A Systematic Review of Zoonotic Enteric Parasites Carried by Flies, Cockroaches, and Dung Beetles

Filth flies, cockroaches, and dung beetles have been close neighbors with humans and animals throughout our joint histories. However, these insects can also serve as vectors for many zoonotic enteric parasites (ZEPs). Zoonoses by ZEPs remain a paramount public health threat due to our close contact with animals, combined with poor water, sanitation, and hygiene access, services, and behaviors in many global regions. Our objective in this systematic review was to determine which ZEPs have been documented in these vectors, to identify risk factors associated with their transmission, and to provide effectual One Health recommendations for curbing their spread. Using PRISMA guidelines, a total of 85 articles published from 1926 to 2021 were reviewed and included in this study. Qualitative analysis revealed that the most common parasites associated with these insects included, but were not limited to: Ascaris spp., Trichuris spp., Entamoeba spp., and Cryptosporidium spp. Additionally, prominent risk factors discovered in the review, such as poor household and community WASH services, unsafe food handling, and exposure to domestic animals and wildlife, significantly increase parasitic transmission and zoonoses. The risk of insect vector transmission in our shared environments makes it critically important to implement a One Health approach in reducing ZEP transmission.


Introduction
Flies (Diptera), cockroaches (Blattodea), and dung beetles (Coleoptera) share their environment with humans, animals, and other insects. While their presence can be beneficialfor example, through pollination, management of other pests, as a food source, and as an organic disposal system for decaying matter-they can also pose risks to human and animal health. Our close ecological connection to these insects presents the public health risk of disease transmission when one or more vectors are infected or contaminated with pathogenic organisms, such as zoonotic enteric parasites (ZEPs) [1][2][3][4][5]. ZEPs can be transmitted through direct contact with an insect vector harboring or carrying a parasite, or by accidental fecal-oral ingestion from contaminated food, water, hands, surfaces, and fomites [1].
Flies, particularly filth flies, are synanthropic and can be found anywhere humans are present, particularly in areas with poor water, sanitation, and hygiene services and practices [6,7]. Of the 46 species of flies that are associated with unclean environments or conditions of "filth", 21 species are considered "disease-causing flies" or known to be vectors of foodborne pathogens [8] (p. 199). Many species of filth flies are coprophagic, feeding on the fecal waste of animals and humans. While these insects often favor indoor spaces, they frequently move back and forth between contaminated environmental settings and human living spaces. This repeated contact introduces the risk of exposure to enteric diseases of public health concern [1,8]. Filth flies are drawn to damp, organic matter  Table S1. A formal protocol was not prepared and the review was not registered.

Screening Process and Study Selection
Following PRISMA guidelines, titles were screened first for eligibility based on full and legible citations and journal article titles only [17]. Then, in groups of two reviewers at a time, the titles and abstracts were assessed. Inclusion criteria consisted of titles that were: (a) peer-reviewed journal articles; (b) from any publication year; with (c) primary research documenting the presence of a recognized or probable ZEP in an insect vector, either through natural or experimental infection; and the (d) ZEP has a primarily enteric transmission route. Exclusion criteria comprised: (a) any publication that was not a peerreviewed journal article; (b) titles written in a language other than English without relevant information provided in an English abstract; (c) reviews or models that did not contain primary research; (d) research on vectors other than filth flies, cockroaches, or dung beetles; (e) research on enteric or gastrointestinal parasites that are not considered zoonotic or likely to be zoonotic; or (f) research that included negative results. When an abstract was not available for the first round of screening, the title was included in the next round for full text review. Titles that studied zoonotic enteric parasites not on our initial list of search terms were included after review by the team against the criteria outlined above.
Full text documents were retrieved by the authors and through the assistance of university librarians. Each full text title was reviewed by at least two authors based on the eligibility criteria above and subsequently marked for inclusion or exclusion. The senior author (AB) served as a tie-breaker when needed. If more than one title addressed the same study or data, the more complete publication was retained for inclusion. When the full text of the article was written in a language other than English, the titles were retained if the relevant inclusion criteria were met in an available English abstract. A second round of review was performed on the excluded full-text articles as a quality control measure in order to ensure a comprehensive list of final studies for inclusion.

Data Extraction
A qualitative analysis was conducted on the included studies by the reviewers to account for the wide variety of publication styles and research methods presented. From the included studies, data were extracted to determine the publication year, the location of the study site, the source or location of the samples, the vector(s) analyzed, the parasite(s) analyzed, specific prevalence rates, if provided, and the means through which the vector was infected (natural or experimental). Information on risk factors for human or animal transmission outlined in the article was also noted.

Results
The full search resulted in 10,063 accessible titles. We removed 5261 duplicate records, 8 titles that were not legible with the use of automation tools, 311 records with missing citations, and 78 books, book chapters, abstracts, thesis/dissertations, and conference proceedings. At this point, 4406 records remained for the title and abstract screening, and 4099 were excluded using the eligibility criteria outlined above. We attempted to find the full-text versions of 307 titles, but 4 records were not accessible through institutional library channels. A total of 303 articles were read in their entirety, if written in English, or the abstract was reviewed if the full text was not available in English. All titles at this stage were screened against the inclusion/exclusion criteria, and 85 titles were incorporated into the final tally of the study results. Full-text articles were excluded for language (n = 19, missing vector or vector not tested (n = 57), missing zoonotic enteric parasite or not testing for parasite (n = 55), article was a review or did not have primary findings (n = 59), the publication was not a journal article (n = 9), the title was an additional duplicate not removed at the earlier stage (n = 9), or other reasons such as negative results (n = 10). A PRISMA flow diagram of the screening process is available in Figure 1. publication was not a journal article (n = 9), the title was an additional duplicate not removed at the earlier stage (n = 9), or other reasons such as negative results (n = 10). A PRISMA flow diagram of the screening process is available in Figure 1. Studies were conducted worldwide, across countries on every continent, except for Antarctica (n = 85; Table 1). The most common included the United States (n = 13), Nigeria (n = 7), Ethiopia (n = 5), and Poland (n = 5). More broadly, studies were conducted in the continental regions of North America (n = 13), South America (n = 13), Europe (n = 20), Africa (n = 17), Asia (n = 19), and Australia (n = 2). Publication dates ranged from 1926 through 2021. Several of the studies (n = 11) identified met the inclusion criteria based on an English abstract and were conservatively included in the results. However, the full text could not be analyzed due to language limitations of the authors. Studies were conducted worldwide, across countries on every continent, except for Antarctica (n = 85; Table 1). The most common included the United States (n = 13), Nigeria (n = 7), Ethiopia (n = 5), and Poland (n = 5). More broadly, studies were conducted in the continental regions of North America (n = 13), South America (n = 13), Europe (n = 20), Africa (n = 17), Asia (n = 19), and Australia (n = 2). Publication dates ranged from 1926 through 2021. Several of the studies (n = 11) identified met the inclusion criteria based on an English abstract and were conservatively included in the results. However, the full text could not be analyzed due to language limitations of the authors. Table 1. Characteristics of included studies examining zoonotic enteric parasites of public health concern in flies, cockroaches, and/or dung beetles.

Strongyloides stercoralis
Within the studies, parasitic pathogens were examined in or on cockroaches. These studies documented the Ascaris spp. Several species of the dung beetle were studied by the included titles to determine if they could harbor, and potentially spread, zoonotic enteric parasites. The dung beetles were from the Scarabaeidae and Geotrupidae families, which primarily feed on fecal or decaying matter. Species came from the Onthophagus genus (e.g., O. fracticornis; Preyssler, 1790), the Bubas genus (e.g., B. bison; Linnaeus, 1767), the Aphodius genus including A. rufus (Moll, 1782) and A. fimetarius (Linnaeus, 1758), and the Anoplotrupes genus (e.g., A. stercorosus; Scriba, 1791), among others.
Within the titles that examined dung beetles, several parasite pathogens were found to have positive results. These zoonotic enteric parasites found on dung beetles included: Numerous risk factors were mentioned for human and/or animal infection or exposure to zoonotic enteric parasites through insect vectors (Table 2). These included poor or inadequate water and sanitation services at home or in the community space (n = 27), having an open defecation site (n = 12) or unmanaged animal waste (n = 16) nearby, insufficient environmental hygiene or the absence of services such as garbage removal (n = 26), seasonal or climatic conditions preferred by the insect vector (n = 14), improper and unsafe food hygiene and storage (n = 23), insect behaviors and feeding practices (n = 29), direct animal contact (n = 22), and ingestion of infected vectors (n = 9). Table 2. Risk factors for exposure to and/or transmission of zoonotic enteric parasites from flies, cockroaches, or dung beetles, as addressed in the included studies.

Discussion
This review highlights the risk of ZEP transmission from insect vectors of interest, including flies, cockroaches, and dung beetles. Flies and cockroaches represent a significant hazard of being exposed to parasites in households and community spaces due to their synanthropic nature [1]. Close cohabitation with humans, especially in the household setting, poses an increased risk of transmission of ZEPs that can be compounded by other factors such as poor sanitation and hygiene. Alternatively, while dung beetles have demonstrated the capability to harbor parasites of public health concern, their preferences for pastures, forest floors, and other natural habitats, coupled with their species-specific dung removal patterns, could actually be of benefit in the removal of zoonotic parasites from the environment [102,103].
The included studies in this review were largely centered on filth flies, which feed and reproduce via human and animal fecal waste as well as through organic waste and garbage [6,7]. Similar to cockroaches, they are drawn to human food items where they may deposit parasitic organisms they have collected via external or internal contamination [1,7]. The mechanical transmission of ZEPs from these insect vectors in food preparation areas are a danger to health and safety in a variety of settings such as homes, restaurants, and hospitals. Food contamination from these insect vectors may be a neglected global threat to human and animal health.

Recommendations
One Health studies that simultaneously investigate parasite presence in humans, animals, food, and environmental reservoirs and vectors can demonstrate which groups and exposure pathways may be the biggest threat. For example, a recent publication conducted by a member of this research team found the zoonotic enteric parasites Cryptosporidium spp. and Giardia spp. among human, animals, flies, and drinking water in households in Mongolia [100]. The highest prevalence rate was round in the fly vectors (14.8%). This information, coupled with a household risk factor survey, demonstrated an association between ZEP presence and unimproved drinking water, not having a handwashing site at the home, domestic animal ownership, and rural location [100]. Researchers Dehghani and Kassiri even presented a question regarding the possible role of flies and cockroaches in the ongoing COVID-19 (SARS-CoV-2) pandemic due to their potential for environmental contamination [107]. More holistic research into water, sanitation, and hygiene (WASH) services and behaviors as well as food safety in personal and community spaces in connection with the prevalence of zoonotic enteric parasites in people, animals, and insect vectors who share these environments can shed light on how and where exposures are occurring. Armed with more robust One Health contexts for ZEP transmission routes, public and veterinary health professionals can collaborate with community members on targeted prevention and control efforts.

Limitations
This review identified studies of ZEPs in cockroaches, filth flies, and dung beetles from all over the world, yet due to the authors' language barriers and lack of qualified translators, only English titles had the full text assessed. English abstracts from several titles illustrated parasite prevalence in vectors of interest and when possible, were included in the final analysis. However, the authors believe that valuable and important work in this subject area is likely to be available in additional languages and found through searching supplemental databases and sources. Furthermore, it is likely that titles of importance were left out of the results due to our search and screening parameters. For example, in one title, the authors spoke of a ZEP in cockroaches but referenced the initial presence data from another source that did not appear in our database results [104,105].
The breadth of parasites analyzed in the included studies demonstrate a wide range of species and hosts. In an effort to outline each pathogen, epidemiological details associated with every parasite were omitted. Information on exposure pathways and disease presentation associated with these zoonotic diseases would be helpful for public health professionals, veterinarians, and medical entomologists tasked with using this review for action against ZEP transmission. Similarly, validated information on the current systematic taxa of the pathogens included in the studies could be of further assistance in understanding more about these zoonotic enteric parasites.

Conclusions
One Health research collaboration is needed to build a better global assessment of ZEPs in insect vectors and the risks posed to human, animal, and environmental health. Implementing a joint approach to tackle these complex exposure pathways using experts and stakeholders in the disciplines of public health, epidemiology, veterinary sciences, biology, medical entomology, environmental health, and more can lead to targeted public and veterinary health education messages for the prevention and control of zoonotic enteric parasites.