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

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 non-inflammatory 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 well-defined equilibrium controlled by the host fish.