Discussion

Acute endophthalmitis, characterized by reduced visual acuity, eye pain, hypopyon and vitreous infiltration, is a rare complication of cataract surgery, occurring in 0.04% of >8.5 million surgeries performed in the United States between 2013 and 2017. ^{Reference Pershing, Lum and Hsu6} The clustering of 6 cases following cataract surgery in a single clinic within 1 month suggested a common source of infection. The recovery of the same strain of B. contaminans, an unusual human pathogen, from all 6 patients provided further compelling evidence of a common source outbreak. ^{Reference Lind, Olsen and Angelsen1} An initial investigation to identify the source of this outbreak included culture of samples from liquid drugs and products, medical devices, and ‘multiple environmental sites’ in the clinic, including sink drains. ^{Reference Lind, Olsen and Angelsen1} No source of Burkholderia infection was identified, prompting our subsequent investigation, which focused on the clinic’s ventilation system because this was found to have pooled water and soil in air intake vents. Cultures of water and biofilm from the ventilation ductwork, performed at the University of Michigan, identified the same B. contaminans strain (ST102) as had been recovered from endophthalmitis patient cultures at the University Hospital of North Norway.

Species within the B. cepacia complex are well recognized as healthcare-associated opportunistic pathogens, most often causing infection by virtue of their capacity to contaminate aqueous pharmaceutical and personal care products and disinfectants. ^{Reference Tavares, Kozak, Balola and Sá-Correia7} Not infrequently, however, the source of nosocomial outbreaks remains obscure, despite extensive investigation to identify a common source. Among B. cepacia complex species, B. contaminans is (as its name implies) most notorious for its proclivity to contaminate medical products and cause outbreaks. Within this species, ST102 is a strain with a global distribution that has been recovered from persons with cystic fibrosis and has been involved in outbreaks traced to a variety of contaminated products, including nasal spray ^{Reference Dolan, Dowell, LiPuma, Valdez, Chan and James8} and liquid docusate (Table 1). ^{Reference Glowicz, Crist and Gould9}

In the outbreak described here, analyses of draft genomes showed that the B. contaminans ST102 isolates recovered from the clinic ventilation system were essentially identical (≤5 SNPs within the 5.8 Mb core genome) to those recovered from cultures of eyes from patients with endophthalmitis. Several other B. contaminans ST102 isolates, not related to this outbreak, showed greater genome sequence variation, strongly implicating the ventilation system as the source of infection.

The ventilation system, which provided 8 air exchanges per hour, received regular maintenance, and had operated without incident for 13 years. Local flooding due to an unusually early thaw resulted in pooling of water in air intake ducts. Whether contamination of the operative field and patient eyes occurred directly from airborne particles from the ventilation system or via an intermediary (eg, topical ophthalmic medication or surgical instrument contaminated with bacteria from the ventilation system) could not be ascertained with certainty. Active air sampling was not routinely utilized in this clinic, and air sampling with settle plates was not performed in our investigation. Notably, however, a vent providing fresh air to the clinic was located directly over the operating field, and multiple cultures of all liquid pharmaceuticals used during surgery did not yield Burkholderia. Furthermore, contamination of the clinic ventilation system, which provided air into the clinic, by a contaminated medical product used in the clinic or by an infected patient is highly improbable.

In summary, our study provides strong evidence that a fresh air ventilation system was the source of an outbreak of postsurgical endophthalmitis due to B. contaminans. Investigations of outbreaks of healthcare-associated infections involving B. cepacia complex species that fail to identify a contaminated healthcare product should consider examination of air handling systems. Our study also demonstrates the utility of whole-genome sequence analyses in epidemiologic investigations of outbreaks involving bacterial strains that are widely distributed in the natural environment and indistinguishable by MLST analysis alone.