What are the primary bacterial causes and treatment options for otitis media?

Bacterial Causes of Otitis Media

The three primary bacterial pathogens causing acute otitis media are Streptococcus pneumoniae, nontypeable Haemophilus influenzae, and Moraxella catarrhalis, which together account for the vast majority of bacterial cases. 1

Primary Bacterial Pathogens

The "Big Three" Organisms

• Streptococcus pneumoniae remains one of the two most common bacterial causes, though its relative frequency has evolved since pneumococcal conjugate vaccine (PCV7) introduction in 2000 1

• Nontypeable Haemophilus influenzae (90% nontypeable, 10% type b) now rivals S. pneumoniae as the most frequently isolated pathogen, particularly in vaccinated children 1

• Moraxella catarrhalis accounts for 7-20% of acute otitis media cases, with the majority of strains producing beta-lactamase 1

Less Common Bacterial Causes

• Streptococcus pyogenes (group A β-hemolytic streptococci) causes less than 5% of acute otitis media cases 1

• Staphylococcus aureus occurs less commonly in acute otitis media but is a predominant organism in chronic otitis media 1, 2

• Alloiococcus otitidis is thought to cause acute otitis media, though additional studies are needed to determine its true significance 1

Evolution of Bacterial Patterns Post-Vaccination

Impact of PCV7 (2000-2010)

• In the first few years after PCV7 introduction, H. influenzae became the most frequently isolated pathogen, replacing S. pneumoniae 1

• By 2003-2006,44% of acute otitis media cases were caused by H. influenzae and 28% by S. pneumoniae 1

• PCV7 strains of S. pneumoniae virtually disappeared from middle ear fluid of vaccinated children by 2007-2009 1

• However, non-PCV7 serotypes of S. pneumoniae increased, making isolation rates of S. pneumoniae and H. influenzae nearly equal 1

Current Bacterial Distribution

• Data from 2007-2009 showed approximately equal isolation rates between S. pneumoniae and H. influenzae in vaccinated populations 1

• With PCV13 introduction, patterns of nasopharyngeal colonization and infection continue to evolve 1

Bacterial Detection Rates

Comprehensive Microbiologic Testing

• When using stringent diagnostic criteria, careful specimen handling, and sensitive microbiologic techniques, bacteria and/or viruses can be detected in up to 96% of acute otitis media cases 1

• The breakdown is: 66% bacteria and viruses together, 27% bacteria alone, and 4% virus alone 1

• Historical series from the United States and Europe reported bacterial pathogens in 69-72% of middle ear fluids 1

• The University of Pittsburgh Otitis Media Study Group reported bacterial pathogens in 84% of middle ear fluids 1

Antibiotic Resistance Patterns

Beta-Lactamase Production

• Approximately 20-30% of H. influenzae strains produce beta-lactamase, reducing amoxicillin efficacy 1, 3

• The majority of M. catarrhalis strains (>90%) produce beta-lactamase 1, 2

Penicillin-Resistant S. pneumoniae

• Increasing isolation of penicillin-resistant and multidrug-resistant S. pneumoniae strains has been documented 1, 4

• Intermediate penicillin resistance in S. pneumoniae increases the risk of bacteriologic failure with amoxicillin and oral cephalosporins 1

• Resistance to macrolides in S. pneumoniae impairs the bacteriologic efficacy of these agents 1

Geographic Variation

• Composite in vitro susceptibility rates to amoxicillin among the three major pathogens ranged from only 62% in the USA to 89% in Central and Eastern Europe 1

• S. pneumoniae was isolated more commonly in Central and Eastern Europe, while H. influenzae was more common in Israel and the USA 1

First-Line Antibiotic Treatment

Recommended Initial Therapy

• High-dose amoxicillin (80-90 mg/kg/day) remains the first-line antibiotic of choice for treating acute otitis media in patients not allergic to penicillin 5

• This recommendation remains unchanged despite evolving resistance patterns 1

Second-Line Options

• Amoxicillin-clavulanate should be used when initial amoxicillin therapy fails after 48-72 hours, when the child received amoxicillin in the past 30 days, or with concurrent purulent conjunctivitis 1, 6

• Amoxicillin-clavulanate is specifically indicated for acute bacterial otitis media caused by beta-lactamase-producing isolates of H. influenzae and M. catarrhalis 6

Alternative Agents

• Azithromycin is indicated for acute otitis media caused by H. influenzae, M. catarrhalis, or S. pneumoniae in pediatric patients 7

• Azithromycin provides reasonable in vitro coverage against penicillin-resistant S. pneumoniae and beta-lactamase-producing H. influenzae, though it is more active against the latter 4

Common Pitfalls to Avoid

• Do not obtain throat or nasopharyngeal cultures to predict the causative organism of otitis media—these are neither sensitive nor specific compared to middle ear fluid cultures 1

• Avoid swabs for specimen collection—middle ear fluid obtained by tympanocentesis or from drainage through myringotomy tubes is the only representative specimen 1

• Do not assume bilateral disease has the same pathogen—cultures from the two ears are disparate in approximately 20% of cases 1

• Recognize that resistance is now the main reason for treatment failure, not inadequate dosing of previously effective antibiotics 1

• Reassess patients at 48-72 hours if symptoms worsen or fail to respond to initial antibiotic treatment 1