Emerging 19A isolate responsible for many acute mastoiditis cases in children

November 2007

The emergence of the serotype 19A isolate marks a shift in the predominant cause of acute pneumococcal mastoiditis and other invasive infections in children, according to new study findings.

This shift, according to researchers from Baylor College of Medicine, Houston, follows the introduction of the pneumococcal conjugate vaccine (Prevnar, Wyeth) in 2000.

“It turns out that there was another type of [isolate] called serogroup 19F,” Sheldon L. Kaplan, MD, professor and vice chairman for clinical affairs, department of pediatrics, Baylor College of Medicine, told Infectious Diseases in Children.

“This 19F was responsible for almost 70% of the other mastoiditis cases in another past study we had.”

Now, however, because it is included in the PCV7 vaccine routinely given to children, the 19F isolate is no longer seen in patients with mastoiditis, according to Kaplan, who is also the chief of infectious disease service at Texas Children’s Hospital.

Kaplan and colleagues conducted a study to review the effect of the PCV7 vaccine in pediatric patients at Texas Children’s Hospital. Their results were presented during a late-breaker session at the 45th Annual Meeting of the Infectious Diseases Society of America, held in San Diego.

Identifying dominant serotype

The researchers retrospectively reviewed the medical records for children who presented with pneumococcal mastoiditis between January 1995 and June 2007.

Pneumococcal isolates were collected prospectively, and antibiotic susceptibility was performed using the Kirby Bauer disk diffusion method or microbroth dilution. The isolates were then serotyped using the capsular swelling method, and pulsed field gel electrophoresis was performed on the 19A isolates.

Kaplan and colleagues identified 41 children (mean age, 23 months; range, 3 months to 12 years) with acute pneumococcal mastoiditis. In these patients, the 19A serotype was found to be the most common (n=19).

Prior to the introduction of PCV7, from 1994 to 2000, none of the 11 cases of acute mastoiditis identified were caused by the 19A serotype, according to the researchers.

Between January 2000 and October 2006, five of 15 cases of pneumococcal mastoiditis were attributed to 19A. However, by November 2006 to June 2007, all of the 13 cases researchers identified were attributed to serotype 19A.

The researchers also found that a serotype 19A isolate was recovered from each of the 11 children who had received three or four doses of PCV7.

“We were a little surprised in that this 19A isolate is the only serotype now that we see causing acute mastoiditis due to pneumococcus,” Kaplan said.

Effects of serotype 19A

According to the researchers, acute pneumococcal mastoiditis cases that were attributable to the 19A isolates were more likely to result in subperiosteal abscess than cases not attributed to 19A (19 of 19 cases attributed to 19A vs. three of 22 cases not attributed to 19A; P<.0001).

Furthermore, patients with acute mastoiditis due to 19A were also more likely to require mastoidectomy than patients with mastoiditis not caused by 19A (17 of 19 patients vs. 10 of 22 patients; P=.004).

“This 19A seems to cause more severe mastoiditis than isolates that are not 19A,” Kaplan said. “The 19A isolates also tended to be more antibiotic resistant than the non-19A [isolates].”

Data showed that 13 (68%) of the 19A isolates were not susceptible to penicillin, trimethoprim-sulfamethoxazole, clindamycin and erythromycin, according to the researchers.

Pulsed field gel electrophoresis results showed that 12 (63%) of the serotype 19A isolates were closely related, differing by three or fewer bands. The researchers found that two additional isolates were possibly related to this strain cluster, but the remaining five isolates were unrelated.

According to Kaplan, physicians must be sure to culture mastoiditis drainage to determine which organism they are dealing with. Until more is known about the serotype 19A isolates, there is not much else that physicians can do to prevent these cases of acute mastoiditis from occurring.

“In our study, the 19A isolate has sort of taken over,” he said. “We know that there’s something special about these 19 isolates that is associated with mastoiditis. We just don’t know what it is.”

Dr. Kaplan received a grant for a surveillance study from Wyeth.

For more information:

• Kaplan SL. Acute pneumococcal mastoiditis in children and the emergence of multi-drug resistant serotype 19A isolates. #LB-3. Presented at: the 45th Annual Meeting of IDSA; Oct. 4-7, 2007; San Diego.