Clostridium difficile is a fastidious, gram-positive, spore-forming bacterium responsible for infectious diarrhea and pseudomembranous colitis with significant morbidity and mortality. Patients at highest risk for C. difficile infection include hospitalized individuals >65 years old with recent antibiotic exposure. Risk factors for C. difficile in these individuals include depletion of protective gut flora by antibiotics^1-3 and diminished immune response to C. difficile due to age and medical comorbidities.^4,5 Most epidemics occur in the hospital setting and in long-term care facilities,^6,7 but outpatient acquisition is also described. With the emergence of hypervirulent strains in both North America and Europe, the impact of C. difficile has broadened to affect a growing community-based population and younger individuals, even without previous exposure to antibiotics.⁸ Though historically a rare entity in Asia, this pathogen can spread quickly and will likely grow in frequency in areas currently considered to be low prevalence. This article describes the pathophysiology and clinical aspects of C. difficile infection, and reviews its emergence in Asia.

C. difficile colonizes the large intestine of humans and domestic and wild mammals. Both toxigenic and nontoxigenic strains exist, but only toxigenic forms produce disease in humans. Pathogenicity is dependent on the presence of one or both of two closely related diarrhea-producing toxins, named toxin A (TcdA) and toxin B (TcdB).⁸ All toxigenic strains to date contain TcdB, with or without the presence of TcdA. TcdA and Tcd B share a common molecular mechanism of action: inactivation of Rho GTPases through enzymatic glucosylation of a conserved threonine residue. This pathway leads to actin depolymerization and cell death, and stimulates an inflammatory cascade that exacerbates tissue damage, diarrhea, and pseudomembranous colitis (Fig. 1).^9,10 A third pathogenic toxin, binary toxin, is produced by some strains of C. difficile. This toxin has been shown to enhance virulence of C. difficile through irreversible adenosine diphosphate-ribosylation of actin, inducing the formation of long host-cell microtubule protrusions that facilitate bacterial attachment.¹¹

The ability of C. difficile to cause enteritis is based upon two host features: colonization resistance and immune response to C. difficile. The large intestine is protected from invasive pathogens by indigenous flora composed of approximately 4,000 bacterial species,¹² collectively called the fecal microbiome. These microbes collectively provide colonization resistance against pathogenic species through competition for essential nutrients and attachment sites to the gut wall.¹³ Antibiotics disrupt the barrier microflora and diminish colonization resistance, thereby providing a niche for colonization by intestinal pathogens.^1-3 Reduction of Bacteroides and Firmicutes phyla by antibiotics appears to be particularly important in the pathophysiology of C. difficile.¹⁴

The fecal flora of the newborn and infant lacks colonization resistance. As a result, 60% to 70% of healthy infants are asymptomatic carriers of C. difficile during the first 12 months of life.¹⁵ During this infantile carrier state, serum immunoglobulin G (IgG) and IgA antitoxins first appear and protect against subsequent C. difficile disease. These antibodies may persist and bind C. difficile toxins in the lumen to prevent diarrhea and colitis. Kyne et al.⁴ reported that serum antitoxin A IgG was higher in hospitalized patients who remained asymptomatic following C. difficile colonization, compared to those who developed acute infection.

Moreover, patients who mounted an appropriate antibody response during an initial episode of C. difficile infection were at decreased risk for recurrent infection.⁵ Conversely, Solomon et al.¹⁶ demonstrated that patients with a low serum antitoxin A IgG were significantly more likely to die during the first 30 days of infection. Advanced age, malnutrition, female gender, and medical comorbidities tend to diminish host protective response to C. difficile in adults,² and may be associated with more severe infection.

The geographic distribution of prevalent C. difficile strains is shown in Table 1. In 2003, the North American Pulse Field type 1 (NAP1)/ribotype 027 strain emerged as a source of C. difficile epidemics in Canada and the United States. This strain contains a mutation in the C. difficile toxin inhibitory gene tcdC, leading to increased toxin A and B production.^17-19 It also produces binary toxin. Due to these virulence factors, the 027 strain has been associated with higher morbidity, recurrence rates, and presence in the community. From 1998 to 2009, the number of United States hospitalizations with a principal diagnosis of C. difficile infection increased from 25,200 to 110,600, reaching a plateau from 2008 to 2009.²⁰ Similar to ribotype 027 in North America, ribotype 078 has been on the rise in Europe since 2005. This strain is also associated with increased community-acquired disease, younger age, and lack of preceding antibiotic therapy.²¹

In Asia, C. difficile is reported as a low prevalence hospital pathogen, and its true prevalence remains unknown. A Korean study of 17 hospitals in 2008 found an increase in incidence from 1.7 cases/1,000 adult admissions in 2004 to 2.7 cases/1,000 adult admissions,²² considerably lower than the rate of 8.75 cases/1,000 adult admissions in United States hospitals over the same period.²⁰ A 2007 to 2008 study in a single hospital in Shanghai, China found a similar incidence of C. difficile infection of 1.7 cases/1,000 admissions.²³ Though ribotype 027 is quite rare in Asia, variant tdcA^-/tcdB⁺ ribotype 017 has emerged as the predominant strain in east Asia, accounting for between 23% and 48% of toxigenic strains in Korea,^24,25 China,^26,27 Japan, and Taiwan. Ribotype 018 has been responsible for C. difficile outbreaks in Tokyo and Korea.²⁸ Speciation of C. difficile in Hong Kong has revealed toxinotypes unique to east Asia. For example, Cheng et al.²⁹ reported that of 345 C. difficile isolates, ribotype 002 was the most prevalent, representing 10.1% of strains, with ribotype 017 representing only 0.6% of strains. Moreover, 70% of strains did not belong to any of the 23 ribotypes prevalent in North America and Europe, and 11.6% were nontypable.²⁹

A prospective study of Indian patients with acute diarrhea in 2012 demonstrated C. difficile in 8% of hospitalized patients and in 1.3% of outpatients.³⁰C. difficile was identified in 29% of patients with antibiotic associated diarrhea in Pakistan.³¹ Surveys of hospitalized patients in Singapore reported a prevalence of 3.0 to 6.6 active C. difficile cases per 10,000 inpatient days.^32,33 In Malaysia and Thailand, recent reports suggest that C. difficile is more prevalent than previously appreciated, with rates of toxin A and B positivity of 14%³⁴ and 44% to 46%,³⁵ respectively, in patients with antibiotic-associated diarrhea. A recent report from the Philippines demonstrated that 44% of colitis cases were C. difficile positive, representing a paradigm shift as most cases in that country were previously attributed to amoebic or parasitic infections.³⁶

The clinical presentation of C. difficile ranges across a wide spectrum from asymptomatic carrier state to toxic megacolon. Typical signs and symptoms of acute C. difficile infection include watery diarrhea (≥3 unformed stools/24 hours), anorexia, nausea, and leukocytosis with a neutrophilic predominance. Disease severity is used to guide antibiotic therapy.³⁷ Disease is characterized as severe if associated with hypoalbuminemia (<3 g="" dl="" and="" leukocytosis="" exceeding="" 15="" 000="" cells="" mm="" sup="">3 or abdominal tenderness.³⁸ Immunocompromised state, presence of inflammatory bowel disease,^39-41 and acute kidney injury⁴² related to C. difficile also portend a worse prognosis and should be treated as severe in practice. The presence of associated leukocytosis above 35,000 cells/mm³, fever, hypotension, mental status changes, elevated serum lactate levels >2.2 mmol/L, end-organ failure, or admission to the Intensive Care Unit, define severe-complicated disease,³⁸ with predicted 20% to 30% mortality. Rarely, C. difficile may result in an ileus with abdominal distention but little to no diarrhea. This presentation tends to herald a more severe course and should also be treated as severe-complicated disease.³⁸

The diagnosis of C. difficile infection is based on the presence of typical signs and symptoms, in addition to identification of C. difficile organisms/toxin in stool or pseudomembranous colitis on colonoscopy (Fig. 2). Several laboratory tests are available for the diagnosis of C. difficile (Table 2). Nucleic acid amplification tests are most commonly employed in the United States, as they have the highest sensitivity and specificity, and provide quick results. A step-wise approach using a screening glutamate dehydrogenase assay followed serially by confirmatory immunotesting for toxins A and B may also be employed for diagnosis, but has lower sensitivity than polymerase chain reaction-based testing.³⁸ Toxin-based testing should always include both toxin A and B, as the predominant ribotypes in Asia are toxin A negative.^24,43

C. difficile is spread via the fecal-oral route by ingestion of acid-resistant spores. Therefore, appropriate hand-hygiene of healthcare workers by washing with soap and water to help remove spores and isolation of patients with acute diarrhea can limit spread in the hospital. Much effort has been focused on patient therapies to prevent symptomatic disease. Probiotics are generally well-tolerated and have been found in a recent Cochrane review including 23 randomized controlled trials to be associated with decreased incidence of C. difficile-associated diarrhea.⁴⁴ Novel C. difficile vaccines based on inactived toxins A and B are also currently in development, and have been shown in early clinical trials to diminish the effects of C. difficile through enhancement of antitoxin A and B response.^45-48

The approach to treatment of C. difficile is outlined in Table 3. The inciting antibiotics should be stopped if possible to allow regeneration of the normal gut microflora, and an antibiotic with activity against C. difficile should be started. Initial therapies based on severity of disease include metronidazole for mild-moderate disease, vancomycin for severe disease, or a combination of the two for severe-complicated disease.⁸ All antibiotics are given orally, with the exception of metronidazole, which is active by the intravenous route owing to an active enterohepatic circulation.⁸ In 2011, fidaxomicin was shown to be non-inferior to vancomycin for the first or second episode of C. difficile^49,50 and was approved in the United States for the treatment of mild-to-moderate C. difficile. The major advantage of fidaxomicin is its lower recurrence rate (15.4%) as compared to vancomycin (25.3%).⁵⁰ However, widespread use of this antibiotic has been constrained by both cost⁵¹ and limited trial data. Treatment for the first episode of recurrent C. difficile is identical to that for initial treatment. Antibiotic therapies for subsequent recurrences include prolonged pulse-dosed vancomycin tapers, with an additional 14 days of rifaximin⁵² or fidaxomicin⁵³ following the vancomycin taper (Table 3). Fecal microbiota tranpslantation, by which donor feces is infused into a patient's gastrointestinal lumen, results in a cure rate of approximately 90% in recurrent C. difficile infection.⁵⁴ In the first randomized controlled trial of stool transplantation for recurrent C. difficile, van Nood et al.⁵⁵ recently demonstrated that duodenal infusion of healthy donor feces was significantly more likely to result in cure (81%) than vancomycin with bowel lavage (31%) or vancomycin alone (23%) for the treatment of recurrent C. difficile. Fecal transplant is becoming the preferred treatment for patients with multiple recurrences of C. difficile infection.

C. difficile, a pathogen responsible for severe infectious colitis that leads to significant morbidity and mortality worldwide, has established a foothold in Asia. The recent uptrend in C. difficile prevalence with increasingly pathogenic strains in North America and Europe likely heralds a similar pattern in Asia over time. Though testing for C. difficile is common in some parts of Asia, the paucity of literature supports the need for further clinical and laboratory awareness of this disease. Prompt recognition of this pathogen will support the essential development of infection control practices to thwart the propagation of C. difficile in Asia.