Volume 5, Number 5—October 1999
Filth Flies Are Transport Hosts of Cryptosporidium parvum
To the Editor: Infection with Cryptosporidium parvum, a zoonotic and anthroponotic coccidian parasite (1), may be fatal for persons with impaired immune systems (2), for whom a low number of oocysts can initiate life-threatening diarrhea (1). Insects such as promiscuous-landing synanthropic flies (i.e., coprophilic filth flies) are recognized transport hosts for a variety of parasites (3-5), but not for C. parvum. We assessed the role of synanthropic flies in the mechanical transmission of C. parvum oocysts.
Bovine diarrheic feces (20-ml specimens) containing 2.0 x 105 oocysts/ml were placed in petri dishes in each of five 4-liter paper cages with approximately 250 pupae of laboratory-reared house flies (Musca domestica F58WTZ strain). Three days after the flies emerged, fecal specimens were collected on glass microscope slides placed in each cage. Thirty flies aspirated from each cage on days 3, 5, 7, 9, and 11 after emergence were eluted, and the eluants were processed by the cellulose acetate membrane (CAM)-filter dissolution method (6). Digestive tracts dissected from randomly selected flies and the glass slides with fly excreta were examined by immunofluorescent antibody (IFA) (7), and C. parvum oocysts were counted (8). Maggots of M. domestica were reared in fly larvae medium (PMI FEEDS, Inc., St. Louis, MO) contaminated with calf diarrheic feces (50 ml) containing 2.0 x 105 C. parvum oocysts/ml. Resulting pupae were eluted, the eluants were processed by the CAM-filter dissolution method (6), and C. parvum oocysts were identified by IFA (7) and counted (8). Diarrheic fecal specimens from a C. parvum-uninfected calf were used as negative controls in similar experiments. Randomly selected samples containing fly-derived C. parvum oocysts were processed with acid-fast stain (AFS) (8) to check for normal cellular morphologic features.
Ten Victor-type flying-insect traps (Woodstream, Lititz, PA) were baited with rotten fish and placed inside a barn (approximately 880 m2) in which a male Holstein calf infected with C. parvum (AUCP-1 strain) was housed. The traps were emptied weekly, the flies were counted and identified (5,9), and the inside surfaces of the traps (containing fly excreta), along with the flies, were eluted with 200 ml of eluting fluid (6). The eluting fluid was filtered through a CAM (Millipore, Bedford, MA) (6,8), which was then processed (6), and C. parvum oocysts were identified by IFA (7) and counted (8).
The mean number of C. parvum oocysts per droplet of M. domestica was 4 to 20 (mean 7.0 + 3.2), and the number of droplets increased over time. All flies harbored C. parvum oocysts on their external surfaces. On average, 14.0 + 6.8 fly excreta were counted per 1.0 cm2 of glass slide. From 1 to 8 C. parvum oocysts were detected in digestive tracts of flies exposed to feces with oocysts. C. parvum oocysts were also numerous on maggot and pupa surfaces; approximately 150 and 320 oocysts were recovered per maggot and pupa, respectively.
Wild-caught flies belonged to the families Calliphoridae (96% of total flies), Sarcophagidae (2%), and Muscidae (2%). An average of eight flies was caught per trap, and more than 90% of flies harbored C. parvum oocysts. The number of trap-recovered C. parvum oocysts per fly was 2 to 246 (mean 73 oocysts per fly).
Synanthropic flies that breed in or come in contact with a fecal substrate contaminated with C. parvum oocysts can harbor these oocysts both externally and internally and will mechanically deposit them on other surfaces. Therefore, synanthropic flies can serve as mechanical vectors for C. parvum oocysts and under poor sanitary conditions could be involved in the transmission of human and animal cryptosporidiosis. The biology and ecology of synanthropic flies indicate that their potential for mechanical transmission of C. parvum oocysts can be high. The morphologic and AFS and IFA staining characteristics of C. parvum oocysts recovered from the exoskeletons of flies and identified in their fecal spots suggest that oocysts are still viable.
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