Early Release – Phormia regina Fly as Vector for Ignatzschineria spp. Bacteremia in Persons Experiencing Homelessness, Canada, 2025 – Volume 32, Number 7—July 2026 – Emerging Infectious Diseases journal

Figure 1

Figure 1. Leg wounds in case-patient 1 in pair of Ignatzschineriaspp. bacteremia cases in persons experiencing homelessness, Vancouver, British Columbia, Canada, 2025. A) Bilateral leg wounds at time of admission;…

Case 1 involved a 49-year-old man, a PEH with active substance use disorder (SUD) and chronic bilateral leg wounds, who sought care for worsening pain, swelling, and wound myiasis (Figure 1, panel A, B). We collected 2 aerobic and 2 anaerobic blood cultures, and within 24 hours, all 4 collection bottles grew gram-negative bacilli. After an additional 24 hours, small gray colonies appeared on sheep blood agar and clear colonies appeared on MacConkey agar (Appendix Figure 2). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI Biotyper; Bruker, https://www.bruker.com) identified the isolates as I. larvae, I. indica, and Fusobacterium varium. Whole-genome sequencing (WGS) confirmed the species as I. larvae and I. indica (Appendix Figure 3).

We conducted antimicrobial susceptibility testing by using gradient strip diffusion method for several antibiotics (Table). We referenced the 2025 Clinical and Laboratory Standards Institute breakpoints for other non-Enterobacterales for interpretation of MICs (8). The genotypic antimicrobial resistance prediction for the I. larvae isolate was concordant with phenotype for all tested antimicrobials (Table). The I. indica isolate did not grow for phenotypic testing, but no genotypic resistance was detected for antibiotics of interest.

The patient underwent surgical debridement of both leg wounds and a split thickness skin grafting from the upper thigh. Computed tomography imaging of the right lower leg demonstrated periosteal reaction and irregularity of the underlying bone cortex. He was treated for osteomyelitis with 6 weeks of amoxicillin/clavulanate. After antimicrobial therapy, the leg wounds healed well and showed healthy granulation tissue. We performed no repeat imaging.

Figure 2

Figure 2. Leg wounds in case-patient 2 in pair of Ignatzschineriaspp. bacteremia cases in persons experiencing homelessness, Vancouver, British Columbia, Canada, 2025. A, B) Lateral (A) and anterior (B) right…

Shortly after case-patient 1 was admitted to hospital, a second patient (case-patient 2), a 36-year-old man who was a PEH and SUD, sought care for fevers and an erythematous right shin containing necrotic wounds heavily infested with fly larvae (Figure 2, panel A, B). His blood cultures grew I. larvae and Pasteurella multocida, which we confirmed by using WGS. The genotypic antimicrobial resistance prediction was concordant with phenotype for all antibiotics tested (Table). His orthopedic and plastic surgery physicians recommended wound care without surgical intervention. The plan was to complete 6 weeks of ceftriaxone for osteomyelitis, but the patient self-initiated discharge before completing therapy.

Figure 3

Figure 3. Development of black blow fly (Phormia regina) from larvae collected from case-patient 2 in pair of Ignatzschineriaspp. bacteremia cases in persons experiencing homelessness, Vancouver, British Columbia,…

When case-patient 2 sought care, we recognized a pattern of myiasis-associated bacteremia. We collected fly larvae from the patient in a sterile container, stored them on blood agar, and observed them over time to document their development into adult flies (Figure 3). The third instar larvae (9–12 mm) were creamy white with distinct bands along the body covered in short spines. The prothoracic spiracles had >10 openings. We observed 2 incomplete peritremes on the posterior with three inner slits directed toward the median line ventrally. The adult flies (10–12 mm) were metallic green. Considered together, those features were suggestive of the Calliphoridae family and the Phormia genus (9). We confirmed the fly to be P. regina by sequencing the cytochrome c oxidase subunit I gene using universal primers (Appendix Figure 4) (10). To confirm the presence of Ignatzschineria spp. in the fly larvae, we washed a larva 3 times in phosphate-buffered saline, macerated the anterior portion of the larva in phosphate-buffered saline, and subjected the homogenate to mechanical disruption with glass beads before DNA extraction and 16S rRNA sequencing. We uploaded sequencing data to the CZ ID platform (https://czid.org) for metagenomic analysis (S.E. Simmonds et al., unpub. data, https://doi.org/10.1101/2024.02.29.579666). We identified Ignatzschineria spp. at a 16S rRNA read abundance of 0.1% from the larva bacterial microbiome, providing molecular evidence of vector colonization.

We have established a relationship between P. regina fly wound myiasis and I. larvae bacteremia by identifying P. regina flies as the probably causative agent of myiasis, isolating Ignatzschineria bacteria in patients’ blood cultures, and detecting the organism within the larval microbiome. Our findings fulfill the criteria for sequence-based determination of causation as proposed by Fredricks and Relman (11). Moreover, the larval microbiome results were consistent with the findings of Deguenon et al. (12), who identified Ignatzschineria bacteria in the microbiome of wild P. regina flies. Because P. regina blow flies are ubiquitous and widely distributed across North America, our results suggest that exposure risk could be far broader than previously assumed.

Both patients were at heightened risk for Ignatzschineria bacteremia because of their PEH status, SUD, chronic wounds, and competing health and social priorities (i.e., resuming everyday life versus receiving treatment and follow-up) (13). We hypothesize that the complexity of wounds and the magnitude of infestation might increase the risk for Ignatzschineria infection. Those cases also illustrate the value of a One Health perspective, in which human vulnerability, environmental exposure, and interactions with urban flies converge to create conditions for pathogen transmission. Clinicians should maintain a high index of suspicion and obtain blood cultures in patients with wound myiasis and systemic symptoms.

Accurate identification of Ignatzschineria bacteria remains a diagnostic challenge in the clinical microbiology laboratory. In several cases, Ignatzschineria isolates were initially misidentified as other bacteria using phenotypic and molecular methods (7,13). We observed that the Bruker MALDI Biotyper successfully identified I. indica and I. larvae/ureiclastica but could not differentiate between I. larvae and I. ureiclastica because of the high degree of genetic similarity. Of note, the VITEK MS version 3.2 knowledge base (bioMérieux, https://www.biomerieux.com) does not include Ignatzschineria species. Most case reports have relied on molecular methods such as 16S rRNA gene sequencing for genus identification, but only WGS can truly provide species-level resolution (14). Those findings highlight a broader diagnostic gap in detecting emerging zoonotic pathogens and demonstrate the value of WGS for achieving species-level resolution.

Information on the antimicrobial susceptibility of Ignatzschineria is currently limited. Reported testing methods include disk diffusion, automated instruments, gradient strip diffusion, or some combination of those methods (1). Most published cases demonstrate that Ignatzschineria bacteria are susceptible to β-lactam antibiotics, which comprise the mainstay of therapy (1). Treatment typically includes appropriate antimicrobial therapy combined with larvae removal, wound care, debridement, amputation, or some combination of these interventions (1,7). Overall, patient outcomes have been favorable when both infection and underlying wounds are addressed (13).