Histoplasmosis — fellow's quick read

Personal study digest for a new ID fellow. Recommendations are from the 2025 IDSA update, which is Part 1 of the first histoplasmosis revision since 2007 and covers ONLY asymptomatic Histoplasma pulmonary nodules (histoplasmomas) and mild or moderate acute pulmonary histoplasmosis. Severe/disseminated and chronic cavitary disease are NOT covered here — those remain under the 2007 guideline. The "What's changed" section is reviewer synthesis of newer evidence, each claim cited. Not a substitute for the full guideline.

In one line

Most acute pulmonary histoplasmosis in an immunocompetent host is self-limited and needs NO antifungal. The 2025 update's central move is to treat by host immune risk and disease severity/duration — not reflexively — and when you do treat, the agent is itraconazole.

◆When to suspect / diagnose

Inhaled Histoplasma capsulatum; hyperendemic in the Ohio and Mississippi River valleys. Clinical spectrum runs from asymptomatic to severe, set by inoculum and cell-mediated immunity.
Severity drives everything (Table 1): asymptomatic (imaging + antigen/antibody evidence of recent/active infection) · mild (symptoms that don't interfere with normal activities) · moderate (symptoms interfering with activities; may need low-flow O₂ or admission) · severe (respiratory failure — not in this update).
"Asymptomatic but active" nodule = new/progressive radiographic abnormality, positive urine or serum Histoplasma antigen, complement-fixation titer ≥1:32 or rising, or an H-band by immunodiffusion.
Excluded from this update: newborns, African histoplasmosis, ocular histoplasmosis syndrome.

◆Workup the fellow drives

Non-calcified vs calcified nodule. Calcified nodules are healed disease — do NOT treat. Non-calcified nodules may reflect recent/active infection.
Stratify host immune risk against Table 2 (high / moderate / low risk for disseminated/severe disease). This — not the radiograph alone — decides treat vs observe.
Antigen (urine + serum) and antibody (CF, immunodiffusion) to establish activity; these anchor the "asymptomatic active" definition above.

◆Treatment — the 4 updated recommendations

Scenario	Recommendation	Strength (GRADE)
Asymptomatic non-calcified nodule / known untreated prior infection (immunocompetent)	Against routinely treating to prevent reactivation. Calcified-only nodules: no treatment.	Conditional · very low certainty
Mild acute pulmonary (immunocompetent)	Against routine antifungal treatment (self-limited).	Conditional · very low certainty
Moderate acute pulmonary (immunocompetent)	Either treat or not — shared decision by severity/duration and harms.	Conditional · very low certainty
Mild OR moderate acute pulmonary (immunocompromised, moderate–high dissemination risk)	Treat with antifungal (itraconazole preferred).	Conditional · very low certainty

When to lean toward treating (mild/moderate immunocompetent): illness >1 month, progressive infiltrates, or enlarging hilar/mediastinal adenopathy. Natural-history anchor: in a large outbreak, >75% were ill ≤1 week and all recovered within 2 months untreated (Brodsky, Am J Med 1973).

◆Itraconazole — dose, duration & TDM

Formulation	Dose	Duration
Conventional itraconazole (capsule/solution), adults	200 mg PO TID × 3 days (load), then 200 mg BID	6–12 weeks
Children	5 mg/kg/dose (max 200 mg) TID × 3 days, then 5 mg/kg BID (max 400 mg/day)	6–12 weeks
SUBA-itraconazole (capsules, adults)	130 mg TID × 3 days, then 130 mg BID	6–12 weeks

Do TDM on itraconazole. Goal trough >1 mg/L (efficacy) and <3–4 mg/L (toxicity), by chromatographic assay. In recent series ~20% needed dose adjustment for sub-/supra-therapeutic levels and ~28% had side effects. Long half-life → a non-trough/random level is usable.

◆Special populations

Immunocompromised (Table 2, high risk): ≥2 mg/kg/day (or ≥20 mg/day >10 kg) prednisone-equivalent ≥2 wk; advanced/untreated HIV (CD4 <200); HSCT <100 d or on GVHD therapy; CAR-T <90 d; SOT with rejection treatment; TNF-α inhibitors (infliximab, etanercept, adalimumab) and IL-12/23 blockers. Treat mild/moderate disease if moderate–high risk.
Pregnancy: treat only after weighing benefit vs harm with MFM + ID. If treatment is needed, avoid azoles in the first trimester and use liposomal amphotericin B instead (azole teratogenicity).
Children: weight-based itraconazole as above; SUBA-itraconazole is off-label in children (adult-approved only) but weight-based dosing may be considered if old enough to swallow capsules, with pharmacy input.

◆Key decisions a fellow owns

Don't treat what heals on its own — immunocompetent + mild/asymptomatic + non-progressive = observe. Calcified nodules never get antifungals.
Let host status flip the call — the same mild/moderate presentation in a moderate–high-risk immunocompromised host is a treat.
Own the itraconazole level — erratic absorption; check a level, chase >1 mg/L, watch drug–drug interactions.
Know the scope boundary — severe/disseminated and chronic cavitary histoplasmosis are NOT in this update; treat those per the 2007 guideline (L-AmB induction → itraconazole).

What's changed since 2007

Reviewer synthesis of newer evidence — not the guideline. Each claim cited; flags where practice is still moving or the update deliberately stops.

First IDSA histoplasmosis update in 18 years, and it's only Part 1 — GRADE-based; covers asymptomatic nodules + mild/moderate acute pulmonary only. Additional parts are planned (salvage/intolerance therapy; severe-disseminated & chronic cavitary disease; role of steroids/NSAIDs). Arnold, CID 2025;81(Suppl 3):i27–i32 · PMID 40667709 · 10.1093/cid/ciaf256
From a duration rule to a risk-based framework — 2007 pivoted on "treat if symptoms >1 month"; 2025 formalizes "treat less" for immunocompetent mild disease and makes the treat/observe call turn on a new immunocompromise risk table (Table 2) plus severity. vs Wheat, CID 2007;45:807–825 · PMID 17806045 · 10.1086/521259
SUBA-itraconazole is the notable formulation shift — MSG-15 RCT (endemic mycoses, n=88; histoplasmosis n=51): bioequivalent to conventional itraconazole with less level variability and fewer serious treatment-emergent AEs (12% vs 50%, P<.001); day-42 success 69% vs 67%. Spec, Open Forum Infect Dis 2024;11:ofae010 · PMID 38440302 · 10.1093/ofid/ofae010 · NCT03572049
Itraconazole TDM target formalized — goal trough >1 mg/L (efficacy) and <3–4 mg/L (toxicity) by chromatographic assay; combined itraconazole + hydroxy-itraconazole >2 mg/L may behave like an itraconazole level >1. per IDSA 2025 remarks
Pipeline (recency-sensitive): olorofim (F2G, orotomide) is active against H. capsulatum, but received an FDA Complete Response Letter (June 2023) and remains FDA-UNAPPROVED as of mid-2026; Phase 3 ongoing (NCT05101187). Fosmanogepix (Gwt1 inhibitor) also in development. Confirm current status. FDA history · drugs.com
Disseminated-disease evidence is moving too (out of this update's scope) — single high-dose L-AmB 10 mg/kg for AIDS-disseminated histoplasmosis gave day-14 response 84% vs 74% control (P=.69, phase 2); a confirmatory phase 3 (INDUCTION, NCT05814432) is recruiting and a factorial phase 3 (Histo-FACT, NCT07261150) is planned — the "AMBITION-for-histo" storyline for the parts not yet updated. Pasqualotto, CID 2023;77:1126–1132 · PMID 37232940 · 10.1093/cid/ciad313

◆Anki cards minted this run

Immunocompetent + mild/asymptomatic pulmonary histoplasmosis → suggest against routine antifungal (self-limited).
Same mild/moderate disease in an immunocompromised (moderate–high risk) host → treat (itraconazole preferred).
Itraconazole TDM target for histoplasmosis = trough >1 mg/L and <3–4 mg/L (SUBA-itraconazole pearl).
Histoplasmosis in pregnancy needing treatment → avoid first-trimester azoles, use liposomal amphotericin B.

Sources: Arnold SR, Spec A, et al. 2025 IDSA CPG update on histoplasmosis: asymptomatic pulmonary nodules and mild/moderate acute pulmonary histoplasmosis. CID 2025;81(Suppl 3):i27–i32 (PMID 40667709; doi:10.1093/cid/ciaf256), with companion articles Baddley et al and Lentz et al (CID 2025). · Wheat LJ, et al. IDSA histoplasmosis 2007 update. CID 2007;45:807–825 (PMID 17806045; doi:10.1086/521259). · Spec A, et al. MSG-15 SUBA-itraconazole vs conventional itraconazole in endemic mycoses. Open Forum Infect Dis 2024;11:ofae010 (PMID 38440302; doi:10.1093/ofid/ofae010; NCT03572049). · Pasqualotto AC, et al. Single high-dose L-AmB in HIV/AIDS disseminated histoplasmosis. CID 2023;77:1126–1132 (PMID 37232940; doi:10.1093/cid/ciad313). · Brodsky AL, et al. 1970 Earth Day histoplasmosis outbreak. Am J Med 1973;54:333–342. · Olorofim FDA status (CRL June 2023; NCT05101187) — drugs.com FDA history; F2G. Currency claims are reviewer synthesis; verify recency-sensitive items (olorofim approval status; whether later parts of the 2025 update have published).