What are the types, causes, and preventive measures of hospital-acquired infections (nosocomial infections) in patients?

Hospital-Acquired Infections: Types, Causes, and Prevention

Definition and Timing

Hospital-acquired infections (HAIs), also called nosocomial infections, are infections that occur 48 hours or more after hospital admission and were not present or incubating at the time of admission. 1, 2, 3

• For infections appearing after discharge, the same 48-hour criterion applies—infections within 48 hours post-discharge may be nosocomial 3
• Exceptions exist for infections with known incubation periods (e.g., legionellosis) and surgical site infections (30 days post-surgery, or 1 year with prosthesis/implant) 4
• A bacterial strain is presumed nosocomial if isolated from a patient hospitalized ≥48 hours or transferred from another healthcare facility 1, 3

Major Types of Hospital-Acquired Infections

1. Hospital-Acquired Pneumonia (HAP)

• The second most common nosocomial infection after bloodstream infections, and the most common in ICUs 2
• Incidence: 5-20+ cases per 1,000 hospital admissions 2
• Mortality rate: approximately 20%, with attributable mortality 5-13% 2
• Ventilator-associated pneumonia (VAP) develops in mechanically ventilated ICU patients after ≥48 hours of ventilation 2, 3
• Non-ventilator HAP occurs in non-ventilated patients after 48 hours of hospitalization 2

2. Catheter-Associated Urinary Tract Infections (CAUTIs)

• One of the four most common nosocomial infection sites 4, 5
• Frequently caused by endogenous flora colonizing normally sterile sites 4

3. Bloodstream Infections (BSIs)

• Particularly catheter-related bloodstream infections 4, 5
• Principal organisms: coagulase-negative staphylococci (37%), S. aureus (12.6-13%), enterococci (13.5%), gram-negative bacilli (14%), and Candida species (8%) 6, 7

4. Surgical Site Infections (SSIs)

• One of the four most frequently affected sites 4, 5
• Highest prevalence in surgical wards (60.87% of HAIs) 8
• Surgical wound infections represent 43.48% of all HAIs 8

Causative Organisms

Early-Onset HAP (within 5 days)

• Methicillin-susceptible Staphylococcus aureus 2
• Streptococcus pneumoniae 2
• Haemophilus influenzae 2

Late-Onset HAP (after 5 days) and High-Risk Infections

• Multidrug-resistant organisms (MDROs): 2
  • Pseudomonas aeruginosa 2, 8
  • Acinetobacter baumannii 2, 8
  • Methicillin-resistant S. aureus (MRSA)—>50% of ICU S. aureus isolates 2, 6, 9
  • Extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae 2, 9, 7
  • Klebsiella pneumoniae (including ESBL producers) 6, 8
  • Escherichia coli (including CTX-M β-lactamase producers) 1, 6, 8

Other Common Pathogens

• Coagulase-negative staphylococci (S. epidermidis): most common overall (37% of catheter infections) 6, 7
• Enterococci: 13.5% of infections, with vancomycin resistance increasing from 0.5% (1989) to 25.9% (1999) 6, 7
• Candida species: 8% of catheter infections, with 48% being non-albicans species 6, 7

Major Causes and Risk Factors

Patient-Related Factors

• Age extremes: infants, young children, and persons >65 years 1, 4
• Severe underlying disease, immunosuppression, or cardiopulmonary disease 1
• Depressed level of consciousness 1, 3
• Underlying chronic lung disease 3

Healthcare-Related Factors

• Prolonged hospitalization: particularly ICU stays >5 days 2, 4
• Invasive devices: endotracheal intubation, mechanical ventilation, urinary catheters, intravenous catheters 1, 3, 4
• Previous antibiotic use: major risk factor for MDROs 2, 9, 7
• Previous colonization with multidrug-resistant pathogens 2
• Thoracoabdominal surgery 1
• Previous episodes of large-volume aspiration 3

Transmission Routes

• Endogenous route (majority): colonization of normally sterile sites by patient's own flora due to barrier breakdown 4
• Exogenous route: transmission from other patients or environment via aerosols, hand contamination, or contaminated equipment 4
• Aspiration: most bacterial nosocomial pneumonias occur via aspiration of oropharyngeal or upper GI tract bacteria 1
• Inhalation: Legionella spp., Aspergillus spp., and influenza virus 1
• Direct inoculation: RSV via contaminated hands to conjunctivae or nasal mucosa 1

Antimicrobial Resistance Drivers

• Hospital areas with highest antibiotic use have highest resistance rates 9
• Overuse or inappropriate use of broad-spectrum antibiotics (e.g., third-generation cephalosporins, vancomycin) 9, 7
• Higher proportion of naturally resistant species in nosocomial infections (e.g., P. aeruginosa) 1
• Higher proportion of acquired resistance traits within species 1

Preventive Measures

Core Infection Control Strategies

1. Surveillance and Monitoring

• Implement ongoing surveillance systems to monitor infections with resistant organisms 7
• Calculate interval between hospitalization and sampling dates to assess acquisition time 1
• Define ratio of acquired to imported cases to reflect control program efficacy 1
• Stratify data by specimen type (blood culture, surgical wound, respiratory specimens) and medical activity 1

2. Hand Hygiene and Barrier Precautions

• Proper handwashing to prevent cross-contamination 1, 9
• Prompt institution of barrier precautions when infected or colonized patients are identified 7
• Appropriate patient isolation 9

3. Equipment Management

• Appropriate disinfection or sterilization of respiratory-therapy devices 1
• Proper reprocessing (cleaning, disinfection, sterilization) of respiratory-care equipment 1
• Adherence to standard guidelines on disinfection and sterilization 9

4. Pneumonia-Specific Prevention

• Decrease aspiration by patients 1
• Prevent cross-contamination via hands of personnel 1
• For allogeneic HSCT recipients: use rooms with high-efficiency particulate air (HEPA) filters 1
• Severely immunocompromised patients should use N95 respirators when leaving rooms during dust-generating activities 1

5. Legionnaires Disease Prevention

• Routine culturing of potable water systems in organ-transplant units as part of comprehensive prevention programs 1
• Initiate investigation when one definite or possible case is identified in inpatient HSCT recipients, or two or more cases in outpatient HSCT unit visitors during the 2-10 day period before illness onset 1

6. Antimicrobial Stewardship

• Appropriate use of antimicrobials through antibiotic control programs 7
• Restriction of widely used broad-spectrum antibiotics (e.g., third-generation cephalosporins) and vancomycin 9
• Rotate antibiotics used for empiric therapy 9
• Use combinations of drugs from different classes 9
• For most HAP/VAP patients with good clinical response: 7-8 day antibiotic course 2

7. Vaccination and Prophylaxis

• Use available vaccines to protect against particular infections 1
• For RSV: determine case-by-case whether to administer palivizumab to high-risk infants and children <24 months born prematurely 1
• For influenza outbreaks: use oseltamivir (in addition to amantadine/rimantadine) for prophylaxis of uninfected patients; use oseltamivir or zanamivir (in addition to amantadine/rimantadine) for treatment of acutely ill patients 1

8. Education and Occupational Health

• Continuing infection-control educational programs for healthcare workers 1
• Occupational health program for healthcare providers 9

Critical Pitfalls to Avoid

• Do not assume all infections in hospitalized patients are nosocomial: MRSA can be wrongly considered community-acquired if previous hospital stay was unrecorded 1, 3
• Do not delay diagnosis: infections like typhoid fever or listeriosis can be wrongly considered nosocomial if diagnosis is delayed beyond 48 hours 1
• Do not ignore transfer history: patients transferred from other healthcare facilities require separate categorization 1
• Do not underestimate catheter-related infections requiring immediate removal: infections by Bacillus species, P. aeruginosa, S. maltophilia, C. jeikeium, vancomycin-resistant enterococci, or established Acinetobacter infections respond poorly to antimicrobials alone and require immediate catheter removal 6

Economic and Clinical Impact

• HAIs are the sixth leading cause of death in the United States 5
• ICU patients are 5-10 times more likely to acquire nosocomial infections than other hospital patients 9
• Prevalence in ICUs: up to 42.8% versus 12.1% in other services 4
• Healthcare costs: $10,000-$40,000 per HAP case 2
• Mortality related to nosocomial infections can reach 70% in certain ICU settings 4
• About 5% of patients admitted to acute-care hospitals acquire nosocomial infections 7