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Clustering of Candida infections in the neonatal intensive care unit: concurrent emergence of multiple strains simulating intermittent outbreaks.

BACKGROUND
Clusters of Candida albicans and Candida parapsilosis infections were noted intermittently in our neonatal intensive care unit (NICU). We attempted to determine whether these clusters represented single strain outbreaks or coincidental emergence of unrelated strains.


METHODS
A retrospective examination of the frequency of candidemia during a 9-year period, two point prevalence studies of colonization and assessment of strain relatedness of individual infant isolates during and in between clusters during a 2-year period with karyotyping and restriction endonuclease analysis of genomic DNA (REAG).


RESULTS
C. albicans and C. parapsilosis infections emerged in a scattered pattern (1 to 2 cases every few months) with intermittent clustering of 3 cases/month. The colonization rate was 50% 5 weeks after an apparent cluster, equally distributed between C. albicans and C. parapsilosis, and 17.6% (exclusively with C. parapsilosis) 4 months after absence of invasive disease. Utilizing REAG or karyotyping singly we noted 12 and 16 DNA banding patterns, respectively, among 23 infant isolates. Few patterns were observed repeatedly over 2- to 20-month periods, implying recurrent emergence of the same strains. Combining karyotyping with REAG revealed a different epidemiologic pattern. It identified 20 distinct composites with identical composites in 3 infant pairs. All infants with identical composites were in the NICU concurrently. The frequency of strain relatedness was comparable among clustered cases (16.7%), scattered cases (7.7%) and simultaneously colonized infants (16.7%).


CONCLUSIONS
These findings illustrate that Candida infections clustered periodically in our NICU but that these clusters were often caused by unrelated strains with infrequent cross-infection during and between clusters. With suboptimal typing this pattern of emergence can be mistaken for same strain outbreaks.