A 3-year-old boy is brought to the office due to a 1-day history of fever and irritability. The mother states that the boy has been tugging at his right ear. The patient has had 2 previous episodes of acute otitis media. Temperature is 38.1 C (100.6 F). Otoscopic examination shows a perforated right tympanic membrane with erythema and purulent exudate. Cultures from the exudate yield small, oxidase-positive, gram-negative coccobacilli that grow on factor X– and factor V–supplemented media, consistent with Haemophilus influenzae. The patient's immunizations are up to date. Which of the following best explains this patient's susceptibility to the pathogen causing his current infection?
This patient with fever and an inflamed, perforated tympanic membrane with purulent otorrhea has acute otitis media. Middle ear infections occur most commonly in young children (age 6 months to 2 years) and are typically due to Streptococcus pneumoniae or Haemophilus influenzae.
H influenzae is a small, oxidase-positive, gram-negative coccobacillus typically classified by the antigens in its polysaccharide capsule. However, there are multiple strains of H influenzae that do not produce a capsule; these nontypeable strains are part of the normal upper respiratory tract flora but can also cause acute otitis media, sinusitis, and bronchitis.
Vaccination against H influenzae is with the protein-conjugated Hib vaccine, which confers immunity only to the more invasive type b strain (Choice A). The vaccine is recommended in all children to prevent Hib-associated bacteremia, pneumonia, epiglottitis, and meningitis. In countries with widespread Hib vaccination (eg, United States), most H influenzae strains isolated from children with respiratory mucosal infections (including acute otitis media) are nontypeable.
(Choice B) The Hib vaccine contains capsular polysaccharide components to induce anticapsular antibodies (humoral immune response). Because the antibody response is weak in children, the vaccine is also conjugated with toxoid protein, which stimulates a helper T-cell response to help activate B-cell antibody formation. A cell-mediated cytotoxic T-cell response is not induced.
(Choice C) Poor neutrophil function or a low neutrophil count predisposes to bacterial disease. However, neutrophil dysfunction (eg, chronic granulomatous disease, Chédiak-Higashi syndrome) typically presents with recurrent, severe bacterial infections such as pneumonia and abscesses. Ear infections due to H influenzae infection despite immunization with the Hib vaccine are more likely due to infection with a nontypeable strain than defective neutrophils.
(Choice E) Exotoxins are secreted by certain bacteria (eg, Clostridium botulinum, Corynebacterium diphtheriae) to promote cell death but are not generally produced by any strain of H influenzae. The Hib vaccine is directed against capsular polysaccharide, not toxin.
Educational objective:
Nontypeable strains of Haemophilus influenzae are part of the normal upper respiratory tract flora and are a common cause of acute otitis media, sinusitis, and bronchitis. Because nontypeable strains do not form a polysaccharide capsule, immunity is not conferred by vaccination with the H influenzae type b vaccine.