Histoplasmosis (Systemic Mycoses)

Question 1

What agent causes histoplasmosis?

Answer 1

Dimorphic fungi Histoplasma capsulatum (thick capsule)

Question 2

How is histoplasmosis transmitted?

Answer 2

• Inhalation (soil-borne)
  
  • > lungs > mononuclear phagocytic system > dissemination
• GI

Question 3

Where (geographically) is histoplasmosis most commonly seen?

Answer 3

• Western hemisphere (31 states)
  
  • Ohio river valley
  • Missouri river valley
  • Rich, moist environments

Question 4

Which species are most commonly affected by histoplasmosis?

Answer 4

Mammalian infection

• Dogs
• Cats

Question 5

Who is the poster child for histoplasmosis?

Answer 5

• Dogs:
  
  • Young (< 4 years)
  • Weimaraner, Pointer, and Brittany (more likely to be outside)
• Cats:
  
  • As common in cats as in dogs
  • Females > males

Question 6

What are the clinical signs in a patient with histoplasmosis?

Answer 6

Most infections are subclinical

Non specific signs

• Dogs
  
  • GI disease (diarrhea, tenesmus)
  • Lung involvement
  • Hepato (HUGE) and splenomegaly
• Cats
  
  • Dyspnea
  • Lymphadenomegaly
  • Hepato and splenomegaly

Question 7

What lab work changes will you see in a patient with histoplasmosis?

Answer 7

Non-specific

Cats: pancytopenia

Question 8

What is the best way to diagnose a patient with suspected histoplasmosis?

Answer 8

• Find the organism
  
  • Cytology (rectal scrape, airway wash)
  • Histopathology of lesion
  • Galactomannan antigen test *
    
    • Antigen in the cell wall of proliferating yeasts (released into tissues and blood)
    • Excreted in urine, so test urine + serum antigen
    • High cross-reactivity with Blastomyces (same treatment)
    • Antigen decreases with therapy and increases with relapse
• Radiographs
  
  • Linear or diffuse interstitial pattern
  • Nodular pattern
  • Hilar lymphadenopathy (LN around trachea)
• Culture (takes time and biosafety concern)
• Serology (Abs not reliable)

Question 9

How do you treat a patient with histoplasmosis?

Answer 9

HARD TO TREAT FUNGAL DISEASES (6 months)

• Fluconazole (cheaper and better choice now)
  
  • Crosses BBB and gets into eye (good for ocular disease)
• Itraconazole (high doses)
• Amphotericin B IV
  
  • Highly toxic but effective
  • For more severe cases
  • More rapid response