Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2014
Publication Date: 9/1/2014
Citation: Olafson, P.U., Lohmeyer, K.H., Edrington, T.S., Loneragan, G.H. 2014. Survival and fate of Salmonella enterica serovar Montevideo in adult horn flies (Diptera: Muscidae). Journal of Medical Entomology. 51(5):993-1001.
Interpretive Summary: Post-harvest interventions to reduce levels of foodborne pathogens on cattle hides and carcasses has decreased the occurrence of Escherichia coli O157:H7 in ground beef, but these same approaches have not been as successful at eliminating Salmonella. Recent studies revealed that Salmonella sequestered in bovine peripheral lymph nodes could be a major source of ground beef contamination, as these nodes are located within fatty tissue and may inadvertently be processed along with the beef carcass. A transdermal route of Salmonella entry was proposed and recently modeled, suggesting that the bacteria may be introduced via abrasions on the hide and/or biting or opportunistic arthropods. Several important characteristics of the horn fly lend itself to the transdermal model of Salmonella contamination, and the objective of this study was to evaluate the survivability of Salmonella in horn flies over time. A low or high dose of a strain of S. enterica serovar Montevideo that expresses the green fluorescent protein was consumed by horn flies, and its persistence was monitored in whole adults and excreta. Viable S. Montevideo was recovered from adults up to 72 h after ingestion, regardless of the initial dose. Fly excreta collected hourly over a 12 h period after ingestion of either dose revealed maximal recovery of Salmonella-positive excreta during the 7 – 12 h after consuming the bacteria. Together, these support a role of horn flies as environmental reservoirs and biological vectors of Salmonella. Transmission of Salmonella by horn flies may occur via mechanical inoculation of their excreta along with contaminated cattle feces into feeding sites or abrasions on host skin. These results support a growing body of literature implicating flying insects in the dissemination of foodborne pathogens, emphasizing the importance of insect control in livestock production facilities.
Technical Abstract: Contamination of cattle peripheral lymph nodes with Salmonella enterica is proposed to occur via a transdermal route of entry. If so, bacteria may be introduced to cattle by biting arthropods. Biting flies, such as horn flies (Haematobia irritans irritans (L.); Diptera: Muscidae), are intriguing candidates for transmitting Salmonella to cattle since they provide a route of entry when they breach the skin barrier during blood feeding. Using a green fluorescent protein-expressing strain of Salmonella Montevideo (S.Montevideo-GFP), the current study demonstrated that horn fly grooming subsequent to tactile exposure to the bacteria resulted in acquisition of the bacteria on mouthparts, as well as microbial ingestion. Consumption of a blood-meal containing ˜102, ˜104 or ˜106 S.Montevideo-GFP resulted in horn fly colonization for up to 72 h post-ingestion (PI). Epifluorescent microscopy indicated that the bacteria were not localized to the crop but were observed within the endoperitrophic space, suggesting that regurgitation is not a primary route of transmission. S.Montevideo-GFP were cultured from excreta of 100% of flies beginning 6-7 h PI of a medium or high dose meal and > 12 h PI in excreta from 60% of flies fed the low dose meal. Animal hides and manure pats are sources for horn flies to acquire the Salmonella and mechanically transmit them to an animal while feeding. Mean quantities of 5.65 – 67.5 × 102 CFU per fly were cultured from fly excreta passed within one day after feeding, suggesting the excreta can provide an additional microbial source on the animal's hide.