Coccidioidomycosis in Horses: Valley Fever Fungal Infection

Quick Answer
  • Coccidioidomycosis, also called valley fever, is a noncontagious fungal infection horses usually get by breathing in dust that contains Coccidioides spores.
  • It is most associated with dry, desert-like areas of the southwestern United States, plus parts of Mexico and Central and South America.
  • In horses, reported signs can include weight loss, cough, fever, musculoskeletal pain, skin abscesses, and in some cases abortion or bone infection.
  • Diagnosis often takes more than one test. Your vet may recommend an exam, bloodwork, chest imaging, and tissue sampling or biopsy to look for the organism.
  • Treatment options vary. Mild cases may be monitored closely, while chronic or multisystem disease often needs months of antifungal medication and repeat rechecks.
Estimated cost: $400–$8,000

What Is Coccidioidomycosis in Horses?

Coccidioidomycosis is a fungal infection caused by Coccidioides species, often called valley fever. In horses, it is considered uncommon, but it can become serious when it causes ongoing lung disease or spreads beyond the lungs. The infection is not contagious from horse to horse.

Most horses are exposed by breathing in fungal spores carried on dust. Because of that, the disease is linked to dry, arid regions, especially the southwestern United States. Some horses may have little to no illness after exposure, while others develop chronic respiratory signs or more widespread disease.

In horses, published veterinary references describe weight loss, cough, fever, musculoskeletal pain, and skin abscesses as common reported signs. More severe cases can involve bone inflammation or the placenta, which may lead to abortion. That wide range is one reason this condition can be easy to miss early on.

If your horse lives in or has traveled through an endemic desert region and develops a lingering cough, unexplained weight loss, or fever, it is worth bringing up valley fever with your vet.

Symptoms of Coccidioidomycosis in Horses

  • Chronic cough
  • Weight loss or poor body condition
  • Fever
  • Musculoskeletal pain or lameness
  • Skin abscesses or draining swellings
  • Reduced appetite and low energy
  • Abortion in a pregnant mare
  • Rapid breathing or increased effort to breathe

See your vet immediately if your horse is struggling to breathe, has a high fever, is rapidly losing weight, becomes lame without a clear reason, or a pregnant mare shows signs of illness or pregnancy loss. Valley fever can look like other problems, including bacterial pneumonia, abscess disease, cancer, or other fungal infections.

Milder signs like a lingering cough, reduced stamina, or subtle weight loss still deserve attention, especially in horses living in desert regions or after dusty weather. Early evaluation gives your vet more options and may help avoid delays in diagnosis.

What Causes Coccidioidomycosis in Horses?

This disease is caused by Coccidioides fungi that live in soil. In dry, shallow desert soil, the fungus forms infectious spores that can become airborne when the ground is disturbed. Horses are thought to become infected mainly by inhaling those spores in dust.

Risk is highest in arid and semiarid areas, especially in the southwestern United States. Dust storms, construction, arena dust, drought after rainy periods, and other events that stir up soil may increase exposure. Horses do not catch valley fever from another horse, and people do not get it from routine contact with an infected horse.

Not every exposed horse becomes sick. Some likely have mild or inapparent infection, while others develop chronic lung disease or more widespread illness. General stress, poor overall health, or immune compromise may make fungal disease more likely to take hold, but horses can be affected even without an obvious predisposing problem.

Rarely, infection may involve the placenta or spread to bone and other tissues. That is why your vet may think beyond the lungs if a horse has fever plus lameness, draining swellings, or reproductive loss.

How Is Coccidioidomycosis in Horses Diagnosed?

Diagnosis usually starts with a careful history and exam. Your vet will want to know where your horse lives, whether there has been travel to desert regions, how long signs have been present, and whether the problem seems limited to the lungs or involves other body systems.

Because the signs overlap with many other conditions, testing is often layered. Common first steps may include a complete blood count, chemistry panel, and imaging such as thoracic ultrasound or chest radiographs if respiratory disease is suspected. If there are skin masses, draining tracts, enlarged lymph nodes, or abnormal bone areas, your vet may recommend needle aspirates, cytology, or biopsy.

A confirmed diagnosis is made by identifying the organism in tissue, where it appears as characteristic spherules. Serology can also help support the diagnosis, and veterinary references describe antibody tests such as AGID and EIA as useful tools. Still, a positive antibody test does not always prove active disease by itself, because some animals may test positive after prior exposure without current illness.

In practice, your vet may combine geography, clinical signs, imaging, lab work, serology, and tissue results to decide how likely valley fever is. That stepwise approach helps separate it from pneumonia, abscess disease, neoplasia, tuberculosis-like granulomas, and other causes of chronic illness.

Treatment Options for Coccidioidomycosis in Horses

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$400–$1,500
Best for: Horses with mild, stable signs, uncertain diagnosis, or pet parents who need to phase testing and treatment over time with their vet.
  • Farm call or haul-in exam
  • Basic bloodwork such as CBC and chemistry panel
  • Focused diagnostics based on the main complaint
  • Close monitoring of temperature, appetite, breathing, and body condition
  • Environmental dust reduction while your vet continues the workup
  • Discussion of whether watchful waiting is reasonable in a mild, stable case
Expected outcome: Variable. Some mild infections may improve, but horses with chronic respiratory signs or disease outside the lungs often need more active treatment.
Consider: Lower upfront cost range, but there is a higher chance of delayed diagnosis or delayed antifungal treatment if the disease is progressing.
Most Common

Recommended Standard Treatment

$1,500–$5,000
Best for: Most horses with confirmed or strongly suspected valley fever that have ongoing cough, fever, weight loss, skin lesions, or musculoskeletal pain but are stable enough for outpatient care.
  • Complete exam and repeat rechecks
  • CBC, chemistry panel, and inflammatory monitoring
  • Thoracic imaging and targeted sampling of lesions when possible
  • Serology plus tissue sampling or biopsy if accessible
  • Long-term oral antifungal treatment directed by your vet
  • Monitoring for medication tolerance, liver values, weight, and clinical response over months
Expected outcome: Fair to guarded, depending on how early the disease is found and whether it is limited to the lungs or has disseminated.
Consider: This is often the most practical middle path, but treatment can last for months and medication plus recheck costs add up over time.

Advanced / Critical Care

$5,000–$8,000
Best for: Horses with severe respiratory disease, suspected bone involvement, pregnancy-related complications, marked debilitation, or cases that have not responded as expected.
  • Referral or hospital-based evaluation
  • Expanded imaging, repeated lesion sampling, and specialist consultation
  • Hospitalization for horses with respiratory compromise, severe weight loss, pregnancy complications, or multisystem disease
  • Intravenous fluids, intensive supportive care, and advanced monitoring as needed
  • Longer and more complex antifungal plans with serial lab monitoring
  • Management of complications such as abortion, osteomyelitis, or severe disseminated disease
Expected outcome: Guarded. Some horses improve with aggressive care, but advanced disease carries a more uncertain outlook and may require prolonged treatment.
Consider: Provides the broadest diagnostic and treatment options, but the cost range and time commitment are much higher, and some horses still have a guarded outcome.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Coccidioidomycosis in Horses

Bring these questions to your vet appointment to get the most out of your visit.

  1. Based on where my horse lives or has traveled, how likely is valley fever compared with other causes of these signs?
  2. Which tests are most useful first, and which ones can wait if I need to stage costs over time?
  3. Do you recommend chest imaging, serology, or a biopsy in my horse's case?
  4. Are my horse's signs more consistent with disease limited to the lungs or spread to other tissues?
  5. If we start antifungal treatment, how long might therapy last and what side effects should I watch for?
  6. What monitoring will my horse need during treatment, including bloodwork or repeat imaging?
  7. What changes at home could reduce dust exposure while my horse recovers?
  8. What signs would mean my horse needs urgent re-evaluation or referral?

How to Prevent Coccidioidomycosis in Horses

There is no vaccine or guaranteed prevention method for valley fever in horses. Current veterinary references describe the best prevention as reducing exposure to desert soil and airborne dust in areas where Coccidioides is known to live.

That usually means focusing on practical dust control. Wet down arenas and dry lots when possible, avoid unnecessary soil disturbance in very dusty conditions, and consider limiting turnout or exercise during dust storms or heavy wind events. If your property is in an endemic area, stable management that reduces airborne dust may help lower exposure risk.

Travel history matters too. A horse that normally lives outside the Southwest can still be exposed during transport, competition, breeding, or seasonal stays in endemic regions. Keep a record of where your horse has been, because that information can help your vet connect symptoms with possible fungal exposure later.

Prevention is really about risk reduction, not elimination. If your horse develops a persistent cough, fever, weight loss, unexplained lameness, or draining skin lesions after living in or traveling through dusty desert regions, prompt veterinary evaluation is the most useful next step.

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