Bombus terrestris as a suboptimal host for Vairimorpha (Nosema) ceranae: Evidence of limited intracellular development following experimental exposure.

Vairimorpha (Nosema) ceranae, a microsporidian parasite that primarily affects honeybees, has caused concern due to its role in compromising bee health. Although the DNA of this microsporidium has been detected in Bombus terrestris and other bumblebees, evidence of active parasite development in these hosts remains limited. Here, we experimentally inoculated B. terrestris workers with V. ceranae spores to investigate whether the parasite can establish intracellular development in this host. We combined molecular and histopathological approaches, including conventional PCR, parasite-specific PTP3 RT-qPCR, light microscopy, and transmission electron microscopy, to assess parasite presence and development within the host. V. ceranae DNA was detected in 17 of 18 inoculated workers (94.4%), whereas PTP3 transcription was detected in 6 of 18 individuals (33.3%). Histopathological and ultrastructural analyses revealed sparsely distributed intracellular parasite developmental stages within the ventricular epithelium. Together, these complementary observations provide evidence consistent with limited intracellular development of V. ceranae in B. terrestris. However, the low frequency of PTP3-positive individuals and the sparse distribution of parasite stages suggest restricted parasite development under the experimental conditions used. These findings highlight the importance of combining molecular and histopathological approaches when assessing V. ceranae detection in alternative bee hosts and provide evidence that B. terrestris may support limited intracellular development of this microsporidium.