Myxobolus cycloides on the swimbladder of chub Leuciscus cephalus: a controlled, host-specific localisation

Abstract

Of 150 wild stock chub, Leuciscus cephalus L. captured in Lower Austrian watercourses, 112 revealed disc like plasmodia of Myxobolus cycloides Gurley, 1893 on the caudal chamber of the swim bladder. Other cyprinid species from the same waters lacked M. cycloides or other myxosporeans in this specific localisation. In chub, the intensity of infection (number of discs on the swim bladder) showed a logarithmic, age-dependent increase. The plasmodia of M. cycloides were situated in the connective tissue -mainly along blood vessels -and exhibited a delicate envelope of host tissue, thus forming a characteristic myxosporean cyst. Occasionally single trophozoites seemed to merge. A general process of fibroblast proliferation leading to encapsulation and degradation of the parasite was observed. This process was initiated by the formation of small multiple encapsulations within the spore containing trophozoid, before thickening of the outer cyst wall occurred. The general noninflammatory course of the M. cycloides infection, and the obvious good health of the investigated chub suggest that this myxosporean in its host specific localisation cannot be regarded as a serious pathogen -on the contrary: parasite multiplication and degradation seemed to occur in a welldefined equilibrium controlled by the host fish.

Comparative morphological and molecular studies on Myxobolus spp. infecting chub from the River Danube, Hungary, and description of M. muellericus sp. n.

“…Ellipsoidal spores (Figs. 1a & 2a) with an average size of 14.1 × 10.4 µm had a large intercapsular process, and corresponded to data described and depicted by Lom & Dyková (1992).Histology: The histology of this species was not studied in detail during this survey, as the morphological data we obtained corresponded in all respects to those described by Holzer & Schachner (2002).Molecular data: 18S rDNA sequences of 5 Myxobolus cycloides samples examined were 99.9 to 100% similar to each other.Remarks: The species was originally described from a systematically closely related cyprinid fish, Rutilus rutilus; therefore, its occurrence in chub is probable. …”

“…Histology: The histology of this species was not studied in detail during this survey, as the morphological data we obtained corresponded in all respects to those described by Holzer & Schachner (2002).…”

“…infecting chub. For M. cycloides infection, Holzer & Schachner (2002) reported a special swimbladder location. Molnár (1994) suggested that most myxozoans had a well-defined tissue and organ specificity.…”

“…infecting chub are available for 3 species. Lom (1961) recorded the frequent occurrence of M. dujardini in L. cephalus and L. leuciscus, while Holzer & Schachner (2002) described intensive infection of the swimbladder of chub with M. cycloides. Both of the 2 species above have been described from Rutilus rutilus, another leuciscine fish that is closely related to chub.…”

Specificity of Infection Sites in Vertebrate Hosts

“…Some myxosporeans, like Myxobolus cycloides forms large flat plasmodia in the connective tissue of the swimbladder wall (Holzer and Schachner 2002). Swimbladder inflammation caused by extrasporogonic stages of S. dykovae in common carp often occurs when the so-called K-stages of the parasite block the capillaries of rete mirabile.…”