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Historical review of piscine trypanosomiasis109hosts such as goldfish. However, due to immunephenomenon, the infection passes on to the next phase.c) Phase III- Chronic phase characterized by areducing number of flagellates in the blood and lastsfor a few weeks or for an indefinite period. As it is thelongest phase, it is the one most frequently observed.There are no division stages in the blood during thisphase.d) Phase IV- There is a complete absence of flagellatesin the peripheral blood and the fish appear to be free oftrypanosomes. However, sometimes, the flagellatesmay be found in their preferred sites, themesonephros, pseudobranchia or rete mirabile of theeye. The disappearance of flagellate is a manifestationof premunition, of a non-sterile rather than of a sterileimmunity as these fish appearing apparently asinfection free may later suffer a sudden, severerelapse.TRANSMISSIONThe parasites are transmitted by the blood suckingleeches. Naturally infected Pontobdella and Piscicolawere allowed to feed on clean fish blood by earlyworkers to facilitate transmission of trypanosomes.Brumpt (1904) described the development oftrypanosomes in large numbers in the crop ofHemiclepsis marginata which had fed on infected fishand large bullheads could also be infected by the sametrypanosome-infected leeches. P. geometra couldtransmit flagellates of carps, pike and tench(Kesseylitz, 1906). The trypanosomes of goldfish inthe leech Hemclepsis marginata were studied byRobertson (1911) and Tanabe (1925) observed thedevelopment of trypanosomes of the Japanese loach inleeches. Qadri (1962c) traced the developmentalstages (crithridial, trypaniform and metacyclic) in T.danilewskyi of C. carpio in H. marginata within 7-11days. Needham (1969) transmitted T. tincae fromTinca tinca through the same vector. Tandon (1986)reported the attachment of H. marginata asiatica tothe trunk of W. attu. Lewis and Ball (1979) describedthe attachment of T. cobitis to the crop wall of H.marginata.SEQUENCE OF STAGES IN THE VECTORFish trypanosomes are transmitted by leeches duringtheir blood meal. According to its ecology, each leechspecies may transmit various flagellate speciesindiscriminately as exemplified by the EuropeanPiscicola geometra and Hemiclepsis marginata.A series of morpho-physiological changes aretriggered when the trypanosomes in fish blood areingested by a leech which is manifested by adevelopmental sequence of amastigote,sphaeromastigote, epimastigote (possibly alsopromastigote) and trypomastigote forms in the leechdigestive tract. The latter are termed metacyclic formssince they are infective and capable of initiating thelife cycle in the fish host. Trypanosome species maydiffer in the relative abundance or absence of some ofthe above forms, in the forms in which the mainproliferation occurs, and in the presence or frequencyof proliferation by multiple division.Reliable data on the preferences of fish trypanosomesfor various leech species and parts of their digestivetract are lacking. As a rule, the dividing forms occupythe crop and its caeca, while the non dividingmetacyclic trypomastigotes accumulate in theproboscis sheath. Information on stages found in thestomach or in the intestine of freshwater leeches maybe based on incorrect definition of parts of the leechdigestive tract (proboscis in its sheath, large crop withmany voluminous caeca, stomach and the shortintestine). Thus Brumpt (1906), who proposeddividing several species of trypanosomes into threegroups according to their movements through the partsof the digestive tract, mentioned that all of themdeveloped in the stomach and one (T. granulosumfrom eels) even in the anterior intestine. The skatespecies, T. scylli and T. rajae were also reported toinvade the anterior intestine, a region supposed tosecrete powerful digestive enzymes capable of killingthe flagellates.Freshwater fish trypanosomes, on entry into the leechcrop, begin transforming into division stages(Robertson, 1911; Tanabe, 1925; Qadri, 1962c; Letch,1980). Initially, tadpole-like epimastigotes are formedby unequal division of bloodstream trypomastigotes,there are a few sphaeromastigotes and evenpromastigotes, but it is the epimastigotes that areresponsible for proliferation. The crop is then filled bya mass of intermediate stages which produceextremely long, often even filiform trypomastigotes,which may exceed 50um in size, with a kinetoplasthalfway between nucleus and posterior end. Theseforms do not divide. The epimastigotes are attached tothe epithelium of the crop by the greatly enlarged tipsof their flagella. Later the trypomastigotes prevail andmove into the proboscis sheath- starting with day 5,but usually much later, and exceptionally even as late