Co-infection with a viral pathogen (Theiler's murine encephalomyelitis virus) tended to improve host tolerance but significantly enhanced resistance to Heligmosomoides bakeri

The host's ability to clear parasites (resistance) and their capacity to reduce the impact of a given parasite burden (tolerance) is often described under conditions of a single pathogen challenge. We developed a mouse infection model to investigate the impact of co-infection on host resistance and tolerance to Heligmosomoides bakeri. C57BL/6 mice were infected with the helminth parasite H. bakeri and Theiler's murine encephalomyelitis virus (TMEV), that both reside in the small intestine. Two infection protocols were used to also investigate the impact of the order of pathogen administration on host resistance and tolerance (H-V protocol: helminth first and V-H protocol: virus first). Non-infected controls, H.bakeri only and TMEV only infected animals were also included. Our data showed that co-infection in the H-V protocol, resulted in significantly improved resistance to H.bakeri, as measured by 30 lower number of H. bakeri recovered (p = 0.023) and a 57 reduction in eggs in colon (p = 0.035) compared to mice infected with H. bakeri only. It also led to a 39 improvement in tolerance against H. bakeri infection compared to mice infected with H. bakeri only (p = 0.052). The positive impact on the resistance to H. bakeri was evident but less pronounced in mice in the V-H protocol. Our hypothesis that resistance and tolerance of animals are penalised from increased pathogen load is rejected with important implications for disease control. © The Author(s) 2026.