zoonoses and communicable diseases common to ... - PAHO/WHO

AEROMONIASIS 7shown that the clinical strains are very diverse and that no single clone is responsiblefor most of the infections (Von Graevenitz and Altwegg, 1991).Over the last decade, researchers have tried to define the virulence factors of thisgenus, both in terms of structural characteristics and the extracellular products theysecrete. Considered important among the structural characteristics is a type of pilus,the “flexible” or curvilinear pilus. It is expressed when stimulated by certain environmentalconditions that give the bacteria the ability to colonize. Another structuralcharacteristic that was first discovered in autoagglutinating strains of A. salmonicidais the S layer, which is outside the cell wall. The loss of this layer—which can beseen with an electron microscope—decreases pathogenicity for fish 1,000 to 10,000times. A similar layer was later discovered in certain strains of A. hydrophila and A.sobria in infected fish and mammals, but their functional role seems to differ substantiallyfrom the same S layer in A. salmonicida (it does not make the surface ofthe bacteria hydrophobic).The substances externally secreted by aeromonas include beta-hemolysin that isproduced by certain strains of A. hydrophila and A. sobria. It has been determinedthat this hemolysin has enterotoxigenic effects on lactating mice and ligated ilealloops of rabbits. Purified beta-hemolysin inoculated intravenously into mice is lethalat a dose of 0.06 µg. The cytotonic enterotoxin that causes an accumulation of fluidin the ligated ileal loop of the rabbit, as well as other effects, has also beendescribed. Between 5% and 20% of the strains produce a toxin that cross reacts withthe cholera toxin in the ELISA test (Janda, 1991).Based on tests conducted in mice and fish (the latter are much more susceptible),it can be concluded that A. hydrophila and A. sobria are more virulent than A.caviae. In addition, there is a great difference in the virulence of the strains withineach species (Janda, 1991). These variations cannot be attributed to a single virulencefactor. In addition, it was not possible to detect a common mechanism in thepathogenic capacity of Aeromonas spp. in humans or in animals.An enzyme (acetylcholinesterase) isolated from fish infected by A. hydrophilaproved to be highly active against the central nervous system. The toxin was lethalfor fish at a dose of 0.05 µg/g of bodyweight; no lesions were observed in the tissues.The same toxin was obtained from six different strains (Nieto et al., 1991).A comparison was made of 11 environmental strains and 9 human strains. All theenvironmental strains and four of the human strains proved to be pathogenic fortrout, at a dose of 3 x 10 7 colony forming units (CFU). Only the human strainscaused death or lesions through intramuscular inoculation of mice. The virulentstrains produced more hemolysis and cytotoxins in cultures at 37°C than at 28°C(Mateos et al., 1993).Geographic Distribution: The motile aeromonas appear worldwide. Their principalreservoir is in river and estuary waters, as well as in salt water where it meetsfresh water. Population density is lower in highly saline waters and waters with limiteddissolved oxygen. It has sometimes been possible to isolate Aeromonas fromchlorinated water, including muncipal water supplies. These bacteria are more prolificin summer than in winter (Stelma, 1989).Occurrence in Man: Aeromoniasis generally occurs sporadically. There is noevidence that water or foods contaminated by Aeromonas spp. have been the sourceof outbreaks (as happens with other agents, such as enterobacteria). The only cases