Histoplasmosis in Dogs: The Fungal Disease Ohio Dog Owners Need to Know About

• Read next: Most Ohio Dog Owners Have Never Heard of It. That's the Problem.

• Also in this series: The Fungal Disease That Doesn't Care Where You Live
  

---

Last week, I wrote about Blastomycosis — a serious fungal infection that's endemic to the Ohio River Valley and disproportionately affects sporting and hunting breeds. If you haven't read that post, it's worth your time, especially if your dog spends time outdoors near water.

But Blastomycosis isn't the only fungal threat lurking in Ohio soil.

In fact, it isn't even the most common.

Research on dogs in Cincinnati found Histoplasma capsulatum—the fungus responsible for Histoplasmosis—in 22% of healthy-appearing dogs that were tested. Blastomyces, by comparison, was found in just 2%.

Histoplasmosis is more common. It's just as serious. And most dog owners have never heard of it.

Let's change that.

What Is Histoplasmosis?

Histoplasmosis is a systemic fungal infection caused by Histoplasma capsulatum, a dimorphic fungus that lives in soil. Like Blastomycosis, it's usually contracted by inhaling microscopic fungal spores. Once inhaled, the fungus can spread from the lungs through the bloodstream and affect multiple organs throughout the body.

Where Histoplasmosis differs is where the fungus thrives.

It grows best in soil that's been enriched by bird or bat droppings—a detail that makes certain environments much riskier than others.

Where The Risk Lives

Histoplasmosis is endemic throughout the Ohio, Mississippi, and Missouri River valleys, placing Ohio dog owners squarely inside the risk zone.

Unlike Blastomycosis, which is broadly associated with moist, decomposing soil near waterways, Histoplasmosis is strongly linked to areas where bird or bat droppings have accumulated over months or years.

High-risk locations include:

• Caves and caverns

• Old barns and farm structures

• Attics and crawl spaces

• Trees used as bird roosts (especially starlings, pigeons, and blackbirds)

• Vacant or abandoned buildings

• Construction and excavation sites

When contaminated soil is disturbed—whether by digging, demolition, wind, or an enthusiastic dog sniffing the ground—fungal spores become airborne.

Inhalation is the primary route of infection.

Unlike Blastomycosis, however, dogs can also become infected by ingesting contaminated material, allowing the fungus to establish itself in the intestinal tract before spreading elsewhere in the body.

For sporting dogs, hunting dogs, and any dog that spends time outdoors in Ohio, the risk is both real and close to home.

Which Dogs Are Most Vulnerable?

Young, large-breed dogs appear to be at the greatest risk, much like Blastomycosis. Pointers, Weimaraners, and Brittany Spaniels are overrepresented in reported cases, likely because of the environments they work in—not because they're genetically predisposed.

Dogs doing exactly what they were bred to do—running fields, working cover, exploring woods, or investigating old farm structures—simply experience greater exposure.

Dogs younger than four years old appear to be affected more often, although Histoplasmosis can occur at any age. Unlike some fungal diseases, males and females appear to be affected equally.

Repeated exposure also matters.

A single exposure may never result in illness, but dogs that repeatedly hunt the same fields or live on properties with decades of bird roost accumulation may face a much higher cumulative risk.

Symptoms: Vague Until They're Not

One of the biggest challenges with Histoplasmosis is that the early signs are frustratingly nonspecific. Many dogs initially look like they have any number of common illnesses, which is one reason the disease is frequently overlooked.

Early symptoms may include:

• Fever

• Lethargy and depression

• Weight loss

• Loss of appetite

• Pale gums or mucous membranes

As the disease progresses, symptoms often reflect the organs that have become involved.

Respiratory involvement:

• Chronic cough

• Labored or rapid breathing

• Exercise intolerance

Gastrointestinal involvement:

One feature that distinguishes Histoplasmosis from Blastomycosis is its tendency to affect the gastrointestinal tract.

Signs may include:

• Diarrhea, sometimes bloody

• Vomiting

• Abdominal discomfort

• Enlarged liver

Because these symptoms resemble inflammatory bowel disease, intestinal parasites, or other gastrointestinal disorders, diagnosis is often delayed while the fungal infection continues to spread.

Disseminated disease:

When the infection spreads throughout the body, additional signs may include:

• Enlarged lymph nodes

• Bone pain or lameness

• Eye abnormalities

• Skin lesions

• Neurological signs in advanced cases

When to Call Your Veterinarian

If your dog has spent time around old barns, bird roosts, caves, construction sites, or other high-risk environments and develops persistent fever, unexplained weight loss, chronic diarrhea, coughing, or unusual lethargy, don't assume it's "just a stomach bug" or kennel cough.

Tell your veterinarian exactly where your dog has been and ask whether fungal disease should be considered. Early diagnosis dramatically improves the chances of successful treatment.

Diagnosis

Diagnosing Histoplasmosis requires putting several pieces together: your dog's symptoms, travel and environmental history, imaging, and laboratory testing.

Your observations matter.

Be sure to tell your veterinarian if your dog spends time around old barns, bat roosts, bird colonies, caves, excavation sites, or similar environments. Those details can significantly shorten the path to diagnosis.

Your veterinarian may recommend:

• A physical examination and detailed environmental history

• Chest X-rays (which may show a distinctive "miliary" pattern of tiny lung nodules)

• Urine antigen testing through MiraVista Diagnostics

• Bloodwork to evaluate anemia, liver values, and overall health

• Fecal testing because gastrointestinal disease is common

• Cytology or biopsy of affected tissues

• Fungal culture in select cases

Like Blastomycosis, Histoplasmosis is commonly misdiagnosed during the early stages because its symptoms overlap with so many other diseases.

If your dog lives in—or has traveled through—an endemic region and isn't improving with standard treatment, fungal testing deserves a conversation.

Treatment

Treatment is similar to Blastomycosis and requires commitment, patience, and regular monitoring.

Itraconazole remains the first-line treatment for most dogs and is typically continued for several months. Rather than stopping treatment after symptoms improve, veterinarians use repeat antigen testing to confirm the infection has cleared before discontinuing medication.

Dogs with severe or disseminated disease may require hospitalization, amphotericin B therapy, oxygen support, and additional supportive care.

The gastrointestinal form of Histoplasmosis can be especially challenging because intestinal involvement often indicates more widespread disease and may require a longer recovery period.

Even after successful treatment, follow-up testing remains important because relapse can occur.

Prognosis

Dogs diagnosed early, before the infection has spread extensively, often respond well to treatment.

The prognosis becomes more guarded when multiple organs are affected, particularly when severe respiratory disease, neurological involvement, or advanced gastrointestinal disease is present.

Even after antigen testing confirms the infection has cleared, periodic monitoring remains important because relapse is possible.

Prevention: Awareness Is Your Best Defense

There is no vaccine against Histoplasmosis.

The best prevention is recognizing the environments where the fungus lives and acting quickly if your dog develops suspicious symptoms afterward.

Pay particular attention if your dog spends time:

• Around old barns or abandoned buildings

• In caves or areas with bat colonies

• Beneath large bird roosts

• Near construction or excavation projects

• On rural properties with long-term bird activity

If your dog develops unexplained fever, weight loss, coughing, chronic diarrhea, or lethargy after visiting these environments, mention that exposure to your veterinarian.

Sometimes that single piece of information is what puts fungal disease on the diagnostic list.

A Quick Comparison

Feature	Blastomycosis	Histoplasmosis	Aspergillosis
Fungus	Blastomyces dermatitidis	Histoplasma capsulatum	Aspergillus spp.
Where it's found	Moist soil near rivers, lakes, wetlands	Soil contaminated with bird or bat droppings	Decaying vegetation, compost, hay, grain, moldy organic matter
Highest-risk dogs	Sporting and hunting breeds, young large-breed dogs	Sporting and hunting breeds, young large-breed dogs	German Shepherds (disseminated), dogs with heavy environmental exposure
How dogs are infected	Inhalation of spores	Inhalation or ingestion of spores	Usually inhalation
Primary organs affected	Lungs, eyes, skin, bones, lymph nodes	Lungs, GI tract, liver, lymph nodes	Nasal passages or multiple organs (disseminated)
Hallmark symptoms	Cough, fever, eye disease, lameness	Weight loss, diarrhea, fever, lethargy	Chronic nasal discharge, nosebleeds, facial pain (nasal) or vague systemic illness (disseminated)
Diagnosis	Urine antigen, imaging, cytology	Urine antigen, imaging, biopsy	CT/rhinoscopy, biopsy, fungal culture/PCR
Treatment	Itraconazole ± amphotericin B	Itraconazole ± amphotericin B	Topical antifungals for nasal disease; systemic antifungals for disseminated disease
Can it relapse?	Yes	Yes	Yes
Can it be prevented?	Reduce exposure; no vaccine	Reduce exposure; no vaccine	Reduce exposure; no vaccine

The Bigger Picture

Blastomycosis captured attention because of a young German Shorthaired Pointer fighting for his life.

Histoplasmosis is quieter.

It tends to develop more gradually, masquerades as other illnesses, and is statistically more common in our region.

Neither disease announces itself clearly. Both can become life-threatening without prompt treatment. And both disproportionately affect the active, outdoor dogs we love most.

Ohio is a wonderful place for dogs that were bred to hunt, explore, and work.

Knowing what's in the soil doesn't mean keeping them inside.

It means recognizing the risks, spotting the warning signs early, and giving them the best chance to recover if the unexpected happens.

---

As always, this post is for informational purposes and is not a substitute for veterinary care. If you have concerns about your dog's health, contact your veterinarian.