i) Clinostomes. (Family Clinostomatidae) The metacercariae of Clinos-
tomum and Euclinostomum spp. are notable particularly for their large
size-hence the common term "white grub" or "yellow grub" for the mature
metacercariae in situ. In fry the growth of such a large parasite causes
bulging and distortion of the body profile. As well as spoiling the appearance
of the fish there is some evidence that in intensive culture, fish infected with
clinostomes are also more susceptible to handling stresses (Balarin pers.
comm.). The adult worms lie in the pharynx of a piscivorous bird and Ardea
goliath has been recorded as being a particularly prevalent carrier of the
infection.
ii) Neascus metacercariae. (Larval genus of Superfamily Strigeoidea, all
belong to Family Diplostomatidae) A group of neascus metacercariae stim-
ulate the accumulation of large numbers of melanocytes in the host capsule.
Since they usually accumulate in the skin, the resulting black spots become
very obvious even to the casual observer. However, although they have been
reported from a wide range of tilapias from east and west Africa (Paperna
and Thurston 1968; Sommerville unpublished;) they probably have only a
limited effect on the fish host per se and their real significance is that they
render fish unmarketable where infection is heavy.
iii) Haplorchis sp. (Family Heterophy idae) Metarcercariae of this genus
are found in a wide range of freshwater food fishes including tilapias. They
have a wide geographic distribution and have already been a cause for con-
cern in the Philippines, Israel, Africa, India, Egypt, China and Japan (Som-
merville in press).
Haplorchis pumilio was first recorded from tilapias by Witenberg in
1929 in Palestine. Like other species of this genus it has a wide distribution,
summarized by Sommerville (in press) who has also recently found it in an
intensive S. spilurus culture system in Kenya. Its cercariae were found in a
high proportion of Melanoides tuberculata: the dominant mollusc in the
tanks, drains, and ponds of the farm system. Experimental infections of
tilapia fry showed that mortalities could result directly from penetration of
the skin and muscle of fry by large numbers of cercariae and the migrations
of metacercariae to accumulate at fin bases. It is assumed that the mortalities
associated with mass penetration are related to loss of skin function, but
other organs may be damaged as the parasites penetrate over the entire skin
surface and migrate indiscriminately through tissues.
With less severe infections, experimental studies (Sommerville unpublished)
have shown that the infected fish suffer no disadvantage when compared
with uninfected fish as far as growth rate, condition or food conversion are
concerned. However, the presence of this parasite in cultured food fish
cannot be tolerated because of the risk it represents to human health when
inadequately cooked fish are eaten. The main pathological effect when man
is the final host is associated with the distribution of the eggs in the circula-
tory system, where they lodge in end-arteries, causing infarction. This can
cause a variety of syndromes but the most significant are related to obstruc-
tion of myocardial, cranial or optic vessels. Since it is non-specific in its final
host, piscivorous vertebrates other than birds, e.g., otters, dogs, cats, or even
monitor lizards, can act as final hosts. Other larval heterophyids have been
reported in tilapias in the Middle East, Paperna (1960) reporting Heterophyes
sean pound
(Sean Pound)
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