an obligate parasite it can survive off the host for a limited period and since
it lays its eggs on vegetation, it is a serious problem in standing waters. It can
readily seek refuge around a pond if fish are removed for treatment. It can
cause mortalities in its own right but usually it is responsible for facilitating
secondary bacterial and fungal invasion by its feeding method, which causes
open wounds.
Ergasilus spp. have been reported to cause epizootics in polyculture
fish ponds in Israel (Sarig 1971) where carps, mullets, and tilapias are farmed
together. Although the tilapias do become infected in such conditions, their
parasite burdens are usually very low compared to those of the other two
groups of fish species, possibly reflecting the higher resistance to skin pene-
tration of the tilapias.
Bacterial DiseasesTilapias are generally kept in water with a high organic load. This may be
due either to deliberate fertilization to increase the production of food
organisms in the water or to agricultural or sewage run-off. Such water
conditions allow most of the recognized aquatic bacteria to occur at signi-
ficant levels and under intensive culture conditions especially heavy mortality
can result from infections with any of a wide range of recognized facultative
pathogens.
Three clinical syndromes are associated with bacterial diseases of tilapias:
a predominantly skin lesion syndrome associated with pathogenic myxo-
bacteria; haemorrhagic septicaemia; and chronic granulomatosis associated
with Mycobacterium tuberculosis infection.
Myxobacteria, or 'slime' bacteria, are aquatic bacteria commonly found
as commensals on fish skin. A few species are capable of becoming patho-
genic under particular circumstances of environmental stress. The most
common stressors inducing myxobacterial infections are high or low temper-
atures, but traumatic damage and low water quality, such as that caused by
excessive silt or high ammonia levels associated with filtration failure in
recirculation systems, can also induce the qualitative or quantitative changes
in mucus secretion from the skin or gills which appear to trigger infection.
Flexibacter columnaris is the commonest myxobacterial pathogen in
tilapia culture and is usually associated with high temperature stress. The
lesions are opaque white, raised and whorl-like, or may take the form of a
saddle-back of grey white epithelial necrosis over the dorsum of the back
with a red haemorrhagic rim. These develop rapidly to form craterifom
ulcers. The affected fish become very dark, slow moving and die quickly.
Myxobacterial infection of the gills, particularly of fry is usually associated
with heavy mucus production and the bacteria may be doing little more than
obstructing gaseous exchange since treatment with surfactants, whew
bacteriostatic or not, serves to remove both the mucus and the problem,