Parasite Index
5.1 Infestation statistics
5.3.1 Digenea
Digeneans, commonly known as flukes, are members of the phylum Platyhelminthes (class Trematoda and subsclass Digenea) (Brusca and Brusca 2003). They are among the most common and abundant parasitic worms, second only to parasitic nematodes in their distribution (Roberts and Janovy 2000). Khalil and Polling (1997) listed 55 genera from 26 families of trematodes, occurring in freshwater fish in Africa. Digenetic trematodes (digeneans) are relatively host specific, heteroxenous flatworms which require typically a snail as the first intermediate host (Paperna 1996). Adult digeneans usually have oval, dorso- ventrally flattened bodies with a smooth, spiny or corrugated surface. A sucker is usually present at the antero-ventral mouth and a second sucker (acetabulum) is usually found mid-ventrally, both used for locomotion and attachment. The digestive system consists of a mouth opening, pharynx, short oesophagus and two blind caeca. Most adult trematodes contain both male and female reproductive organs.
Digeneans have a heteroxenic life cycle and can infect fish either in the adult form or as the metacercaria larvae (Bartoli and Boudouresque 2007). Metacercarial infections in fish have been recorded during various freshwater studies in Africa (Ortlepp 1935; Paperna 1964; Lombard 1968; Paperna and Thurston 1968; Khalil 1969; Prudhoe and Hussey 1977; Mashego 1982; Britz et al. 1984a, b; Van As and Basson 1984; Britz et al. 1985; Mashego and Saayman 1989; Paperna 1996;
Luus-Powell 2004). Piscivorous birds, crocodiles, water monitor lizards and occasionally humans are the definitive hosts of the metacercariae found in fish.
Most of the metacercariae form cysts in the flesh, on the gills and under the skin of fish, while others occur freely in the gall bladder, swimbladder, brain and the eyes.
At low levels of infestation the host suffers no ill effects. However when infestation levels reach high levels, the fish may become morbid and loose condition.
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Furthermore, according to Bartoli and Boudouresque (2007) the more the digenean load increases, the more the movement of the fish are disturbed which increases the probability of its capture by the final host. In addition, species of the genus Diplostomum can be significant pathogens causing a range of disease symptoms (i.e. exophthalmia, local hemorrhage, lens cataract and growth reduction) which may lead to fish mortality (Chappell et al. 1994; Dorucu and Yspir 2001).
Table 5.5: Total number of digeneans recorded at the four sampling sites
0 50 100 150 200 250 300
Abundance
Site A Site B Site C Site D
Digeneans
Winter Spring Summer Autumn
Figure 5.7: Seasonal variation in abundance of digeneans at the four sampling sites
Surveys Site A Site B Site C Site D Winter 330 15 2136 469
Spring 741 109 881 133
Summer 525 95 2797 195
Autumn 641 53 2109 61
Mean 559.25 68 1980.75 214.5
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Various types of diplostomid metacercariae were recorded from different organs such as eyes, gills, brain and swimbladder and one type of clinostomid metacercaria from the branchial region of O. mossambicus during this study (Addendum C, Tables 1 – 4 and Figures 5.2B, C, D). All the digeneans were however grouped and is referred to as Digenea larvae as no adult digeneans were recorded during this study. The larval digenetic trematodes were identified to genus level only for most species, as no reproductive organs were noted in larval forms making it difficult to identify them to species level.
Neutraclinostomum sp. metacercariae (Figure 5.2E) were recorded from the branchial region of fish at sites A, C and D. No Neutraclinostomum sp. was recorded from site B indicating the possible absence of the specific snail intermediate host or that the water quality at this site might have an effect on the cercariae shed by the snails, but this aspect requires further investigation.
Clinostomid infestations, commonly referred to as yellow grub, are not pathogenic to the host although mass mortality of fish at a fish station in South Africa was attributed to these parasites (Britz et al. 1985). Furthermore, the large conspicuous cysts will make the fish unattractive to human consumers, especially when large numbers are present in the muscle of fish. Clinostomids can utilize mammalians as definitive hosts and infections have been reported from domestic animals such as cats and also humans (although not from southern Africa) (Britz et al. 1985). Raw or undercooked fish as well as partially sun-dried fish may thus be a potential health hazard to human consumers.
As mentioned above, various types of diplostomid metacercariae were recorded from different organs. Diplostomum tregenna larvae were recorded from the brain (Figure 5.2B), sometimes in very large numbers (up to 125). The digenea metacercariae from the skin and fins were encysted between the integument and body musculature as well as on the fins of the fish (Figure 5.2A). The cyst is usually black in colour (formed by the parasites). According to Mashego and Saayman (1989) the black colour is due to dentritic pigmentation on the outer cyst
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wall and is responsible for the clearly visible black spot on the fish and is thus commonly referred to as ‘Black spot’.
Very high numbers of digenean larvae were recorded at site C, followed by site A and then site D, while lower numbers were recorded from site B (Table 5.5). The difference in abundance of larval digeneans between the two sites with poorer water quality (sites B and C) is difficult to explain. The lower numbers at site B may be due to possible lower numbers of the snail intermediate host at this site or the possible influence of certain water constituents on the cercariae shed by the snails.
The abundance of digenea larvae was however much higher at site C compared to the other sites (Figure 5.7). It seems that the poorer water quality at the latter site had no effect on the snail intermediate host or the transmission of digenea cercariae to the fish intermediate host. The definitive hosts (piscivorous birds) were present in large numbers at all the sites.