Potentially Deadly Airborne Spores Infect Pets in 31 Areas

Story at-a-glance -

  • Histoplasmosis is a fungal infection that is most often seen in outdoor dogs and cats, and hunting dogs
  • There can be a wide range of symptoms from the infection, depending on which organs of your pet’s body are affected
  • In mild cases of respiratory histoplasmosis, often the pet is able to clear the infection without medical intervention
  • More severe infections may require hospitalization initially, and a long course of antifungal therapy
  • Functional medicine and integrative veterinarians also use adjunctive therapies to help histoplasmosis patients recover more quickly and comfortably

By Dr. Karen Shaw Becker

Histoplasmosis is an infection caused by the Histoplasma capsulatum fungus, which is found in soil, most commonly in warm, moist, humid conditions. In the U.S., the fungus has been reported in 31 states, primarily in the midwest and south, especially along the Mississippi, Missouri and Ohio rivers. Areas with bird and/or bat droppings are also friendly environments for the fungus.

Histoplasma fungi produce tiny airborne spores that are heat-resistant and can be inhaled by pets. The spores engage the immune system, reproduce in immune system cells and then travel throughout the body in the bloodstream. They ultimately wind up in the respiratory tract, gastrointestinal (GI) tract, lymph nodes, bone marrow, lungs, liver and spleen.

Both dogs and cats can acquire histoplasmosis, but the infection is most often seen in outside dogs and cats and particularly hunting dogs. A few cases of infection in strictly indoor animals have been reported, but it’s rare and it is suspected that potting soil or dirt brought in from outside is the source of infection in these animals. Pets of any age can be affected, but most infections occur in animals under the age of four.

Symptoms of Histoplasmosis

Symptoms of infection range from none to severe. The nature and extent of symptoms depends on the organ systems infected by the fungi. Histoplasmosis may be confined to just the lungs or just the GI tract, or it can become systemic, infecting the whole body. Symptoms include:

Loss of appetite

Bloody stool



Weight loss

Mucous or fat in the stool

Difficult or
labored breathing

Muscle wasting



(usually chronic)

Enlarged tonsils

Straining to



Enlarged lymph nodes

As the infection progresses, pets can become emaciated, have elevated heart and respiration rates, lameness, and ulcerated sores on the skin or around the eyes. In addition to the GI and respiratory tracts, histoplasmosis can affect a number of other organ systems including the liver, spleen, lymph nodes, bones and bone marrow, adrenal glands, kidneys, testes, and the mouth, tongue and eyes.

Diagnosing Histoplasmosis

Your veterinarian will take a history of your pet’s overall health and symptoms and perform a thorough physical examination. He or she will note important changes such as abnormal lung sounds, pale mucous membranes, changes in your pet’s heart or respiratory rate, an enlarged liver or spleen, lymph node enlargement, lameness or joint swelling, and any eye abnormalities.

A complete blood count (CBC) will be ordered, along with a serum biochemical profile to check internal organ function, and a urinalysis. However, while blood tests can confirm the presence of histoplasmosis antibodies, this only means your pet has been exposed to the fungus, and not necessarily that she’s developed disease as the result of exposure.

The presence of antibodies will prompt further differential testing to confirm an infection, and can include urine screening for the fungus, chest X-rays, aspiration of the liver and spleen, taking lung fluid samples, bone X-rays or biopsies, GI biopsies, and a spinal tap. Since the symptoms of histoplasmosis are also present with a number of other diseases, it’s critically important that a confirming diagnosis of histoplasmosis is made.


Treatment Options

In mild cases of the respiratory form of histoplasmosis, treatment may not be necessary, because your pet’s body may mount an effective immune response and clear the infection on its own. If the infection is severe, aggressive treatment with antifungal medication may be needed to save your pet’s life. Medications used to treat histoplasmosis include itraconazole, fluconazole, ketoconazole and amphotericin B.

In mild cases, antifungals are given orally for four to six months. That may seem like a long time, but whereas antibiotics can resolve bacterial infections quite quickly in most cases, antifungal therapy takes considerably longer. Typically, veterinarians prescribe antifungal drugs for a minimum of four months.

In severe cases of histoplasmosis requiring hospitalization, intravenous (IV) antifungal medications may also be given, along with oxygen, IV fluids and nutritional support as necessary.

Unfortunately, like all drugs, antifungals have a number of potential adverse side effects, especially involving the GI tract. They are also expensive, especially for large and giant breed dogs. Amphotericin B also carries a high risk for kidney damage, so most people opt to use some of the safer antifungal drugs.

Your integrative veterinarian will probably suggest adjunctive therapies, like nebulization therapy if the fungus is in the lungs, or ozone therapy if there is a systemic infection. Many animals also benefit from IV vitamin C therapy, which provides tremendous antioxidant and immune system support.

As with any illness, the sooner your pet is diagnosed and treated for histoplasmosis, the better. The prognosis is good if the correct treatment is given before the animal becomes debilitated.