Treating one infection may worsen another

Treating one infection may worsen another

A study of wild mice, which typically carry several parasitic infections at a time, finds treating one infection may worsen another. Led by the University of Edinburgh, the study is the first of its kind to suggest multiple infections may compete with each other and attempts to eliminate one could give another a foothold that results in poorer health.

Study leader Amy Pedersen, of Edinburgh's School of Biological Sciences, and colleagues, write about their findings in a paper due to appear in the 7 July print issue of the Proceedings of the Royal Society B.

In a statement to the press this week, Pedersen says their study "shows for the first time that treating infections in isolation can have knock-on effects for other diseases that may be present."

In their background information the team explains how in nature, infections rarely occur by themselves, it is far more common for animals to carry several parasites at the same time.

It would be reasonable to assume, therefore, that interactions among the parasites may have important knock-on effects for severity and spread of diseases and reactions to how they are treated.

"However, our current view of parasite interactions in nature comes primarily from observational studies, which may be unreliable at detecting interactions," note the authors.

They thus decided to carry out a "pertubation experiment" in wild wood mice infected with nematodes (gut worm).

They treated them for the gut worm infection (using an anthelminthic) over several weeks, during which time they also tested for levels not only of the worm but also of dozens of other common parasites.

They found during the treatment period, while levels of the gut worm fell, levels of other parasites rose.

The parasite communities seemed "remarkably stable to perturbation", write the researchers.

Only one of the non-targeted parasites responded to the anti-worm treatment, and even this reaction did not not last long.

The team also found "strong, but short-lived, increases in the abundance of Eimeria protozoa, which share an infection site with the dominant nematode species, suggesting local, dynamic competition".

They conclude that their study, shows "a rare and clear experimental demonstration" of how worms and co-infecting parasites interact, offering important insights into how similar drug treatments may affect humans and other species.

Pedersen says:

"More work is needed to understand the effect of drug treatment for disease where individuals are prone to, or likely to be carrying a range of infections."

The study highlights the complexities of the communities host animals carry.

Recently, scientists revealed just how extensive the microbial populations that live inside us are: there are 3.3 million gut flora genes compared to the 23,000 or so in the human genome.

And in another intriguing study published in 2012, researchers in the US showed how there may be a way to fight the malaria parasite by genetically modifying gut bacteria in the malaria mosquito.

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Section Issues On Medicine: Disease