trypanosomosis in freshwater fish
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- trypanosomosis in freshwater fish
International Common Names
- English: trypanosomiasis; trypanosomiasis in freshwater fish; trypanosomosis
Pathogen/sTop of page Trypanosoma danilewskyi
OverviewTop of page
Trypanosomes usually have a free flagellum at the anterior end (Fig. 33). About 190 species of piscine trypanosomes have been described and host specificity of the vast majority of species is not known. Some species are monomorphic while others are pleomorphic. It is likely that many of the species are invalid; however, this can only be confirmed after careful experimental studies (Woo, 1994). Piscine trypanosomes are always transmitted by leeches and most species are not known to cause disease and/or mortality in the host; hence they will not be discussed in the present review.
Fig. 33. Trypanosoma danilewskyi in the blood of an experimentally infected goldfish: note the monomorphic trypomastigotes, a non-nucleated abnormal form, and a pre-division stage with two kinetoplasts, two free flagella and a nucleus
Evidence of trypanosomiasis in naturally infected fishes
There are several trypanosomes that are presumed to cause lesions in organs and/or changes in the blood of naturally infected fish. Fish from the field are often infected with a variety of pathogens and may have also been subjected to stress factors (e.g. toxic pollutants, nutritional deficiencies). Hence, it is difficult to ascribe abnormalities to any one particular cause. At best, these field reports can only be considered as preliminary and some are briefly included here.
Neumann (1909) found inflammation of the brain, fatty degeneration in organs, anaemia and eosinophilia in skates (Raja punctata) that had high numbers of Trypanosoma variabile. According to Smirnova (1970), the number of red cells, haemogloblin contents and serum protein levels of Trypanosoma lotae-infected burbot (Lota lota) were lowered, while the number of phagocytic white cells was elevated.
Fish infected with Trypanosoma vittati and Trypanosoma maguri had low numbers of red cells and haemogloblin (Tandon and Joshi, 1973). However, the numbers of white cells and immature or abnormal red cells were higher in the infected fish. Clarias batrachus and Channa punctatus infected with Trypanosoma batrachi and Trypanosoma aligaricus had lower red cell counts and lower haemogloblin (Gupta and Gupta, 1985). The numbers of agranulocytes and granulocytes were higher in infected fishes, except for neutrophils, which were significantly lower. Blood glucose levels were lower in C. punctatus, C. batrachus, Heteropneustes fossilis and Mystus seenghala infected with trypanosomes (Tandon and Joshi, 1974). Also, serum alkaline phosphatase and cholesterol levels were lowered in infected Cirrhina mrigala, C. batrachus, Mastacembelus armatus, M. seenghala and Wallago attu (Tandon and Chandra, 1977a, b).
Since naturally infected fish can have mixed infections, it is recommended that the parasite first be cloned. The cloning is done using either susceptible laboratoryreared fish (e.g. Woo, 1979) or an appropriate culture medium (e.g. Jones and Woo, 1991). The cloned parasite is then identified (Woo, 1994) and cultured. To determine pathogenicity, all three strains (field, cultured and cloned strains) should be used to experimentally infect hatchery-reared fish. This approach would satisfy Koch's postulates for determining the aetiological agent for a disease.
[Derived from: Woo, PTK, ed., 2006. Fish diseases and disorders, Volume 1: Protozoan and Metazoan infections. (2nd edition) Wallingford, UK: CAB International]
Host AnimalsTop of page
|Animal name||Context||Life stage||System|
|Ameiurus nebulosus (brown bullhead)|
|Anguilla anguilla (European eel)||Wild host|
|Carassius auratus auratus (goldfish)||Domesticated host|
|Catostomus catostomus||Experimental settings|
|Catostomus commersonii (white sucker)|
|Cyprinus carpio (common carp)||Wild host|
|Etheostoma caeruleum||Experimental settings|
|Heteropneustes fossilis (stinging catfish)|
|Tinca tinca (tench)||Wild host|
Hosts/Species AffectedTop of page
Trypanosoma danilewskyi was first described from the blood of the common carp (Cyprinus carpio) in Europe (Laveran and Mesnil, 1904). The parasite has since been found in carp, goldfish (Carassius auratus), tench (Tinca tinca) and eel (Anguilla sp.) in Europe (Thompson, 1908; Pavlovskii, 1964; Lom, 1973) and in Saccobranchus fossilis in India (Qadri, 1962). The parasite is not host specific and is infective when inoculated experimentally into Barbus conhus, Danio malabaricus, C. commersoni, Notropis cornutus, Etheostoma caeruleum and Ictalurus nebulosus (Woo and Black, 1984).
T. danilewskyi-like trypanosomes have been isolated from numerous fish species and from different localities. In vitro and in vivo studies clearly indicate that there are significant differences between the isolates, specifically in their nutritional requirements, cultural characteristics and changes in virulence (on subpassage in fish). These are some of the reasons why it is considered here that it is premature to synonymize T. danilewskyi with Trypanosoma carassii, as Lom and Dykova had done.
It is suggested here that the parasite is most probably a species complex, as was shown with Ichthyobodo, and that future biological studies should help to establish the taxonomic status of the various isolates. These should include developmental (in the vector, in the fish host), antigenic (e.g. using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), Western immunoblots), cross-protection (e.g. using recovered fish, complement-fixing antibodies), molecular (e.g. using SSG-rDNA) and susceptibility (e.g. using genetically known carp stocks) studies.
DistributionTop of page
See under "Animals affected"
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 10 Jan 2020
PathologyTop of page
Course of infection and clinical signs
The parasite multiplies rapidly in goldfish when they are kept at 20°C and more slowly at 30 or 10°C (Lom, 1973; Woo et al., 1983; Islam and Woo, 1992). The parasitaemia increases after infection and in fish maintained at 20°C it starts to decline at about 28 days after infection. These fish eventually recover from the infection and are immune to homologous challenge (Woo, 1981b; Islam and Woo, 1991a; Overath et al., 1999).
The parasite causes anorexia in experimentally infected goldfish (Islam and Woo, 1991c) and this is most evident during high parasitaemia. Fish that survive the disease return to normal feeding.
Pathology and mortality
Trypanosoma danilewskyi also causes anaemia in goldfish (Robertson, 1911; Dykova and Lom, 1979; Islam and Woo, 1991b). As in salmonid cryptobiosis, the severity of the anaemia in trypanosomiasis is directly related to the number of parasites in the blood, and it is also partly caused by a lytic factor and haemodilution (Islam and Woo, 1991b). The lytic factor(s) is secreted by living parasites and it lyses red cells in the absence of specific antibodies. It is likely that the mechanism of the anaemia in trypanosomiasis is similar to that in salmonid cryptobiosis and that the virulent factor is a protease.
Mortality of infected goldfish is related to the size of the inoculum, and a small inoculum produces low mortality (Woo, 1981b; Islam and Woo, 1991a). Infectivity and virulence of one parasite strain (isolated from carp) increased on repeated transfer in goldfish (Lom, 1979). However, the virulence of two other strains of T. danilewskyi (isolated from eels and tench) decreased on repeated transfer in goldfish. No explanations were provided for the difference and, since no experimental details were given, the study should be carefully repeated.
Goldfish that have recovered from acute infections are protected on subsequent homologous challenge (Lom, 1973; Woo, 1981b; Islam and Woo, 1991a; Overath et al., 1999) and plasma from these immune fish contains neutralizing antibodies (Woo, 1981b). Passive immunization with IgM purified from recovered fish confirms that the infection is controlled by antibodies (Overath et al., 1999). The immunity is non-sterile as low numbers of parasites are in the blood of recovered fish. Intraperitoneal injection of corticosteroid into recovered goldfish significantly increases the number of trypanosomes in the peripheral blood of immune fish (Islam and Woo, 1991a; Overath et al., 1999). Oestradiol can modulate the susceptibility of infected goldfish. Oestradiol-implanted fish had significantly higher parasitaemias and mortality than sham-operated fish. Also, mitogen-induced proliferation of circulating lymphocytes from oestradiaol-implanted fish was impaired compared with those from sham-operated fish (Wang and Belosevic, 1994b). Excretory/secretory products from T. danilewskyi, when injected with Freund's complete adjuvant, confer protection (Bienek et al., 2002). This is encouraging and protective antigen(s) need further characterization.
As indicated earlier, cultured T. danilewskyi is infective to goldfish (Wang and Belosevic, 1994a) and it survives well under in vitro conditions in normal goldfish serum. However, if its surface proteins are removed, using trypsin, the parasite is lysed via the alternative pathway of complement activation. The trypsinized parasite regains its resistance to lysis after 24 h when cultured in a trypsin-free medium (Plouffe and Belosevic, 2004). The significance of this is not well understood.
DiagnosisTop of page
During the acute phase of the disease, parasites are readily detected by examination of a drop of freshly collected blood under the light microscope. Taxonomic identification of the parasite is best done with alcohol- formalin-fixed blood smears stained with Giemsa’s stain (Woo, 1969). The haematocrit centrifuge technique (Woo, 1969) can be used to detect infections (e.g. early after infection or in the chronic phase of the infection), which are not usually detectable using the wet-mount technique. There are no serological techniques for piscine trypanosomiasis; however, serodiagnostic techniques developed for salmonid cryptobiosis can easily be adapted for trypanosome infections.
ReferencesTop of page
Bienek DR; Plouffe DA; Wiegertjes GF; Belosevic M, 2002. Immunization of goldfish with excretory/secretory molecules of Trypanosoma danilewskyi confers protection against infection. Developmental and Comparative Immunology, 26(7):649-657.
Dykova I; Lom J, 1979. Histopathological changes in Trypanosoma danilewskyi Laveran & Mesnil, 1904 and Trypanoplasma borelli Laveran & Mesnil, 1902 infections of goldfish, Carassius auratus (L). Journal of Fish Diseases, 2(5):381-390.
Laveran A; Mesnil F, 1904. Trypanosomes and Trypanosomiases (translated by D. Nabarro). Chicago, Illinois, USA: WT Keener.
Lom J, 1973. Experimental infection of goldfish with blood flagellates. In: Progress in Protozoology. 4th International Congress in Protozoology, Clermont-Ferrand, France, p. 255 (abstract).
Lom J, 1979. Biology of the trypanosomes and trypanoplasms of fish. In: Lumsden WHR, Evans DA, eds. Biology of the Kinetoplastida, Vol. 2. London, UK Academic Press, 269-337.
Neumann RO, 1909. Studien über protozoische Parasiten im Blut von Meeresfischen. Zeitschrift für Hygiene und Infektionskrankheiten, 64:1-112.
Pavlovskii EN, 1964. Key to Parasites of Freshwater Fish of the USSR. Academy of Sciences of the USSR, Zoological Institute. Translated from Russian by Israel Program for Scientific Translations, Jerusalem, Israel, 919 pp.
Plouffe DA; Belosevic M, 2004. Enzyme treatment of Trypanosoma danilewskyi (Laveran and Mesnil) increases its susceptibility to lysis by the alternative complement pathway of activation in goldfish, Carassius auratus (L.). Journal of Fish Diseases, 27:277-285.
Qadri SS, 1962. An experimental study of the life cycle of Trypanosoma danilewskyi in the leech, Hemiclepsis marginata. Journal of Protozoology, 9:254-258.
Robertson M, 1911. Transmission of flagellates living in the blood of certain freshwater fishes. Philosophical Transactions of the Royal Society of London, Series B, 202:29-50.
Smirnova TL, 1970. Trypanosoma in the blood of Lota lota L. - Trypanosoma lotae sp. n. Parasitologyia, 4:296-297.
Tandon RS; Chandra S, 1977. Physiology of host parasite relationship: effects on serum alkaline phosphatase levels of fish hosts parasitized by trypanosomes. Zeitschrift fur Parasitenkunde, 52(3):195-198.
Tandon RS; Joshi BD, 1973. Studies on the physiopathology of blood of fresh water fishes infected with two new forms of trypanosomes. Zeitschrift fur Wissenschaftliche Zoologie, 185A(Heft 3/4):207-221.
Thompson JD, 1908. Cultivation of the trypanosome found in the blood of the goldfish. Journal of Hygiene, 8:75-82.
Wang R; Belosevic M, 1994. Cultivation of Trypanosoma danilewskyi (Laveran & Mesnil, 1904) in serum-free medium and assessment of the course of infection in goldfish, Carassius auratus (L.). Journal of Fish Diseases, 17(1):47-56; 8 ref.
Woo PTK, 1969. The haematocrit centrifuge for the detection of trypanosomes in blood. Canadian Journal of Zoology, 47:921-923.
Woo PTK, 1994. Flagellate parasites of fishes. In: Kreier JP, ed. Parasitic Protozoa. Vol. VIII, 2nd edn. London, UK: Academic Press, 1-80.
Qadri SS, 1962. An experimental study of the life cycle of Trypanosoma danilewskyi in the leech, Hemiclepsis marginata. In: Journal of Protozoology, 9 254-258.
Distribution MapsTop of page
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