Characterizing the Xenoma of Vairimorpha necatrix Provides Insights Into the Most Efficient Mode of Microsporidian Proliferation

Microsporidia are a group of obligated intracellular parasites that can infect nearly all vertebrates and invertebrates, including humans and economic animals. Microsporidian Vairimorpha necatrix is a natural pathogen of multiple insects and can massively proliferate by making tumor-like xenoma in host tissue. However, little is known about the subcellular structures of this xenoma and the proliferation features of the pathogens inside. Here, we characterized the V. necatrix xenoma produced in muscle cells of silkworm midgut. In result, the whitish xenoma was initially observed on the 12^th day post infection on the outer surface of the midgut and later became larger and numerous. The observation by scanning electronic microscopy showed that the xenoma is mostly elliptical and spindle with dense pathogen-containing protrusions and spores on the surface, which were likely shedding off the xenoma through exocytosis and could be an infection source of other tissues. Demonstrated with transmission electron microscopy and fluorescent staining, the xenoma was enveloped by a monolayer membrane, and full of vesicle structures, mitochondria, and endoplasmic reticulum around parasites in development, suggesting that high level of energy and nutrients were produced to support the massive proliferation of the parasites. Multiple hypertrophic nuclei were found in one single xenoma, indicating that the cyst was probably formed by fusion of multiple muscle cells. Observed by fluorescence in situ hybridization, pathogens in the xenoma were in merongony, sporogony, and octosporogony, and mature stages. And mature spores were pushed to the center while vegetative pathogens were in the surface layer of the xenoma. The V. necatrix meront usually contained two to three nuclei, and sporont contained two nuclei and was wrapped by a thick membrane with high electron density. The V. necatrix sporogony produces two types of spores, the ordinary dikaryotic spore and unicellular octospores, the latter of which were smaller in size and packed in a sporophorous vesicle. In summary, V. necatrix xenoma is a specialized cyst likely formed by fusion of multiple muscle cells and provides high concentration of energy and nutrients with increased number of mitochondria and endoplasmic reticulum for the massive proliferation of pathogens inside.

Development of a continuous cell line from larval Atlantic cod (Gadus morhua) and its use in the study of the microsporidian, Loma morhua

In vitro cell culture methods are crucial for the isolation, purification and mass propagation of intracellular pathogens of aquatic organisms. Cell culture infection models can yield insights into infection mechanisms, aid in developing methods for disease mitigation and prevention, and inform commercial-scale cultivation approaches. This study details the establishment of a larval cell line (GML-5) from the Atlantic cod (Gadus morhua) and its use in the study of microsporidia. GML-5 has survived over 100 passages in 8 years of culture. The line remains active and viable between 8 and 21°C in Leibovitz-15 (L-15) media with 10% foetal bovine serum and exhibits a myofibroblast phenotype as indicated by immuno-positive results for vimentin, α-smooth muscle actin, collagen I and S-100 proteins, while being desmin-negative. GML-5 supports the infection and development of two microsporidian parasites, an opportunistic generalist (Anncaliia algerae) and cod-specific Loma morhua. Using GML-5, spore germination and proliferation of L. morhua was found to require exposure to basic pH and cool incubation temperatures (8°C), in contrast to A. algerae, which required no cultural modifications. Loma morhua-associated xenoma-like structures were observed 2 weeks postexposure. This in vitro infection model may serve as a valuable tool for cod parasitology and aquaculture research.

Integrative Approach for the Reliable Detection and Specific Identification of the Microsporidium Loma morhua in Atlantic Cod (Gadus morhua)

Microsporidia are fungal parasites that infect diverse invertebrate and vertebrate hosts. Finfish aquaculture supports epizootics due to high host density and the high biotic potential of these parasites. Reliable methods for parasite detection and identification are a necessary precursor to empirical assessment of strategies to mitigate the effects of these pathogens during aquaculture. We developed an integrative approach to detect and identify Loma morhua infecting Atlantic cod. We show that the spleen is more reliable than the commonly presumed gills as best organ for parasite detection in spite of substantial morphological plasticity in xenoma complexes. We developed rDNA primers with 100% sensitivity in detecting L. morhua and with utility in distinguishing some congeneric Loma species. ITS sequencing is necessary to distinguish L. morhua from other congeneric microsporidia due to intraspecific nucleotide variation. 64% of L. morhua ITS variants from Atlantic cod have a 9-nucleotide motif that distinguishes it from Loma spp. infecting non-Gadus hosts. The remaining 36% of ITS variants from Atlantic cod are distinguished from currently represented Loma spp., particularly those infecting Gadus hosts, based on a 14-nucleotide motif. This research approach is amenable to developing templates in support of reliable detection and identification of other microsporidian parasites in fishes.