The latest Paper of the Month for Parasitology is ‘Parasitic nematodes simultaneously suppress and benefit from coccidian coinfection in their natural mouse host’
by Melanie Clerc, Andy Fenton, Simon A Babayan, Amy B Pedersen

Infections with parasitic nematodes are a major threat to global health and affect millions of people across the globe. Although most of the time not lethal, nematode worms have evolved sophisticated strategies to alter or circumvent the host’s natural immune response, allowing them to establish chronic infections that cause great morbidity in infected individuals. More often than not, people living in nematode-endemic regions also suffer from infections with other parasites, making coinfection the norm rather than exception. Further, the ability of nematodes to alter the host’s immune response means their ubiquitous presence may be altering the progression of disease caused by these other infections. As such, if we can understand how different parasite species interact with each other during coinfection, we may be able to manipulate the within-host parasite community in such a way to maximise the benefits for host health. Laboratory mouse models have long been used to shed light on the underlying processes involved in within-host parasite interactions. However, lab mice, which are kept under closely controlled conditions, and have been bred to eradicate variation between animals, are very different from a mouse living in the wild, where nothing is controlled. This creates a problem, because humans also don’t live in controlled environments and vary extensively between each other on a variety of scales. Hence, translating knowledge from inbred laboratory mice to natural populations of humans or other animals can be challenging.

We used wild-derived, captive wood mice (Apodemus sylvaticus) to study how two naturally-occurring gastrointestinal parasites (one a nematode, Heligmosomoides polygyrus, and one a protozoan, Eimeria hungaryensis) interact with each other, and what role the host immune response plays in mediating their interaction. Using these wild-derived host and parasite species allowed us to create a simplified within-host parasite community, whilst still retaining much of the important host and parasite genetic and demographic variation found in nature.

We found a clear benefit of coinfection for the nematode. Coinfected mice carried over twice as many worms as mice infected with the nematode only. Further, these worms produced more eggs, and over a longer time period, when coinfecting with the protozoan compared to when infecting alone. However, we did not find any benefit of coinfection for the protozoan. Because we used the same host and parasite species in the controlled lab experiment as we find in the wild, we could directly compare the immune response between the two settings. Interestingly, although we did not find a signal of coinfection in any of the immune markers we measured in the lab, those same immune markers were affected by coinfection in the wild. As such the interaction between the two parasites was associated with changes in the host’s immune response under wild, resource-limited conditions, but not under controlled, resource-replete laboratory environment. Overall this shows that moving away from traditional lab mouse studies can generate novel, and nuanced, insights that are potentially more readily transferable to human populations.

The paper ‘Parasitic nematodes simultaneously suppress and benefit from coccidian coinfection in their natural mouse host’ is available free for a month.

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