Discussion

This study describes the successful containment of a cluster of IC caused by fluconazole-trailing C. parapsilosis in the NICU using multiple infection-control interventions. The study highlights the role of sink drains as important reservoirs for C. parapsilosis, underlining the importance of maintaining HHC in healthcare providers, environmental hygiene, and regular monitoring to prevent nosocomial transmission in the NICU.

According to a previous systematic review, lower incidence rates of hospital-acquired infection can be achieved with HHC rates of over 60%. ^{Reference Mouajou, Adams, DeLisle and Quach18} During the study period, the overall HHC rate consistently exceeded 70%, although the possibility of overestimation due to the Hawthorne effect cannot be disregarded. ^{Reference McCambridge, Witton and Elbourne19} Notably, the rate before performing a sterile or aseptic procedure was significantly lower at 25% before the HH education. The observed low HHC rate before performing a sterile or aseptic procedure might have been a contributing factor to the cluster, including three cases of catheter-related BSI. Furthermore, the HHC rates in physicians were markedly lower than those in nurses as previously reported, ^{Reference Ojanperä, Ohtonen, Kanste and Syrjälä20,Reference Bredin, O’Doherty, Hannigan and Kingston21} averaging around 50%, and showed no significant improvement after HH education. Conversely, the HHC rate in nurses improved significantly to 90% following the education but regressed to 72% after 1 year, returning to the baseline level. Despite the lower number of observed HH opportunities for physicians compared to nurses, and the variability in the number of opportunities among the WHO’s five moments for HH, this study demonstrated that the HH education should be prioritized for physicians and the appropriate moments for HH, and should be implemented regularly to maintain high HHC rate.

Environmental investigations revealed that only sinks near the incubators used by patients infected or colonized with a fluconazole-trailing C. parapsilosis strain at admission were contaminated with that strain, suggesting that the sinks were the source of the cluster. Sinks are a major reservoir of pathogenic microorganisms. ^{Reference Kanamori, Weber and Rutala22,Reference Baba, Kanamori and Katsumi23} Previous studies have indicated a correlation between nosocomial infection of C. parapsilosis and contaminated water environments. ^{Reference Miyake, Gotoh and Iwahashi2,Reference Qi, Fan and Xia24} Candida species that colonize sink drains are frequently dispersed through the splattering of water, and this dispersion may lead to the contamination of medical devices or the hands of healthcare providers. ^{Reference Jencson, Cadnum, Piedrahita and Donskey25} C. parapsilosis is a common cause of outbreaks in NICUs because it forms biofilms that adhere to environmental surfaces and medical devices and is spread via the hands of healthcare providers. ^{Reference Miyake, Gotoh and Iwahashi2,Reference Pammi, Holland, Butler, Gacser and Bliss26,Reference van Asbeck, Huang, Markham, Clemons and Stevens27} Therefore, in addition to monitoring and preventing contamination of sinks with C. parapsilosis, maintaining HHC in healthcare providers is crucial in preventing the acquisition of C. parapsilosis and the occurrence of IC in the NICU.

While it can be challenging to entirely eradicate pathogens from contaminated sinks, previous studies have demonstrated that implementing risk mitigation measures, such as sink cleaning and disinfection, is effective in interrupting clusters. ^{Reference Parkes and Hota28} In this study, sink surfaces contaminated with Candida species were successfully decontaminated using AHP. Furthermore, UV-C irradiation was applied in the milk preparation room, predicated on the hypothesis of potential environmental contamination. The milk preparation room, utilized for both formula and donor milk preparation by nursing staff and expression of milk by mothers, presents a risk for pathogen transmission to infants through milk, milk preparation equipment, and environmental surfaces when milk handling procedures and HH practices are inadequate. ²⁹ UV-C devices are frequently used as an adjunct to standard cleaning in healthcare settings and are effective against Candida species on environmental surfaces. ^{Reference Rutala, Kanamori, Gergen, Sickbert-Bennett and Weber30,Reference Cadnum, Shaikh and Piedrahita31} Meanwhile, the most effective cluster mitigation strategy involves the replacement of sink components, despite the costs and labor-intensive nature. ^{Reference Parkes and Hota28} In this study, the drainage pipes, which are particularly challenging to decontaminate using disinfectants, were replaced.

Owing to the trailing effect, the automated analyzer can occasionally misread the MIC and report pseudo-resistance. ^{Reference Zhang, Wang and Xiao13,Reference Ono, Suematsu, Sawamura, Yamagishi and Mikamo32} Thus, confirmation might be required when high MICs are observed, particularly in regions where resistant strains are uncommon. To date, knowledge regarding the molecular mechanisms responsible for the trailing effect is limited, and the clinical implications of the trailing effect remain unclear. ^{Reference Rueda, Puig-Asensio and Guinea12,Reference Marcos-Zambrano, Escribano, Sánchez-Carrillo, Bouza and Guinea33} In this study, four out of five infants infected with the fluconazole-trailing C. parapsilosis strain were successfully treated with amphotericin B or micafungin. One case of peritonitis resulted in fatality despite combined treatment with amphotericin B and fluconazole, although no definitive evidence indicated that fungal infection directly contributed to the cause of death. In premature infants, who are particularly susceptible to IC, the potential influence of the trailing effect should be considered when selecting antifungal agents, as it may affect their efficacy. ^{Reference Rueda, Puig-Asensio and Guinea12}

Strain typing of Candida species is useful for identifying nosocomial transmission by elucidating the clonality among strains, and for determining the infection route by linking clinical isolates with environmental reservoirs. ^{Reference Miyake, Gotoh and Iwahashi2} Although several discriminating molecular typing methods have been developed for Candida spp., including random amplified polymorphic deoxyribonucleic acid typing and amplified fragment length polymorphism fingerprinting, these methods require highly trained staff and are time-consuming, expensive, and unsuitable for real-time surveillance. ^{Reference Qi, Fan and Xia24,Reference Vatanshenassan, Boekhout and Mauder34} In this study, phylogenetic analysis based on FT-IR spectroscopy helped detect the clonal spread of a fluconazole pseudo-resistant strain among patients. This finding was substantiated through microsatellite genotyping. While not considered the gold standard, this method represents the most widely used high-resolution approach for typing C. parapsilosis. ^{Reference Sabino, Sampaio, Rosado, Videira, Grenouillet and Pais35} FT-IR spectroscopy has previously proven to be useful for typing bacterial and fungal species (including C. albicans and C. auris) to investigate hospital outbreaks. ^{Reference Vatanshenassan, Boekhout and Mauder34,Reference Sandt, Sockalingum and Aubert36} To our knowledge, however, this is the first study to examine its application to C. parapsilosis. In this study, the results obtained through FT-IR spectroscopy and microsatellite genotyping were consistent for the patient-derived isolates, whereas discordance was observed for the environment-derived isolates. FT-IR spectra can fluctuate depending on the conditions during cultivation and sample preparation, and a standard method for the phylogenetic analysis of C. parapsilosis has not yet been established. ^{Reference Vatanshenassan, Boekhout and Mauder34,Reference Sandt, Sockalingum and Aubert36} Although further optimization and evaluation are needed to enhance its discriminatory power, FT-IR spectroscopy is rapid, easy to use, and inexpensive compared to other typing methods. ^{Reference Vatanshenassan, Boekhout and Mauder34} FT-IR spectroscopy is a promising tool for the early detection of nosocomial transmission of C. parapsilosis.

Our study had some limitations. First, owing to the simultaneous implementation of multiple measures in the study, it was challenging to ascertain which measure exhibited the highest efficacy. Second, the cause of the trailing presented by C. parapsilosis in this study is unknown because molecular investigations related to antifungal susceptibility have not been conducted. Third, the transmission of C. parapsilosis via the hands of healthcare providers remains unconfirmed because we did not examine the contamination of healthcare providers. Nevertheless, a cluster of invasive candidiasis caused by fluconazole-trailing C. parapsilosis, presenting with fluconazole trailing, in the NICU was successfully treated by implementing targeted infection-control interventions for contaminated sinks, identified using an environmental investigation, and the milk preparation room, and by improving HHC among NICU staff. Ensuring environmental hygiene and constant staff education to maintain a high HHC level is necessary to limit the spread of C. parapsilosis strains in the NICU.