Plesiomonas shigelloides Infection, Ecuador, 2004–2008

Juan C. Escobar, Darlene Bhavnani, Gabriel Trueba, Karina Ponce, William Cevallos, and Joseph Eisenberg

Author affiliations: University of Michigan, Ann Arbor, Michigan, USA (J. Eisenberg, D. Bhavnani); Universidad San Francisco de Quito, Quito, Ecuador (G. Trueba, W. Cevallos, K. Ponce, J.-C. Escobar)

Abstract

Diarrheal risk associated with Plesiomonas shigelloides infection was assessed in rural communities in northwestern Ecuador during 2004–2008. We found little evidence that single infection with P. shigelloides is associated with diarrhea but stronger evidence that co-infection with rotavirus causes diarrhea.

Plesiomonas shigelloides (family Enterobacteriaceae) has been implicated in gastroenteritis outbreaks in travelers to tropical regions and in persons who have ingested contaminated food or water (1–3). For persons native to tropical regions, however, case–control studies have found little or no association between P. shigelloides infection and diarrhea (4–6). Although these studies have been conducted in areas where mixed infections are generally common, to our knowledge, none examined co-infections. We assessed the pathogenicity of P. shigelloides in the context of co-infections and across all age groups in a province in northwestern Ecuador.

The Study

During 2004–2008, serial case–control studies were conducted in 22 remote communities in Esmeraldas Province, Ecuador. Complete study design and laboratory procedures for pathogen detection have been described (7). Briefly, each community was visited 4–6 times on a rotating basis; each visit lasted for 15 days, during which all cases of diarrhea were identified by a visit to each household every morning. Household residents with cases had >3 loose stools in a 24-hour period, and controls had no symptoms of diarrhea during the past 6 days. Fecal samples were collected from 3 healthy controls per person with diarrhea. These samples were plated on selective agar media, and 5 lactose-fermenting colonies were screened by PCR for enterotoxigenic Escherichia coli (ETEC), enteropathogenic E. coli, and enteroinvasive E. coli (EIEC). Lactose-negative isolates that were identified as either Shigella spp. or E. coli were also screened by PCR for the same molecular marker used for EIEC. All non–lactose-fermenting pathogens, including P. shigelloides, were biochemically identified by API 20E system (bioMèrieux, Marey l’Etoile, France). Because shigellae are phylogenetically similar to E. coli pathotypes, we combined data from persons infected with E. coli and those infected with shigellae in our analysis. We tested for Giardia lamblia by using an ELISA kit (RIDASCREEN Giardia; R-Biopharm, Darmstadt, Germany), and rotavirus was detected with an enzyme immunoassay kit (RIDA Quick Rotavirus; R-Biopharm). We chose a molecular method for detecting E. coli pathotypes because they cannot be differentiated solely on the basis of biochemical tests; the metabolic homogeneity of P. shigelloides, however, makes this organism easily and clearly identifiable by biochemical test. Similarly, immunologic methods used for Giardia spp. and rotavirus detection are specific and sensitive enough to accurately detect these pathogens, and use of molecular methods would be justified only for deeper analysis. Institutional review board committees at the University of California, Berkeley; University of Michigan; Trinity College; and Universidad San Francisco de Quito approved all protocols.