Transovarial Transmission of Foodborne Pathogens by the Housefly, Musca domestica

Introduction: Bacterial foodborne pathogens can be transmitted by flies either horizontally, by contact with the environment or other flies, or vertically (transovarially) from mother to progeny. Although scientific evidence supports the horizontal transmission of foodborne pathogens by flies, transovarial transmission has received little consideration.

Purpose: Determine the frequency of transovarial transmission of Salmonella enterica and Listeria monocytogenes by the housefly, Musca domestica. 

Methods: Groups of 20-30 lab reared adult houseflies were kept in jars in a Percival growth chamber at 25°C. Flies were fed with fly food (milk with 2% sugar) containing 0, 10⁴, or 10⁸ cells/ml of either S. enterica or L. monocytogenes for 24 hours and with fly food containing no bacteria for an additional 48 hours.  Small pieces of meat were then added to the jars for fly oviposition. Groups of ~100 eggs were collected, surface disinfected, placed in specific enrichment media, and incubated at 37°C for 24 hours. Enriched media was used to detect specific foodborne pathogens using the BAX system Q7, according to manufacturer’s instructions. Fourteen and six replicates were performed for S. enterica and L. monocytogenes, respectively. Data were analyzed using the Fisher Exact Test (SAS v9.3).

Results: The frequency of transovarial transmission of S. enterica by houseflies fed with 10⁴ and 10⁸ cells/ml was 7% and 38% (P = 0.065), respectively, whereas the frequency of transovarial transmission of L. monocytogenes was 17% and 67% (P = 0.018), respectively. Neither S. enterica nor L. monocytogenes were detected on eggs of flies fed with only sugar and milk (0 cells/ml).

Significance: These results show an unforeseen mode of transmission that is likely to have epidemiological ramifications for the spread of foodborne pathogens by flies, emphasizing the public health significance and the regulatory importance of the presence of flies in food.