provided environmental conditions are also improved. Both gill myxobac-
terial infection and dorsal fin-rot (also associated with myxobacteria) are
usually associated with low water temperatures and Avault et al. (1968)
stress the importance of maintaining overwintering temperatures of at least
14°C for this reason.
Gram-negative bacterial septicaemias are the commonest bacterial causes
of mortality in tilapia culture. Infection may arise from introduction of
infected fish into a system but more often it is a function of the environ-
mental conditions of that culture system.
Aeromonas hydrophila infection is the most common of the septicaemias
and usually manifests itself by causing affected fish to darken, lose appetite,
and cluster around exit screens. When examined they may have ulcers, or
more frequently, areas of hyperaemia at the base of the pectoral and pelvic
fins, and at the margins of the orbit (Haller 1974; Scott 1977). Internally the
liver is usually pale and there may be focal haemorrhages over the visceral
and peritoneal surfaces. Histologically there is haemopoietic necrosis and
focal necrosis in the liver, heart or skeletal muscles, with accumulations of
gram-negative bacteria ranged along strands of fibrin. The cellular inflam-
matory infiltrate is rarely marked but when it is present it generally comprises
macrophages which often contain ingested melanosomes. Generally the
condition is predisposed by handling trauma, poor nutrition, heavy parasitism
or excessive fertilizing of ponds but sometimes severe outbreaks may occur
without any obvious predisposing factor.
Occasionally pseudomonads, such as Proteus spp., p-haemolytic strep-
tococci (Wu 1970) or Edwardsiella tarda (Roberts unpublished) may be asso-
ciated with the condition. These are particularly common in ponds newly
fertilized with human or animal faeces. Coliforms and Salmonellae of human
health significance may also be ingested from such sources and there is there-
fore a risk of outbreaks of human gram-negative infections, although con-
sidering the extent to which night soil is used to fertilize fish ponds in Asia,
their significance as a source of human infection would seem unlikely to be
high.
Edwardsiella tarda infection provides a particularly intractable problem in
intensive systems. Although clinically and histologically this condition is a
typical bacterial septicaemia, losses are usually sporadic. It is extremely
difficult to eradicate from the system and over a period of time such infec-
tions can be responsible for the loss of significant numbers of fish. Food
medication with oxytetracycline or potentiated sulphonamides eliminates
losses during the feeding period but they often recur once treatment has
ceased. Ruthless culling of all poor growing, sluggish or darkened fish from
the system and maintenance of best possible water quality and feeding
standards are the most effective means of preventing losses.
Experimental infections with Aeromonas salmonicida (Almeida et al.
1968; Roberts unpublished) have shown that both the pigmented and the
achromogenic strains of this common pathogen of temperate fishes can cause