Histoplasmosis
Case Study
Case Study
A 63-year-old male with a recent liver transplant on tacrolimus and mycophenolate presented with several weeks of fatigue, cough, malaise, and severe hyponatremia (Na 115).
Presented with:
- Stable vitals
- Normal oxygenation
- Normal initial chest X-ray
- Leukopenia and thrombocytopenia
- CT later showing bilateral ground glass opacities
- Hepatosplenomegaly
- Ascites
Despite treatment for community-acquired pneumonia, he worsened.
He developed:
- Progressive hypoxemia
- Cytopenias Diffuse pulmonary infiltrates
- Recurrent desaturations
- Respiratory failure requiring intubation
Urine Histoplasma antigen returned positive. Serum fungal markers were positive. He was diagnosed with disseminated histoplasmosis and started on amphotericin B.
This was not simply pneumonia. It was systemic immune failure meeting an opportunistic organism.
Histoplasma
Histoplasmosis is a fungal infection caused by Histoplasma capsulatum. Histoplasma capsulatum is a dimorphic fungus endemic to the Ohio and Mississippi River valleys.
It lives in soil contaminated with bird or bat droppings.
Infection begins with inhalation of spores into the alveoli.
At body temperature:
- The organism converts to yeast form.
- Alveolar macrophages phagocytose it.
At this moment, the entire course of disease depends on the host immune response.
Isolated Pulmonary Infection
What Should Happen
In an immunocompetent host:
- CD4+ T cells activate
- Interferon-gamma is released
- Macrophages become activated
- Intracellular killing increases
- Granulomas form
- Infection becomes contained
The fungus is walled off. Containment is the goal.
Many patients are asymptomatic or have mild, self-limited pulmonary illness.
What Happens in a Transplant Patient
Our patient was on tacrolimus and mycophenolate for his recent liver transplant. These medications suppress T-cell activation and proliferation.
Without effective T-cell signaling:
- Macrophages ingest histoplasma
- But cannot effectively kill it
- The organism survives intracellularly
- Replication continues inside macrophages
Histoplasma does not need to escape the immune system. It uses immune cells as transportation.
Infected macrophages travel through:
- Lymphatic System
- Bloodstream
The infection spreads from:
Lung → Mediastinal nodes → Liver → Spleen → Bone marrow → Adrenals → CNS (in severe cases)
Dissemination of Histoplasmosis
Histoplasmosis achieves widespread dissemination throughout the body primarily through the bloodstream, leveraging the efficiency of the Mononuclear Phagocyte System (MPS).
The MPS, formerly known as the Reticuloendothelial System (RES), is a critical component of the innate immune response and tissue homeostasis. It is a diffuse, interconnected network of highly specialized immune cells, most notably macrophages and their precursors, monocytes. These cells are strategically positioned in various organs to filter and clear foreign and waste materials from the circulation and tissues.
The key anatomical sites where the MPS is concentrated include:
- Spleen: Contains numerous splenic macrophages that filter blood, removing old or damaged red blood cells and blood-borne pathogens.
- Liver: Houses Kupffer cells, which are resident macrophages lining the sinusoids, making the liver the body's primary blood filter for gut-derived material and systemic pathogens.
- Lymph Nodes: MPS cells here filter lymph fluid, clearing debris and microbes from the tissues before the fluid returns to the bloodstream.
- Bone Marrow: The site of monocyte production and contains macrophages involved in hematopoiesis.
The central and most vital function of this system is phagocytosis. Phagocytosis is a complex cellular process of 'cell eating' where specialized cells engulf and internally destroy foreign particles, cellular debris, and pathogens. In the context of the MPS, this process is continuously at work to:
- Clear Foreign Materials and Pathogens: Engulfing invading bacteria, viruses, and, as in the case of histoplasmosis, fungal elements. H. capsulatum yeasts are ingested by macrophages but have the unique ability to survive, replicate, and even thrive within the acidic environment of the phagolysosome, effectively using the MPS cells as a vehicle for systemic spread.
- Remove Toxins: Neutralizing and clearing circulating toxins and immune complexes.
- Recycle Senescent Cells: Efficiently removing and processing old or damaged red blood cells (erythrophagocytosis) and apoptotic cells to recycle essential components like iron.
In the disease process of histoplasmosis, the pathogen exploits the very system designed to destroy it.
After inhalation, the conidia are phagocytosed by pulmonary macrophages. Within these cells, the fungus transforms into its pathogenic yeast phase and evades destruction. The yeast-laden macrophages then travel via the lymphatic system and eventually enter the bloodstream, facilitating widespread dissemination to the various MPS-rich organs, leading to multi-organ involvement characteristic of severe or chronic progressive histoplasmosis.
Common features include:
- Hepatosplenomegaly
- Pancytopenia
- Elevated liver enzymes
- Ascites
- Adrenal involvement
- Systemic inflammatory response
Our Patient's Presentation:
He developed:
- Progressive thrombocytopenia
- Leukopenia
- Hepatosplenomegaly
- Ascites
- Severe hyponatremia
This was not isolated lung disease.
This was systemic fungal proliferation.
The Hyponatremia Clue
His initial severe hyponatremia was one of the earliest red flags.
Disseminated histoplasmosis can cause:
- SIADH
- Adrenal infiltration leading to insufficiency
- Cytokine-mediated alterations in sodium regulation
Even when adrenal imaging is normal, functional impairment can occur.
Hyponatremia in an immunosuppressed patient with pulmonary findings should widen the differential beyond bacterial pneumonia.
SIADH
Definition:
Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is characterized by excessive ADH activity despite dilute plasma.
Pathophysiology:
- Primarily a disorder of water retention, not primary sodium loss.
- Excessive ADH over-activates V2 receptors in the kidney, leading to water reclamation and concentrated urine.
- This causes dilutional hyponatremia.
- Total body sodium stores are usually preserved, resulting in euvolemic hypotonic hyponatremia with inappropriately concentrated urine.
Causes:
- CNS triggers (stress, pain, nausea)
- Pulmonary issues (pneumonia, ventilation)
- Ectopic ADH production (classic: Small Cell Lung Cancer)
- Certain drugs
Presentation:
Euvolemic hyponatremia with neurological symptoms proportional to the speed and magnitude of sodium decline.
Diagnosis:
- Confirm true hypotonic hyponatremia (low serum osmolality), ruling out pseudohyponatremia and hypertonic shifts.
- Confirm SIADH physiology: inappropriately high urine osmolality (>100 mOsm/kg) and non-low urine sodium.
- Exclude endocrine mimics: adrenal insufficiency and hypothyroidism.
- Identify the underlying cause.
Management Goals:
- Stop water accumulation (fluid restriction)
- Safely raise serum sodium (hypertonic saline-3%)
- Treat the root cause (Histoplasmosis)
Case Study
Pt was initially admitted from OSH on NC. Rapid was called for hypoxemia (40% on 6L with improvement to 80% on 15L NRB). Transferred to CCMU and was placed on HFNC but ultimately required intubation. He remained intubated for 3 days and was extubated to HFNC and ultimately weaned to NC. Two days after extubation, he had recurrent desats to 70% despite 15L NC. Placed on HFNC with high settings (75% 50L) with limited ability to wean due to severe hypoxia (PaO2 50s). Pt ultimately reintubated. Pt required escalated support for hypercarbia and hypoxia ultimately resulting in deep sedation and paralysis. Pt continued to have severe hypercarbia (PaCO2- 90) and worsening metabolic acidosis.
Why the Lungs Continued to Worsen
Once dissemination occurs, pulmonary injury escalates for several reasons:
- Ongoing intracellular replication in alveolar macrophages
- Cytokine-driven inflammatory response
- Capillary leak and interstitial edema
- Necrotizing nodules from high fungal burden
His CT showed:
- Diffuse ground glass opacities
- Tree-in-bud nodularity
- Necrotic-appearing nodules
- Exudative pleural effusion
This reflects high organism burden and inflammatory lung injury.
Treatment Strategy
First-line therapy: IV Liposomal Amphotericin B
Mechanism:
- Binds ergosterol in fungal cell membranes
- Creates membrane pores
- Leads to fungal cell death
Amphotericin is fungicidal and acts quickly.
After stabilization, transition to long-term oral itraconazole (often ≥12 months). Itraconazole alone is too slow for severe disease.
Considerations on Amphotericin
Amphotericin is life-saving. But it is also nephrotoxic.
Monitor closely for:
- AKI
- Hypokalemia
- Hypomagnesemia
- Infusion reactions
Our patient had overlapping risks:
- Tacrolimus nephrotoxicity
- Systemic inflammation
- Amphotericin exposure
Layered physiology increases vulnerability.
The ECMO Conversation
ECMO was contacted due to continued hypercarbia and hypoxia. ECMO supports oxygenation and ventilation.
It does not treat:
- Intracellular fungal proliferation
- Bone marrow involvement
- Systemic dissemination Immune dysfunction
ECMO is a bridge to recovery or transplant.
In this case, the patient had severe disseminated fungal infection with multisystem involvement. This was not isolated reversible lung failure. His was deemed not a lung transplant candidate. Therefore, ultimately he was deemed a poor candidate for ECMO.
Based on his low likelihood of meaningful recovery and worsening clinical trajectory, there was a goals of care discussion with his family. Decision was made to transition to comfort care measures. Patient passed comfortably with family at bedside.
Key Takeaways
This patient initially appeared stable. Normal oxygenation. Normal initial imaging. Fatigue and hyponatremia.
He decompensated within days:
- Diffuse lung injury
- Pancytopenia
- Ascites
- Intubation
- ECMO discussion
Histoplasmosis is not “just pneumonia.”
In immunosuppressed patients, it is an intracellular pathogen that hijacks macrophages, spreads through the mononuclear phagocyte system (MPS), and reveals itself only when organ systems begin to fail.
This is not just about fungi. It is about recognizing when immune suppression changes the rules of infection.
Contact Us
Help up improve the Crit Bit Corner. Follow the link below to help us improve the newsletter or suggest future Crit Bit topics!
References
Assi, M., & Miller, R. (2013). Endemic fungal infections in solid organ transplantation. American Journal of Transplantation, 13(Suppl 4), 250–261. Gajurel, K., Dhakal, R., & colleagues. (2017). Histoplasmosis in transplant recipients. Clinical Transplantation, 31(10), e13087. Hakim, L., Krishnan, S., & Kutty, R. G. (2024). Histoplasmosis: More than just pneumonia—Widely disseminated histoplasmosis in a lung transplant recipient. Chest, 166(4 Suppl), 1477A–1478A. https://doi.org/10.1016/j.chest.2024.06.939 Kirmani, N., Durkin, M., & Liang, S. (2020). Washington manual infectious disease subspecialty consult (3rd ed.). Wolters Kluwer Health. Lung transplantation: General guidelines for recipient selection. (2024). In UpToDate. Pathogenesis and clinical features of pulmonary histoplasmosis. (2024). In UpToDate. Pathogenesis and clinical manifestations of disseminated histoplasmosis. (2024). In UpToDate. Diagnosis and treatment of pulmonary histoplasmosis. (2024). In UpToDate. Diagnosis and treatment of disseminated histoplasmosis in patients without HIV. (2024). In UpToDate.