Aeromonas Hydrophila: Public Health Risk and Critical Analysis
THE ISSUE
Aeromonas species has been isolated from fresh water,
sewage, and wastewater
and from a variety of food products including vegetables, a variety
of seafood,
and drinking water. Since contact with human hosts is potentially
common,
its ability to become a human pathogen is dependent on a result of
complex
interactions between environmental, bacterial, and host factors.
Aeromonas
hydrophila is included on the U.S. EPA's Candidate Contaminant List.
A crit-
ical analysis of important factors for Aeromonas hydrophila include:
methods
of water analysis; documentation of actual waterborne disease; determination
of clones with virulence factors; regrowth in water; and presence
in source
waters.
RESEARCH STRATEGY
A public health risk and critical analysis was conducted by Arthur
Chuape,
M.D., and Stephen C. Edberg, PH.D., A.B.M.M. (Yale University School
of
Medicine).
Infection
Gastrointestinal infection due to contact with water has been reported
to occur
in both the immunocompetent and immunosuppressed patient. In
general,
there are few data available regarding the frequency of these
Aeromonas infec-
tions. Aeromonas hydrophila was first proposed as a potential
cause of enteric
disease as early as 1937 after Miles and Hainan isolated it from
human feces.
Ever since then, its enteropathogenic role in human disease has
been contro-
versial, with some considering these species as transient, nonpathogenic
flora
of the gut. For example, Morgan et al. induced diarrhea in only
2 of 57
healthy human volunteers after known Aeromonas strains with virulence
fac-
tors such as cytotoxin, enterotoxin, and hemolysin were administered
to them.
Other observations such as similar fecal carriage rates in symptomatic
and
asymptomatic subjects and the lack of an animal model have led
some to
doubt its role in causing enteric disease.
Since the 1980's there have been some clinical studies also supporting
that at
least some strains of Aeromonas spp. are intestinal pathogens.
Immunologic
responses in patients with Aeromonas infections, for example,
may provide
evidence that Aeromonas is an intestinal pathogen. Jiang and
collaborators
have demonstrated a 4-fold rise in intestinal sIgA response in 11
of 12 symp-
tomatic adults infected with either A. sobria or A. hydrophila.
In another study
by Kuijper, 5 of 11 patients infected with a cytotoxin-producing
strain of
Aeromonas spp. developed acute severe diarrhea (3 patients
required hospital-
ization because of their symptoms) and had a positive cytotoxin-neutralizing
assay. All 5 patients were over 60 years of age. This compared
to 3 of 50
healthy patients who had low serum titers of cytotoxin-neutralizing
antibod-
ies. Two other case reports have similarly shown a rise in
neutralizing anti-
body to Aeromonas toxins in patients with cholera-like symptoms.
Despite the above findings, significant serum antibody responses
to
Aeromonas have not been demonstrated in symptomatic patients
shedding
Aeromonas species in the same and other studies. However,
this has been
attributed to the frequent occurrence of an intestinal, but
not serum immune
response, to infection by pathogens infecting the intestinal
tract. Other stud-
ies have demonstrated isolation of Aeromonas spp. either
as a sole isolate (or
in combination with other intestinal pathogens) in symptomatic
patients, par-
ticularly in young children. An Australian prospective, case-control
study of
1156 children found enterotoxigenic Aeromonas spp. isolated
from 10.20/0 of
children with diarrhea compared to 0.6 of controls. Aeromonas
was the sole
pathogen recovered in 6.5 of these children. In another study
of Indian chil-
dren Aeromonas spp. was the sole isolate from 45 (1.8) of
2,480 subjects with
acute gastroenteritis. All of the strains, irrespective of
species, were entero-
toxin positive and hemolysin positive. Most of the patients
were under 5 years
of age. No bacterial enteric pathogens were isolated from
512 matched con-
trols.
Finally, a Nigerian study of children with and without diarrhea
found twelve
(1.9) of 616 symptomatic children had fecal carriage of Aeromonas
compared
to 9 (3.0) children with diarrhea. Although this finding was
not statistical-
ly significant, when analyzing isolates from children </= 5years
of age alone,
there was a significant difference in the rate of isolation of
Aeromonas spp.
between children with diarrhea (3.8) and asymptomatic children
(0). Other
studies have similarly revealed a greater incidence of Aeromonfl5-associated
diarrhea in younger children, ages 6 months to 5 years.
MAJOR FINDINGS AND SIGNIFICANCE
A critical review of Aeromonas hydrophila infections, and its potential
for reg-
ulation indicates:
-
There is not enough scientific evidence to demonstrate that A. hydrophila
is
a significant public health risk.
- The ID50 for virulent A. hydrophila in human feed studies shows
that an
enormous dose is required to cause infection.
- There is no method suitable for the routine monitoring of Aeromonas
hydrophila, particularly the "virulent" strains.
Chuape, A., and Edberg, S.C, Aeromonas hydrophila: Public Health
Risk and
Critical Analysis, to be published in Critical Reviews in
Microbiology, antici-
pated in 1999.
Order reprints from publisher.
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