Diseases of TilapiasDr. Roberts summarized the present importance of diseases in the com-
mercial culture of tilapias. There are very few serious disease problems
and these are localized in nature, for example, infestation with Lernaea in a
few Southeast Asian and Indian locations. The myxobacterial diseases and
bacterial septicaemias are generally diseases of bad husbandry and can be
avoided but such disease problems will probably increase as tilapia culture
expands. Vaccines may be available in the future for septicaemias.
Although only one virus has been isolated. from tilapias at present and
produces no serious pathological effects, more viruses are bound to occur in
intensively cultured fish. There are also risks for the future with respect to
public health. The misuse of antibiotics in fish ponds receiving human wastes
could create antibiotic-resistant strains of human pathogens. Also, a parasite
such as Haplorchis could ruin a culture industry if it can be shown that
infection is caused by farmed fish.
The transportation of broodstock and fry has attendant risks of disease
transfer. If a large fry supply industry develops, then control measures will
have to be undertaken. These must be realistic, however, not like the extreme
quarantine measures recently applied to the introduction of coho salmon to
the U.K. for experimental work, which included complete confinement in
recirculation systems for long periods and sterilization of effluents. A
realistic series of measures is needed, including routine monitoring of brood-
stock health, disinfection of fry with formalin and malachite green both
before despatch and on receipt, isolation from natural waters and other
stocks on receipt (quarantine) and the destruction, preferably by burning, of
all packing materials. All inspection, certification and licensing should be
matters for official government scientists or other trained personnel. Stirling
University has been involved in training fish disease experts from Southeast
Asia and Africa and in liaison work with Thailand where the Government is
developing a routine disease diagnostic system for the catfish industry.
Dr. Pullin asked whether the possibility of shipping disinfected tilapia eggs
had been investigated as it was impossible to give whole fish a clean bill of
health. The shipment of disinfected eyed ova is routine in the salmonid
culture industry and greatly reduces the risks of transfer of all diseases apart
from intraovarian viruses. It was agreed that tilapia eggs can be incubated in
water provided that they are kept moving by slow aeration or agitation and
that transportation of disinfected eggs in self-contained units could be
viewed as a future method.
Dr. Lovshin drew attention to the difficulties of treating tilapia diseases,
particularly in the rural areas of developing countries where even basic
chemicals such as formalin and potassium permanganate are either unavailable
or very costly. Prof. Roberts stated that the best treatments in any disease
situation were usually improvement in water quality and reduction in
stocking density. It was agreed that formalin is the most useful general
chemical for disease treatment, but that salt can also prove useful against
ectoparasites. Details of treatment methods applicable to tilapia culture in
rural areas have still to be worked out.