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Articles » Bad Bug Book » Aeromonas hydrophila

Aeromonas hydrophila

1. Name of the Organism: Aeromonas hydrophila, Aeromonas caviae, Aeromonas sobria & (Aeromonas veronii?) Aeromonas hydrophila is a species of bacterium that is present in all freshwater environments and in brackish water. Some strains of A. hydrophila are capable of causing illness in fish and amphibians as well as in humans who may acquire infections through open wounds or by ingestion of a sufficient number of the organisms in food or water.

Not as much is known about the other Aeromonas spp., but they too are aquatic microorganisms and have been implicated in human disease.

2. Nature of Acute Disease: A. hydrophila may cause gastroenteritis in healthy individuals or septicemia in individuals with impaired immune systems or various malignancies.

A. caviae and A. sobria also may cause enteritis in anyone or septicemia in immunocompromised persons or those with malignancies.

3. Nature of Disease: At the present time, there is controversy as to whether A. hydrophila is a cause of human gastroenteritis. Although the organism possesses several attributes which could make it pathogenic for humans, volunteer human feeding studies, even with enormous numbers of cells (i.e. 10^11), have failed to elicit human illness. Its presence in the stools of individuals with diarrhea, in the absence of other known enteric pathogens, suggests that it has some role in disease.

Likewise, A. caviae and A. sobria are considered by many as "putative pathogens," associated with diarrheal disease, but as of yet they are unproven causative agents.

Two distinct types of gastroenteritis have been associated with A. hydrophila: a cholera-like illness with a watery (rice and water) diarrhea and a dysenteric illness characterized by loose stools containing blood and mucus. The infectious dose of this organism is unknown, but SCUBA divers who have ingested small amounts of water have become ill, and A. hydrophila has isolated from their stools.

A general infection in which the organisms spread throughout the body has been observed in individuals with underlying illness (septicemia).

4. Diagnosis of Human Illness: A. hydrophila can be cultured from stools or from blood by plating the organisms on an agar medium containing sheep blood and the antibiotic ampicillin. Ampicillin prevents the growth of most competing microorganisms. The species identification is confirmed by a series of biochemical tests. The ability of the organism to produce the enterotoxins believed to cause the gastrointestinal symptoms can be confirmed by tissue culture assays.

5. Associated Foods: A. hydrophila has frequently been found in fish and shellfish. It has also been found in market samples of red meats (beef, pork, lamb) and poultry. Since little is known about the virulence mechanisms of A. hydrophila, it is presumed that not all strains are pathogenic, given the ubiquity of the organism.

6. Relative Frequency of Disease: The relative frequency of A. hydrophila disease in the U.S. is unknown since efforts to ascertain its true incidence have only recently been attempted. Most cases have been sporadic rather than associated with large outbreaks, but increased reports have been noted from several clinical centers.

7. Course of Disease and Complications: On rare occasions the dysentery-like syndrome is severe and may last for several weeks.

A. hydrophila may spread throughout the body and cause a general infection in persons with impaired immune systems. Those at risk are individuals suffering from leukemia, carcinoma, and cirrhosis and those treated with immunosuppressive drugs or who are undergoing cancer chemotherapy.

8. Target Populations: All people are believed to be susceptible to gastroenteritis, although it is most frequently observed in very young children. People with impaired immune systems or underlying malignancy are susceptible to the more severe infections.

9. Food Analysis: A. hydrophila can be recovered from most foods by direct plating onto a solid medium containing starch as the sole carbohydrate source and ampicillin to retard the growth of most competing microorganisms.

10. Selected Outbreaks: Literature references can be found at the links below.

Most cases have been sporadic, rather than associated with large outbreaks. MMWR 39(20):1990

Aeromonas species are associated with gastroenteritis and with wound infections, particularly wounds incurred in outdoor settings. On May 1, 1988, isolates of Aeromonas became reportable in California, the first state to mandate reporting of isolates of and infections with these organisms. From May 1, 1988, through April 30, 1989, clinicians and clinical laboratories in California reported 225 Aeromonas isolates from 219 patients. Cases were reported on Confidential Morbidity Report cards to local health departments, which then conducted case investigations and forwarded their reports to the California Department of Health Services. Morbidity and Mortality Weekly Reports For more information on recent outbreaks see the CDC.

11. Education and Background Resources: Loci index for genome Aeromonas hydrophila Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.

12. Molecular Structural Data: None currently available.

Source: U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, U.S. Department of Health and Human Services: Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins Handbook

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