What are the differential diagnoses for a 72-year-old man with four days of fever, diarrhea, and decreased appetite?

Differential Diagnoses for a 72-Year-Old Male with Fever, Diarrhea, and Decreased Appetite

This 72-year-old man with fever and diarrhea for 4 days most likely has an infectious gastroenteritis, with bacterial pathogens (Salmonella, Campylobacter, Shigella, C. difficile, or STEC) being the primary considerations, though viral causes (particularly norovirus) and non-infectious etiologies must also be evaluated. 1

Infectious Causes (Most Likely)

Bacterial Pathogens

The presence of fever with diarrhea strongly suggests an invasive/inflammatory bacterial pathogen. 1 The most commonly identified bacterial causes in North America include:

• Salmonella species - Most common bacterial cause requiring hospitalization in adults, particularly in elderly patients 1
• Campylobacter - Second most common bacterial pathogen causing inflammatory diarrhea 1
• Shigella - Causes dysentery with fever and inflammatory diarrhea 1
• Clostridioides difficile - The most commonly identifiable cause of infectious diarrhea in elderly patients, especially if recent antibiotic exposure (within 8-12 weeks) or recent hospitalization 2, 1
• Shiga toxin-producing E. coli (STEC) - Must be considered even with fever present, though fever may be absent 1
• Yersinia enterocolitica - Consider if persistent abdominal pain develops or if exposure to undercooked pork products 1

Viral Pathogens

• Norovirus - The leading cause of gastroenteritis overall (58% of cases), particularly prevalent and highly contagious in elderly populations and institutional settings 1, 2
• Rotavirus - Less common in adults but possible 1

Parasitic Infections

• Giardia lamblia - Consider with prolonged symptoms or contaminated water exposure 2
• Cryptosporidium - Particularly if immunocompromised or contaminated water exposure 2

Enteric Fever

• Typhoid/Paratyphoid fever (Salmonella typhi/paratyphi) - Consider if travel history to endemic areas or contact with travelers; note that diarrhea is uncommon in enteric fever 1

Non-Infectious Causes (Important to Consider in Elderly)

Medication-Related

• Drug-induced diarrhea - Drug-related fever accounts for 6% of fever of unknown origin cases in elderly patients 3
• Recent antibiotic use predisposing to C. difficile 2

Inflammatory/Autoimmune

• Inflammatory bowel disease (IBD) - Can present with fever and diarrhea 1
• Microscopic colitis - Causes secretory diarrhea, more common in elderly 4
• Temporal arteritis/Giant cell arteritis - The most frequent specific diagnosis in fever of unknown origin in elderly patients (part of multisystem diseases accounting for 31% of cases) 3

Malignancy

• Colorectal cancer - Can present with inflammatory diarrhea and constitutional symptoms 4
• Lymphoma - Tumors account for 12% of fever of unknown origin in elderly 3

Other Gastrointestinal Disorders

• Ischemic colitis - More common in elderly with vascular disease 5
• Chronic pancreatic insufficiency - Specific to elderly, can cause malabsorption and weight loss 5
• Small intestinal bacterial overgrowth - Syndrome specific to elderly without anatomic abnormality 5

Key Clinical Considerations

Age-Specific Factors

The elderly have the highest percentage of hospitalizations and deaths from gastroenteritis despite lower overall incidence compared to children. 1 In this age group:

• Atypical presentations are common - Infections may present with nonspecific symptoms like functional decline, altered mental status, weakness, or falls rather than classic fever 6
• Fever may be absent in 20-30% of elderly patients with serious infection 6
• Lower fever threshold - An elevation of 2°F from baseline should be considered significant 6
• Reduced nutritional reserve makes malabsorption and deficiencies more devastating 5

Critical Red Flags Requiring Immediate Workup

This patient meets criteria for comprehensive stool testing because he has fever accompanying diarrhea, which indicates likely invasive/inflammatory pathogen. 1 Blood cultures should be obtained given his age >65 years with systemic manifestations (fever, decreased appetite suggesting possible sepsis). 1

Common Pitfalls to Avoid

• Don't assume viral gastroenteritis without testing - Bacterial causes are equally common and require different management 1
• Don't overlook C. difficile - It's the most common identifiable cause in elderly, and one-third of colonized nursing home residents develop symptomatic disease within 2 weeks of antibiotic exposure 2
• Don't forget non-infectious causes - Multisystem diseases (including temporal arteritis) are the most frequent cause of fever of unknown origin in elderly (31%), exceeding infections (25%) 3
• Consider STEC even with fever - Though fever is less common, STEC must be ruled out as antibiotics are contraindicated and can cause harm 1