Pathogen dimorphism
The causative agent of coccidioidomycosis is the fungus Coccidioides immitis. Its main biological feature is dimorphism. Depending on environmental conditions, it exists in two fundamentally different developmental phases:
- Mycelial (saprophytic) phase
- Develops in the external environment (in soil) or during laboratory cultivation at a temperature of 25 °C.
- Structure: the fungus forms thin, colorless, septate hyphae.
- Reproduction: occurs through the formation of characteristic barrel-shaped arthrospores.
- Significance: it is the mycelial phase with arthrospores that is infectious and poses a hazard to humans.
- Tissue (parasitic) phase
- Develops exclusively inside the macroorganism (found in sputum, organs, and tissues).
- Structure: the fungus forms large spherules (12 to 200 µm in diameter).
- Contents: numerous small endospores measuring 2–5 µm mature inside the spherules.
- Life cycle: upon maturation, endospores exit the ruptured spherules, grow, and transform into new spherules, completing the cycle within the host.
- Significance: the tissue form is not contagious to others.
Epidemiology and pathogenesis
Coccidioidomycosis has a clear geographic distribution: the infection is endemic to arid areas of the southwestern United States and Latin American countries. The reservoir and primary source of the pathogen is soil containing arthrospores.
Infection occurs predominantly via the airborne dust route (aerogenic mechanism). Humans inhale arthrospores along with dust; contact transmission occurs less frequently. The incubation period ranges from one to six weeks.
Stages of infection development in the body:
- Fungal arthrospores enter the respiratory tract.
- The pathogen transforms into the parasitic tissue form (spherule).
- Endospores are massively released from matured spherules and spread throughout the body.
- Secondary foci of infection form in various systems and organs.
Clinical forms and immunity
Coccidioidomycosis can present in several clinical variants:
- Primary pulmonary infection. Often presents with flu-like symptoms.
- Allergic manifestations. Skin reactions may occur in the form of erythema nodosum or erythema multiforme.
- Disseminated forms. The most severe clinical course, accompanied by profound lesions of the skin, bones, joints, and internal organs.
In response to the invasion of C. immitis, cellular immunity is established in the body. The immune response proceeds via delayed-type hypersensitivity (DTH).
Microbiological diagnostics
Important: since C. immitis is classified as a high-consequence (highly virulent) microorganism, all handling of it must be performed strictly in specialized laboratories equipped with laminar flow cabinets. Test specimens include sputum, pus, blood, urine, or cerebrospinal fluid.
Comprehensive diagnostics includes the following methods:
- Microscopic method
Aiming to detect characteristic spherules and endospores in the specimen. Hematoxylin and eosin staining, Van Gieson staining, Wright's stain, Gram-Weigert stain, and the PAS reaction are used for visualization.
- Mycological (culture) method
Inoculation is performed on Sabouraud agar and standard nutrient media, incubating at 25 °C. Grayish fluffy colonies grow in about a week. They reveal the mycelial phase of the pathogen (hyphae, chlamydospores, arthrospores).
- Serological method
Determination of specific antibodies in blood serum using precipitation tests, indirect hemagglutination assays (IHA), and complement fixation tests (CFT).
- Immunodiagnostics (skin tests)
Cutaneous allergic tests are performed using allergens from different developmental phases: coccidioidin (from the mycelial phase) or spherulin (from the tissue phase).
- Biological assay (in vivo)
Experimental infection of animals (white mice or hamsters). The mycelial suspension is administered intratesticularly to subsequently monitor the transformation of the fungus into the tissue phase (formation of typical spherules).
First-line drugs for etiotropic therapy include fluconazole, itraconazole, or amphotericin B.
Briefly: answers to topical questions
Which internal organs (aside from the skin, bones, and joints) are most frequently affected in the disseminated form of coccidioidomycosis?
Sources do not specify which exact internal organs are affected most frequently. They only indicate disseminated multiorgan involvement with the formation of infiltrates and abscesses; peritonitis may develop.
To memorize the phases of the dimorphic fungus: SMArt — Saprophytic phase, Mycelium, Artrospores (in the environment). PETs — Parasitic phase, Endospores, Tissue form, spherules (in the body).
Class and exam questions
Is a patient with coccidioidomycosis contagious to others?
No, the patient is completely non-contagious. The hazard is posed solely by arthrospores (the mycelial phase), which form in the external environment at 25 °C, whereas the human body harbors the non-contagious tissue phase (spherules).
Where and how can one contract this infection?
The disease is endemic to arid regions of the southwestern United States and Latin America. Infection occurs via the airborne dust route by inhaling infected dust containing soil-derived arthrospores.
What is the dimorphism of Coccidioides immitis?
This is the ability of the fungus to change its morphology. In soil (at 25 °C), it grows as a mycelium with barrel-shaped arthrospores, whereas upon entering the human body, it transforms into large spherules filled with endospores.
Why is a biological assay in animals performed and how exactly?
The bioassay is performed on white mice or hamsters to confirm the diagnosis. A mycelial suspension is administered intratesticularly to observe how the fungus transforms into the characteristic parasitic phase (spherules).
What else to study on this topic
- Mechanisms of delayed-type hypersensitivity (DTH) development in fungal infections
- Biosafety requirements and design of laminar flow cabinets for handling especially dangerous fungi
- Differences between primary pulmonary coccidioidomycosis and bacterial and viral pneumonias
- Principle of the PAS reaction method and specific features of fungal structure staining
- Mechanism of action of amphotericin B and azoles on the cell wall of dimorphic fungi
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «Coccidioidomycosis» and 4,383 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the section «Systemic endemic mycoses (histoplasmosis, blastomycosis, coccidioidomycosis, cryptococcosis)»
Entire section «Systemic endemic mycoses (histoplasmosis, blastomycosis, coccidioidomycosis, cryptococcosis)»Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.