A newly published study points to a reason why it's sometimes so difficult to permanently alter the types and numbers of microbes living in or on an animal. This is a problem that veterinarians run across all too frequently. We may try to change the microbial array that lives in animal’s gut to deal with digestive disorders, alter the skin flora of a patient with chronic skin infections, or reduce intestinal parasites in a herd of grazing animals, but often we find that as soon as we stop treatment, everything quickly reverts back to the way that it was prior to our intervention.

As was reported in Proceedings of the Royal Society B:

Co-infection, where an individual host harbours multiple parasite species, is the rule rather than the exception in nature … The parasites infecting an individual host can be viewed as an ecological community, within which species may interact directly through chemical or physical interference…

Understanding parasite interactions and the effects of perturbations is particularly important in disease ecology. A change in the abundance of a target parasite species, for example as a result of drug administration, could affect other (non-target) parasites in the same host, negatively or positively affecting host health depending on the type of parasite interaction.

Using a novel drug-based perturbation experimental approach, we provide a rare experimental demonstration of interaction between co-infecting parasite species in a wild vertebrate population, but also show that, overall, the within-host–parasite communities examined were relatively stable to drug-based perturbation. Mice given Ivermectin showed a reduction in nematode infection probability, while concurrently showing more than a 15-fold increase in Eimeria intensity compared with controls, suggesting significant competition between these two GI parasite taxa. Moreover, this interaction appeared to be highly dynamic, with Eimeria intensity rapidly returning to pre-perturbation levels as nematode re-infection occurred.

In effect, this study showed that when you treat one parasite in wild mice, another takes advantage of the situation and flourishes, but once treatment stops, the old balance quickly returns. Now these results can’t be directly transferred to our companion animals or to non-parasitic conditions, but I do think that the general principles are still important.

For example, when it comes to intestinal parasites, our primary goal should be to eliminate disease, not infection. This is most important in grazing animals, where resistance to dewormers is becoming an increasingly worrisome problem. We should not be aggressively treating clinically normal animals with low level infections. It is impossible to eliminate intestinal parasites from these populations and overtreatment does more harm than good.

Finally, this study helps us understand that when dealing with a community of microbes in an animal, affecting permanent change is very difficult. Ecosystems like these resist alterations to the status quo, so long-term (perhaps life-long) treatment may be necessary when a patient’s biome is causing symptoms of disease.

Dr. Jennifer Coates


Stability of within-host-parasite communities in a wild mammal system. Knowles SC, Fenton A, Petchey OL, Jones TR, Barber R, Pedersen AB. Proc Biol Sci. 2013 May 15;280.

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