Flies are a vector for spreading foodborne pathogens pertinent to fresh produce, such as Shiga toxigenic Escherichia coli and Salmonella; however, most studies focus on concentrated animal feeding operations, which do not reflect low-density animal farming practices that often adjoin fruit and vegetable acreage. In this study, we determined the prevalence of Salmonella in flies collected biweekly on an integrated animal and produce operation over two growing seasons. Eleven of 889 pooled samples tested positive for Salmonella. Flies from the Calliphoridae, Muscidae, Sarcophagidae, and Tachinidae families were associated with Salmonella carriage, but fly family was not a significant factor for isolation of Salmonella (P = 0.303). Fly species were a significant factor (P = 0.026), with five Pentacricia aldrichii pools testing positive for Salmonella. With the exception of single specimen isolation, prevalence ranged from 2.2 to 15.2%. With the exception of the Tachinidae family, these results reflect a strong association of flies that are commonly associated with feces or are pests of animals. Trap location was not significantly associated with isolation of Salmonella-positive flies (P = 0.236). Overall, the population of flies was not as abundant as studies conducted with produce grown close to concentrated animal feeding operations, indicating a reduced risk of transmission; however, similar to these studies, fly families that are commonly isolated from fecal and decaying matter were most frequently associated with Salmonella isolation. Further work is warranted to elucidate the foodborne pathogen transmission rates to produce and subsequent survival over time.
Pooled fly samples (1.2%) tested positive for Salmonella.
Flies of Calliphoridae, Muscidae, Sarcophagidae, and Tachinidae families were Salmonella positive.
Flies were not as abundant as studies conducted close to concentrated animal feeding operations.
Produce (fruits, nuts, and vegetables) accounted for 4,423,310 (45.9%) cases of foodborne illness in the United States between 1998 and 2008, 985,807 (27%) of which were caused by bacterial sources (20). Although the overall burden of foodborne illness has declined, Salmonella remains one of the most common causes of outbreaks, and its association with seeded vegetable outbreaks included the organism in a recent list of top five pathogen-food pairings most frequently involved in outbreaks from 2009 to 2015 (10, 23). Also, produce was ranked fourth as a contributing food category to the burden of foodborne disease in the United States, as reported in a 2012 study (5). The cause of this sustained association is likely multifaceted, including elevated consumption of fresh produce for the nutritional benefits, increased production to meet demand, and importation of produce from various climates to enable year-round consumption. However, it is the consumption of produce without application of a kill step, coupled with the inefficiency of pathogen reduction by home washing (8, 17), which emphasizes the need for pathogen control at the farm level.
Produce can be contaminated by improper management of manure (3, 24), irrigation water (11, 13, 14), and wildlife or domestic animals (16). Filth flies were also shown to serve as vectors of pathogens such as Salmonella and Shiga toxigenic Escherichia coli on animal farms (2, 31). The link between fly species and fruit and vegetable contamination has been previously reported, specifically in apples and spinach (15, 32). Fruit flies (Drosophila melanogaster) were shown to be easily contaminated by a nonpathogenic strain of E. coli and were then able to transfer that strain to uncontaminated apples at wound sites (15). A study evaluating the use of fly regurgitation spots as a site for foodborne pathogen contamination found that flies acted as a vector for the transmission and growth of E. coli O157:H7 from contaminated cattle manure to the spinach leaf surface (32). Given the association with livestock feces, flies have been investigated as potential indicators for foodborne pathogens within animal-rearing operations (9). In addition, it has been suggested that comanagement of produce operations with nearby land used for livestock rearing resulted in a significantly higher prevalence of Salmonella within the produce operation (27).
There is a dearth of data with respect to the diversity of flies that may be isolated close to produce growing regions intermingled with low-density animal operations (e.g., beef cow–calf operations with approximately one cow per acre). Better understanding of fly dispersion within mixed farming operations, combined with the prevalence of key foodborne pathogens, will provide insight into the potential of flies to act as mechanical vectors within these farming systems. The objectives of this study were to determine the distribution of flies throughout a farm with low-density beef cattle and fruit and vegetable production and assess the prevalence of Salmonella-containing fly species.
MATERIALS AND METHODS
Sample collection occurred on the University of Tennessee, Plateau AgResearch and Education Center (Crossville, TN), a farm with fruit and vegetable, forage, and beef cattle production. Figure 1 demonstrates where beef cattle were located in relation to the produce plot for this trial and placement of all fly collection traps. Crops included strawberries, tomatoes, cabbage, and cantaloupe. Beef cattle were low density, with approximately one animal per acre of pasture and no more than 35 head in any one pasture during data collection.
Fly collection and identification
Thin-walled, polyvinyl chloride pipes (10 cm width) were cut into sections (30 cm length). Approximately seven holes (2.5 cm) were drilled into the side of the trap. A Starbar Quikstrike Fly Abatement Strip (Wellmark International, Schaumburg, IL) was placed inside the pipe and hung from a polyvinyl chloride cap used to cover the trap. A 0.95-L freezer bag was secured to the bottom of the trap with a band clamp to catch flies. Traps were hung 1 m above the ground at designated collection points (Fig. 2). A total of 10 traps were distributed in fixed locations across the farm (Fig. 1), ranging in distance from 46.9 to 370.4 m from cattle pastures.
Sample collection bags were retrieved from the trap, and a new bag secured with a band clamp. Collection days occurred approximately twice per month from August to October in 2014 (8 collection periods) and April to October in 2015 (23 collection periods). The abatement strips were replaced each collection day. Crop, field, and localized weather observations from this site were not significantly different by months across which collection seasons overlapped (P > 0.05; data not shown). Collection bags were transported at 4°C and stored at −20°C until the flies were identified to species.
Identification of fly species
Fly samples were identified to family, genus, and species by using dichotomous keys (18, 29). Each fly was examined under a light microscope and characterized according to specific morphological qualities. Flies of the same species that were collected from the same trap and sampling date were pooled together for isolation of Salmonella.
Samples were macerated and enriched following a characteristic enrichment procedure detailed in the U.S. Food and Drug Administration's Bacteriological Analytical Manual through primary enrichment, after which a modified selective enrichment was conducted to increase isolation efficacy (30). Pooled fly samples were emptied into a 50-mL centrifuge tube (Corning Inc., Corning, NY) containing 25 mL of buffered peptone water (BD, Sparks, MD). Approximately 10 sterile glass beads (5 mm; Walter Stern Inc., Port Washington, NY) were aseptically transferred into the centrifuge tube and vortexed on the highest setting for 1 min or until complete maceration. The centrifuge tubes were incubated at 35°C for 24 ± 2 h as a nonselective primary enrichment. After incubation, 10 mL of the nonselective enrichment was deposited in a centrifuge tube containing 10 mL of 2× concentrated tetrathionate broth (BD) for the detection of Salmonella. These selective enrichments were incubated at 35°C for 24 ± 2 h.
The 2× concentrated tetrathionate broth enrichments were streaked for isolation onto xylose lysine Tergitol 4 agar plates (BD) and incubated at 35°C for 24 ± 2 h. The presence of black colonies indicated presumptive Salmonella positive on the basis of hydrogen sulfide gas production. Aliquots (1.4 mL) of each sample were saved in 1.5-mL sterile, flat-top microcentrifuge tubes (Thermo Scientific, Asheville, NC) and stored at −20°C for further testing.
Confirmation of positive samples
Presumptive-positive Salmonella isolates were confirmed via Wellcolex latex agglutination testing (Remel Products, Lenexa, KS) for serogroups A, B, C, D, E, G, and Vi by corresponding color changes on the basis of sample homologous antigen agglutination.
The statistical analysis program JMP Pro Version 13.2 (SAS Institute Inc., Cary, NC) was used to analyze the data. Single linear regressions were performed to assess the relationships between pathogen isolation, trap location, and fly taxa. The relationship was considered significant when P < 0.05.
RESULTS AND DISCUSSION
A total of 889 pooled samples consisting of 2,665 individual fly specimens across seven families and 58 genera were collected during the sampling period (August to October 2014 and April to October 2015). The community was dominated by the Calliphoridae family (378 pooled samples; 42.52%), followed closely by the Muscidae (267 pooled samples; 30.03%) and Sarcophagidae (219 pooled samples; 24.63%) families (Table 1). Fourteen different species were identified within our collection window. The most prevalent fly samples were classified as Phormia regina (142 pooled samples; 15.97%) and Pollenia rudis (125 pooled samples; 14.06%) of the Calliphoridae family, followed by Musca domestica (43; 4.61%) and P. aldrichii (33; 3.71%) of the Muscidae family (Table 1).
The prevalence of flies observed in this study differed greatly from those observed in leafy greens planted adjacent to a beef cattle feedlot in Nebraska (7). Although the observed months of both the Berry et al. and this study included sampling events in June through September, this study evaluated more collection events over time (8 in year 1 and 4 in year 2 compared with 7 in year 1 and 12 in year 2). Also, in that study, a total of 7,360 flies were collected throughout two growing seasons in the Nebraska study, which is far greater number than the 2,665 flies collected in this study. This demonstrates greater populations in concentrated animal feeding operations compared with lower density cattle farming. Overall, 96.6% belonged to M. domestica, Musca autumnalis, and Stomoxys calcitrans and Sarcophagidae and Calliphoridae families in the Nebraska study. In contrast, Phormina regina and P. rudis from the Calliphoridae family were isolated most frequently among all species throughout this study, but the Calliphoridae, Muscidae, Sarcophagidae families overall predominated in both studies. M. domestica, M. autumnalis, and S. calcitrans are all pests of cattle, with higher densities in concentrated animal feeding operations potentially resulting in higher prevalence in the Nebraska-based study. Also, differences in baiting and trapping methods, as well as fly management practices, could also influence outcomes.
Of 889 pooled samples, 11 (1.24%) were confirmed positive for Salmonella, and all belonged to serogroup B. All Salmonella-positive samples were isolated in a relatively narrow 3-week collection period, between 28 August 2014 and 18 September 2014 of the first growing season, with no Salmonella-positive pools isolated in 2015 (Fig. 3). Salmonella-positive samples were not significantly associated with fly family (P = 0.3039). Of 11 positive pooled samples, 2 (2 of 11, 18.2% of positive pooled samples; 2 of 378, 0.53% of total pooled samples per family) were from the Calliphoridae (e.g., blow fly) family, 6 (6 of 11, 54.5%; 6 of 267, 2.25%) were from the Muscidae (e.g., house fly) family, 2 (2 of 11, 18.18%; 2 of 219, 0.91%) were from the Sarcophagidae (e.g., flesh fly) family, and 1 (1 of 11, 9.1%; 1 of 9, 11.1%) was from the Tachinidae (e.g., parasitic fly) family (Table 1 and Fig. 3). Fly species was significantly linked to Salmonella isolation (P = 0.0262). Positive samples were most commonly isolated from the P. aldrichii (five samples), with no other significant differences observed among species, which is not surprising, given they occurred as isolated incidents (Table 1).
The Calliphoridae, Muscidae, Sarcophagidae, and Tachinidae families show the potential to serve as vectors of human pathogen transmission in a farm environment. These observations are supported by the current scientific literature (2, 22, 28, 34), with one exception. The association of a Salmonella-positive fly of the Tachinidae family is unique and has not been previously reported. As these flies are parasitoids, contamination could have occurred from the host or environmental cross-contamination.
House flies (Muscidae) are commonly associated with transporting foodborne pathogens. Ahmad et al. (1) observed that house flies were capable of not only harboring foodborne pathogens but also transmitting antibiotic-resistant strains in a cattle operation. Although this is not the first study to show the capability of the Muscidae family to harbor and transmit foodborne pathogens in or near livestock environments (2, 6, 7), it is the first to describe repeated isolation of Salmonella in P. aldrichii, to our knowledge. The prevalence of pathogen harborage in sampled Muscidae populations could be linked to seasonal elevations in activity, which is consistent with house fly populations increasing during the summer months (25). With increased fly activity, the possibility of contamination and the potential transmission of foodborne pathogens will likely increase. The low prevalence and short duration of Salmonella-positive fly samples suggests that contamination was associated with seasonal increases in fly activity on the farm and concurrent cross-contamination within the farm environment (4, 21, 26). No Salmonella-positive fly pools were obtained during the 2015 growing season, although the sample collection period started 4 months earlier and fly numbers were similar. This is indicative of the sporadic introduction of foodborne pathogens and opportunistic spread with these vectors.
Trap location was not a significant predictor of Salmonella isolation (P = 0.2363), and positive pools were isolated in 6 of 10 trap locations (Table 2). This indicates that traps just adjacent to low-density livestock were no more likely to be associated with Salmonella-positive samples than those at greater distances. The lack of significance is not surprising, given the low prevalence of Salmonella-positive samples. Further investigation should occur with larger data collection capacity and replication to elucidate if produce in plots closest to livestock operations (associated hedgerows or buffer zones) may be at an increased risk of contact with contaminated vector populations when environmental pressure is greater or if the flight range of flies commonly associated with these operations is great enough to overcome distance as a barrier.
Overall, prevalence of flies with Salmonella across all families was 1.23% (11 of 889). Within the Calliphoridae family, Calliphora sp. and Phaenicia sp. yielded prevalences of 7.7% (1 of 13) and 2.2% (1 of 45), respectively (Table 1). Within Muscidae, P. aldrichii and Synthesiomyia nudiseta had Salmonella prevalences of 15.2% (5 of 33) and 5.9% (1 of 17), respectively. Within Sarcophagidae, one pool was positive for Boettcheria sp. and one for Mantidophaga sp., with a prevalence of 7.7% (1 of 13) and 100% (1 of 1), respectively. Finally, within the Tachinidae family, the positive pool was collected from a fly with an unknown genus and species classification and a Salmonella prevalence of 100% (1 of 1). With the exception of single isolation events, prevalence for Salmonella within pooled samples ranged from 2.2 to 15.2%.
The overall prevalence of Salmonella-positive samples in this study is much lower (1.23%) compared with values reported in the literature. Xu et al. (33) found an average of 11% of flies collected from 32 beef and dairy cattle farms in Georgia were positive for Salmonella; however, isolation was highly variable among farms and ranged from 0 to 78% prevalence. Bacterial abundance in the environment may also have played a role in pathogen harborage. A recent study evaluating the effects of Salmonella Typhimurium dosage on pathogen excretion and persistence showed that flies infected with high doses (approximately 105 CFU) of the pathogen exhibited static abundance in excreta up to 12 h postingestion, while flies infected with a low dose (approximately 104 CFU) saw bacterial abundance decline during the same time period (19). These data suggest that the number of Salmonella isolations in this study could have been the result of a transient, low-level contamination event within this farming environment that was not encountered during the 2015 growing season.
Beyond acting as a vector, note the possibility of flies serving as a sentinel for environmental contamination that could originate from several sources (i.e., feces, soil, plant matter, or water). When houseflies were exposed to hens challenged with Salmonella Enteritidis, the percentage of Salmonella-positive flies increased from 50% at 48 h after exposure to 70% after 7 days, with a subsequent decrease in the following week to 30% (12). More research is needed to determine if fly collection and subsequent pathogen isolation may serve as a tool to evaluate environmental pressure in the produce growing environment.
The low prevalence of Salmonella was isolated from fly samples on a diversified cattle and fresh produce farm; however, of the fly families associated with isolation, blow, house, flesh, and parasitic flies emerged as potential vectors for on-farm Salmonella transmission. Although these results did not suggest a statistically significant ability for these fly families to harbor and transmit foodborne pathogens in this environment, a fly control program, in conjunction with a robust environmental monitoring program and effective pathogen control measures, should be considered as a means of reducing the risk of foodborne pathogen dispersal on raw agricultural commodities. Overall, fly populations observed in this study were lower compared with similar studies in concentrated animal feeding operations. Further research into methods of dispersal, vector mitigation strategies, and commodity-specific constraints or concerns will be necessary to ensure sufficient control of vectors of foodborne disease at the farm level.