Myxobolus buckei sp. n. is described from the spinal column of Leuciscus cephalus (L.), Rutilus rutilus (L.) and Abramis brama (L.) from freshwater rivers in the North of England. The plasmodia develop within the remnants of the embryonic notochord in the intervertebral spaces. The spores are large, measuring (in µm) 14.0 ± 0.7 × 11.5 ± 0.6 (mean ± SD), smooth, round to ellipsoid in valvular view with several sutural edge markings. The polar capsules are pyriform and of equal size, measuring 7.5 ± 0.5 × 4.2 ± 0.2 (mean ± SD), with 11-12 turns of the polar filament arranged perpendicularly to the longitudinal axis of the polar capsule. The parasite has a large intercapsular appendix and large iodinophorous vacuole. The parasite can be differentiated from all known species of Myxobolus Bütschli, 1882 by a combination of the morphological characters defined. Infected fish show marked longitudinal compression of the body compared to uninfected individuals of the same year class, a feature which is pathognomonic for the disease. Histologically, host responses ranged from mild hypertrophy of the zygapophyseal process and expansion of the intervertebral membrane to complete hypertrophy and fusion of the vertebrae. Prominent notochord is present in the intervertebral spaces of infected fish and sporogony of the parasite leads to a vigorous focal inflammatory response involving proliferating fibroblast and osteogenic cells. The parasite causes a radial expansion of the centra and extensive dorsal and ventral outgrowths of the vertebrae leading to compression of the spinal cord and blood vessels running through the neural and haemal spines respectively. The parasite is considered highly pathogenic to juvenile cyprinids.
A new species, Paramarteilia canceri sp. n., is described using light and electron microscopy from the edible crab Cancer pagurus L. captured from the English Channel. No external symptoms were noted, although infected animals were typically lethargic and unresponsive to external stimuli. Organs of infected animals were shrunken and collapsed compared with apparently healthy individuals. Although the infection was systemic, marked host responses were only noted in the hepatopancreas where the parasite induced a pronounced haemocytic infiltration. Prevalence of infection throughout the study was 1.1%, with a maximum monthly prevalence of 3%. The intracellular parasite was typically 15 µm in length and composed of a primary cell containing up to three secondary cells derived by internal cleavage. Each secondary cell contains two bicellular spores. The parasite is readily differentiated from the other described paramyxean species by a combination of the number of secondary and tertiary cells. In light of this new discovery, a revision of the order Paramyxida Chatton, 1911 is proposed based upon comparison to the original descriptions of this parasite group in various species of invertebrate hosts. The proposed classification is based on the number of cells within the spores (tertiary cells), so that only three genera remain within the order, namely Marteilia Grizel, Comps, Bonami, Cousserans, Duthoit et Le Pennec, 1974, Paramarteilia Ginsburger-Vogel et Desportes, 1979 and Paramyxa Chatton, 1911. Subsequent discrimination of species is based on a combination of the number of secondary cells within the primary cell and the number of tertiary cells within secondary cells. It is proposed that the genus Marteilioides Comps, Park et Desportes, 1986 is suppressed and the type species of the genus, M. chungmuensis Comps, Park et Desportes, 1986, is transferred to Marteilia and that the other representative of the genus, M. branchialis Anderson et Lester, 1992, is transferred to Paramarteilia. Further, Paramyxoides Larsson et Køie, 2005 is considered as a junior synonym of Paramyxa and its type and only species, Paramyxoides nephtys Larsson et Køie, 2005, is transferred to Paramyxa.