Aeromonas spp. and Traveler’s Diarrhea: Clinical Features and Antimicrobial Resistance

Traveler’s diarrhea is the most common health problem of international travelers. We determined the prevalence of Aeromonas spp. associated with traveler’s diarrhea and analyzed the geographic distribution, clinical features, and antimicrobial susceptibility. Aeromonas spp. were isolated as a cause of traveler’s diarrhea in 18 (2%) of 863 patients. A. veronii biotype sobria was isolated in nine patients, A. caviae in seven patients, and A. jandai and A. hydrophila in one patient each. Aeromonas spp. were isolated with a similar prevalence in Africa, Latin America, and Asia. Watery and persistent diarrhea, fever, and abdominal cramps were common complaints. All strains were resistant to ampicillin; showed variable resistance to chloramphenicol, tetracycline, and cotrimoxazole; and were susceptible to cefotaxime, ciprofloxacin, and nalidixic acid. The persistence of symptoms made antimicrobial treatment necessary.

The genus Aeromonas has high diversity: at least 16 DNA hybridization groups are recognized (3). Among these genospecies, A. hydrophila, A. caviae, and A. veronii biotype sobria are considered of clinical significance (4,5). The spectrum of infectious diseases caused by Aeromonas species includes gastrointestinal infections as well as extraintestinal infections such as cellulitis, wound infections, septicemia, urinary tract infections, and hepatobiliary and ear infections, among others (6). Although healthy carriers of Aeromonas spp. have been described, several case-control studies have shown that these bacteria cause diarrhea (6). With the incorporation of genotypic techniques, identification of Aeromonas to species level has improved (7). The main objective of this study was to determine the prevalence of Aeromonas spp. as a cause of traveler's diarrhea and to analyze the species' geographic distribution, clinical features, and susceptibility to antimicrobial agents.

Patients
A total of 863 patients with traveler's diarrhea were recruited from the Tropical Medicine Unit of the Hospital Clinic of Barcelona, Spain, during the period January 1999-December 2001. All patients completed an epidemiologic questionnaire; their clinical history was taken, and a physical examination was performed. Traveler's diarrhea was defined as the occurrence of three or more episodes of watery stool within a 24-hour period, with or without other symptoms, or the occurrence of unformed stools accompanied by one of the following: vomiting, nausea, abdominal cramps, fever, chills, prostration, or tenesmus. Persistent diarrhea was defined as that of >14 days' duration.

Microbiologic Tests
A stool sample was collected, sent to the Laboratory of Clinical Microbiology, and processed for bacterial, viral, and parasitologic studies. To isolate Aeromonas spp., blood agar supplemented with ampicillin and a selective media, CIN (cefsulodin-irgasan-novobiocin) agar, were used. After incubation at 37°C for 24-48 hours, an oxidase test was performed on the colonies compatible with bacilli. Biochemical criteria were used to identify Aeromonas. Identification of the species was performed by 16S rDNA-restriction fragment length polymorphism as previously described (7,8). The biotype of A. veronii strains was identified on the basis of a positive reaction to arginine dihydrolase and negative response to bilis-esculin hydrolysis and production of ornithine decarboxylase (9).

Distribution and Geographic Origin of Species Causing Traveler's Diarrhea
Aeromonas spp. were isolated as a cause of traveler's diarrhea in 18 (2%) of 863 patients. A. veronii biotype sobria was isolated in nine patients, A. caviae in seven patients, and A. jandaei and A. hydrophila in one patient each (Table 1). In three of these patients, another enteropathogen was also found: in one patient who had traveled to Mexico, Shigella sonnei was isolated together with A. veronii; in another patient traveling to India, Giardia lamblia was detected together with A. veronii; in the third patient, who had traveled to Thailand, Salmonella Typhimurium was found with A. veronii. The frequency of Aeromonas spp. as a cause of traveler's diarrhea was similar in patients returning from Africa (1.7%), Latin America (1.8%), and Asia (2.3%) ( Table 1).

Clinical Features
The signs and symptoms of Aeromonas enteritis in these 18 patients are summarized in Table 2. Sixteen of the 18 patients had watery diarrhea; these were the cases associated with A. veronii biotype sobria and A. caviae. The patients with enteritis caused by A. hydrophila and A. jandaei had loose stools. Fifty percent of the patients had fever and abdominal cramps, whereas nausea and vomiting were uncommon complaints. Gross blood was observed in the stools of one patient, but this could be attributed to the S. sonnei isolated in the same stool. In 9 of the 18 patients, diarrhea was persistent.

Antimicrobial Susceptibility
The antimicrobial susceptibility of Aeromonas spp. isolates causing traveler's diarrhea is shown in Table 3. All strains were resistant to ampicillin but susceptible to cefotaxime, ciprofloxacin, and nalidixic acid. The susceptibility to chloramphenicol, tetracycline, and trimethoprim-sulfamethoxazole varied. Some 66.6% of A. veronii biotype sobria strains and 71.4% of A. caviae strains were susceptible to chloramphenicol; 55.6% of A. veronii biotype sobria strains and 71.4% of the A. caviae strains were susceptible to tetracycline; and 77.8% of A. veronii biotype sobria and 100% of A. caviae were susceptible to trimethoprim-sulfamethoxazole.

Treatment
While travelling, two patients received treatment, amoxicillin in one case and amoxicillin plus clavulanic acid in the other. Patients with persistent diarrhea were treated with the following antibiotics: norfloxacin (one patient), ciprofloxacin (six patients), and trimethoprimsulfamethoxazole (two patients); all recovered.

Discussion
In this study we describe the prevalence of different types of Aeromonas species associated with traveler's diarrhea in a cohort of travelers to a variety of tropical and subtropical countries. In contrast, other published studies have often been selective in terms of the types of travelers, geographic areas visited, or attempts to isolate specific microorganisms with the aim of testing antibiotic efficiency. In our study, Aeromonas spp. were isolated in 18 (2%) of 863 patients with traveler's diarrhea. A. veronii biotype sobria and A. caviae were the most frequently isolated species. These findings agree with the results of Hänninen et al. (11), who reported that these were the most common Aeromonas spp. associated with traveler's diarrhea in tourists traveling to Morocco. Likewise, Yamada et al. (12) found that A. veronii biotype sobria was the Aeromonas species most frequently implicated as a cause of traveler's diarrhea in Japanese travelers returning from unindustrialized countries. In our study, the geographic distribution of Aeromonas species did not favor any predominant area: species were isolated with a similar prevalence in Africa, Latin America, and Asia. However, all four species (A. veronii biotype sobria, A. caviae, A. jandaei, and A. hydrophila) were isolated from patients returning from India. In India, Aeromonas spp. has been identified as an enteric pathogen in 1.8% of patients with diarrhea (13). In a recent study performed in Dhaka (Bangladesh), Aeromonas spp. were significantly associated with diarrhea, similar to occurrences in other countries (14)(15)(16)(17). In our study, 3 (16.7%) of the Aeromonas isolates were detected together with other enteropathogens. This situation allowed us to consider that the symptoms we observed in the patients with traveler's diarrhea associated with Aeromonas spp. were due to the presence of this Aeromonas organisms. In our study, watery stools, fever, and abdominal cramps were the most common symptoms, which is consistent with other reports (11,18). Albert et al. (18) suggested that isolates of Aeromonas spp. positive for both the alt and ast genes, which encode enterotoxins, were associated with watery diarrhea but that isolates pos-itive only for the alt gene were associated with loose stools.
Fifty percent of the patients with Aeromonas spp. enteritis had persistent diarrhea. Chronic diarrhea lasting more than 1 year caused by A. caviae has been reported (2). A direct link between drinking water and food contaminated with Aeromonas spp. and gastrointestinal disease has been demonstrated (19).
Patients with prolonged enteritis required treatment. A quinolone was the drug of choice, although increased occurrence of quinolone-resistant Aeromonas spp. strains has been reported in industrialized countries (20,21). Regarding the ß-lactam antibiotics, Aeromonas spp. strains analyzed in this study were, as expected, uniformly resistant to ampicillin, whereas third-generation cephalosporins, such as cefotaxime, showed good activity. These results are in accordance with those reported by other authors, showing that third-generation cephalosporins are active against Aeromonas spp. (2,22). The percentage of strains with resistance to chloramphenicol, tetracycline, or trimethoprim-sulfamethoxazole ranged from 22.9% to 45%. These levels of resistance are likely related to the extensive use of these antimicrobial agents in unindustrialized countries.
In summary, A. veronii biotype sobria and A. caviae are the Aeromonas species most frequently associated with traveler's diarrhea; watery diarrhea, fever, and abdominal cramps are the predominant clinical features. The persistence of symptoms makes the use of antimicrobial treatment necessary. In this patient, a Shigella sonnei strain was also isolated.