Valley Fever

The Organism

Valley fever, or coccidioidomycosis, is caused by fungi in the genus Coccidioides. Two recognized species cause the disease: Coccidioides immitis, associated primarily with California’s San Joaquin Valley, and Coccidioides posadasii, associated with Arizona, New Mexico, Texas, Mexico, and other endemic regions. Their geographic distributions are not absolute, and both cause essentially the same clinical disease [1, 2].

In the environment, the fungus grows as branching filaments called hyphae. These fragment into arthroconidia, small infectious spores that can become airborne when contaminated soil is disturbed.

After inhalation into the lungs, an arthroconidium typically develops into a spherule—a rounded structure that enlarges and produces numerous internal endospores. When a mature spherule ruptures, the endospores can develop into additional spherules within the infected tissue [1].

This difference between environmental and tissue forms explains an important feature of Valley fever: infected animals and people ordinarily do not produce the airborne arthroconidia responsible for natural transmission. They are therefore not contagious under normal circumstances.

Where It Lives

Coccidioides is associated with soils in arid and semiarid regions of the southwestern United States and northern Mexico. In the United States, its recognized range includes Arizona, California, New Mexico, Texas, and parts of Utah, with particularly high reported human disease burdens in south-central Arizona and California’s San Joaquin Valley [1].

Climate models can help estimate where environmental conditions may support the fungus. One published model used a mean annual temperature above approximately 10.7°C (51.3°F) and annual precipitation below 600 mm (23.62 inches) as criteria for predicting potentially endemic areas [4].

These are statistical modeling criteria, not strict biological survival limits. Local soil conditions, seasonal weather, microclimates, and the uneven distribution of the organism can influence where it occurs.

Researchers have also detected evidence of Coccidioides in northern Arizona. In a study of 171 soil samples from five northern Arizona counties, fungal DNA was detected in six of 75 samples from Coconino County. Genetic comparisons of seven patient isolates from Coconino County found relationships to populations associated with Maricopa and Pima counties [3].

The investigators characterized northern Arizona as moderately or potentially endemic, while noting that travel to more heavily endemic areas in southern Arizona could explain many infections among northern Arizona residents [3].

Finding fungal DNA in soil is evidence that the organism or its genetic material was present. It does not necessarily establish that viable infectious spores were present in every positive sample.

The relationship between dust storms and Valley fever is more complicated than it first appears.

One study reported that regional dust-storm activity increased substantially between the 1990s and 2000s and found an association with Valley fever incidence [5]. However, another study examining individual dust storms in Maricopa County and Kern County, California, found no consistent increase in reported cases following those events [6].

Both findings can be valid. Dust disturbance is a recognized route of exposure, but the amount of infectious material carried by any particular dust event depends on whether viable Coccidioides is present in the disturbed soil.

Environmental sampling also shows that the fungus is distributed unevenly. Investigators may fail to recover it from soils in regions where human disease is common [7].

How It Spreads

Inhalation of airborne arthroconidia is the principal natural route of Valley fever infection.

Spores may become airborne when contaminated soil is disturbed by wind, digging, construction, farming, grading, vehicles, or other activities. Exposure does not require a dramatic dust storm; ordinary soil disturbance can be sufficient when infectious spores are present [1, 2].

Under normal conditions, Valley fever does not spread from animal to animal, animal to person, or person to person.

Rare exceptions to inhalational acquisition have been reported, including traumatic inoculation, transmission associated with organ transplantation, and an unusual case involving a cat bite [1]. These events do not represent ordinary transmission in livestock or households.

A dog and its owner who both develop Valley fever almost certainly acquired their infections independently through environmental exposure rather than transmitting the disease to each other.

Infection Pattern

Cattle. Clinically apparent Valley fever is uncommon. Infection is more often identified incidentally at slaughter or necropsy, when granulomatous lesions are found in the lungs or thoracic lymph nodes.

A granuloma is an organized collection of inflammatory cells that develops around material or organisms the immune system cannot readily eliminate. These lesions may resemble those caused by bovine tuberculosis.

Historical surveys cited in veterinary references described compatible lesions in approximately 5% to 15% of cattle slaughtered in certain Arizona populations [1]. These findings should not be interpreted as the current prevalence of Valley fever in Arizona cattle.

When cattle develop clinical signs, chronic respiratory disease, including a persistent cough, may occur [1].

Horses. Recognized disease is uncommon, but severe pulmonary or disseminated infection can occur. A survey cited in the veterinary literature found antibodies in approximately 4% of clinically healthy horses in an endemic area [1].

Some published case series have reported very high mortality among horses with severe lung disease, pleural effusion—fluid around the lungs—or disseminated infection. An approximately 90% fatality figure cited for severely affected horses reflects a selected clinical population, not all infected horses [1].

Abortion, mastitis, skin lesions, and abscesses have also been reported.

Dogs. Many infected dogs never develop recognizable illness. An estimate cited in the veterinary literature suggests that approximately 70% of infections may be subclinical, although the proportion varies with the population studied [1].

Clinically affected dogs may develop a persistent cough, fever, lethargy, poor appetite, and weight loss.

In some dogs, infection spreads beyond the lungs. Bones are a common site of dissemination, producing pain, swelling, or lameness. Infection of the central nervous system can cause seizures or other neurologic signs.

Veterinary sources describe substantial disease burdens in endemic Arizona counties, including an estimate of approximately 4% of dogs developing clinical disease annually in a studied population [1]. This should not be generalized to every dog or county without confirming the study population and period.

Alpacas and llamas. New World camelids can develop severe, disseminated coccidioidomycosis, sometimes with fatal outcomes.

In a retrospective Arizona study involving more than 1,100 alpacas, more than 4% were diagnosed with coccidioidomycosis over an approximately 11-year period [8]. This cumulative figure is not an annual incidence rate.

What to Do If You Suspect Valley Fever

Call your veterinarian if an animal develops persistent respiratory illness, unexplained weight loss, bone pain, lameness, or other compatible signs, especially after living in or traveling through an endemic area.

In dogs, horses, and camelids, early diagnosis can be important because treatment may require many months, and disseminated disease can be difficult to control [1].

Tell the diagnostic laboratory that Valley fever is suspected before submitting specimens. This is particularly important if fungal culture might be considered. The environmental mold form can produce highly infectious arthroconidia, creating a serious occupational risk for laboratory personnel.

Specialized laboratories can culture Coccidioides using appropriate biosafety precautions, but routine culture should not be undertaken without proper warning and procedures [1].

An infected animal does not ordinarily pose an infection risk to people or other animals during normal handling.

Use standard protective precautions when handling draining lesions, tissue specimens, or contaminated dressings. Wear gloves and prevent material from drying or becoming aerosolized. The principal concern is unusual exposure to infected material and potential fungal growth outside the host, rather than routine transmission from the animal.

Because people and animals may share environmental exposures, a diagnosis in an animal can be a useful reminder that Valley fever occurs in the area. People with persistent cough, fever, fatigue, or other compatible symptoms should discuss possible environmental exposure with a healthcare professional.

Diagnostic Differential

In cattle, granulomatous lesions in the lungs and thoracic lymph nodes may resemble bovine tuberculosis, other fungal infections, and certain bacterial or parasitic conditions.

The appearance of a lesion alone cannot establish its cause. Histopathology and other laboratory tests are needed to distinguish these diseases [1].

Because bovine tuberculosis is a federally regulated disease, suspicious granulomatous lesions discovered during meat inspection require appropriate evaluation under federal inspection procedures. The federal regulations specifically address coccidioidal granuloma in 9 CFR § 311.36 [9].

In horses, dogs, and camelids, differential diagnoses for chronic cough and weight loss include bacterial pneumonia, other systemic fungal infections, neoplasia, and other chronic inflammatory diseases.

In dogs with bone lesions, Valley fever can resemble bone tumors, bacterial osteomyelitis, or other destructive bone conditions on radiographs [1].

Diagnosis

Serology is commonly used to support a diagnosis of Valley fever in live animals.

Available antibody assays include agar gel immunodiffusion (AGID), enzyme immunoassays, and other methods, depending on the species and diagnostic laboratory [1].

Antibody production takes time. An animal tested early in infection may have a negative result despite being infected. Results must therefore be interpreted alongside clinical signs, imaging findings, exposure history, and the timing of illness.

A positive antibody test indicates an immune response to Coccidioides, but it does not always establish that the organism is responsible for the animal’s current clinical signs.

Direct identification of the organism in tissue or fluid provides stronger evidence of infection. In cytology or histopathology, Coccidioides typically appears as a large spherule, often approximately 20–80 micrometers in diameter, containing smaller endospores. Larger spherules may occasionally be observed [1].

Finding characteristic spherules in a compatible lesion can establish the diagnosis.

In cattle, diagnosis is most often made after slaughter or at necropsy through microscopic examination of lung or lymph-node lesions.

Culture is possible in appropriately equipped laboratories, but it carries a significant biosafety risk because the mold phase produces infectious arthroconidia. Laboratory staff must be alerted whenever Coccidioides is suspected [1].

Risk to Humans

People acquire Valley fever primarily by inhaling airborne spores from the environment. They do not ordinarily acquire it from infected animals.

Arizona reported 14,763 human cases in 2024, corresponding to an incidence of approximately 193.7 cases per 100,000 residents, according to the Arizona Department of Health Services annual report [10].

The report also describes 986 hospitalizations and 86 deaths associated with Valley fever that year [10]. These figures should be interpreted according to the report’s case definitions, including whether hospitalizations and deaths were classified as attributable to or associated with the disease.

Many infections cause no symptoms. Others produce a respiratory illness with fever, cough, fatigue, chest pain, and muscle or joint aches. Symptoms may persist for weeks or months.

A minority of infected people develop disseminated disease, in which infection spreads beyond the lungs to sites such as the skin, bones, joints, or meninges—the membranes surrounding the brain and spinal cord.

Risk of severe or disseminated disease is increased in certain populations, including people with impaired cellular immunity and pregnant patients. Epidemiologic studies have also documented increased dissemination risk among people of African or Filipino ancestry [1].

Occupational exposure is important in agriculture and construction. Activities that disturb dry soil—digging, grading roads, operating equipment, farming, and moving livestock through dusty conditions—may increase opportunities for inhalation [11].

Reducing unnecessary dust exposure and using an appropriately fitted N95 or more protective respirator when indicated can reduce inhalational exposure, although no measure completely eliminates risk.

Workers with persistent respiratory symptoms should tell their healthcare provider about occupational dust exposure and the possibility of Valley fever.

Treatment and Prevention

Cattle are not routinely treated for Valley fever because most infections are clinically silent and are discovered incidentally at slaughter [1].

Dogs, horses, and camelids with clinically significant disease may require prolonged antifungal treatment. Commonly used drugs include fluconazole and itraconazole, with the choice depending on the species, disease location, severity, and response [1].

Treatment may continue for several months to a year or longer.

Dogs with disseminated infection often require prolonged therapy, and some animals with central nervous system involvement may need lifelong treatment. The University of Arizona’s Valley Fever Center for Excellence describes substantial treatment responses in dogs with neurologic disease, although outcomes vary [12].

The overall relapse rate after stopping treatment in dogs is not firmly established. Relapse estimates from human disseminated disease should not be assumed to apply directly to dogs.

No vaccine is currently identified as licensed for prevention of Valley fever in animals or people.

University of Arizona researchers have developed and investigated a vaccine candidate, including work directed toward protection in dogs [13].

It is not feasible to eliminate Coccidioides from the regional environment.

Practical exposure reduction focuses on limiting dust disturbance where possible, improving dust control around pens and arenas, and avoiding unnecessary activities that generate airborne soil particles.

Keeping dogs from digging in potentially contaminated soil may reduce one source of exposure, although it cannot eliminate the environmental risk [11].

What Valley Fever Costs Producers

For cattle, the principal economic consequences are associated with lesions detected during slaughter inspection rather than widespread clinical illness on ranches.

Federal meat inspection regulations address carcasses affected by coccidioidal granulomas. Depending on the distribution of lesions and the animal’s condition, affected tissues or the carcass may be condemned according to the regulatory criteria [9].

Localized lesions do not necessarily require condemnation of the entire carcass. Generalized disease or other specified findings can result in more extensive condemnation.

Valley fever is not managed through a federal livestock eradication program and does not ordinarily result in the herd quarantines, routine movement restrictions, or interstate trade controls associated with bovine tuberculosis or brucellosis.

For horses, dogs, and camelids, costs arise from diagnostic testing, prolonged antifungal medication, repeated veterinary visits, and the possibility of relapse or severe disseminated disease [1, 12].

The largest documented regional economic burden is in human healthcare.

The Arizona 2024 annual report lists approximately $95.5 million in hospital charges associated with Valley fever hospitalizations and a median charge of $64,552 per hospitalization [10].

Hospital charges are billed amounts, not necessarily the actual cost of providing care or the amount paid by patients and insurers.

Where to Report a Case in the Southwest

Valley fever is not a federal livestock eradication-program disease. Routine animal coccidioidomycosis reporting requirements are not identified in the Arizona, New Mexico, or Texas regulations discussed here. Human public-health reporting differs by jurisdiction.

Documented Incidence in the Southwest

Arizona and California account for the great majority of reported US human Valley fever cases.

During 2011–2017, approximately 97% of nationally reported cases came from these two states, with Arizona accounting for about 64.5% [18].

Reported incidence in Arizona has also changed over time. An analysis covering 2005–2022 documented increasing incidence across regions, with particularly large relative increases in the northern Plateaus region [19].

The Plateaus region, which includes Coconino County, experienced a 2020–2022 incidence rate approximately 6.61 times its 2005–2007 rate. Nevertheless, the region accounted for less than 1.5% of the state’s total reported cases [19].

This illustrates the difference between relative growth and absolute disease burden: a region with relatively few cases can experience a large percentage increase without approaching the case counts of heavily affected counties.

The investigators discussed several possible explanations, including changes in population, travel, clinical awareness, testing, and environmental suitability. The surveillance data alone cannot determine how much of the increase represents newly established local transmission [19].

Systematic statewide animal surveillance remains limited.

Arizona

Arizona reported 14,763 human Valley fever cases in 2024, with approximately 93% among residents of Maricopa, Pima, and Pinal counties [10].

Coconino County reported 98 cases, corresponding to approximately 65.4 cases per 100,000 residents [10].

These are counts by residence. Some Coconino County residents may have acquired their infections while visiting or working in more heavily endemic parts of southern Arizona [15].

The 2024 Arizona annual report also discusses a problem involving a batch of diagnostic test kits used from January through March. As many as 800 reported positives may have been false positives [10].

Environmental studies have detected Coccidioides DNA in Coconino County soil [3], providing evidence relevant to the possibility of local environmental exposure.

Flagstaff’s 1991–2020 climate normals include a mean annual temperature of approximately 8.2°C and annual precipitation of 521 mm [20].

Compared with the published climate model, Flagstaff meets the precipitation criterion but falls approximately 2.5°C below the modeled temperature criterion [4]. This does not establish that the fungus cannot occur there. The model describes broad environmental suitability, while actual fungal occurrence depends on local conditions.

Historical veterinary surveys have reported coccidioidal lesions in cattle slaughtered in parts of Arizona, with estimates of approximately 5%–15% in certain sampled populations [1]. Those findings cannot be used to estimate current statewide cattle prevalence.

New Mexico

Human coccidioidomycosis is reportable in New Mexico, but a recent comprehensive state-specific annual incidence series is not available in the surveillance studies discussed here.

In national surveillance for 2011–2017, New Mexico was grouped with Nevada and Utah. Together, these states reported 1,394 cases, approximately 1.5% of the national total, with reported incidence estimates ranging from 2.0 to 3.5 per 100,000 [18].

These combined figures should not be presented as New Mexico’s individual case count.

Recent statewide animal prevalence in New Mexico has not been established by the published studies discussed here. That limitation does not establish that livestock infections are absent.

Texas

Coccidioides occurs in West Texas, and human Valley fever has been recognized in the region [1].

The statewide Texas notifiable-conditions list does not include coccidioidomycosis [17]. Consequently, routine statewide case counts comparable to Arizona’s surveillance figures are not available from that reporting system.

El Paso has historically conducted local surveillance [18].

Current statewide prevalence of Valley fever in Texas cattle, horses, dogs, or camelids has not been established by the published studies discussed here.

References

1. Spickler AR. Coccidioidomycosis. Center for Food Security and Public Health, Iowa State University, September 2021. https://www.cfsph.iastate.edu/Factsheets/pdfs/coccidioidomycosis.pdf↑ a b c d e f g h i j k l m n o p q r s t u v w x
2. About Valley fever (coccidioidomycosis). US Centers for Disease Control and Prevention, reviewed 24 April 2024. https://www.cdc.gov/valley-fever/about/index.html↑ a b
3. Mead HL, Kollath DR, de Melo Teixeira M, Roe CC, Plude C, Nandurkar N, Donohoo C, O’Connor BLW, Terriquez J, Keim P, Barker BM. Coccidioidomycosis in northern Arizona: an investigation of the host, pathogen, and environment using a disease triangle approach. mSphere 2022;7(5). https://doi.org/10.1128/msphere.00352-22↑ a b c
4. Gorris ME, Treseder KK, Zender CS, Randerson JT. Expansion of coccidioidomycosis endemic regions in the United States in response to climate change. GeoHealth 2019. https://doi.org/10.1029/2019GH000209↑ a b
5. Tong DQ, Wang JXL, Gill TE, Lei H, Wang B. Intensified dust storm activity and Valley fever infection in the southwestern United States. Geophysical Research Letters 2017;44(9):4304–4312. https://doi.org/10.1002/2017GL073524↑
6. Comrie AC. No consistent link between dust storms and Valley fever (coccidioidomycosis). GeoHealth 2021;5(12). https://doi.org/10.1029/2021GH000504↑
7. Wagner R, Montoya L, Head JR, Campo S, Remais J, Taylor JW. Coccidioides undetected in soils from agricultural land and uncorrelated with time or the greater soil fungal community on undeveloped land. PLOS Pathogens 2023;19(5). https://doi.org/10.1371/journal.ppat.1011391↑
8. Butkiewicz CD, Shubitz LF. Coccidioidomycosis in alpacas in the southwestern United States. Transboundary and Emerging Diseases 2019;66(2):807–812. https://doi.org/10.1111/tbed.13090↑
9. 9 CFR § 311.36: Coccidioidal granuloma. US federal meat inspection regulations. https://www.law.cornell.edu/cfr/text/9/311.36↑ a b
10. Valley fever 2024 annual report. Arizona Department of Health Services. https://www.azdhs.gov/documents/preparedness/epidemiology-disease-control/valley-fever/reports/valley-fever-2024.pdf↑ a b c d e f
11. Preventing Valley fever. US Centers for Disease Control and Prevention, reviewed 24 April 2024. https://www.cdc.gov/valley-fever/prevention/index.html↑ a b
12. Prognosis and outcome: Valley fever in dogs. Valley Fever Center for Excellence, University of Arizona. https://vfce.arizona.edu/valley-fever-dogs/prognosis-outcome↑ a b
13. A Valley fever vaccine for dogs. Valley Fever Center for Excellence, University of Arizona. https://www.vfce.arizona.edu/sites/default/files/2026-04/VaccineBrochure.pdf↑
14. Arizona Administrative Code § R3-2-402: Mandatory disease reporting by veterinarians and veterinary laboratories. https://www.law.cornell.edu/regulations/arizona/Ariz-Admin-Code-SS-R3-2-402↑
15. Coccidioidomycosis surveillance caveats. Arizona Department of Health Services. https://www.azdhs.gov/documents/preparedness/epidemiology-disease-control/valley-fever/coccidioidomycosis-surveillance-caveats.pdf↑ a b
16. 7.4.3.13 NMAC: Notifiable conditions and reporting timeframes. New Mexico Department of Health. https://www.srca.nm.gov/parts/title07/07.004.0003.pdf↑
17. Texas notifiable conditions, 2025. Texas Department of State Health Services. https://www.dshs.texas.gov/sites/default/files/IDCU/investigation/Reporting-forms/notifiable-conditions-2025.pdf↑ a b c d
18. Benedict K, McCotter OZ, Brady S, et al. Surveillance for coccidioidomycosis — United States, 2011–2017. MMWR Surveillance Summaries 2019;68(7). https://www.cdc.gov/mmwr/volumes/68/ss/ss6807a1.htm↑ a b c d
19. Kruger SE, Ruberto I, Williamson T, Remais JV, Heaney AK, Head JR. Regional increases in incidence of coccidioidomycosis (Valley fever) — Arizona, 2005–2022. MMWR 2026;75(6):85–91. https://www.cdc.gov/mmwr/volumes/75/wr/mm7506a3.htm↑ a b c
20. Flagstaff, Arizona: normals, means and extremes, 1991–2020. Arizona State Climate Office, Arizona State University, 2025. https://globalfutures.asu.edu/azclimate/wp-content/uploads/sites/35/2025/06/Flagstaff.pdf↑