Submitted to: Pathogens in the Environment Workshop Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2004
Publication Date: 7/24/2004
Citation: Trout, J.M., Santin, M.D., Fayer, R. 2004. Identifying and quantifying sources of parasistic organisms. Pathogens in the Environment Workshop Proceedings. p.22-25.
Interpretive Summary: The protozoan parasites Cryptosporidium parvum and Giardia duodenalis (syn. G. lamblia, G. intestianlis) and Enterocytozoon bieneusi are capable of causing infections in both animals and humans and are transmitted via an environmentally resistant stage, the oocyst, cyst, and spore, respectively, which is shed in the feces of the infected host. Many species of livestock and wildlife have been implicated in the transmission of both C. parvum and G. duodenalis, while E. bieneusi has been identified in a few species of livestock and wildlife. Recent implementation of molecular techniques for parasite analysis revealed that there can be different genotypes, or possibly even separate species, that with different abilities to infect humans and animals within groups of organisms that are microscopically identical. Thus, in order to obtain a more accurate assessment of which animal sources present a risk to humans, positive samples must be evaluated by a second method that can determine if a human infective genotype is present. In addition to the challenges posed by the identification of new genotypes of organisms, are the issues of variability in detection protocols. These protocols can vary significantly between laboratories, thus there is little standardization of techniques either in current research or in the published literature. The variation in applying these protocols can yield results with equally high levels of variation and this complicates our ability to develop a realistic assessment of the identity and quantity of parasite sources. To further complicate matters, detection limits or recovery efficiencies are often not tested or reported for a given detection protocol. In order to interpret data on sources of parasites, it is essential to know how well a protocol detects a positive when in fact a parasite is present. Thus all protocols should be tested using samples spiked with known numbers of parasites, and this data should be included or referenced in any publications using the specific protocol. Although they do not produce infectious organisms, mechanical vectors must nonetheless be included on a list of potential sources. Shellfish, birds, and filth flies have all been found to harbor infectious parasites. Thus, although they do not become infected can serve as possible sources of infectious organisms and spread organisms within the environment. Clearly the task of identifying and quantifying sources of parasites is complicated, as well as time and labor intensive. The multitude of potential sources, the genetic diversity within microscopically identical organisms, the dependance on a reliable detection protocol, the variability in genotypes with animal age, and the fact that some parasite may not be shed continuously all serve to underscore the inherent difficulty in this type of research.
Technical Abstract: There are numerous animal sources of environmental contamination with parasites that pose a risk of human infection. Fecal material from livestock, companion animals, wildlife, as well as human sewage all have the potential to contain pathogenic parasites. Dairy and beef cattle and many wildlife species have been found to be infected with Cryptosporidium, Giardia, and microsporidia. Domestic and feral cats can be infected with Toxoplasma gondii. Dogs have been shown to be infected with both Cryptosporidium and Giardia. Identification of these sources has generally been based upon microscopic identification of the environmental stages of the parasites in fecal samples. Quantification has been more challenging in that relatively large numbers of animals must be examined to obtain a representative sampling, and by the fact that geographical differences in parasite distribution within a given host species can exist. The application of DNA sequence analysis to parasite genes and the subsequent discovery of previously unknown species and genotypes of parasites that appear identical by microscopy, has complicated the identification of animal sources as well. These parasites, identical under the microscope, have been shown to belong to different species or genotypes and to have different infectivity potentials for animals and humans. Thus much of the existing data on animal sources of pathogenic parasites may no longer be valid. Certainly, all future work testing animals for the presence of parasites must include molecular analysis to determine if the parasites belong to a species or genotype that represents a risk of human infection.