Tracheal infestation of dipteran larvae manifesting initially as a pseudo-outbreak

INTRODUCTION: Larval infestations of trachea are uncommon and often without symptoms. Culture plates of two patients growing larvae, without evidence of infestation at the culture sites (one wound, one tracheal aspirate), made it unclear if this was an infestation of one site with secondary contamination of the other plates, infestation of both persons from a common source (i.e., saline irrigation), or a pseudo-outbreak from contamination of both plates in the laboratory. Subsequently, two outpatient cultures (vaginal, corneal), incubated in the same incubator 3 days later, yielded larval growth. METHODS: Epidemiology evaluation included repeat cultures of the original two patients; wound and environmental cultures on the surgical ward; and incubation of culture plates from the same manufacturer lots. Saline used for wound irrigation was no longer available for culture. Entomology was asked to perform a hospital survey for evidence of insect/rodent infestation; dead rodents can harbor certain insect species, and droppings can be associated with outbreaks. RESULTS: All other cultures showed no evidence of larvae except from the tracheotomy patient (comatose from a motor vehicle accident) who had larval growth again on his fourth culture. ENT endoscopic evaluation revealed no apparent infestation. Entomology survey did not reveal evidence of laboratory contamination as the initial source of larval growth. Speciation of the larvae revealed Megaselia scaleris. CONCLUSIONS: The tracheotomy patient was determined as the larval source, and larval migration resulted in secondary contamination of additional culture plates in the labʹs incubator. In cultures with larvae and no gross evidence of infestation of wounds, patients with tracheotomy sites should be considered as a probable source of infestation unless proven otherwise, and repeat cultures or endoscopic evaluation should be considered. Since the first positive culture was obtained within 2 hours of admission, the infestation was felt to have occurred at the previous hospital or en route to the new facility.