Cutaneous myxosporidiosis in wild creek chub
Heindrich N. Snyman
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2025;29(2):21.
During routine fish health and environmental monitoring of a localized water body in a semi-urban conservation area in Southwestern Ontario, it was noted that some retrieved wild creek chub (Semotilus atromaculatus) fish had abnormal external appearances and deformities. Fish were routinely retrieved using standardized electrofishing stunning techniques; a single abnormal-appearing fish was humanely euthanized for further analysis. After opening of the coelomic cavity to allow for adequate fixative penetration, the affected fish was placed whole into 95% ethanol for preservation and then submitted to the Animal Health Laboratory for further gross and histological analyses.
On external examination, the fish contained numerous individual to coalescing, firm, white-tan expansile nodules that protruded above the external skin surface. Nodules were most prominent and numerous along the dorsal, lateral, and ventral surfaces of the head, and rostral trunk/abdomen (Fig. 1). These nodules compressed and distorted the local soft tissues, and occasionally resulted in mild displacement or compression of regional structures such as the eye, operculum and branchial cavity. Other than these nodules, the fish was in good body condition with adequate coelomic fat stores and no other internal abnormalities.
The fish was 5-6 cm in length, and was subsequently demineralized and softened by submersion of the whole fish in slow decalcifying solution for 48 hours prior to trimming. Serial cross or longitudinal sections of the head and body with organs in situ were then routinely trimmed into cassettes for processing and generation of routine H&E-stained histological sections.
Numerous individual and clustered, well-demarcated 1.0 mm - 3.2 mm diameter round to short oval myxosporean cysts (Fig. 2A) were scattered throughout the interstitium of the perivertebral and pericerebral skeletal muscle bundles. These cysts often displaced and compressed the adjacent myofiber bundles, in addition to occasional adjacent parenchymal organs (e.g. trunk kidney). Cysts contained an outer 30 µm-75 µm thick fibrous wall that was internally lined by a single layer of macrophages and rimmed by necrotic cellular debris. The centre was filled by abundant, loose to moderately densely packed myxozoan spores, measuring 9 µm-12 µm long and 4.0 µm-5.5 µm wide (Fig. 2B). Spores contained an outer indistinct thin hyalinized wall, with single central round pale basophilic nucleus. Special-stained sections (B&H Gram and Giemsa) highlight the spore morphology; spores ranged from fusiform to pyriform with paired polar capsules and central basophilic nuclei (Figs. 2C, 2D). Apart from regions of overlying epidermal ulceration and intervening peripheral fibrous encapsulation, there was no obvious host-associated inflammatory reaction.
These findings are consistent with a disseminated myxosporean parasitic infection and a diagnosis of myxosporidiosis. Myxosporean parasites are fairly common in wild fish and most infections are well adapted to their host, causing minimal morbidity. However, aberrant host infections can result in a much more robust inflammatory response. Apart from fibrous encapsulation and regions of pressure-induced skin ulceration, the current case did not contain any obvious targeted inflammatory response, and therefore, a host-adapted infection is suspected in this case. Because myxosporeans have a life cycle that requires at least two distinct hosts, they are a threat to cohabitating fish only if the intermediate host is present in their system.
Various species of myxosporean parasites have been described in creek chub; however, these reported infections were restricted to the urinary system, gall bladder, or gills (e.g. Myxobilatus semotilii, Myxobolus muelleri, Myxobolus pellicides, Myxobolus pendula, Sphaerospora paulini, and Chloromyxum catostomi). The presence of two paired polar capsules is considered characteristic for myxosporean parasites belonging to the Myxobolus genus and the inter/intra-muscular location would also be consistent with a Myxobolus species. Further species identification would require molecular sequencing which was not performed in this case.
Although parasitic infections such as this are often disfiguring, they rarely cause significant morbidity. Parasite numbers in this case were quite numerous, but other than impingement or compression of the vertebral column and associated structures, the clinical significance and risk to cohabitating fish in this water body is probably minimal. Histological evaluation however plays an important role in differentiating incidental host-adapted infections from other disease conditions that could have more important implications for population health.
Figure 1. Postmortem findings in a wild caught creek chub with cutaneous myxosporidiosis. There are multiple firm, white-tan expansile cutaneous nodules scattered along the dorsal, lateral, and ventral surfaces of the head and rostral trunk/abdomen.
Figure 2. Histological findings in a wild caught creek chub with cutaneous myxosporidiosis. A. Head. There are numerous individual or often clustered, well-demarcated, round to oval myxosporean cysts. (arrows). H&E stain. B. Myxosporean cyst. Higher magnification of cyst wall and spores. H&E stain. C & D. B&H Gram and Giemsa stain respectively. Special-stained sections highlight the spore morphology, ranging from fusiform to pyriform with paired polar capsules and central basophilic nuclei.
References
1. Wootten R. Chapter 7: Parasitology. In: Fish Pathology 4th ed. Roberts RJ, ed. Wiley-Blackwell. 2012:292-338.
2. Smith, PA, Elliott, DG. Bruno, DW, Smith SA. Chapter 5: Skin and fin diseases. In: Fish Diseases and Medicine. Smith, SA, ed. CRC Press. 2019:122-124.
3. McDonald TE, Margolis L. Synopsis of the parasites of fishes of Canada: Supplement (1978-1993). Canadian Special Publication of Fisheries and Aquatic Sciences #122. Department of Fisheries and Ocean, Canada. NRC. 1995;Bulletin 199:152.