heterophyes as well as Haplorchis pumilio as common parasites of man and
dogs in Egypt and Israel.
iv) Diplostomulae. (Larval genus of Family Diplostomatidae) The meta-
cercariae of these strigeoids lodge in a variety of tissues within their second
intermediate fish host, depending on the particular Diplostomum species
involved. The most noticeable in clinical terms, however, are the metacer-
cariae which are associated with blindness: the so-called eye flukes. There is
a wide range of species of eye fluke, with a remarkable degree of site specific-
ity so that some species localize in the lens, others in the retina and yet
others in the aqueous or vitreous humour. Where only small numbers of
unencysted metacercariae are involved, the sight is not significantly affected,
but when large numbers of metacercariae locate in the eye they can produce
total blindness (Plate 1). This causes loss of reflex pigmentation control
and affected fish are thus more vulnerable to predation than would other-
wise be the case, as well as being unable to find their food by visual means.
c) Cestodes, Nematodes and Acanthocephalans. Cestodes have not as yet
been reported to cause any serious problems in cultured tilapias, although
their presence in wild fish has been reported (Fryer and Iles 1972). Several
nematode species have also been reported (Fryer and Iles 1972; Goldstein
1971) but apart from reports by Paperna (1964) and Scott (1977) of larval
Contracaecum infection causing pathological effects, little is known of their
significance. Scott found that in S. alcalicus (S. grahami) in Kenya up to 7
parasites might be found in the pericardial cavity (Plate 2) and that they had
a significant effect on the growth of the fish. Contmcaecurn is a particularly
unattractive parasite to the consumer as it can occur as large encysted worms
throughout the muscle. Like some other anisakid parasites, it can also be
zoonotic and it would seem important that it be excluded from cultured
fish. Its life cycle involves ingestion of the egg by a free living crustacean
which is ingested by the fish. When this then is eaten in turn by a piscivorous
bird the life cycle is completed, and it would seem that it is only at this stage
that the cycle can be readily broken. Since piscivorous birds are also respon-
sible for many digenean infections of fish and also cause direct losses of
cultured fish by predation, the netting or wiring of sites, tank coverings,
and an active predator elimination policy, would seem to be highly desirable.
Plate 1. Sarotherodon mossambicus with bilateral cataract caused by a heavy diplostornulid
metacercarial infection.