The latest Paper of the Month for Parasitology is “Historical contingency, geography and anthropogenic patterns of exposure drive the evolution of host switching in the Blastocystis species-complex” and is freely available for one month.

Where do new diseases come from? It’s a topical question. The conventional answer is that infectious diseases originate with the switch of a pathogen from one host to a new one. It is, however, not often clear how, when, and why pathogens switch hosts. Historically scientists have believed that parasites tend to switch to hosts that are closely-related to their current hosts: from a monkey to a human, for example. While this does happen, it isn’t necessarily the rule: viruses, like Influenza A, can switch between humans and birds, and people can catch bacteria—for example, those that cause cholera—from fish. More advanced parasites also switch hosts, and indeed, humans acquired Plasmodium falciparum malaria—one of the deadliest diseases of humans—relatively recently from great apes (most likely Gorillas). But are such host switches dumb luck, or are there underlying patterns? To better understand this, we conducted a detailed analysis of how the parasite Blastocystis switches hosts in our paper, “Historical contingency, geography and anthropogenic patterns of exposure drive the evolution of host switching in the Blastocystis species-complex.”

Blastocystis? It’s a single-celled organism. It’s a parasite, and it’s closest relative that you’ve heard of is probably kelp. It also probably infects you or someone you know: it is very common and can be found in about ~10% of people across developed countries and is ubiquitous in several less-developed countries. It lives in the gut and generally isn’t considered very dangerous. It can also infect almost every animal that you might think of: birds, lizards, cats, dogs, cows, kangaroos, (even) frogs…making it a fantastic organism for studying how parasites move between hosts.

We assessed how Blastocystis changes hosts by analyzing DNA sequences of the parasites in different hosts from thousands of reports made by scientists around the world. We found that the most important driver of host-switching in Blastocystis is simply exposure – the more times there is contact between potential hosts, the more likely a switch will occur. More importantly, human activities seem to play a central role in driving the types of exposure that give rise to host switching. Several strains of Blastocystis historically associated with primates, can now can be found in dogs, cats, pigs, and chickens—probably as a result of exposure to humans through domestication. Domestication has also likely influenced how easily Blastocystis can adopt future hosts: stains that can infect both humans and livestock have much broader global distributions and typically infect a greater variety of hosts than those that are restricted to wildlife. Exposure is not all important, but exposure and past host use, which is often linked to exposure, seem to be more important than the relatedness of hosts. Ultimately, our results reinforce the importance of the human-animal interface in the acquisition of parasites, and suggests that the greatest threat of parasite emergence centers around repeated human-animal interactions, regardless of the animals involved.

The paper “Historical contingency, geography and anthropogenic patterns of exposure drive the evolution of host switching in the Blastocystis species-complex” by Justin J. S. Wilcox, John J. Lopez-Cotto and Hope Hollocher, published in Parasitology, is available free for a month.