- METAZOAN HELMINTH PARASITES
a) Monogenea. Monogenean parasites are found on both the gills and
the skin of tilapias and a specific genus for the monogeneans of cichlid
fish, Cichlidogyrus, was erected by Paperna in 1960. Paperna and Thurston
(1968) found 16 different species from rift valley tilapias and Mohd-Shaharom
and Sommerville (unpublished) have performed extensive studies on the
incidence and pathogenesis of these monogeneans in cultured populations.
Cichlidogyrus sclerosus was found to be the most widely spread, being
commonly observed in fish from Southeast Asian origins such as Philippines
(Duncan 1973) and Hong Kong, Singapore and Thailand (Sommerville,
unpublished) as well as from Africa. Studies on the population dynamics and
growth rate effects of C. sclerosus and C. tubicirrus minutus indicate that
they are normally of low pathogenicity with little or no effect on growth
rate. Pathological effects were quite obvious, however, in the form of focal
hyperplasia of the branchial lamellae.
Gyrodactylus spp. are potentially of great significance to tilapias in
culture. Clinical outbreaks of gyrodactyliasis have been recorded by Fryer
and Iles (1972) in pond-reared tilapias in Uganda, where they were associated
with corneal damage and by Sommerville and Haller (unpublished) from
intensively cultured tilapias in Kenya. In the latter case mortalities appeared
to be associated with handling and moving of fish from ponds to tanks.
Mortalities always developed two weeks after transfer, tailing off after 5 to 6
weeks. This pattern was presumed to be related to the viviparous life cycle of
the gyrodactylids. The increase in transmission opportunity was associated
with the higher stocking density of a concrete tank system and the debility
of badly handled fish.
b) Digenea. A great variety of digenean parasites are potentially capable of
causing heavy losses in cultured tilapias. All, however, have a basically similar
life cycle which involves three hosts: the first intermediate host, an aquatic
or amphibious gastropod mollusc; the second intermediate host, a fish and
the final host, a piscivorous vertebrate. The stage occurring in the fish is
known as the metacercarial stage and is generally found encysted in the fish
tissues. Virtually any part of the body can be involved but the metacercwiae
of many species of digeneans are site specific. The molluscan hosts are partic-
ularly common in water supply dams, inlet channels and drainage canals,
where waste food and organic material accumulate. The fish may be infected
on the farm or, as is frequently the case, in ponds or dams where fry are
collected for stocking of rearing tanks. A wide range of metacercariae
is observed in tilapias and because of the difficulty of identifying the larval
form to species, they usually remain unspecified. Sommerville (unpub-
lished) has investigated a wide range of digenean infections in intensively
cultured tilapias and carried out experimental infections of final hosts
to determine species. The principal groups of significance were clinostomes,
neascids, haplorchids and diplostomulae and all were significant in different
ways. Although the following are currently the only named digenean meta-
cercariae to have been associated with disease in cultured tilapias, it seems
likely that many others will be shown to be of significance as controlled
intensive tilapia culture develops: