In 2000, >400 cases of disease caused by Neisseria meningitidis serogroup W135 (MenW135), the largest MenW135 outbreak reported to date, occurred worldwide among Hajj pilgrims and their contacts. To elucidate the origin of the outbreak strains and to investigate their relatedness to major clonal groups, genotypic and phenotypic subtyping was performed on 26 MenW135 outbreak-associated isolates and 50 MenW135 isolates collected worldwide from 1970 through 2000. All outbreak-associated isolates were members of a single clone of the hypervirulent electrophoretic type (ET)-37 complex, designated the "(W)ET-37 clone"; 19 additional MenW135 strains were also members of this clone, and the remaining 31 MenW135 strains were clearly distinct. The 2000 MenW135 outbreak was not caused by emergence of a new MenW135 strain but rather by expansion of the (W)ET-37 clone that has been in circulation at least since 1970; the strains most closely related to those causing the 2000 outbreak have been isolated in Algeria, Mali, and The Gambia in the 1990s.

To resolve discrepancies in slide agglutination serotyping (SAST) results from state health departments and the Centers for Disease Control and Prevention (CDC), we characterized 141 of 751 invasive Haemophilus influenzae isolates that were identified in the United States from January 1998 to December 1999 through an active, laboratory-based, surveillance program coordinated by the CDC. We found discrepancies between the results of SAST performed at state health departments and those of PCR capsule typing performed at the CDC for 56 (40%) of the isolates characterized: 54 isolates that were identified as a particular serotype by SAST were shown to be unencapsulated by PCR, and two isolates that were reported as serotypes b and f were found to be serotypes f and e, respectively, by PCR. The laboratory error most likely to affect the perceived efficacy of the conjugate H. influenzae type b (Hib) vaccine was the misidentification of isolates as serotype b: of 40 isolates identified as serotype b by SAST, 27 (68%) did not contain the correlating capsule type genes. The frequency of errors fell substantially when standardized reagents and routine quality control of SAST were used during a study involving three laboratories. An overall 94% agreement between SAST and PCR results showed that slide agglutination could be a valid and reliable method for serotyping H. influenzae if the test was performed correctly, in accordance with standardized and recommended procedures. An ongoing prospective analysis of all H. influenzae surveillance isolates associated with invasive disease in children less than 5 years old will provide more accurate national figures for the burden of invasive disease caused by Hib and other H. influenzae serotypes.

Since 1990, the frequency of Neisseria meningitidis serogroup C (NMSC) outbreaks in the United States has increased. Based on multilocus enzyme electrophoresis (MEE), the current molecular subtyping standard, most of the NMSC outbreaks have been caused by isolates of several closely related electrophoretic types (ETs) within the ET-37 complex. We chose 66 isolates from four well-described NMSC outbreaks that occurred in the United States from 1993 to 1995 to evaluate the potential of pulsed-field gel electrophoresis (PFGE) to identify outbreak-related isolates specific for each of the four outbreaks and to differentiate between them and 50 sporadic isolates collected during the outbreak investigations or through active laboratory-based surveillance from 1989 to 1996. We tested all isolates collected during the outbreak investigations by four other molecular subtyping methods: MEE, ribotyping (ClaI), random amplified polymorphic DNA assay (two primers), and serotyping and serosubtyping. Among the 116 isolates, we observed 11 clusters of 39 NheI PFGE patterns. Excellent correlation between the PFGE and the epidemiological data was observed, with an overall sensitivity of 85% and specificity of 71% at the 95% pattern relatedness breakpoint using either 1.5 or 1.0% tolerance. For all four analyzed outbreaks, PFGE would have given public health officials additional support in declaring an outbreak and making appropriate public health decisions.

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