A 10-month-old boy is brought to the emergency department due to 2 days of fever, irritability, and vomiting. The patient has had no prior significant medical conditions and has received all recommended vaccinations, including protein-conjugated, 13-valent pneumococcal capsular polysaccharide vaccines (PCV13) at ages 2, 4, and 6 months. On physical examination, the patient is febrile and lethargic, with a bulging anterior fontanelle. Lumbar puncture yields turbid fluid with an abundance of neutrophils. Gram stain of cerebrospinal fluid reveals lancet-shaped, gram-positive diplococci. Which of the following best explains this patient's current condition?

Question

A 10-month-old boy is brought to the emergency department due to 2 days of fever, irritability, and vomiting.  The patient has had no prior significant medical conditions and has received all recommended vaccinations, including protein-conjugated, 13-valent pneumococcal capsular polysaccharide vaccines (PCV13) at ages 2, 4, and 6 months.  On physical examination, the patient is febrile and lethargic, with a bulging anterior fontanelle.  Lumbar puncture yields turbid fluid with an abundance of neutrophils.  Gram stain of cerebrospinal fluid reveals lancet-shaped, gram-positive diplococci.  Which of the following best explains this patient's current condition?

Quizplus · Accepted Answer

Streptococcus pneumoniae, a gram-positive lancet-shaped diplococcus, causes life-threatening invasive infections such as pneumonia and meningitis (eg, fever, vomiting, bulging fontanelle). Its major virulence factor is a polysaccharide capsule that protects the bacterium from phagocytosis and opsonization. Antigenic variation in the polysaccharide capsule differentiates >93 S pneumoniae serotypes. Immunity against one provides little or no protection against others. Therefore, infection with a new serotype typically leads to weeks/months of nasopharyngeal colonization and increased risk of invasive infection. Vaccination against the most common and virulent S pneumoniae serotypes is recommended for children and high-risk adults to reduce colonization and invasive disease. In children, a 13-valent pneumococcal capsular polysaccharide-conjugate vaccines (PCV13) is typically administered; this vaccine contains polysaccharides from 13 serotypes conjugated to an immunogenic carrier protein (nontoxic recombinant diphtheria toxin) to improve immunogenicity. Although vaccination creates protective antibodies against included polysaccharide antigens, it provides little or no protection against strains not in the vaccine; therefore, invasive disease with a nonvaccinated serotype can still occur. (Choice A) Antigenic variation in the bacterial cell wall can lead to short-lived immunity against certain bacteria (eg, Neisseria gonorrhoeae). In contrast, S pneumoniae antigenic variation occurs primarily on the polysaccharide capsule, not the cell wall. (Choice B) Some S pneumoniae strains have no polysaccharide capsule; these strains are minimally virulent and rarely cause invasive disease. (Choice D) T-cell immunodeficiency would likely increase the risk for poor response to vaccinations. However, primary T-cell immunodeficiency is quite rare and typically presents with recurrent upper respiratory infections. This patient is far more likely to be infected with a strain not in the vaccine. (Choice E) The 23-valent pneumococcal polysaccharide vaccine (PPSV23) contains capsular polysaccharide antigens with no protein conjugate; therefore, it elicits a T-cell-independent response and is not immunogenic enough to be used in individuals age <2. PPSV23 is reserved primarily for high-risk adults. Educational objective: Streptococcus pneumoniae is virulent primarily due to a polysaccharide capsule. A conjugant vaccine provides long-lasting immunity against serotypes in the vaccine but provides no significant immunity against nonvaccinated serotypes. Therefore, infection can still occur due to serotypes not covered by the vaccine.

A 10-Month-Old Boy Is Brought to the Emergency Department Due