Nematode parasites of Clarias gariepinus ( Burchell , 1822 ) from the Rietvlei Dam , South Africa

BARSON, M. & AVENANT-OLDEWAGE, A. 2006. Nematode parasites of Clarias gariepinus (Burchell, 1822) from the Rietvlei Dam, South Africa. Onderstepoort Journal of Veterinary Research, 73:87–94 Catfish, Clarias gariepinus, from the Rietvlei Dam near Pretoria, South Africa were examined for nem atode parasites. Two species, Procamallanus laeviconchus in the stomach and Contracaecum spp. larvae in the abdominal cavity, were found. The morphology of these species, based on light and scanning electron microscopy, and how they compare with previously described specimens are discussed. Infection rates were mild compared to previous surveys although Contracaecum spp. had a high prevalence of 86 %.


INTRODUCTION
The study of parasitic diseases of fish and other aquatic organisms in South Africa only started to gain attention over the past few decades as scientists began to realise their significance in fisheries and aquaculture (Safriel & Bruton 1984;Hoffman & Prinsloo 1996).Only a few researchers have published their work on nematode parasites in the country, and these include Prudhoe & Hussey (1977), Mashego (1977Mashego ( , 1982)), Mashego & Saayman (1981), Boomker (1982Boomker ( , 1994a, b), b), Saayman, Mashego & Mokgalong (1991) and Mokgalong (1996).The results of this study will contribute to this existing body of knowledge.
The sharptooth catfish, Clarias gariepinus (Burchell, 1822), the host species investigated in this study, is widely distributed in Africa (Safriel & Bruton 1984;Skelton 2001) and is an excellent species for aquaculture and biological research (Hoffman & Prinsloo 1996).
The study was carried out as a reconnaissance survey of the internal parasites that are found in C. gariepinus from the Rietvlei Dam, which can be used in the fish health assessment index that has been developed for South African fish by Avenant-Oldewage (2001).The objectives of this paper were to specifically identify and classify the nematode parasites collected from C. gariepinus based on their morphological features, and to note their prevalence and mean intensity in the Rietvlei Dam.

Study area
The Rietvlei Nature Reserve (25°41'22" S; 26°37'48" E) lies between Pretoria and Johannesburg in Gauteng Province (Fig. 1).Developed out of the Rietvlei Water Scheme, it is solely responsible for conservation of the Sesmylspruit catchment area, and the Rietvlei Dam (25°32'30" S; 28°16'46" E) currently supplies 27 % of Pretoria's water requirements (Wessels 1998).A smaller dam, the Marais Dam, lies approximately 4 km upstream, and the two are separated by a wetland (Fig. 1).Further upstream, just before the Sesmylspruit enters the reserve, the stream receives effluent from a number of industries and a wastewater treatment plant.

Field collection of fish and parasites and identification
Fish were collected in May 2003 from the Rietvlei Dam using large mesh gill nets.The fish were dissected and the mesenteric cavity examined for par-asites.The gastrointestinal tract was then dissected from the rectum to the oesophagus and all nematodes encountered were carefully detached from the stomach or intestinal mucosa.The internal organs of each fish were also examined for parasites or cysts.The nematodes were fixed in glacial acetic acid and preserved in 70 % ethyl alcohol.Some larval nematodes were stained with Horen's trichome stain according to the method of Khalil (1991).Specimens for scanning electron microscopy (SEM) examination were preserved in absolute alcohol after which they were processed for SEM as detailed by Barson & Marshall (2004).
The parasites were identified based on their observed morphology as well as from drawings.Light micrographs were taken with a Zeiss Axioplan micro- scope, and scanning electron micrographs with a JEOL 6100 scanning microscope.Drawings and measurements were done with a Zeiss 25 standard light microscope equipped with a drawing tube.The descriptions by Yamaguti (1961), Chabaud (1974), Hartwich (1974) and Moravec (1975) were used to aid in the identification of the parasites.Parasite prevalence and mean intensities were measured and calculated as defined by Mar golis, Esch, Holmes, Kuris & Schad (1982).
Voucher specimens were deposited in the zoological collection of the University of Johannesburg (formerly Rand Afrikaans University), South Africa.

RESULTS AND DISCUSSION
Two nematode genera were found in C. gariepinus from the Rietvlei Dam, namely Procamallanus laeviconchus (Wedl, 1862) and larvae of Contracaecum spp.
Procamallanus laeviconchus (Wedl, 1862) (Fig. 2 and 3) This is a small ovoviviparous nematode that is prevalent in most African freshwater fishes, notably in sil uroids (Khalil 1970;Moravec 1975;Mashego 1977;Mashego & Saayman 1981;Boomker 1982Boomker , 1994a,  1992;Khalil & Polling 1997).One out of seven hosts was infected (prevalence 14 %) with an intensity of 13 nematodes.Only female specimens were obtained and described, most of the characteristics matching those described by Moravec (1975) and Boomker (1982).Diagnostic measurements show that the Rietvlei Dam specimens have the same size range as those described by Boomker (1982) although they were somewhat larger with respect to the distance of anus to tail and length of the glandular portion of oesophagus (Table 1).
This study additionally showed the three-dimension al aspect of the head of P. laeviconchus with the SEM (Fig. 3F), the buccal capsule of which still fits the  Moravec (1975).Microscopical observation showed three terminal tail processes (Fig 2D), but these were, however, not observed on the SEM photograph of the posterior end of the worm (Fig. 3G); the specific specimen was probably still in its fourth larval stage.Eggs and motile larvae at various stages of development were observed, some eggs having been squeezed out through the genital opening (Fig. 3B and C).Measurements of L3 larvae are reflected in Table 2 Procamallanus laeviconchus is always found deeply attached to the mucosa of pyloric region of the host's stomach wall and has been shown to cause severe pathological effects (Paperna 1996).Apart from the finding of Mashego & Saayman (1981) who recorded a total of 23 worms in one fish, the intensity of 13 worms in one fish from the Rietvlei Dam is considerably high when compared to the low numbers recorded by Boomker (1982Boomker ( , 1994a) ) from C. gariepinus.Moravec (1975) states that P. laeviconchus infection is widespread in many African fish families.

Contracaecum
Contracaecum larvae are difficult to differentiate into species except when using molecular analysis or alternatively infecting experimental hosts to obtain adult worms.Adult Contracaecum species from fish-eating birds have only been studied and recorded by Ortlepp (1938, cited by Mokgalong 1996), Saayman et al. (1991) and Mokgalong (1996) in South Africa.Canaris & Gardner (1967) listed nine adult Contracaecum species from African waterbirds, while Barson & Marshall (2004) recorded four species from Zimbabwean birds.
While a high prevalence (86 %) and a mean intensity of 16.3 (intensity range 3-44) were recorded from Rietvlei Dam, 100 % infection levels are very common, with intensities as high as 700-2 000 worms per fish (Mashego & Saayman 1981;Boom ker 1982Boom ker , 1994a)).This makes Contracaecum one of the most prevalent fish parasites in South Africa and the fact that its life cycle involves migratory bird species (e.g.cormorants) can justify this observation.Pa perna (1996) urges aquaculturists to control aquatic birds on fishponds as an effective means of reducing Contracaecum infection.

FIG. 1
FIG.1The location of the Rietvlei Dam inside the Rietvlei Nature Reserve.X indicates point of sampling