Parasites in humans influence each other via shared food sources

Parasites in humans influence each other via shared food sources

Humans are often infected by parasites, sometimes by several species at a time.  Such co-infections are more difficult to treat if the parasites interact with each other.  Amy Pedersen in the Institute of Evolutionary Biology at the Unviersity of Edinburgh, and her collaborators at the Universities of Sheffield, Liverpool and Zurich, has compiled a list of the numerous ways in which parasites can interact.  This analysis has shown that the most common way parasites interact is in fact indirect, via the food source they share.

Interactions among parasites

Over 1,400 species of parasites – viruses, bacteria, fungi, intestinal worms and protozoa – are able to infect humans. In most cases, the right medicine against a parasite cures the patient. However, if he or she suffers from an infection by two or more species of parasite at the same time, it soon becomes more difficult to diagnose and treat.  Medication can even exacerbate the medical condition if one pathogen is eliminated off but the second continues flourishes. One reason is the little-understood interactions between the parasites that reside in the same host.  

Results of the network analysis:  parasite-resource interactions are displayed in blue and pink, and parasite-immune system interactions are displayed in green and turquoise.  Left:  There are 10 modules associated with particular microhabitats, and within these modules parasite-resource interactions predominate over parasite-immune system interactions.  Right:  The numbers of within-module links between the immune system, parasite and host resources.

In the study published in Proceedings of Royal Society B, the research team presents a network that explains how different pathogens and parasite groups mutually influence each other in the human body. Surprisingly, the biologists discovered that the parasites are most likely to interact via the food source they share – not the immune response or directly through contact with other parasites. 

Complex overview with clear patterns

Co-infections are very common:  simultaneous infestations by different intestinal worms, for instance, affect around 800 million people worldwide.  Amy Pedersen said:

In order to develop effective treatment approaches for co-infections, we need to understand the structures of the parasite communities in a host – in this case individual humans – and the interactions between the parasites better.

The researchers compiled a list of 305 parasite species, 124 resources in the host and 98 immune system components in a meta-study – then analyzed over 2,900 combinations of all these factors in an unprecedented manner.  The resulting network displays clear patterns: The infected part of the body and the same food resource are the most common contact points that can lead to an interaction between the different parasites.  Owen Petchy, another author of the study said:

We found twice as many parasites that fight for the same energy source as parasites that elicit the same immune response

This suggests that the manner in which the immune system responds to the individual pathogens is of secondary importance to the food resources, despite the fact that other studies pointed towards precisely this.

Personalized medicine in the spotlight

The network-like overview of the various interactions of parasites that can harm humans goes beyond the usual consideration of parasite pairs.  The researchers hope that these results can serve as a basis for the development of new, personalized treatment schemes for infected patients, and studies are already underway to explore these possibilities.


Emily C. Griffiths, Amy B. Pedersen, Andy Fenton and Owen L. Petchey. Analysis of a summary network of co-infection in humans reveals that parasites interact most via shared resources. Proceedings of Royal Society B, March 12, 2014. Doi: 10.1098/rspb.2013.2286

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