Evolutionary genomics of anthroponosis in Cryptosporidium

Generalist pathogens can infect multiple host species. In the case of anthroponotic infectious diseases, the spread is from humans to animals, whereas zoonotic diseases spread from animals to humans. Many Cryptosporidium species can infect multiple hosts, and these infections tend to result in severe diarrhoea. Diarrhoeal pathogens cause more mortality than malaria, measles, and AIDS combined. Globally, Cryptosporidium is the most severe, vaccine non-preventable cause of diarrhoeal mortality for children under five. The zoonotic Cryptosporidium parvum and the anthroponotic Cryptosporidium hominis account for a vast majority of such cases. Because these infections cannot be treated with vaccines, understanding the biology and transmission of these pathogens is important. In a recent paper published in Nature Microbiology, Nader et al. (2019) conducted a phylogenetic comparison of 21 whole genome sequences to investigate the evolutionary genomic changes of Cryptosporidium during its association and specialisation to its human host. In addition, the study characterised the global distribution of Cryptosporidium species and subtypes, summarising the data of 743 peer-reviewed publications of cases in a total of 126 countries. The study shows that hybridisation between four anthroponotic Cryptosporidium subtypes and species has led to genetic exchange between these pathogens, and that these hybridisation events have occurred recently, probably within the past millennium. The genes that are exchanged during hybridisation are undergoing rapid change due to strong natural selection, and in particular genes that are responsible for coding extracellular peptides show evidence of rapid adaptive evolution. The study also identifies a new subspecies, Cryptosporidium parvum anthroponosum, which has only recently emerged through hybridisation. This species is well-adapted to humans and appears to have a distinct way of infecting host through direct human-to-human transmission. The study suggests that this could explain the occasional outbreaks of infections in Europe, which may be transmitted by travellers who have visited developing countries.

Picture: Scanning electron micrograph showing a Cryptosporidium infection of (rat) colon. Credits: Bohumil Sak, Nikola Holubová, Martin Kváč.

Nader JL, Mathers TC, Ward BJ, Pachebat JA, Swain MT, Robinson G, Chalmers RM, Hunter PR, van Oosterhout C, Tyler KM. Evolutionary genomics of anthroponosis in Cryptosporidium. Nature Microbiology. 2019 Mar 4:1. doi.org/10.1038/s41564-019-0377-x

Earlham institute
Quadram Institute Bioscience
John Innes Centre
The Sainsbury Laboratory
University of East Anglia

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