Histoplasmosis

Pathophysiology

1) fungal infection causes by Histoplasma capsulatum 2) immunocompetent patient – infection is self-limited and largely asymptomatic 3) symptomatic in patients with defective cell-mediated mechanisms 4) clinically resembles tuberculosis Primary histoplasmosis – 5) alveolar macrophages phagocytose spores but do not kill them 6) subsequent growth of spore causes more macrophages to move to area of infection 7) result is area of lung consolidation 8) infected macrophages are transmitted to hilar lymph nodes to mediastinal lymph nodes to systemic distribution (liver, bone marrow, spleen, tongue and mouth mucosa, adrenals)

Signs and Symptoms

1) weakness 2) episodic fever 3) weight loss 4) headache 5) malaise 6) cough 7) mental status changes

Characteristic Test Findings

Radiology – 1) chest radiograph may be confused with TB (scattered infiltrates and hilar lymphadenopathy) Laboratory – 2) pancytopenia in severe disease

Histology/Gross Pathology

1) H. capsulatum is a dimorphic fungus – mold at ambient temperature and yeast at body temperature 2) yeast form – central basophilic body and encircling halo pattern inside a rigid cell wall 3) yeast form can cause chronic granulomas 4) in soil H. capsulatum produces micronidia (that reach alveoli owing to their 2.5 micron size) and macronidia 5) affected lung tissue shows subpleural, encapsulated, caseating granulomas, which eventually may calcify 6) fungus can be seen in cells of reticular endothelial system

Associated Conditions

if affects adrenals can cause secondary Addison’s disease.

Biochemistry

1-3 weeks after infection the host produces hypersensitivity and cell-mediated response to H. capsulatum

Inheritance/Epidemiology

1) occurs worldwide 2) acquired by inhaling infectious spores 3) reservoir is bat and bird droppings and soil 4) endemic in Ohio-Mississippi Valley and eastern USA, Mexico, Central America, and northern South America 5) most commonly seen in HIV, transplant, infant, and chronic steroid use patients 6) amount of inoculum is a factor in clinical virulence

Treatment

systemic antifungals like amphotericin

Tips for USMLE

look for a 34 year old kidney transplant recipient who developed intermittent fevers, headache, and cough 10 days after cleaning out his pigeon coop