Infectious diarrhea — fellow's quick read

Personal study digest for a new ID fellow. Recommendations are from the cited IDSA 2017 guideline (diagnosis & management of acute/persistent infectious diarrhea; C. difficile is referred out to the IDSA/SHEA CDI guideline). The "What's changed since 2017" section is reviewer synthesis of newer evidence, each claim cited. Not a substitute for the full guideline.

In one line

Most acute diarrhea is self-limited and viral — the fellow's job is to rehydrate first, test only the few who warrant it (fever, bloody/mucoid stool, severe cramps, sepsis, immunocompromise, travel), and withhold empiric antibiotics in most, reserving them for dysentery, enteric fever, or a severe host — and never for suspected Shiga toxin–producing E. coli.

◆When to test (and when not to)

Send stool studies (Salmonella, Shigella, Campylobacter, Yersinia, STEC, and C. difficile) for diarrhea with fever, bloody or mucoid stools, severe abdominal cramping/tenderness, or signs of sepsis (strong, moderate).
STEC: test EVERY bloody stool simultaneously by culture for O157 AND by a Shiga toxin / toxin-gene assay (non-O157 STEC is missed by culture alone) (strong, low).
Blood cultures in infants <3 mo, suspected enteric fever, systemic/septic illness, immunocompromise, or hemolytic anemia (strong, moderate).
Do NOT use fecal leukocytes or stool lactoferrin to establish a cause (strong, moderate); serology and a WBC/differential don't establish an etiology (strong, low).
C. difficile: single stool, age >2 y, after antibiotics or with healthcare-associated diarrhea (weak, high). Uncomplicated traveler's diarrhea generally needs no testing unless it lasts ≥14 days (→ parasites) (strong, moderate).

◆Workup the fellow drives

Reflex-culture every culture-independent (CIDT / multiplex PCR) positive — the panel detects DNA, not viable organisms (rec 14), so an isolate is still needed for susceptibilities and public-health serotyping/PulseNet (rec 15).
Suspected STEC / HUS: monitor Hgb, platelets, BUN/Cr serially and examine a smear for schistocytes — hematologic/renal changes precede overt HUS (strong, high).
Optimal specimen = a diarrheal (container-shaping) stool; a rectal swab is an acceptable fallback for bacteria (weak, low).
Persistent diarrhea ≥14 days → reconsider parasites (Giardia, Cryptosporidium, Cyclospora) and non-infectious IBD/IBS; in AIDS add Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, MAC, CMV (strong, moderate).

◆Empiric & definitive therapy

Scenario	Antimicrobial approach	Notes
Acute watery diarrhea, no travel	No antibiotics — ORS only (strong, low)	Exception: immunocompromised or ill-appearing young infant
Acute bloody diarrhea, immunocompetent	No empiric antibiotics while awaiting studies (strong, low)	Empiric coverage risks harm if it's STEC
Presumptive Shigella dysentery (febrile, ill)	Empiric fluoroquinolone OR azithromycin (adult); 3rd-gen cephalosporin or azithromycin (child) (strong, moderate)	Guided by local susceptibility + travel — watch for XDR Shigella
Suspected STEC O157 / Shiga toxin 2	AVOID antibiotics AND antimotility agents (strong, moderate)	↑ HUS risk; supportive care + HUS monitoring
Suspected enteric fever / sepsis	Empiric broad-spectrum after blood/stool/urine cultures; narrow on susceptibilities (strong)	Tailor to the acquisition setting if no isolate
Febrile returning traveler (≥38.5°C / sepsis)	Empiric fluoroquinolone or azithromycin (weak, low)	Azithromycin favored for S/SE Asia (FQ-resistant Campylobacter)

De-escalate: modify or stop antimicrobials once a clinically plausible organism is identified (strong, high). This guideline gives syndrome-level empiric guidance, not pathogen-by-pathogen durations — those live in pathogen-specific references.

◆Supportive & ancillary care

Reduced-osmolarity ORS is first-line for mild–moderate dehydration at any age (strong, moderate); isotonic IV fluids (LR/NS) for severe dehydration, shock, altered mental status, or ORS failure (strong, high). Resume an age-appropriate diet during/right after rehydration; continue breastfeeding.
Loperamide: never in children <18 y (strong, moderate); acceptable in immunocompetent adults with watery diarrhea, but avoid with fever or bloody stools (toxic megacolon risk) (strong, low).
Oral zinc shortens diarrhea in children 6 mo–5 y in zinc-deficient/malnourished settings (strong, moderate). Probiotics may modestly reduce symptom duration in immunocompetent adults/children (weak, moderate).

◆Key decisions a fellow owns

Withhold antibiotics in suspected STEC until O157/Shiga-toxin-2 is excluded — the single most consequential "don't."
Reflex-culture CIDT positives so you (and public health) get susceptibilities and a serotype.
Don't treat asymptomatic carriers in low-risk settings — except Salmonella Typhi, or food-handlers/healthcare/childcare workers per local public-health rules (strong, high).
Report and infection-control: notify public health for reportable/outbreak pathogens; ill patients avoid food-handling, swimming, and sexual contact while symptomatic.

What's changed since 2017

Reviewer synthesis of newer evidence — not the guideline. Each claim cited; flags where the 2017 IDSA guideline lags current practice.

XDR Shigella has emerged — the biggest threat to the 2017 empiric plan. Extensively drug-resistant strains (resistant to ampicillin, azithromycin, ceftriaxone, ciprofloxacin, AND TMP-SMX) rose from 0% (2011–15) to 8.5% of US isolates in 2023, concentrated in adult men (~½ HIV-coinfected, ~⅓ hospitalized). No FDA-approved oral agent — treat off susceptibilities; pivmecillinam, fosfomycin, or an oral carbapenem (sulopenem) are investigational. Recency-sensitive — confirm current treatment options. MMWR 2026;75(13):173–178 (doi:10.15585/mmwr.mm7513a1) · CDC HAN-00486, 2023
Azithromycin, not a fluoroquinolone, is the safer directed agent for Campylobacter. US ciprofloxacin resistance is ~29% (NARMS) while macrolide resistance stays ≤3%. CDC NARMS / AR Threats — Campylobacter
Multiplex GI PCR panels are now ubiquitous — fast and sensitive, but they detect DNA (co-detections, carriage) and yield no isolate; the 2017 reflex-culture rule (rec 15) matters more than ever for AST and outbreak subtyping. IDSA 2017 recs 14–15
Traveler's diarrhea has its own graded guidance — single-dose azithromycin preferred for febrile/dysenteric TD (and S/SE Asia); rifaximin or a fluoroquinolone for non-dysenteric; fluoroquinolone use drives ESBL-Enterobacterales gut colonization, a reason to favor azithromycin. Riddle et al, J Travel Med 2017;24(suppl_1):S57–S74
Still avoid antibiotics in STEC — the HUS-risk signal persists; the 2017 "avoid" stance is unchanged. reaffirmed; Shigellosis/STEC reviews, e.g. Lancet 2018;391:801–812 (PMID 29254859)
Out of scope but updated: C. difficile management moved on (fidaxomicin preferred; bezlotoxumab) in the IDSA/SHEA 2021 focused update — this diarrhea guideline defers to it. No newer IDSA infectious-diarrhea guideline since 2017. Johnson et al, Clin Infect Dis 2021;73:e1029–e1044

◆Anki cards minted this run

XDR Shigella — why empiric oral therapy fails; no FDA-approved oral agent (MMWR 2026).
Suspected STEC O157 / Shiga toxin 2 → withhold antibiotics + antimotility (HUS risk).
Immunocompetent acute bloody diarrhea → no empiric antibiotics, and the exceptions.
Multiplex-PCR positive → still reflex-culture (DNA ≠ viable; AST + PulseNet).
Held as near-duplicates: azithromycin for FQ-resistant Campylobacter (deck 1783469209674); loperamide contraindicated in dysentery (deck 1657474252501).

Sources: IDSA 2017 infectious diarrhea guideline — Shane AL, et al. Clin Infect Dis 2017;65(12):e45–e80 (doi:10.1093/cid/cix669); XDR Shigella — Logan N, et al. MMWR 2026;75(13):173–178 (doi:10.15585/mmwr.mm7513a1) & CDC HAN-00486 (2023); Campylobacter resistance — CDC NARMS; Traveler's diarrhea — Riddle MS, et al. J Travel Med 2017;24(suppl_1):S57–S74; Shigellosis review — Kotloff KL, et al. Lancet 2018;391:801–812 (PMID 29254859); C. difficile 2021 focused update — Johnson S, et al. Clin Infect Dis 2021;73:e1029–e1044.