What are the causes of loose stools in infants?

Causes of Loose Stools in Infants

Loose stools in infants are most commonly caused by viral gastroenteritis, with rotavirus and norovirus being the predominant pathogens, though it is critical to distinguish normal breastfed infant stool patterns from true diarrhea and to identify bacterial causes that may require specific treatment. 1, 2

Normal vs. Pathologic Loose Stools

• Passing loose, "pasty" stools by infants consuming human milk is normal and should not be classified as diarrhea. 3 This is a common pitfall—breastfed infants frequently have soft, frequent stools that are physiologic.
• True diarrhea is defined as passage of 3 or more loose or liquid stools per 24 hours, or more frequently than is normal for that individual infant. 3

Infectious Causes: Viral Pathogens (Most Common)

Viral Gastroenteritis

• Norovirus is the most common pathogen in hospitalized children, responsible for 58% of all gastroenteritis cases. 1
• Rotavirus is the second most common viral cause, though it was previously the leading cause before vaccine introduction. 1, 2
• Viral infections are mainly identified in infants aged 0-11 months. 4
• Clinical presentation typically includes acute onset of fever and vomiting, followed 24-48 hours later by watery diarrhea. 1
• Vomiting occurs in 80-90% of infected children and usually lasts less than 24 hours. 1
• Diarrhea typically consists of 10-20 bowel movements per day. 1

Other Viral Pathogens

• Sapoviruses and astroviruses are also identified as causes of acute gastroenteritis in infants, though less frequently than rotavirus and norovirus. 5

Infectious Causes: Bacterial Pathogens

Common Bacterial Causes

• Salmonella is the most commonly isolated bacterial pathogen in infants with acute diarrhea. 2, 4
• Campylobacter and Shigella are also frequently isolated bacterial pathogens. 2, 6
• Diarrheagenic Escherichia coli (DEC) is another important bacterial cause. 4
• Bacterial infections are more prevalent in older children (24-59 months) compared to younger infants. 4

Clinical Indicators of Bacterial Gastroenteritis

• Infants presenting with loose stools, elevated total leukocyte count, and elevated procalcitonin most likely have bacterial gastroenteritis with systemic bacterial infection. 7 This requires immediate stool culture, blood culture, and empirical antibiotic therapy.
• Elevated procalcitonin is highly specific for bacterial infection and indicates systemic bacterial involvement. 7
• Fever, elevated white blood cell counts, and presence of systemic markers of infection suggest bacterial etiology. 7

Infectious Causes: Parasitic Pathogens

• Giardia is the most common parasitic cause, particularly in day care settings. 1, 2
• Cryptosporidium is another parasite that commonly produces acute infectious diarrhea in infants. 2

Epidemiologic Risk Factors

• Day care center attendance significantly increases exposure to enteropathogens through fecal-oral contamination. 5 The incidence of acute diarrheal episodes in day care settings is approximately 0.08 episode per child per month. 5
• Recent exposure to group settings increases likelihood of viral transmission. 8
• Infants have higher incidence and greater severity of diarrheal disease compared to older children due to increased rate of exposure and age-specific differences in host defense mechanisms. 2

Co-infections

• With multiplex diagnostic panels, clinicians can expect to see 12.6% of cases displaying co-infections with two viruses or a virus and a bacterium. 4 This complicates treatment decisions and requires careful clinical correlation.

Age-Specific Considerations

• Infants under 6 months are at higher risk of severe dehydration and require a lower threshold for hospitalization. 1
• The etiology of acute diarrhea varies significantly in children of different age groups, with viral infections predominating in younger infants. 4

Important Clinical Pitfalls to Avoid

• Do not assume all loose stools in breastfed infants represent pathology—frequent passing of formed stools or loose "pasty" stools in breastfed infants is normal. 3
• Do not delay rehydration while pursuing diagnostic testing if viral gastroenteritis is suspected. 8
• Consider milk protein allergy if symptoms persist despite appropriate management. 1
• Bilious vomiting requires immediate imaging to rule out malrotation with volvulus, not presumptive gastroenteritis treatment. 8