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Abstract:
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In the present study, we investigated the
involvement of Aeromonas spp. in eliciting disease
outbreaks in freshwater ornamental fishes across the
state of Kerala, India. We investigated three incidences
of disease, in which the moribund fishes
exhibited clinical signs such as haemorrhagic septicemia
(in gouramy, Trichogaster sp.), dropsy (in Oscar,
Astronotus ocellatus) and tail rot/fin rot (in gold fish,
Carassius carassius). Pure cultures (n = 20 from each
fish; 60 in total) of Aeromonas spp. were recovered
from the abdominal fluid as well as from internal
organs of affected fishes, although they could not be
identified to species level because of the variations in
their phenotypic characters. The molecular fingerprinting
of the isolates using Enterobacterial Repetitive
Intergenic Consensus PCR proved the genetic
diversity of the isolates from the three sites. The
phylogenetic trees constructed using concatenated
sequences (using 16S rRNA, gyrA, gyrB and rpoD
genes) indicated that they were related to Aeromonas
veronii. They exhibited marked cytotoxic and haemolytic
activity, which were responsible for the pathogenic
potential of the isolates. The isolates possessed
multiple virulence genes such as enterotoxins (act and
alt), haemolytic toxins (aerA and hlyA), genes
involved in type III secretion system (ascV, aexT
and ascF–ascG), glycerophospholipid-cholesterol
acyltransferase (gcat) and a type IV pilus (tapA) gene,
as determined by PCR. Virulence of representative
isolates to goldfish was also tested, and we found LD50
values of 104.07–105.35 cfu/fish. Furthermore, the
organisms could be recovered as pure cultures from
the lesions as well as from the internal organs. |