Parasites and people: feeding off each other

Malaria, parasitic worms, Ebola, HIV, even bed bugs—today’s parasites and pathogens are always in the news. As a species, humans affect the lives of everything else on our planet, ranging from the tallest trees to these smallest of organisms. In turn, these changes also affect the parasites that infect us and other animals, and it now seems that the parasites are spreading ever more widely, and sometimes rapidly and aggressively, across the planet.

An article recently published in The Journal of Parasitology looks at why parasites now appear nearly everywhere, and how people have affected their range and dominance. It reviews years of research related to parasite genetics and the ecological links between humans and pathogens. This paper focuses on the genetic diversity of parasites within single hosts and uses this data to illustrate the effects of human activities on parasite evolution.

The authors based their model on several studies of specific parasite populations, including tapeworms, nematodes, and protozoans, infecting a wide variety of hosts. They then mined these studies for information on how parasites have changed over the centuries and how people may have played a role in those changes. The data indicates that parasite spread is linked to human population growth. Over time, parasites have increased their distributions and drug resistance and even changed hosts as they co-evolved with man. The ability of parasites to switch hosts is particularly important, as it has greatly boosted parasite diversity and distribution.

The authors also emphasize what they call “key tipping points in human history,” starting with the spread of Homo sapiens from Africa and continuing through the growth of agriculture, exploration, and industrialization. They list modern-day tipping points such as global warming, ocean acidity, changes in essential nutrients, and loss of biodiversity. Our role in creating, altering, and overcoming these issues can be significant. However, they argue that in some cases, our solutions may be more damaging than the problems. Attempts to hold onto struggling ecosystems or bring them back to health can open the door to pathogens, allowing them to spread disease into new environments and hosts.