Synanthropic Flies—A Review Including How They Obtain Nutrients, along with Pathogens, Store Them in the Crop and Mechanisms of Transmission

SIMPLE SUMMARY: The significance of non-blood feeding cyclorrhaphan flies with some synanthropic, dipteran families (i.e., Calliphoridae, Sarcophagidae, and Muscidae) in transmitting pathogens to humans and their food sources needs a lot of focused research. This review is designed to provide information on how these flies obtain nutrients and pathogens, store these, and then transmit pathogens to new hosts. Most of the studies in the literature have made references to mechanical transmission by these flies with some references mentioning their gut, and a few specifically mentioning the diverticulated crop organ involvement in the carriage of pathogens. New studies, where pathogens have been shown to multiply within the crop storage organ and, also where the crop is the main site for horizontal transmission of pathogen resistance, make the crop a bio-enhanced organ worthy of consideration in future epidemiological studies and models. Emphasis is also placed on nutrient fly resources in nature, such as bat excreta, enabling these flies to obtain additional pathogens. ABSTRACT: An attempt has been made to provide a broad review of synanthropic flies and, not just a survey of their involvement in human pathogen transmission. It also emphasizes that the crop organ of calliphorids, sarcophagids, and muscids was an evolutionary development and has served and assisted non-blood feeding flies in obtaining food, as well as pathogens, prior to the origin of humans. Insects are believed to be present on earth about 400 million years ago (MYA). Thus, prior to the origin of primates, there was adequate time for these flies to become associated with various animals and to serve as important transmitters of pathogens associated with them prior to the advent of early hominids and modern humans. Through the process of fly crop regurgitation, numerous pathogens are still readily being made available to primates and other animals. Several studies using invertebrate-derived DNA = iDNA meta-techniques have been able to identify, not only the source the fly had fed on, but also if it had fed on their feces or the animal’s body fluids. Since these flies are known to feed on both vertebrate fluids (i.e., from wounds, saliva, mucus, or tears), as well as those of other animals, and their feces, identification of the reservoir host, amplification hosts, and associated pathogens is essential in identifying emerging infectious diseases. New molecular tools, along with a focus on the crop, and what is in it, should provide a better understanding and development of whether these flies are involved in emerging infectious diseases. If so, epidemiological models in the future might be better at predicting future epidemics or pandemics.