Plate 14 f-h (p. 73)
Species affected and geographic range
Infections by Hexamita (Spironucleus) are common in cultured tilapia (Landsberg, 1989) as well as in commercially reared South American cichlids in Israel, but, thus far, have not been reported from cichlids in Africa. Occasional hexamitoses occur also in farmed goldfish in Israel. Opalinids (Protoopalina and Zelleriella) and ciliates (Balantidium and Nyctotherus) were reported from African fish of the families Schilbeidae, Mochocidae and Citharinidae (Fantham, 1918; Sandon, 1949).
Amoebae have never been reported from African fish but infections have been reported in grass carp (Ctenopharyngodon idella) and in a neotropic catfish (Pimelodus clarias) (additionally to Salmonids in coldwater habitats - Sawyer et al., 1975). There is also an unconfirmed report on systemic amoebiasis epizooty among tilapia cultured in Auburn, Alabama, US (W. Rogers, per. comm.).
Description, taxonomy and diagnosis:
Hexamita (Spironucleus) [Zoomastigophorea, Diplomonadida] is 7–12 × 4–9 μm, pear shaped or round, flagellate with 3 pairs of anterior flagellae and one pair of posteriorly pointed flagellae. The taxonomic relationships (at the generic and species levels) between parasites found in different hosts are still unresolved. Neither examination of live flagellates in fresh smears, nor fixed and Giemsa stained, can provide sufficient details for taxonomic differentiation (Molnar, 1974).
Balantidium [Ciliophora, Vestibuliferia, Trichostomatida] is a large holociliate with a large round macronucleus, readily seen in fresh and stained bouin fixed smears (with either Giemsa or Hematoxylin) or in histological sections (Molnar & Reinhardt, 1978).
Nyctotherus [Polymenophorea, Heterotrichida] are common ciliates in amphibia and have a very large cytostome.
Opalines [Opalinata] are large (about 250 μm long), rounded, organisms uniformly covered with parallel rows of short flagellae. They are superficially reminiscent of ciliates, but are included among the Sarcomastigophora (flagellates) (Sandon, 1976; Foissner et al., 1979). They have from two to many similar nuclei and in this differ from the ciliophorans which have micro and macro nuclei.
There is a difference of opinion concerning aetiological agents of visceral granuloma in tilapia and goldfish, regarded as amoeba [Sarcodina] by some (Lom & Dykova, 1992) and fungi (Dermocystidium-like) by others (see 4.4).
Life cycles and biology
Both Hexamita and the gut ciliates divide by binary fission, it is not known if these piscine parasites may persist as resting stages in cysts.
The life history of opalinids has been studied in species infecting anurans. It involves sequences of binary divisions and divisions yielding daughter cells (tomonts, which form cysts and in addition, a gamogonous process resulting in a zygocyst.
Hexamitosis (octomitosis) in aquarium held South American cichlids, notably Symphysodon discus and Pterophyllum scalare, often coincides with poor conditions and mortalities. In recent years infection has frequently been diagnosed in tilapia hybrids cultured in Israel. Massive numbers often congest the posterior digestive tract and coincide with food retention. It is, however, not yet certain if Hexamita is a primary pathogen or a synergist in other clinical conditions and bacterial contaminations. Heavy infections in grass carp or South American cichlids cause haemorrhagic enteritis, with injuries to the mucosal epithelium, some necrotic changes in the liver and sometimes haemorrhagic dropsy (ascitis) (Molnar, 1974).
Heavy intestinal infection by Cryptobia iubilans in the South American cichlids Herichthys cyanogutatus and Cichlasoma meekei caused severe inflammation in the entire digestive tract, though the epithelial layer remained by and large intact. Oedema, atrophy and necrosis occurred in the lamina muscularis; lesions extended to the liver and the spleen (Dykova & Lom, 1985). Fish stopped feeding, developed dropsy, and gradually died (Bejerano, pers. comm.). A granulomatous condition in viscera associated with heavy proliferation of Cryptobia-like organisms has been recently found in tilapia hybrids in Israel.
Balantidiosis in grass carp causes haemorrhagic enteritis. In the terminal stage of the disease, hyperaemia and inflammation extends to the entire gut mucosa. Large numbers of parasites may accumulate in the exudate, filling the posterior end of the intestine. Histopathology reveals loss of the superficial epithelium in many places, invasion of the lamina propria by the parasites and multiple ulcerations (Molnar & Reihardt, 1978).
Mortality of aquarium reared, South American cichlids (Symphysodon aequifasciata) coincided with heavy infection by opalinids (Protoopalina symphysodontis), resulting in congestion of the digestive tract (Foissner et al., 1979).
There are no data on hexamitosis in Africa. Species from South American and African cichlids seem to be different, and cross infection may not be possible. Susceptibility of African cichlids to Cryptobia iubilans has yet to be examined; the recent occurrence of granuloma associated with a Cryptobia-like infection in cultured tilapia could have been the consequence of cross infection between American and African cichlids.
Balantidium ctenopharyngodonis has not been found thus far in grass carp introduced to South Africa.
There are several pharmaceutical products recommended against hexamitoses: Flagil, Enheptin, Gabbrocol (Farmitalia), Actinitrazol (Fluka). The first was used to control hexamitosis in South American cichlids. All of these are applied via medicated feeds (1–2 g per 100 kg feeds for 3–7 days - see Reichenbach-Klinke, 1980). Therapeutic control of intestinal ciliate infections has not yet been practised.
Dykova, I. & Lom J., 1985. Histopathological changes due to infections with Cryptobia iubilans Nohynkova, 1984, in two cichlid fishes. Z. Angew. Ichthyol./J. Appl. Ichthyol. 1: 34–38.
Fantham, H.B., 1918. Some parasitic protozoa found in South African fishes and Amphibians. S.Afr. J. Sci., 15: 337–338.
Foissner, W., Schubert, G., & Wilbert, N., 1979. Morphology, infra ciliature and silverline system of Protoopalina symphysodontis nov. sp. (Protozoa: Opalinata), an Opalinidae from the intestine of Symphysodon aequifasciatus Pellegrin (Percoidae: Cichlidae). Zool. Anz. (Jena), 202: 71–85.
Landsberg, J.H., 1989. Parasites and associated diseases of fish in warm water culture with special emphasis on intensification. In: Shilo, M. & Sarig, S. (ed.) Fish Culture in Warm Water Systems: Problems and Trends. CRC Press Inc. Boca Raton, Flo. pp. 195–252.
Lom, J. & Dykova, I., 1992. Protozoan parasites of fishes. Elsevier, Amsterdam, London, New York, Tokyo.
Molnar, K., 1974. Data on “octomitosis” (Spironucleosis) of cyprinids and aquary fishes. Acta Vet. Acad. Scient. Hungar., 24: 99–106.
Molnar, K. & Reinhardt, M., 1978. Intestinal lesions in grass carp Ctenopharyngodon idella (Valenciennce) infected with Balantidium ctenopharyngodonis Chen. J. Fish Dis., 1: 151–156.
Reichenbach-Klinke, H.H., 1980. Krankheiten und Schadigungen der Fische. Gustav Fisher Verlag, Stuttgart, New York.
Sandon, H., 1949. Opalinids from Nile fish. Nature, Lond., 164: 410.
Sandon, H., 1976. The species problem in the opalinids (Protozoa, Opalinata), with special reference to Protoopalina. Trans. Amer. Micros. Soc., 95: 357–366.
Sawyer, T.K.G., Hoffman, G.L., Heath, J.G. & Conrad, J.F., 1975. Infection of salmonid gills by aquatic amoebas (Amoebidae: Thecamoebidae). In: Ribelin, W.E. & Migaki, G. The Pathology of Fishes, Univ. Wisconsin Press, Madison. pp. 143–150.
ILLUSTRATIONS pages 72–73.
Plate 14. Blood and gut Protozoa: a. Trypanosomes in the blood of Liza richardsoni, Kowei lagoon, Southeastern Cape, South Africa. b. A hemogregarine in blood of L. richardsoni of same locality as a. c. Light microscopic histology and electron microscopic view of a presumed hemogregarine cystozoite stages in Red sea grey mullet (L. subviridis). f. Hexamita sp. in giemsa stained smear from tilapia hybrid gut. g. Hexamita (Spironucleus) in giemsa stained smears from gut of angel fish (Pterophyllum scalare). h. Histological section of Hexamita infected tilapia hybrid.
Plate 14. Blood and gut Protozoa (legend p. 72).