296 Organic waste recycling: technology and management
carried in and on fish (Jansen 1970), may be carried passively, infecting only the
gut surface, which can be cleaned by depuration. Studies of human pathogens in
salmonides reared in Arcata wastewater fish ponds, in California, showed
bacteria causing human infections only in the gut (Allen et al. 1976). Results
from kidney, liver and spleen samples indicated that none of the potential
pathogens were present in these fish organs under the fish culture conditions in
Arcata.
Cloete et al. (1983) reported that the skin, gills and intestines of the waste-
grown fish from a pond system treating cattle feedlot effluents contained large
numbers of bacteria, including potential pathogens. However, similar bacterial
numbers, including potential pathogens, were also associated with the skins,
gills and intestines of naturally grown fish. This suggests that the health risk
involved in the consumption of waste-grown fish might not be substantially
different to that of natural fish populations. In both cases, the tissues and blood
appeared to be sterile, which would contribute to a much reduced health risk.
From the above information, it appears that bacteria are normally present in
fish intestines. However, if bacteria are present in very high concentrations in
the fish pond water or the fish body, the natural immunological barrier of fish
could be overcome and the bacteria could invade the fish meat. Buras et al.
(1985) defined a "threshold concentration" as the number of bacteria which,
when inoculated into the fish, causes their appearance in the fish meat. From
their experiments with Tilapia and carp, a list of threshold concentrations is
given in Table 6.6. Depuration experiments were found to be effective when the
fish did not contain high concentrations of bacteria in their meat. With respect to
the microbiological quality of fish pond water, Buras et al. (1987)
experimentally found the 'critical concentration' of standard plate count bacteria
to be 5x10^4 no./mL, in which bacterial concentrations higher than this critical
concentration were found to cause their appearance in the meat of the fish reared
in the ponds.
Hejkalet al. (1983) investigated the levels of bacteria and viruses in fish
reared in experimental wastewater-fish ponds in Arkansas, U.S.A. They found
that even when levels of bacteria exceeded 10^5 /100g in the fish guts, very little
penetrated into the fish muscle tissue; the maximum of 25 fecal streptococci/100
g was found in the fish meat. They finally suggested that while the fish do not
accumulate bacteria in the muscle tissue, contamination of the muscle tissue
during processing is difficult to avoid. Buras et al. (1985) proposed that to
prevent serious public health problems, the threshold values in Table 6.6 should
be considered satisfactory criteria for the design and management of fish ponds
in which wastewater is used. In addition, since the experiments performed with
Polio 1 LSc viruses suggested a very low threshold concentration, and since