17. LEECHES

Plate 26. (I) (p. 169) and Fig. 7 E (p. 175)

Species affected and geographic range
Leeches have so far only been reported from a few fish in Africa; Bagrus docmac, Barbus altianalis, B. tropidolepis, carp and Protopterus aethiopicus. However, leeches apparently attack a wider range of fish (Claridae, Synodontidae, Mormyridae and Cichlidae) and in a greater number of water systems as is evident from the distribution of leech-transmitted trypanosomes in African fish (see 9.1). Most records of leeches removed from fish in Africa are of Batrachobdelloides tricarinata. This leech occurs from the Jordan system in Israel, infecting Clarias lazera, throughout tropical West and East Africa to Zululand in Southern Africa (Oothuizen, 1989). Piscicolid leeches are common parasites of Mugilidae in the riverine-estuarine system of the southern Cape Province (Swartskop and Kowie estuaries) in South Africa.

Description, taxonomy and diagnosis
Leeches feeding on fish are Rhynchobdellae and belong either to the Glossiphoniidae or the Piscicolidae (Mann, 1962). Most named records of Glossiphoniid leeches from African fish and many of those found free in the habitat were proven to be synonymous with B. tricannata. There is one record of another fish-feeding glossiphonid, Hemiclepsis quadrata (Moore, 1939), from Ethiopia (Oosthuizen, 1987). Apart from the piscicolid leeches (as yet undescribed) of Cape grey mullets, the only other African record of a piscicolid leech is of a species of Phyllobdella removed from Barbus (Moore, 1939).

Rhynchobdellae have a small pore-like mouth on the oral sucker from which a proboscis may be protruded, no jaw is present, the blood is transparent (Gnathobdellae which feed on higher vertebrates have a large mouth with jaws and red blood). Differentiation even between Piscicolidae and Glossiphoniidae is not easy for the nonexpert:

Experts would prefer leeches live, to be fixed to their own specifications. Leeches can survive for a considerable time, even when mailed in a vial inside wet cottonwool. If fixed, it is best in 70% ethanol and preferably the leech should be relaxed first with menthol, ether, or by refrigeration, sometimes, if not too small, under glass slide pressure.

Life history and biology
Leeches, once engorged with the blood of the host, detach and rest on a protected substrate in the water (under stones or in plant debris) until their next meal. Reproduction takes place after several meals. B. tricarinata in tropical and subtropical waters breeds throughout the year (Oosthuizen, 1989). Glossiphoniidae exhibit parental care. The leeches produce a thin walled cocoon and immediately after deposition place their bodies over it and the hatching offspring attach themselves in a brood pouch which forms on the parent's ventral wall. They remain in the pouch until the first feeding stage when they are often brought to their first host by the parent. The entire life cycle may last, depending on availability of hosts, from 24 days to several months. B. tricarinata feeds on fish, and if fish are unavailable will also feed on tadpoles and adult anurans (Oosthuisen, 1991). Some glossiphonids also feed on freshwater molluscs but, although B. tricarinata enters the mantles of molluscs, it does not feed on these hosts. B. tricarinata has been found feeding naturally and induced experimentally to feed on hosts of diverse fish families; Clarias spp., Bagrus spp., Oreochromis spp. Barbus spp. goldfish and Protopterus aethiopicus (Oosthuizen, 1989; Paperna, unpublished). However, populations of a given geographical region or a water system show preference for, or will exclusively occur on, one particular host, for example Clarias lazera in the Jordan system, Israel and Bagrus docmac in Lake Victoria (Paperna, unpublished).

Pathology and epizootiology
Heavy leech infections have variable effects on fish hosts. In Lake Victoria, 19% of all studied Bagrus docmac harboured leeches, with a mean of 26 per infected fish. Leeches were attached to the external perimeters of the mouth region and pathological changes were restricted to bite signs and mild tissue changes even in the most heavily infected individuals harbouring over 100 leeches. More severe damage was evident in piscicolid infection inside the mouth (mainly on the palate and the corners of the jaws) of grey mullets. Infections occurred in several mullet species (in almost all examined L. dummurelli and L. tricuspidata, and occasionally also in Liza richardsoni, and Mugil cephalus) but only in mature fish (over 200 mm in length). The number of leeches per fish was variable and could reach up to 67. Lesions only partially corresponded to observed infection loads and developed in the mouth in the area with the highest predilection for leech attachment, i.e. in the anterior part of the mouth roof and and the articulation zone of the lower and upper jaw. Damage to skin comprised of bite wounds, haemorrhages and erosion of the mucous membranes. In heavy infections leeches were also attached in the craters of the deeper lesions. In some heavy infections the gular membrane also became perforated.

Natural hyperinfection in an American catfish (Ictalurus catus) studies in USA, induced epithelial hyperplasia together with inflammatory changes and haemorrhages in the dermis (Paperna & Zwerner, 1974). Fish confined to ponds seem to be more vulnerable to leech attack. Heavy leech infection of eyes and nostrils of cultured carp, which resulted in mortalities, have been reported from Ghana (Ghana Ministry of Agriculture, 1965). An increase in B. tricarinata was noted in pond cultivated Clarias lazera in Israel, but fish were removed before leech populations reached high levels. In Iraq, Hemiclepsis marginata infection coincided with incidences of mortality in pond reared carp (Khalifa, 1985). Leech attacks were reported from Clarias spp. and tilapia farmed in Southeast Asia. The leeches involved were unidentified local piscicolids or Glossiphoniids which induced pathology similar to that reported above (Kabata, 1985).

Leeches are vectors of haemoprotozoans (see 9.1, 9.2). Piscicola geometra was shown to transmit SVC virus to Carp (see 2.0) (Ahne, 1985). Feeding wounds may become contaminated by opportunistic bacteria and fungi (Kabata, 1985).

Treatment
Species of Piscicola were eradicated from pond fish by application of 0.5 ppm Dipterex (Dylox) or 0.25–0.8 ppm (Al) Masoten (Neguvon). Analogous organophosphates such as Malathion are effective too. In higher temperatures the higher dose is recommended. Removal of leeches from fish was also achieved by a 15 min. flush treatment with 5 ppm CuCl₂; 0.5 ppm copper sulphate for 5–6 hours or 25 ppt NaCl for one hour (Prost, 1974; Kabata, 1985) [not effective for euryhaline leeches of grey mullets]. It was also recommended that ponds should be treated with unslaked lime (2500–3000 kg per ha) before stocking.

REFERENCES

Ahne, W., 1985. Argulus foliaceus L. and Piscicola geometra L. as mechanical vector of spring viraemia of carp virus (SVCV). J. Fish Dis., 8:241–242.

Ghana Ministry of Agriculture, 1965. Annual report of the fisheries division, for the biennium 1964–65: 100 pp.

Kabata, Z., 1985. Parasites and Diseases of Fish Cultured in the Tropics. Taylor & Francis, London & Philadelphia.

Khalifa, K.A., 1985. Leeches on freshwater farmed fish in Iraq. J. Wildl. Dis., 21:312–313.

Mann, K.H., 1962. Leeches (Hirudinea). Their structure, physiology, ecology and embryology. International Series of Monographs on Pure and Applied Biology. Pergamon Press, Oxford, London, New York, Paris.

Moore, J.P., 1939. Additions to our knowledge of African leeches (Hirudinea). Proc. Acad. Nat. Sci. Philadelphia, 90:297–360.

Oosthuizen, J.H., 1987. Redescription of Hemiclepsis quadrata (Moore, 1939) n. comb. (Hirudinea: Glossiphoniidae). Syst. Parasitol., 10: 73–78.

Oosthuizen, J.H., 1989. Redescription of the African leech Batrachobdelloides tricarinata (Blanchard, 1897). Hydrobiol., 184: 153–164.

Oosthuizen, J.H., 1991. An annotated check list of the leeches (Annelida: Hirudinea) of the Kruger National Park with a key to the species. Koedoe, Pretoria, 34: 25–38.

Paperna, I. & Zwerner D.E., 1974. Massive leech infection on a white catfish (Ictalurus catus): a histological consideration. Proc. Helm. Soc. Wash. 41: 64–67.

Prost, M.M., Studnicka, M. & Niezgoda, J., 1974. Efficacy of some methods controlling leeches in water. Aquaculture, 3:287–294.