Coccidioides (Valley Fever): Spherules, Arthroconidia, and Lab Diagnosis

A construction worker in Arizona develops fever, cough, and chest pain a couple of weeks after a dust storm blew through an excavation site. It looks like pneumonia, but antibiotics do nothing. The clue is not in the chest film but in the geography and the dust: he has inhaled the spores of a desert-soil fungus, and in his lungs those spores have swollen into large, round structures packed with tiny endospores. This is coccidioidomycosis, or Valley fever, and the fungus behind it, Coccidioides, is the one dimorphic fungus that does not become a yeast in the body. It becomes a spherule.

Coccidioides is a thermally dimorphic fungus and the cause of coccidioidomycosis (Valley fever, or San Joaquin Valley fever). It is a member of the classic dimorphic fungi, but it breaks the usual rule in a way that is the single most tested fact about it: instead of converting from mold to yeast at body temperature, it converts from mold to a spherule, a large structure filled with endospores. For where it sits among the other dimorphic fungi, see the dimorphic fungi overview.

Two species, one disease

Coccidioides is now recognized as two species that are clinically identical and handled the same way: Coccidioides immitis, found mainly in California (the San Joaquin Valley), and Coccidioides posadasii, found elsewhere across the endemic range. Because they cause the same disease, look the same, and are managed the same way, they are usually discussed together simply as Coccidioides.

What Coccidioides looks like

Coccidioides looks completely different in its two forms, and each has a diagnostic structure.

In the environment (mold form, 25 to 30°C). In desert soil it grows as a mold with septate hyphae that break up into arthroconidia: small, thick-walled, barrel-shaped spores formed by fragmentation of the hyphae, classically alternating with empty (disjunctor) cells along the filament. These arthroconidia are light, airborne, and highly infectious. They are the form that infects people and the form that makes laboratory cultures dangerous.

In tissue (the spherule, 37°C). Once inhaled, each arthroconidium rounds up and enlarges into a spherule: a large (typically 20 to 80 µm, sometimes up to 200 µm), thick-walled, round structure. The spherule fills with hundreds of small endospores. When the mature spherule ruptures, it releases the endospores, each of which can form a new spherule, and the cycle repeats in the tissue. This spherule-with-endospores is the diagnostic tissue form and the reason Coccidioides is the dimorphic fungus that does not make a yeast.

The exam-trap exception: every other classic dimorphic fungus becomes a yeast in tissue. Coccidioides becomes a spherule packed with endospores. If you remember one thing, remember that Coccidioides is dimorphic (mold in the environment, spherule in tissue), but its tissue form is a spherule, not a yeast.

How people get Valley fever

Infection follows inhalation of arthroconidia when arid soil is disturbed, by wind, dust storms, farming, construction, excavation, or archaeological digging. The organism lives in the alkaline desert soils of the endemic regions: the southwestern United States (Arizona, California's San Joaquin Valley, and neighboring states), northern Mexico, and parts of Central and South America. Outbreaks follow soil disturbance and dust storms, and cases rise in dry seasons after wet winters (a "grow then blow" pattern). It is not transmitted from person to person. Occupational and recreational soil exposure in an endemic area is the key epidemiological clue.

Pathogenesis

Coccidioides is a primary pathogen: it can infect healthy people, not only the immunocompromised, although immunosuppression greatly increases the risk of severe and disseminated disease.

The sequence follows the two-form life cycle. Inhaled arthroconidia reach the alveoli and convert into spherules. As each spherule matures and ruptures, it releases endospores that spread locally and form new spherules, amplifying the infection in the lung. The host mounts a granulomatous inflammatory response. In most people, cell-mediated immunity contains the infection, and the disease is mild or self-limited. When cell-mediated immunity is weak, the organism disseminates through the blood to the skin, bones and joints, and, most seriously, the meninges.

Certain groups are at markedly higher risk of dissemination: people with impaired cell-mediated immunity (HIV/AIDS, transplant, high-dose steroids), pregnant women (especially in the third trimester), and, for reasons still studied, people of African and Filipino ancestry.

Clinical forms of coccidioidomycosis

1. Asymptomatic or primary pulmonary infection. About 60% of infections are silent or mild. Symptomatic primary disease is a flu-like or pneumonia-like illness (fever, cough, chest pain, fatigue) developing 1 to 3 weeks after exposure. This is "Valley fever." Some patients develop hypersensitivity signs such as erythema nodosum or erythema multiforme, and the combination of fever, joint aches, and erythema nodosum is sometimes called "desert rheumatism."
2. Chronic pulmonary coccidioidomycosis. A minority develop persistent pneumonia, lung nodules, or thin-walled cavities that can mimic tuberculosis or lung cancer.
3. Disseminated coccidioidomycosis. In a small proportion, especially the high-risk groups above, the organism spreads beyond the lungs to the skin (verrucous lesions), bones and joints, and the meninges. Coccidioidal meningitis is the most serious form, is difficult to treat, and requires lifelong therapy.

Laboratory diagnosis

A critical safety warning first. Coccidioides is one of the most hazardous fungi in the laboratory. The mold form's arthroconidia are extremely infectious by inhalation, and it is a recognized cause of laboratory-acquired infection and a select agent. If coccidioidomycosis is suspected, the laboratory must be informed, and any mold culture must be handled in a biological safety cabinet, with work on confirmed or strongly suspected cultures done under BSL-3 conditions. Cultures must never be opened or examined on an open bench.

Direct microscopy and histopathology. The diagnosis can be made rapidly by finding the spherule filled with endospores in sputum, other respiratory specimens, pus, or tissue, seen on a KOH preparation or on stained tissue sections (PAS, Gomori methenamine silver). Seeing a mature endospore-filled spherule is diagnostic. A useful safety point: the spherule (tissue form) is not infectious to handle; it is the mold-form arthroconidia that are dangerous.

Culture. Coccidioides grows within a few days to about two weeks on Sabouraud dextrose agar and routine fungal media at 25 to 30°C as a white to tan, cottony mold. Because this mold form is so hazardous, once growth suggestive of Coccidioides appears, the culture is handled with maximum caution and identified by nucleic acid probe, DNA sequencing, or antigen methods rather than by extensive manipulation. Converting the mold to the spherule form in the laboratory is difficult and is not routinely done.

Serology. Antibody testing is central to diagnosis and follow-up. Early infection produces IgM (detected by tube precipitin or immunodiffusion), followed by IgG (detected by complement fixation). The complement fixation (CF) titer is clinically important: a high or rising CF titer suggests severe or disseminated disease, and CF antibody in the cerebrospinal fluid supports the diagnosis of coccidioidal meningitis. Enzyme immunoassays are widely used for screening.

Antigen and molecular tests. A urine and serum Coccidioides antigen test is available and is most useful in severe or disseminated disease and in the immunocompromised, though it can cross-react with other endemic mycoses such as histoplasmosis. PCR on clinical specimens is increasingly available.

Skin test. The coccidioidin or spherulin skin test indicates prior exposure and delayed-type hypersensitivity. Like the histoplasmin test, it is an epidemiological and prognostic tool (loss of skin-test reactivity in disseminated disease is a poor sign), not a routine diagnostic test.

Treatment

Treatment depends on severity and site, and this section gives drug-of-choice and class-level guidance only.

Mild, uncomplicated primary pulmonary infection in a healthy person often resolves on its own and may need only observation. When treatment is required, an azole, usually fluconazole or itraconazole, is the drug of choice for mild to moderate disease. Severe or rapidly progressive pulmonary disease, disseminated disease, and infection in the immunocompromised are treated with a polyene, amphotericin B (liposomal), often followed by an azole. Coccidioidal meningitis requires lifelong azole therapy (high-dose fluconazole), because relapse is common if treatment stops. Specific doses and durations are decided by the treating clinician. No vaccine is available; prevention relies on reducing soil-dust exposure in endemic areas.

How to remember

Spherule, not yeast, is the whole point. Every other classic dimorphic fungus becomes a yeast in tissue. Coccidioides becomes a spherule packed with endospores. This is the single most tested fact about it.

Arthroconidia in the air, spherules in the tissue. In the desert soil (cold) the mold makes barrel-shaped arthroconidia that are inhaled. In the body (hot) they become spherules. Mold form infects, spherule form is the tissue diagnosis.

Dust and desert equals Valley fever. Arid soil in the southwestern USA, disturbed by wind, farming, or construction. A pneumonia that will not respond to antibiotics in someone from Arizona or the San Joaquin Valley should raise coccidioidomycosis.

The arthroconidia are the biohazard, not the spherule. The airborne mold-form arthroconidia cause laboratory-acquired infection, so cultures go in a safety cabinet. The tissue spherule is safe to handle.

Meningitis means lifelong treatment. Coccidioidal meningitis is the most feared form and needs lifelong azole therapy because it relapses.

Key exam facts

Where students get confused

Coccidioides does not form a yeast in tissue. It forms a spherule packed with endospores. It is still dimorphic (mold in the environment, spherule in the body), but the tissue form is a spherule, which is exactly why it is the classic exception among the dimorphic fungi.

The spherule is safe to handle; the mold is not. The tissue spherule is not the infectious form. The danger is the mold-form arthroconidia, which are airborne and cause laboratory-acquired infection. This reverses the intuition that the "tissue form" is the dangerous one.

Valley fever is not tuberculosis, even when it looks like it. Chronic pulmonary coccidioidomycosis can produce cavities and a wasting illness that mimics TB. Geography and travel history (arid southwestern soil) are the discriminating clue.

It infects healthy people, not just the immunocompromised. Coccidioides is a primary pathogen. Immunosuppression, pregnancy, and certain ancestries raise the risk of dissemination, but a healthy person can get primary Valley fever.

A falling skin test in disease is a bad sign, not a good one. Loss of delayed-type hypersensitivity (skin-test reactivity) in a patient with coccidioidomycosis signals failing cell-mediated immunity and disseminated disease, the opposite of recovery.

References

1. Tille PM. Bailey and Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
2. Procop GW, Church DL, Hall GS, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Philadelphia: Wolters Kluwer; 2017.
3. Nguyen C, Barker BM, Hoover S, et al. Recent advances in our understanding of the environmental, epidemiological, immunological, and clinical dimensions of coccidioidomycosis. Clin Microbiol Rev. 2013;26(3):505-525. https://doi.org/10.1128/CMR.00005-13
4. Centers for Disease Control and Prevention. Valley Fever (Coccidioidomycosis). https://www.cdc.gov/valley-fever/