Deadly competition between parasites

There’s an interesting paper out in Nature Communications: Coinfection alters population dynamics of infectious disease. The paper is the study of Plantago lanceolata a common weed where I live in the UK. It’s found across Europe to India and Africa, and as an invasive plant in North America, China and Australia. It’s an example of why scientific names are so important, because I’d call it Plantain, but it’s nothing like the plantains that people eat as Musa.

P. lanceolata is host to a mildew, Podosphaera plantaginis. It looks like a white powder on the leaves of plants. In fact it’s burrowing roots into the leaf and growing spores to spread elsewhere above it. The parasite is not good for the plant, but it needs a live host to survive. Yet sometimes infections take a turn for the worse. What causes that?

The key is co-infection. Sometimes multiple strains of parasite infect a plant. When that happens things get much more complex. With a simple infection, the parasite only needs to overcome the plant. However, if a plant has multiple parasites, then they compete with each other. Normally a gentle approach would be enough for a parasite, but when there are multiple infections then a more aggressive attacker can pull resources from its competitors. At least that’s how you’d expect it to work.

To find out if it’s true Anna-Liisa Laine and her team based at the University of Helsinki carried out experiments and field surveys in the Åland Islands, southwest of Finland. This is part of a longer-term study on infection, so they had well-known plant populations to examine. In September (2012) they took a leaf from up to 10 plants per population and examined the DNA. They also infected leaves in the lab, and in garden plots.

I think the most striking result they got is in the graph below. The bars measure spore activity, and green and blue bars measure infections from single strains of mildew. The red is what you find when there’s a coinfection.

Anna-Liisa Laine, who led the project said: “Here we find that coinfection by different strains of the same pathogen species completely change infection dynamics. These results are really just scraping the surface of how complex infection dynamics can be under coinfection. In our current work we’ve discovered that ribwort plantain populations in Finland contain hundreds of viruses. We’re now measuring how this within host disease community affects infection dynamics for a wide range of pathogen species.”

You can pick up the paper as an Open Access publication from Nature Communications.

Susi H., Barrès B., Vale P.F. & Laine A.L. (2015). Co-infection alters population dynamics of infectious disease., Nature Communications, DOI: http://dx.doi.org/10.1038/ncomms6975