Version May 2024
INTRODUCTION — Plesiomonas shigelloides (formerly known Aeromonas shigelloides) is an oxidase-positive, facultatively anaerobic gram-negative bacillus commonly found in aquatic environments. It is an important pathogen of fish and has been implicated as a cause of enteric disease in humans, especially following the consumption of raw seafood. It has also been isolated from a number of extraintestinal sites [1,2].
Issues related to P. shigelloides infections are discussed here. The approach to diarrhea in adults and children is discussed in detail elsewhere. (See "Approach to the adult with acute diarrhea in resource-abundant settings" and "Diagnostic approach to diarrhea in children in resource-abundant settings".)
MICROBIOLOGY AND PATHOGENESIS — Molecular phylogenetic studies have shown that the ancestry of Plesiomonas is most closely aligned with the family Enterobacteriaceae [3]. Therefore, Plesiomonas has been re-categorized from the family Vibrionaceae to the family Enterobacteriaceae, in which it is the only oxidase-positive member [4]. The species name "shigelloides" is derived from the fact that many strains cross-react antigenically with Shigella, particularly Shigella sonnei [5].
Outbreaks of diarrheal disease have been associated with contaminated water and oysters containing P. shigelloides, and reduction in the severity and duration of symptoms following appropriate antimicrobial therapy has been observed [6-8]. However, efforts to confirm virulence in experimental settings (including one human volunteer study) have been inconclusive [2,9,10]. In a case-control study in Ecuador, detection of P. shigelloides alone was similar between patients with diarrhea and controls; co-infection with P. shigelloides and another gastrointestinal pathogen was associated with diarrhea [11]. This finding is consistent with the concept that isolation of P. shigelloides sometimes reflects its status as a "fellow traveler" with known pathogens, rather than the causative agent for the observed illness. Nonetheless, it is likely that certain strains of P. shigelloides are capable of causing diarrhea. Host susceptibility also influences the risk of illness. Plesiomonas has a type VI secretion system and carries a variety of putative virulence factors; however, their role in causing disease remains uncertain [12-16].
EPIDEMIOLOGY
Distribution — P. shigelloides is primarily a freshwater aquatic organism, with increased rates of isolation in the warmer months of the year [17]. The organism requires a minimum temperature of 8ºC and the absence of salt for growth, which limits its aquatic habitats [1]. Thus, P. shigelloides is generally found in fresh or estuarine (brackish) waters rather than marine environments. It is most frequently observed in tropical or subtropical areas, although it has been isolated from surface waters in Europe as far north as Sweden [18-20].
P. shigelloides is widely distributed among warm and coldblooded animals, including cats, dogs, pigs, poultry, cattle, vultures, toads, snakes, lizards, freshwater and tropical fish, newts, and shellfish, including shrimp and oysters [2,21-24]. P. shigelloides has also been identified in gull feces in southwestern Lake Michigan bathing beaches [25].
There have been several well documented outbreaks of diarrheal disease associated with P. shigelloides in Japan, China, and Cameroon [6,7,26,27]. In a study including 2500 cases of neonatal diarrhea in Bangladesh, 1.6 percent of cases were associated with infection with P. shigelloides [28]. In another study of 3536 patients with acute diarrhea in southeast China, P. shigelloides was isolated in 2.9 percent of cases [29]. In a study of travelers' diarrhea in Bangkok, Plesiomonas was isolated from 14 percent of patients compared with 8 percent of healthy controls [30].
Risk factors and sources of infection — Risk factors for P. shigelloides gastroenteritis include travel to tropical regions, consumption of raw or undercooked seafood, and/or exposure to reptiles or contaminated water sources [4,8,31]. Cases have also been associated with near drowning or other freshwater exposures.
Individuals who are immunocompromised, have hepatobiliary disease (including alcoholic cirrhosis), or hemochromatosis are at increased risk of extraintestinal infections. Infections have also been reported in patients with HIV [32,33] and inflammatory bowel disease [34,35].
Nosocomial transmission has also been reported in a study from China [29].
CLINICAL MANIFESTATIONS
Incubation period — The incubation period for illness ranges from 24 to 48 hours.
Gastrointestinal infection — Patients with diarrhea due to P. shigelloides most frequently develop watery or secretory diarrhea, but some have an invasive, dysenteric illness (ie, bloody or mucoid diarrhea) [2,21,23]. Severe abdominal pain and cramping are common in adults. In a study of Bangladeshi children, presenting symptoms included watery diarrhea (84 percent), dysentery (16 percent), vomiting (71 percent), and fever (8 percent) [36]. Diarrhea lasted 14 days or longer in 13 percent of cases. Symptoms are typically self-limited; in some patients, a subacute to chronic presentation lasting between two weeks and three months occurs.
Extraintestinal infection — Case reports of extraintestinal infections with P. shigelloides include:
●Bacteremia (which may be preceded by enteric symptoms) [18,37-40]
●Cellulitis and skin abscesses following trauma in fresh water [41]
●Peritonitis (including infection associated with continuous ambulatory peritoneal dialysis) [42,43]
●Meningoencephalitis (usually in neonates, with an associated case fatality rate of 80 percent) [44,45]
●Eye infections [46,47]
●Pneumonia [48]
●Prosthetic joint infection following consumption of raw oysters [49]
●Septic abortion [50]
Extraintestinal infections have frequently involved immunocompromised hosts, including neonates and patients with underlying hepatobiliary disease or hemochromatosis [37,38,44,51-54].
DIAGNOSIS — P. shigelloides infection can be diagnosed when the organism is isolated by culture of the relevant specimen. Molecular testing for P. shigelloides is also included on some diagnostic multiplex platforms for enteric pathogens. However, if P. shigelloides is detected by molecular testing in a patient who is seriously ill, we favor additional efforts to isolate the organism by culture to enable susceptibility testing to inform therapy. (See 'Treatment' below.)
Using traditional microbiologic techniques, P. shigelloides grows readily on nonselective media, including blood agar and other media used for isolation of pathogens from wound and blood.
In stool cultures, the organism is a nonlactose fermenter and therefore should be identified readily with routine stool culture on MacConkey agar. Colonies are oxidase positive, which provides a rapid differentiation from Shigella (with which Plesiomonas may cross-react on serologic testing). Some strains can be isolated from cefsulodin-irgasan-novobiocin (CIN) medium, normally used to isolate Yersinia and Aeromonas species [1].
Molecular tests for Plesiomonas appear accurate; as an example, reproducibility of positive tests for Plesiomonas with the FilmArray GI Panel was high (17 of 18 tests) [55].
TREATMENT — In general, gastroenteritis caused by P. shigelloides tends to be mild and self-limited. For diarrheal disease, supportive care (eg, with oral or intravenous rehydration) is the primary therapeutic intervention. Data informing the optimal approach to antibiotic therapy are limited. Our approach depends on the severity of illness (or the potential for severe illness):
●For patients with diarrhea that is mild to moderate and does not require intravenous volume repletion, we recommend only supportive therapy with oral rehydration, without the addition of an antimicrobial agent.
●For more severe diarrheal cases requiring intravenous volume repletion, for extraintestinal infections, and in immunocompromised patients, older adults, or very young children, we suggest antimicrobial therapy. For those who are severely ill, we suggest empiric therapy with a carbapenem. For patients who are not severely ill but still warrant antibiotic therapy, a fluoroquinolone can be used as empiric therapy. Once susceptibility testing results are available, the empiric regimen can be switched to an active agent with a narrower spectrum of activity to complete the duration, if necessary. Given concerns about potential adverse effects of fluoroquinolones, we favor using other active agents for directed therapy, if possible.
Duration of therapy is dictated by clinical response. For most patients with diarrhea severe enough to require antimicrobial therapy, three to five days of therapy should be adequate. Patients with extraintestinal infections warrant more prolonged antimicrobial therapy, in the range of one to two weeks or more, depending on underlying conditions and clinical response.
However, clinical efficacy data for antibiotics are extremely limited. One retrospective study suggested that treatment with an appropriate antimicrobial agent shortens the course of diarrhea compared with no treatment or treatment with antibiotics to which the organism is not susceptible [56]. However, another retrospective series found no difference in the duration of fever or diarrhea among treated or untreated children with P. shigelloides [57].
Despite these ambivalent data, we favor antibiotic therapy in patients at risk for poor outcomes with infection.
Given the paucity of direct clinical data, our preference for empiric regimens is based on in vitro susceptibility patterns. Almost all strains remain susceptible to carbapenems [2]. Most are susceptible to fluoroquinolones, although reports of resistance are emerging globally [2]. Most strains of P. shigelloides carry a variety of beta-lactamases and therefore are resistant in vitro to ampicillin, carbenicillin, piperacillin, and ticarcillin [1,57-59]. Varying degrees of in vitro resistance have been reported to tetracycline, trimethoprim-sulfamethoxazole, erythromycin, and aminoglycosides [2].
SUMMARY AND RECOMMENDATIONS
●Plesiomonas shigelloides is a facultatively anaerobic gram-negative bacillus common in the aquatic environment. It has emerged as a cause of enteric disease in humans, especially following the consumption of raw seafood. Not all strains appear to be capable of causing diarrhea. Host susceptibility may also influence the risk of illness. (See 'Microbiology and pathogenesis' above.)
●The risk of P. shigelloides infection can be reduced by avoiding contact with untreated water supplies and consumption of raw or undercooked shellfish (particularly during warmer summer months). Individuals who are immunocompromised or have hepatobiliary disease are at increased risk of infection associated with water-related injuries. (See 'Epidemiology' above.)
●Patients with diarrhea due to P. shigelloides most frequently develop watery or secretory diarrhea, but some have an invasive, dysenteric illness. Other symptoms include abdominal pain, cramping and vomiting; fever occurs occasionally. Extraintestinal manifestations have also been reported. (See 'Clinical manifestations' above.)
●P. shigelloides can be detected on routine cultures of stool, wound, and blood cultures. It is also included in some multiplex diagnostic panels for enteric pathogens. (See 'Diagnosis' above.)
●The gastroenteritis caused by P. shigelloides tends to be mild and self-limited and, in almost all instances, can be treated symptomatically with rehydration alone. For severe diarrhea requiring intravenous rehydration, for patients at risk for severe diarrhea (eg, extremes of age, immunocompromising condition), and for extraintestinal disease, we suggest antimicrobial therapy (Grade 2C). For empiric therapy of patients who are severely ill, we suggest a carbapenem (Grade 2C). For empiric therapy of patients who are less severely ill but still warrant antibiotic therapy, we suggest a fluoroquinolone (Grade 2C). Once susceptibility testing is available, an active agent with a narrower spectrum of activity should be chosen to complete the course, if necessary. (See 'Treatment' above.)
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