Tissue tropism and molecular data of Myxobolus ellipsoides ex S. cephalus (Myxozoa: Myxobolidae) infecting European chub Squalius cephalus

In the River Traun in Austria, diseased European chub Squalius cephalus were observed for several years. In 2019, an investigation of the condition revealed the presence of several myxozoan species in different tissues, without evidence of other pathogens. The most prevalent and abundant myxozoan parasite in the different organs was Myxobolus ellipsoides ex S. cephalus, a parasite formerly only reported to infect the fin of European chub. To further investigate tissue tropism and molecular data of this parasite, samples from 11 different organs of 13 European chub were collected 1 yr later and examined with various methods. Myxospore morphology was assessed by microscopy and compared to the literature. A specific PCR protocol targeting the 18S rRNA gene of M. ellipsoides ex S. cephalus and subsequent sequence analyses detected 11 different 18S variants clustering into 2 groups. To differentiate M. ellipsoides ex S. cephalus unambiguously from other myxozoan parasites in the tissues, histological methods and in situ hybridization with a species-specific probe targeting the 18S rRNA of the parasite were applied. DNA of M. ellipsoides ex S. cephalus could be detected by PCR in each of the examined fish in at least 2 of the sampled organs, but not in any blood sample. In 2 fish, M. ellipsoides ex S. cephalus myxospores were detected in plasmodia in the kidney. Our findings present new data regarding tissue tropism and molecular diversity of M. ellipsoides ex S. cephalus in European chub and provide a basis for further studies investigating possible health impacts by this parasite.