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Serotype distribution of Haemophilus influenzae isolates from post-vaccinated Mexican children: comparison between antisera and PCR-based typing methods

Abstract number: P1323

Gonzaga R., Diaz Garcia J., Alfaro A., Villaseñor Sierra A., Gracia R., Santos J., Gomez de Leon P.

Objective: The world wide use of Haemophilus influenzae (Hi) type b conjugate vaccine has resulted in a significant decline in the incidence of invasive disease in children <5 years of age. The 3 doses scheme of polyribosyl ribitol phosphate (PRP)-T was licensed in our country in 1999. However it has been speculated that this may lead to a greater prevalence of nonencapsulated (NE) and encapsulated non-b strains. Surveillance studies are increasingly important in Mexico for monitoring the emergence of such strains. In this study we compare the serotype distribution of post-vaccine Hi isolates using PCR-based serotyping method to that obtained by slide agglutination serotyping (SAS), currently considered as the gold standard.

Methods: One hundred and eleven Hi strains were screened by the SAS method using polyvalent and monovalent antisera. A PCR assay for the bexA locus was used to detect encapsulated strains. The encapsulated isolates were genotyped using serotype-specific primers for the a, b, e, and f Hi types.

Results: In this study, the SAS method revealed 88 (79.3%) NE Hi strains, of which 6 were found to be types a (1), f (4) and e (1) when the PCR capsule genotyping was performed. Additional discrepancies were observed when 7 NE isolates detected by the PCR method were misidentified as serotypes b (6) and f (1) by SAS. The overall 7.2% (8/111) of the strains contained the f serotype-specific sequence. There was 11.7% discrepancy between the SAS and the PCR-based genotyping methods.

Conclusions: The data showed that Hi serotype f is circulating among post-vaccinated Mexican children. The gold standard method for serotyping the Hi strains still offers equivocal results. The genotyping PCR assay provides a more accurate alternative for determining Hi serotypes.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Subject:
Location: Barcelona, Spain
Presentation type:
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