What are the causes and types of infectious diarrhea in adults?

Causes and Types of Infectious Diarrhea in Adults

Infectious diarrhea in adults is caused primarily by viral pathogens (most commonly norovirus), followed by bacterial pathogens (Salmonella, Shigella, Campylobacter, and pathogenic E. coli), and less frequently by parasites (Giardia, Cryptosporidium, and Entamoeba histolytica). 1, 2

Viral Causes

• Norovirus is the most common cause of diarrheal disease outbreaks and represents the leading viral pathogen across all age groups in adults 2
• Chronic and severe norovirus infection can occur in immunosuppressed patients, particularly following organ transplantation 2
• Adenovirus (types 40,41) causes illness lasting ≥1 week, longer than other viral pathogens 3

Bacterial Causes

The major bacterial pathogens include:

• Campylobacter, Salmonella, Shigella, and pathogenic E. coli strains (including enterotoxigenic E. coli) are the most important bacterial causes 1, 2, 4
• Yersinia enterocolitica presents with diverse manifestations including nonbloody diarrhea, bloody diarrhea, or pseudoappendicular syndrome mimicking appendicitis 1
• Clostridium difficile is of increasing concern, especially in patients with recent antimicrobial therapy 2
• Vibrio vulnificus infections occur specifically in patients with chronic liver disease (cirrhosis, alcoholic liver disease, hepatitis), iron overload states (hemochromatosis, hemolytic anemia, chronic renal failure), or other immunocompromising conditions 1, 2

Clinical Context for Bacterial Testing

• Among adults presenting to emergency departments with diarrhea, only 17% who submitted stool specimens had a bacterial enteric infection 1
• Testing should be restricted to patients with bloody stools, fever, or abdominal tenderness to increase the likelihood of identifying a bacterial pathogen 1

Parasitic Causes

• Giardia, Cryptosporidium, and Entamoeba histolytica are the most common parasitic causes 1, 2
• Cyclospora and Cystoisospora (formerly Isospora belli) are additional parasitic pathogens 2
• Approximately 10% of traveler's diarrhea is caused by parasitic infections, which can persist for weeks to months, with giardiasis being the most common 2
• Microscopic examination for ova and parasites is unlikely to include Cryptosporidium and Cyclospora testing unless specifically requested 2

Special Populations at Higher Risk

Immunocompromised Patients

• Diarrhea in immunocompromised patients may involve bacterial, viral, parasitic, and fungal pathogens depending on the underlying immune status 1, 2
• HIV-infected patients are at risk for enteroaggregative E. coli, Cryptosporidium, microsporidia, Cystoisospora belli, CMV, and Mycobacterium avium complex 2
• Cryptosporidium, Cyclospora, Cystoisospora, and microsporidia cause more severe, chronic, or relapsing disease in patients with impaired cell-mediated immunity 2
• Campylobacter, Shigella, and Listeria infections are more likely to be severe or recurrent in HIV-infected patients 1

Risk Factors for Invasive Bacterial Infections

• Invasive nontyphoidal Salmonella infection occurs more frequently in patients with young or advanced age, HIV infection, cytotoxic chemotherapy, malnutrition, hemoglobinopathies, recent malaria, and cirrhosis 1, 2
• Aneurysms of the aorta and aortitis can occur in elderly patients with invasive nontyphoidal salmonellosis or yersiniosis 1
• Yersinia enterocolitica higher risk groups include young African American and Asian children (especially during winter months), diabetics, and those with chronic liver disease, malnutrition, or iron-overload states 1, 2

Clinical Classification by Presentation

Acute Watery Diarrhea

• Predominantly caused by viral pathogens and enterotoxigenic bacteria 5, 6
• Most cases are self-limited and resolve without specific intervention 7, 6

Acute Bloody Diarrhea (Dysentery)

• Dysentery is defined by frequent, scant stools with visible blood and mucus and is always classified as severe disease 5
• Characterized by fever, severe abdominal cramping, and often systemic illness 5
• Primary pathogens include Shigella, Salmonella, Campylobacter, and enteroinvasive/enterohemorrhagic E. coli 1, 5
• Antimicrobial therapy may confer clinical benefit for Salmonella, Shigella, and Campylobacter in dysentery 5

Traveler's Diarrhea

• The majority of traveler's diarrhea is caused by bacterial pathogens (primarily enterotoxigenic E. coli), with viral pathogens to a lesser extent 2
• Typically lasts <7 days 2
• Approximately 10% is caused by parasitic infections that can persist for weeks to months 2

Chronic Infectious Diarrhea (>14 days)

• Parasites are the most likely infectious cause of chronic diarrhea in immunocompetent individuals 1, 8
• Giardia is the most common parasitic cause, with infection typically lasting 2-6 weeks but can become chronic 1
• Bacterial causes are rare in immunocompetent individuals but include Salmonella, Campylobacter, Yersinia, Aeromonas, and Plesiomonas 1, 8
• Recurrent C. difficile infection requires separate testing 1

Critical Pitfalls to Avoid

• In patients with diarrhea lasting ≥30 days, testing for HIV may be appropriate 2
• Multipathogen nucleic acid amplification tests can detect multiple pathogens simultaneously, but the clinical significance of detecting multiple pathogens or nucleic acid alone (versus viable organisms) remains unclear 2
• Diagnostic evaluation should be restricted to those with severe symptoms (significant volume depletion or dysentery) or altered hosts (immunosuppression or chronic illness) 7
• Testing less ill patients with probable viral illnesses is unrevealing and wasteful of resources 7