Systemic mycoses
Also known as: endemic mycoses, systemic fungal infections
Systemic mycoses are deep fungal infections caused by dimorphic fungi that are acquired by inhaling spores from the environment. Each organism is tied to a specific geographic region, and infection usually begins in the lungs before it can disseminate.
The systemic mycoses are a small group of infections that share three features: the causative fungi are dimorphic, they are geographically restricted, and they infect people with intact immune systems. Dimorphism means the organism grows as a mold in the environment at ambient temperature and converts to a yeast form in tissue at body temperature — captured by the phrase "mold in the cold, yeast in the heat." Infection begins when spores are inhaled, so the lung is the primary site; the host response is granulomatous, and the picture frequently mimics tuberculosis.
Four organisms account for most of the group. Histoplasma capsulatum is endemic to the Mississippi and Ohio River valleys, is associated with bird and bat droppings and cave exposure, and appears as small yeasts packed inside macrophages. Blastomyces dermatitidis occupies the Great Lakes and eastern river valleys and forms a broad-based budding yeast, with a marked tendency to produce skin and bone lesions. Coccidioides species are found in the arid southwestern United States and central California, where the inhaled arthroconidia form spherules filled with endospores rather than budding yeasts; the illness, known as Valley fever, often includes erythema nodosum and arthralgias. Paracoccidioides brasiliensis is endemic to Latin America and buds in a pattern likened to a ship captain's wheel.
Most infections in healthy hosts are mild or asymptomatic and resolve on their own, sometimes leaving a calcified lung nodule. Disseminated disease — spreading to skin, bone, the reticuloendothelial system, or the central nervous system — occurs mainly in patients with impaired cell-mediated immunity. That is the practical distinction from the opportunistic mycoses caused by organisms such as Candida, Aspergillus, and Cryptococcus, which rarely cause invasive disease in an intact host.
USMLE Step 1 tests systemic mycoses in the mycology material alongside general mycology. Questions are almost always pattern recognition: a travel or residence history plus a described tissue morphology. Knowing which fungus goes with which region, which one appears inside macrophages, which one shows broad-based budding, and which one forms spherules resolves most of them.
Key takeaways
- Systemic mycoses are caused by dimorphic fungi that grow as molds in the environment and as yeasts in tissue.
- Infection is acquired by inhaling spores, so disease starts in the lungs and can resemble tuberculosis.
- Histoplasma is found in the Mississippi and Ohio River valleys and grows inside macrophages.
- Blastomyces shows broad-based budding, while Coccidioides forms spherules filled with endospores in the arid Southwest.
- Unlike opportunistic fungi, these organisms infect healthy hosts, though dissemination is largely a problem of impaired cell-mediated immunity.