Piauçus (Leporinus macrocephalus), were raised in 300 m² ponds (density of 10 fish/m²) presenting asphyxia signals and daily mortality of 27 fishes. Specimens with 8-cm total body length, were collected for necropsy. Mucus of body surface and pieces of organs were collected and examined microscopically, in wet mounts, stained or in histological sections. The smears examination showed the presence of several spores in the secondary lamellae of the gill filaments, identified as Henneguya leporinicola n.sp (Myxozoa: Myxobolidae). Histopatological study showed epithelial hyperplasia and fulfilling of the spaces between the secondary lamellae, congestion and teleangiectasia sinusoidal. It was also observed hyperplasia of the goblet cells and several cysts of parasite with 70.3mum diameter. Such cysts were situated among the secondary lamellae, covered or not by the hyperplasic epithelium. With this diagnostic, three applications of formalin solution 10 ml/m³ were carried out. Fifteen days after that, fish were examined again to ascertain whether the treatment was efficient on disease caused by the protozoa. The tissue alterations present in the gills after the treatment were just a moderate sinusoidal congestion and a slight epithelial hyperplasia on the base of the secondary lamellae.

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The purpose of the study was to evaluate the susceptibility of 4 important cultivated fishes to sporozoan parasites. Fishes were collected bimonthly from a pond for a period of 1 year. Myxobolus colossomatis and Henneguya piaractus were found in the internal organs and gills, respectively. The combined incidence of parasitism by both myxozoa was 97.3% in pacu (Piaractus mesopotamicus), 33.3% in hybrid tambacu (Piaractus mesopotamicus x Colossoma macropomum), 5.6% in tambaqui (Colossoma macropomum) and 0% in carp (Cyprinus carpio). Pacu was the most susceptible fish and was parasitized 79.2% in the gills, 66.7% in the kidney and 50% in the spleen. Histopathological evaluation of the gills showed hemorrhages, inflammatory reaction with mononuclear cells and fibroblasts and hyperplasia of basal and goblet cells.Key words: Myxozoa, susceptibility, histopathology, cultivated fish. RESUMO

The following lrealments were designed lO each lank, in lriplicale: 8, 16,32 and 64 lrophonls. A fler 8 days of exposure lhe fishes were ki Iled and lhe number oflrophonls was counted al a stereoscopic microscope. This assay showed to be teasible at lropical condilions and was able lO show lhat "lambacu" was more susceplible lhan "piauçu".KEY WüRDS. /ch/hyoph/hirius mll/tifi/iis, lropical fish, experimenlal infeClion, susceplibility Jchtyophthirills multifiliis Fouquet, 1876 (Protozoa, Ciliophora) é um ectoparasito que penetra completamente na epiderme de peixes de água doce e alimenta-se de células do hospedeiro, o qual geralmente sucwllbe às condições patológicas da infecção (KUDü 1966).Para a infecção experimental o parasito 1. multifiliis deve ser cultivado, necessitando de passagens sucessivas na espécie de hospedeiro escolhido, A manutenção do parasito em laboratório implica em certas diíiculdadesjá que as diferentes fases do seu ciclo de vida podem ser afetadas por condições desfavoráveis da água, pela imunidade ou pela morte do hospedeiro (DICKERSON et aI. 1985).