Flies as Vectors of Parasites Potentially Inducing Severe Diseases in Humans and Animals

Abstract

Many fly species feed regularly on feces or their larvae develop inside it. Therefore, it is not astonishing that parasites in the form of durable cysts or eggs or even as fragile trophozoites may be found in fly feces or attached to their legs and bodies. These parasites may then be transported to humans and animals or to their food, from where the parasites may enter (via oral uptake) the inner organs and can induce an infection. These infections might be severe in the case of pathogenic parasites or if high numbers of parasites are transferred. Since many articles in the past reported—often only from occasional findings—that parasites might be transported by licking flies, their role as regular vectors of microorganisms and parasites is therefore eminent. The activity of flies as vectors of pathogens was mostly neglected especially in countries that are proud of their hygiene standards. However, in recent times of intensive globalization of goods and humans and of shrinking distances between rural and downtown centers, flies and their potential load of microorganisms and parasites must be constantly controlled in order to avoid outbreaks of highly transmissible agents of diseases (e.g., viruses, bacteria, parasites such as Cryptosporidium oocysts). This chapter reviews the results of former reports and elucidates the situation near some German mid-sized towns with their rural surroundings. By investigation of flies caught close to pigpens, cattle barns, poultry houses, rabbit cages, and horse stables as well those from dog meadows and/or from recreation sites in or close to downtown sites, it is shown that certain flies are specific for these different sites. In all cases it was noted that a considerable number of flies carried parasites besides numerous bacteria. Some of them carried specimens of parasitic species which have zoonotic importance. When animal feces from these places was investigated, it was found that the parasitic species were the same as those that had been determined before in/on flies. In additional experiments, where laboratory bred, parasite-free flies were placed onto food (milk) with eggs of Trichuris suis and Ascaris suum, it was proven that these flies took up large amounts of these two worm egg types. These results in addition to the finding of parasites on flies caught in the wild as well as the findings of existing publications make it clear that flies have an extremely high potential as vectors of agents of disease that “might lurk unexpectedly before the doors of our well-believed islands of safety and hygiene.”