) with recommended regimens for each source. Panel B: Healthcare-associated infection risk factor assessment showing MRSA, Pseudomonas, ESBL, and Candida risk factors with corresponding antimicrobial additions. Panel C: Special clinical situation protocols showing neutropenic fever (cefepime), asplenia (ceftriaxone), meningitis (ceftriaxone plus vancomycin plus dexamethasone), and necrotizing fasciitis (vancomycin plus piperacillin-tazobactam plus clindamycin). Panel D: Pharmacokinetic considerations in sepsis showing the impact of increased volume of distribution on drug levels, loading dose principles, and time-dependent versus concentration-dependent antibiotic dosing strategies.</image>

### Section 4: Necrotizing Soft Tissue Infections

Necrotizing soft tissue infections are rapidly progressive, life-threatening infections that destroy skin, subcutaneous tissue, fascia, and potentially muscle, with mortality rates ranging from 20 to 40 percent even with appropriate treatment. The clinical hallmark that should raise immediate suspicion is pain that is disproportionate to the visible physical examination findings, as the infectious process spreads along fascial planes beneath relatively normal-appearing skin. Rapid progression over hours, rather than days, distinguishes necrotizing infections from simple cellulitis. Cutaneous findings that suggest deep tissue necrosis include dusky or violaceous discoloration, bullae or hemorrhagic blisters, and palpable crepitus from gas-producing organisms. Systemic toxicity with fever, tachycardia, hypotension, and altered mental status is common at presentation. The Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score provides a laboratory-based risk assessment tool, though clinical suspicion should not be abandoned when the score is low.

Necrotizing soft tissue infections are classified by microbiology into several types. Type I (polymicrobial) infections are the most common and involve a mixture of aerobic and anaerobic organisms, often occurring in the perineum (Fournier gangrene), abdominal wall, or postoperative wounds. Type II (monomicrobial) infections are caused by a single virulent organism, most commonly group A Streptococcus (Streptococcus pyogenes) or Staphylococcus aureus, including community-acquired MRSA. Type III infections are caused by Clostridium species, particularly Clostridium perfringens, producing the classic gas gangrene with rapid tissue necrosis, gas production, and overwhelming toxemia. Vibrio vulnificus necrotizing fasciitis occurs after saltwater or raw shellfish exposure, particularly in patients with liver disease, and progresses with alarming rapidity.

The management of necrotizing soft tissue infections requires simultaneous aggressive resuscitation and emergent surgical intervention. Hemodynamic resuscitation with crystalloid fluids and vasopressors follows standard sepsis protocols, as most patients present with septic shock. Empiric antibiotic coverage should be broad and include vancomycin for MRSA coverage, piperacillin-tazobactam for gram-negative and anaerobic coverage, and clindamycin, which is added specifically because it inhibits bacterial protein synthesis and toxin production, an effect that is particularly important against the exotoxins produced by group A Streptococcus and Clostridium species. Emergent surgical debridement is the definitive treatment, and any delay in operative intervention is directly associated with increased mortality. The initial debridement must be aggressive, excising all necrotic and questionably viable tissue back to healthy, bleeding margins. Planned re-exploration at 24 to 48 hours is standard, as the extent of necrosis often exceeds what is apparent at initial surgery.

The LRINEC score uses six laboratory parameters to stratify the risk of necrotizing fasciitis. C-reactive protein of 150 mg/L or greater receives 4 points. White blood cell count between 15,000 and 25,000 receives 1 point, and above 25,000 receives 2 points. Hemoglobin below 11 g/dL receives 2 points. Serum sodium below 135 mEq/L receives 2 points. Serum creatinine above 1.6 mg/dL receives 2 points. Serum glucose above 180 mg/dL receives 1 point. A total score of 6 or greater indicates a high risk for necrotizing fasciitis. However, the LRINEC score has significant limitations, including modest sensitivity, and should be used as an adjunct to clinical judgment rather than as a rule-out tool. The decision to pursue surgical exploration should be based on clinical suspicion, and a low LRINEC score should never delay surgical consultation when the clinical picture is concerning.

<image>Panel A: Clinical photographs showing the progression of necrotizing fasciitis from initial dusky erythema through bullae formation, skin necrosis, and crepitus, with emphasis on the discordance between minimal visible findings and severe pain. Panel B: Classification diagram of necrotizing soft tissue infections showing Type I (polymicrobial), Type II (monomicrobial Streptococcus or Staphylococcus), Type III (Clostridial gas gangrene), and Vibrio vulnificus with their associated clinical settings. Panel C: Emergent management algorithm showing simultaneous resuscitation, triple antibiotic therapy (vancomycin, piperacillin-tazobactam, clindamycin), and surgical debridement with planned re-exploration. Panel D: LRINEC score calculator showing the six laboratory parameters with point values and the threshold score of 6 indicating high risk, accompanied by a note on its limited sensitivity.</image>

### Section 5: Severe Pneumonia

Severity assessment in community-acquired pneumonia is essential for determining the appropriate level of care and guiding antibiotic selection. The CURB-65 score is the most widely used bedside tool, incorporating five variables: Confusion (new onset), Urea greater than 7 mmol/L (or BUN greater than 20 mg/dL), Respiratory rate of 30 or greater, Blood pressure below 90 mmHg systolic or 60 mmHg diastolic, and age 65 years or older. Each variable receives 1 point, and a score of 2 or greater suggests the need for hospital admission, while a score of 3 or greater indicates consideration for intensive care unit admission. The Pneumonia Severity Index (PSI/PORT score) is a more comprehensive tool that incorporates demographics, comorbidities, physical findings, and laboratory data to stratify patients into five risk classes predicting 30-day mortality. Clinical judgment should always supplement scoring tools, as individual patient factors may warrant a higher level of care than the score alone would suggest.

Community-acquired pneumonia treatment is stratified by the level of care required. Outpatient management for previously healthy patients without recent antibiotic use consists of amoxicillin or doxycycline, with azithromycin as an alternative when local pneumococcal resistance rates are low. Inpatient, non-ICU management typically consists of a beta-lactam (ceftriaxone) combined with a macrolide (azithromycin), or alternatively a respiratory fluoroquinolone (levofloxacin or moxifloxacin) as monotherapy. ICU-level pneumonia requires ceftriaxone plus azithromycin as the standard regimen, with the addition of vancomycin or linezolid when MRSA is suspected based on risk factors or positive nasal screening. When Pseudomonas aeruginosa is a concern, an antipseudomonal beta-lactam replaces ceftriaxone.

Hospital-acquired and ventilator-associated pneumonia represent infections that develop 48 or more hours after hospital admission and are caused by a broader spectrum of organisms including MRSA and gram-negative bacilli such as Pseudomonas aeruginosa, Acinetobacter, and ESBL-producing Enterobacteriaceae. MRSA coverage with vancomycin or linezolid is recommended for patients with risk factors for MRSA or in units with high MRSA prevalence. Antipseudomonal coverage with cefepime, piperacillin-tazobactam, or a carbapenem is standard. The duration of antibiotic therapy for hospital-acquired pneumonia is typically 7 days if the patient is demonstrating clinical improvement, and de-escalation from broad-spectrum to targeted therapy should occur as soon as culture results are available to reduce the risk of antibiotic resistance and Clostridioides difficile infection.

Parapneumonic effusions and empyema are important complications of pneumonia that require specific management beyond antibiotics alone. Simple parapneumonic effusions are exudative fluid collections that develop in the pleural space adjacent to the pneumonia and may resolve with antibiotic treatment alone without drainage. Complicated parapneumonic effusions have biochemical features suggesting progression toward empyema and require drainage; indicators include pleural fluid pH below 7.2, glucose below 60 mg/dL, positive Gram stain or culture, and loculations on imaging. Empyema, defined as frank pus in the pleural space, requires definitive drainage with a chest tube, and patients who fail to improve with tube thoracostomy may require video-assisted thoracoscopic surgery or open thoracotomy for decortication. All patients with significant parapneumonic effusions should undergo diagnostic thoracentesis for Gram stain, culture, pH, glucose, and cell count to guide management.

<image>Panel A: CURB-65 scoring tool with the five variables (Confusion, Urea, Respiratory rate, Blood pressure, age 65) and the disposition algorithm based on total score from outpatient through ICU admission. Panel B: Community-acquired pneumonia treatment ladder showing antibiotic regimens for outpatient, inpatient non-ICU, and ICU levels of care with MRSA and Pseudomonas risk additions. Panel C: Hospital-acquired pneumonia management diagram showing MRSA and antipseudomonal coverage options, 7-day treatment duration, and de-escalation principles based on culture data. Panel D: Parapneumonic effusion progression from simple (antibiotics only) through complicated (drainage indicated by pH below 7.2, low glucose, positive culture) to empyema (chest tube or surgical drainage), with diagnostic thoracentesis indications.</image>

### Section 6: Meningitis

Bacterial meningitis presents with the classic triad of fever, neck stiffness, and altered mental status, though this complete triad is present in fewer than 50 percent of cases, making clinical suspicion essential for diagnosis. Headache, typically severe and diffuse, is the most sensitive symptom. Photophobia is common and reflects meningeal irritation. A petechial or purpuric rash in a febrile patient with meningeal symptoms strongly suggests meningococcemia and should prompt immediate empiric treatment. The classic physical examination signs of meningeal irritation include Kernig sign (pain with passive knee extension when the hip is flexed) and Brudzinski sign (involuntary hip and knee flexion with passive neck flexion), though both have limited sensitivity and their absence should never be used to exclude meningitis.

Lumbar puncture is the diagnostic procedure of choice and should be performed as expeditiously as possible. However, specific clinical features mandate cranial computed tomography before lumbar puncture to exclude mass lesions or elevated intracranial pressure that could predispose to herniation. Indications for CT prior to lumbar puncture include immunocompromised status, history of central nervous system disease, new-onset seizures, papilledema on fundoscopic examination, focal neurologic deficits, and significantly altered level of consciousness. Critically, antibiotics must not be withheld while awaiting CT or lumbar puncture, as the delay in antimicrobial therapy significantly worsens outcomes. Cerebrospinal fluid analysis in bacterial meningitis typically demonstrates elevated white blood cell count with a neutrophilic predominance, decreased glucose (less than 40 mg/dL or a CSF-to-serum ratio less than 0.4), and elevated protein. Gram stain may identify the causative organism in 60 to 90 percent of untreated cases.

Empiric antibiotic treatment for bacterial meningitis must be administered immediately, ideally within 30 minutes of presentation for suspected cases. For adults under 50 years of age, the standard regimen is ceftriaxone plus vancomycin, providing coverage for Streptococcus pneumoniae (including penicillin-resistant strains) and Neisseria meningitidis. For adults aged 50 years and older, ampicillin is added to cover Listeria monocytogenes, which has an increased incidence in this age group, pregnant women, and immunocompromised patients. Dexamethasone at 0.15 mg/kg intravenously every 6 hours for 4 days should be administered before or simultaneously with the first dose of antibiotics, as it has been shown to reduce mortality and neurologic sequelae in pneumococcal meningitis. For immunocompromised patients, the regimen expands to include ampicillin plus ceftazidime (or meropenem) plus vancomycin to cover Listeria, Pseudomonas, and resistant pneumococci.

Viral encephalitis, particularly herpes simplex encephalitis, must be considered in the differential diagnosis of any patient presenting with altered mental status, fever, and focal neurologic findings or seizures. Herpes simplex virus is the most common cause of sporadic encephalitis in the developed world and has a predilection for the temporal lobes, producing temporal lobe-specific symptoms including olfactory hallucinations, personality changes, and temporal lobe seizures. Magnetic resonance imaging typically demonstrates temporal lobe edema and hemorrhage. Cerebrospinal fluid analysis shows a lymphocytic pleocytosis with elevated protein and normal glucose, and HSV PCR is the definitive diagnostic test. Treatment with intravenous acyclovir at 10 mg/kg every 8 hours must be initiated empirically whenever encephalitis is suspected, as untreated herpes simplex encephalitis carries a mortality rate exceeding 70 percent, while early treatment reduces mortality to approximately 20 percent.

<image>Panel A: Clinical presentation diagram showing the classic triad of fever, neck stiffness, and altered mental status with photographs of petechial rash in meningococcemia and demonstration of Kernig and Brudzinski signs. Panel B: Decision algorithm for CT before lumbar puncture, listing the specific indications (immunocompromised, CNS disease, seizures, papilledema, focal neuro, altered consciousness) with the critical instruction to administer antibiotics before CT if delay is anticipated. Panel C: Empiric antibiotic regimens stratified by age and immune status showing ceftriaxone plus vancomycin for adults under 50, addition of ampicillin for those over 50 or immunocompromised, and the timing of dexamethasone administration. Panel D: Comparison of bacterial meningitis and herpes simplex encephalitis showing CSF findings (neutrophilic versus lymphocytic), imaging (normal versus temporal lobe involvement), treatment (antibiotics versus acyclovir), and MRI demonstrating temporal lobe edema in HSV encephalitis.</image>

### Section 7: Endocarditis

Infective endocarditis is an infection of the endocardial surface of the heart, most commonly involving the cardiac valves, and presents with a wide range of clinical manifestations depending on the virulence of the organism, the valve involved, and the duration of infection. Fever is the most common presenting symptom, occurring in up to 90 percent of cases. A new or changing cardiac murmur indicates valvular involvement and is present in the majority of patients, though its absence does not exclude the diagnosis. Embolic phenomena are characteristic and include stroke from cerebral emboli, splinter hemorrhages (linear subungual hemorrhages), Janeway lesions (painless erythematous lesions on the palms and soles), and Osler nodes (painful, tender nodules on the fingers and toes). Risk factors include intravenous drug use (which predisposes to right-sided, particularly tricuspid valve endocarditis), prosthetic heart valves, poor dentition, structural heart disease, and prior endocarditis. Blood cultures are positive in approximately 90 percent of cases when obtained before antibiotic exposure.

The modified Duke criteria provide a standardized diagnostic framework for infective endocarditis. The two major criteria are microbiological evidence (positive blood cultures with typical endocarditis organisms from two separate cultures or persistently positive cultures) and endocardial involvement (echocardiographic evidence of vegetation, abscess, or new valvular dehiscence, or a new regurgitant murmur). Minor criteria include a predisposing heart condition or intravenous drug use, fever of 38 degrees Celsius or higher, vascular phenomena (arterial emboli, septic pulmonary infarcts, mycotic aneurysm, intracranial hemorrhage, Janeway lesions), and immunologic phenomena (Osler nodes, glomerulonephritis, Roth spots). Definite endocarditis is diagnosed with 2 major criteria, 1 major plus 3 minor criteria, or 5 minor criteria. Transesophageal echocardiography is superior to transthoracic echocardiography for detecting vegetations and should be obtained when clinical suspicion is high despite a negative transthoracic study.

Empiric antibiotic treatment for endocarditis depends on whether the infection involves a native or prosthetic valve and should be initiated after blood cultures are obtained. For native valve endocarditis, vancomycin plus ceftriaxone provides coverage for the most common organisms including Staphylococcus aureus, viridans group streptococci, and enterococci pending culture results. Prosthetic valve endocarditis carries a broader microbial spectrum and requires vancomycin plus gentamicin plus rifampin to cover coagulase-negative staphylococci and other resistant organisms. Intravenous drug use-associated endocarditis is most commonly caused by Staphylococcus aureus (including MRSA) and typically involves the tricuspid valve, presenting with septic pulmonary emboli manifesting as multiple peripheral lung nodules on chest imaging. The duration of treatment for endocarditis is prolonged, typically 4 to 6 weeks of intravenous antibiotics, reflecting the difficulty of eradicating organisms embedded within the fibrin-platelet matrix of vegetations.

Surgical intervention in endocarditis is indicated for specific complications that cannot be managed with antibiotics alone. Heart failure resulting from acute valvular insufficiency is the most common indication for surgery and should be addressed urgently, as medical management alone carries high mortality. Uncontrolled infection, manifested as perivalvular abscess, fistula formation, or persistent fever and bacteremia despite appropriate antibiotics for more than 7 to 10 days, requires surgical debridement and valve replacement. Prevention of embolism is considered when large vegetations (greater than 10 millimeters) are present, particularly on the mitral valve, or when recurrent embolic events occur despite appropriate antibiotic therapy. Prosthetic valve endocarditis generally has a lower threshold for surgical intervention because of the difficulty in sterilizing an infected prosthesis with antibiotics alone.

<image>Panel A: Clinical manifestations of endocarditis showing cardiac murmur on auscultation, and photographs of embolic phenomena including splinter hemorrhages, Janeway lesions (painless palmar lesions), and Osler nodes (painful fingertip nodules). Panel B: Modified Duke criteria organized into major criteria (blood cultures and echocardiographic findings) and minor criteria (predisposing conditions, fever, vascular phenomena, immunologic phenomena) with diagnostic thresholds. Panel C: Echocardiographic images comparing transthoracic and transesophageal views of a mitral valve vegetation with the superior resolution of transesophageal imaging. Panel D: Surgical indications algorithm showing heart failure from valvular insufficiency, uncontrolled infection with abscess, and embolic prevention with large vegetations, each with corresponding clinical triggers.</image>

### Section 8: Immunocompromised Host

The evaluation of the febrile immunocompromised patient begins with identifying the type of immune deficiency, as different defects predispose to different categories of pathogens. Neutropenia, most commonly resulting from chemotherapy, impairs the first line of defense against bacteria and fungi, with the risk of infection increasing sharply as the absolute neutrophil count falls below 500 cells per microliter and becoming critical below 100 cells per microliter. Cellular immune deficiency, as seen in HIV/AIDS and solid organ transplant recipients on immunosuppressive therapy, primarily predisposes to intracellular organisms, viruses, and opportunistic infections. Humoral immune deficiency, including anatomic or functional asplenia and agammaglobulinemia, specifically increases susceptibility to encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis). Mixed immune deficiency, as occurs with chronic high-dose corticosteroid therapy, creates vulnerability across multiple pathogen categories.

Neutropenic fever is a medical emergency that requires immediate intervention because the absence of neutrophils means that even minor infections can progress to overwhelming sepsis within hours. The definition is an absolute neutrophil count below 500 cells per microliter with a single temperature of 38.3 degrees Celsius or higher, or a sustained temperature of 38.0 degrees Celsius over one hour. The workup should include blood cultures from all lumens of central venous catheters and from a peripheral site, urinalysis and urine culture, and a chest radiograph, though it is important to recognize that chest radiographs may be falsely negative in neutropenic patients who cannot mount an adequate inflammatory response. Empiric treatment with a broad-spectrum antipseudomonal beta-lactam (cefepime, piperacillin-tazobactam, or a carbapenem) must be initiated within 60 minutes. The addition of vancomycin is indicated for specific circumstances including suspected catheter-related infection, skin or soft tissue infection, hemodynamic instability, or severe mucositis.

HIV/AIDS-associated opportunistic infections follow a predictable pattern based on the CD4 T-lymphocyte count, providing a framework for the differential diagnosis. At CD4 counts below 200 cells per microliter, patients are at risk for Pneumocystis jirovecii pneumonia (presenting with subacute dyspnea, dry cough, and bilateral interstitial infiltrates), Toxoplasma gondii encephalitis (presenting with focal neurologic deficits and ring-enhancing lesions on brain imaging), and Candida esophagitis. Below 100 cells per microliter, Cryptococcus neoformans meningitis becomes a concern, presenting with headache and often subtle meningeal signs. Below 50 cells per microliter, disseminated Mycobacterium avium complex and cytomegalovirus retinitis and colitis are added to the differential. Importantly, tuberculosis and bacterial pneumonia can occur at any CD4 count and should not be overlooked.

Solid organ transplant recipients experience a predictable timeline of infectious complications that reflects the evolution of their immunosuppressive regimen. In the early post-transplant period (less than 1 month), infections are primarily nosocomial in nature, including surgical site infections, catheter-related bloodstream infections, hospital-acquired pneumonia, and Clostridioides difficile colitis. During the intermediate period (1 to 6 months), the intensity of immunosuppression reaches its peak, and patients are most vulnerable to opportunistic infections including cytomegalovirus reactivation, Pneumocystis pneumonia, and invasive fungal infections. In the late post-transplant period (beyond 6 months), patients on maintenance immunosuppression experience community-acquired infections at a higher rate and with greater severity than immunocompetent individuals, while also remaining at risk for late opportunistic infections. The approach to any febrile transplant patient should include broad empiric antimicrobial coverage and prompt infectious disease consultation.

<image>Panel A: Immune deficiency classification diagram showing neutropenia (bacteria, fungi), cellular deficiency (intracellular organisms, viruses, opportunistic), humoral deficiency (encapsulated bacteria), and mixed deficiency (all categories) with representative clinical scenarios. Panel B: Neutropenic fever protocol showing the definition (ANC below 500, temperature 38.3 or higher), required workup (blood cultures, urinalysis, chest radiograph), empiric antipseudomonal therapy, and specific indications for adding vancomycin. Panel C: HIV/AIDS opportunistic infection spectrum organized by CD4 count thresholds: below 200 (PCP, Toxoplasmosis, Candida), below 100 (Cryptococcus), and below 50 (MAC, CMV), with representative imaging findings for each. Panel D: Transplant infection timeline showing early nosocomial infections, intermediate opportunistic infections with CMV and PCP, and late community-acquired infections, with the immunosuppression intensity curve overlaid.</image>

### Section 9: Urinary Tract Infections

Complicated urinary tract infections are distinguished from simple cystitis by the presence of factors that increase the risk of treatment failure, ascending infection, or systemic complications. All male urinary tract infections are considered complicated by definition due to the anatomic factors that usually prevent bacterial colonization of the male urinary tract. Pregnancy mandates treatment of even asymptomatic bacteriuria because untreated bacteriuria progresses to pyelonephritis in up to 40 percent of pregnant women and is associated with preterm labor. Structural abnormalities including indwelling catheters, urinary stones, and urologic malformations impair normal urinary drainage and create a niche for persistent infection. Immunocompromise from diabetes, chronic steroid use, or other causes increases the risk of complicated infections. Urinary tract obstruction, identified by hydronephrosis on imaging, represents the most dangerous complication and may require emergent urologic intervention.

Acute pyelonephritis presents with fever, flank pain, and costovertebral angle tenderness, often accompanied by lower urinary tract symptoms of dysuria and frequency. Laboratory findings include pyuria and bacteriuria on urinalysis, and urine culture should be obtained to guide targeted therapy. Imaging with computed tomography is indicated when the patient fails to respond to appropriate antibiotics within 48 to 72 hours, when a complicating factor such as obstruction is suspected, or when the diagnosis is uncertain. Outpatient management with oral fluoroquinolone or a single dose of intramuscular ceftriaxone followed by oral antibiotics is appropriate for otherwise healthy patients who can tolerate oral intake and have reliable follow-up. Inpatient management with intravenous antibiotics is indicated for patients with sepsis, intractable vomiting, inability to take oral medications, urinary tract obstruction, or failure of outpatient therapy.

Urosepsis represents sepsis arising from a urinary source and requires implementation of standard sepsis protocols alongside source-specific interventions. Fluid resuscitation and broad-spectrum antibiotics should be initiated immediately according to the hour-1 bundle. Appropriate empiric antibiotic choices include ceftriaxone, a fluoroquinolone, or piperacillin-tazobactam, with broader coverage for patients with healthcare-associated risk factors or prior resistant organisms. The critical distinction in urosepsis management is the identification and treatment of urinary tract obstruction, which represents a closed-space infection that cannot be resolved with antibiotics alone. Obstructive urosepsis requires emergent decompression, accomplished either by retrograde ureteral stent placement by urology or by percutaneous nephrostomy tube placement by interventional radiology. Urology consultation should be obtained immediately when obstruction is identified.

Catheter-associated urinary tract infections are among the most common healthcare-associated infections and are defined by the combination of an indwelling urinary catheter (in place for more than 2 days), compatible signs and symptoms (fever, suprapubic tenderness, altered mental status in catheterized patients), and a positive urine culture. Prevention is the most important management strategy, centering on minimizing the duration of catheterization through daily assessment of continued need and prompt removal when the catheter is no longer indicated. Treatment involves removal or exchange of the existing catheter, as biofilm on the catheter surface harbors organisms that are resistant to antibiotic penetration, followed by a 7-day course of antibiotics guided by culture results. A critical point is that asymptomatic bacteriuria in catheterized patients should not be treated with antibiotics, as treatment does not improve outcomes and promotes antibiotic resistance. The only exception is asymptomatic bacteriuria in pregnant women, which should always be treated.

<image>Panel A: Classification of complicated urinary tract infections showing male sex, pregnancy, structural abnormalities (catheters, stones), immunocompromise, and obstruction with hydronephrosis as risk factors requiring expanded treatment approaches. Panel B: Pyelonephritis evaluation pathway from clinical presentation (fever, flank pain, CVA tenderness) through laboratory studies and imaging indications, with decision points for outpatient versus inpatient management. Panel C: Urosepsis management protocol showing the intersection of standard sepsis bundle with urologic-specific interventions including CT imaging for obstruction and emergent decompression via ureteral stent or nephrostomy tube. Panel D: Catheter-associated UTI prevention and management diagram showing daily catheter necessity assessment, the distinction between symptomatic infection requiring treatment and asymptomatic bacteriuria (do not treat), and catheter exchange protocol.</image>

### Section 10: Other Serious Infections

Spinal epidural abscess is a neurosurgical emergency that classically presents with the triad of fever, back pain, and progressive neurologic deficit, though the complete triad is present in only a minority of patients at initial presentation. Risk factors include intravenous drug use, diabetes mellitus, spinal procedures (epidural injections, surgery), and distant sites of infection serving as a hematogenous source. The diagnosis requires a high index of suspicion, as the initial presentation of fever and back pain is common and nonspecific. Progressive neurologic symptoms including radiculopathy, weakness, sensory deficits, and bowel or bladder dysfunction should prompt emergent magnetic resonance imaging, which is the imaging modality of choice. Treatment consists of intravenous antibiotics providing coverage for Staphylococcus aureus (the most common organism) and, in most cases, emergent surgical decompression and drainage. Delay in diagnosis and treatment directly correlates with worse neurologic outcomes, and patients who develop paralysis before treatment rarely recover full function.

Septic arthritis presents as an acutely inflamed, warm, swollen, and exquisitely tender joint, most commonly the knee, and requires urgent joint aspiration for diagnosis and management. Synovial fluid analysis revealing a white blood cell count exceeding 50,000 cells per microliter with a neutrophilic predominance is highly suggestive, though lower counts do not exclude the diagnosis, particularly in immunocompromised patients. Staphylococcus aureus is the most common causative organism across all age groups. Crystal analysis should be performed to evaluate for concurrent gout or pseudogout, which can coexist with infection. Treatment requires both intravenous antibiotics and joint drainage, which may be accomplished by serial needle aspiration or surgical irrigation. Orthopedic consultation should be obtained emergently, as inadequately treated septic arthritis leads to rapid cartilage destruction and permanent joint dysfunction.

Ludwig angina is a rapidly progressive cellulitis of the submandibular space, typically originating from dental infections of the second and third molars. The infection spreads through the sublingual and submylohyoid spaces, producing massive swelling of the floor of the mouth and tongue, drooling, muffled voice, and trismus. The primary danger is airway compromise from progressive swelling that displaces the tongue posteriorly and superiorly, potentially causing complete airway obstruction. Treatment requires early and proactive airway management, often necessitating awake fiberoptic intubation or surgical airway in cases of severe swelling, followed by intravenous broad-spectrum antibiotics and, when a drainable collection is identified, surgical drainage. Delay in airway management is the most common cause of mortality in Ludwig angina.

Fournier gangrene is a specific form of necrotizing fasciitis affecting the perineum and external genitalia, characterized by rapid tissue destruction and high mortality rates of 20 to 40 percent. Risk factors include diabetes mellitus, immunocompromise, alcoholism, and local trauma or recent perineal procedures. Patients present with perineal pain and swelling that rapidly progresses to skin necrosis, crepitus from gas-producing organisms, and systemic sepsis. Treatment mirrors that of necrotizing fasciitis elsewhere and requires emergent surgical debridement combined with broad-spectrum intravenous antibiotics covering gram-positive, gram-negative, and anaerobic organisms. The extent of debridement is often far greater than the surface appearance suggests, as the infection spreads rapidly along fascial planes. Delay in surgical intervention is the strongest modifiable predictor of mortality, and multiple serial debridements are typically required.

<image>Panel A: Spinal epidural abscess clinical triad (fever, back pain, neurologic deficit) with MRI sagittal image demonstrating posterior epidural collection with spinal cord compression and corresponding T2-weighted signal changes. Panel B: Septic arthritis evaluation showing the acutely swollen knee joint, arthrocentesis technique, and synovial fluid analysis criteria (WBC greater than 50,000, neutrophilic predominance, Gram stain, crystal analysis). Panel C: Ludwig angina progression showing submandibular swelling causing tongue elevation, airway algorithm from fiberoptic intubation through surgical airway, and CT demonstrating floor of mouth soft tissue swelling with gas. Panel D: Fournier gangrene clinical presentation showing perineal necrosis and crepitus, CT demonstrating subcutaneous gas in the perineum, and intraoperative photograph following aggressive debridement.</image>

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## Summary

- Sepsis is defined as infection with organ dysfunction quantified by a SOFA score increase of 2 or more; septic shock adds vasopressor requirement and lactate greater than 2 despite fluids
- The hour-1 sepsis bundle requires lactate measurement, blood cultures, broad-spectrum antibiotics, 30 mL/kg crystalloid for hypotension, and vasopressor initiation for persistent hypotension
- Norepinephrine is the first-line vasopressor for septic shock with a target mean arterial pressure of 65 mmHg or greater
- Necrotizing fasciitis is recognized by pain out of proportion to examination, rapid progression, and systemic toxicity; emergent surgical debridement is the definitive treatment and should not be delayed
- Bacterial meningitis treatment is ceftriaxone plus vancomycin plus adjunctive dexamethasone; add ampicillin for patients aged 50 or older to cover Listeria
- Neutropenic fever requires immediate antipseudomonal monotherapy with cefepime, piperacillin-tazobactam, or a carbapenem; add vancomycin for specific indications
- Pyelonephritis requires admission when complicated by sepsis, obstruction, or inability to tolerate oral medications; outpatient treatment uses fluoroquinolone or ceftriaxone
- Septic arthritis requires joint aspiration (WBC greater than 50,000 with neutrophilic predominance), Staphylococcus aureus is the most common organism, and joint drainage is essential
- Spinal epidural abscess presents with fever, back pain, and neurologic deficit; emergent MRI and surgical decompression are required
- Source control through drainage, debridement, and removal of infected devices is essential for successful treatment of sepsis

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## Key Terms

| Term | Definition |
|------|------------|
| Sepsis | Life-threatening organ dysfunction caused by a dysregulated host response to infection, quantified by SOFA score increase of 2 or more |
| Septic shock | Sepsis requiring vasopressor therapy to maintain MAP of 65 mmHg or greater with lactate exceeding 2 mmol/L despite adequate fluid resuscitation |
| SOFA | Sequential Organ Failure Assessment, a scoring system evaluating dysfunction across six organ systems |
| LRINEC | Laboratory Risk Indicator for Necrotizing Fasciitis, a scoring tool using six laboratory parameters |
| CURB-65 | Pneumonia severity scoring system incorporating Confusion, Urea, Respiratory rate, Blood pressure, and age 65 |
| Neutropenic fever | Temperature of 38.3 degrees Celsius or higher with an absolute neutrophil count below 500, constituting a medical emergency |
| Ludwig angina | Rapidly progressive submandibular space cellulitis with risk of airway compromise, typically from dental origin |
| Source control | Physical interventions to eradicate infectious foci including drainage, debridement, and device removal |

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