Fungal Infection (Histomplasmosis) in Dogs

Histoplasmosis was found at the Panama Canal in 1905 according to an American doctor, Sam Darling, who found the fungus during an autopsy in a patient thought to have died of tuberculosis. This infection is now uncommon in humans, but still prevalent in dogs in both acute and chronic form. The histoplasmosis infection is also classified into two types, which are pulmonary and disseminated. Although histoplasmosis can affect any breed, age, or gender; the infection is found more often in young males of the hunting breeds, such as Weimaraners, Pointers, and Spaniels. This is not thought to be caused by a genetic defect, more of a consequence of the breed’s’ digging and hunting behaviors. This fungus can be found anywhere worldwide, but it is most often found in the dirt along the Missouri, Mississippi, and Ohio rivers. Infection is caused by inhaling spores, where it can travel through the respiratory tract, lungs, and lymph nodes. It can also invade the intestinal tract if the spores are ingested and may spread through the entire body where it can start localized infections in the liver, spleen, skin, bones, bone marrow, or even in the eyes.

Fungal infection (histoplasmosis) is an infection caused by ingesting a fungus called Histoplasma capsulatum. This fungus is usually prevalent in dirt that is contaminated by bat or bird droppings and although it grows well in the soil, it also grows at temperatures and forms which make it possible to live and thrive in body tissues as well. 

Histoplasmosis causes different symptoms depending on the form and type of the infection. Symptoms may also differ depending on the age and health of the dog infected. The most common signs in any infection of Histoplasma capsulatum are fever, tiredness, and lack of appetite. The symptoms specific to each form and type are:

Pulmonary Histoplasmosis

• Fever (can reach as high as 104 degrees Fahrenheit)
• Raspy cough
• Breathing trouble
• Harsh lung noises
• Pale gums and mucous membranes
• Swollen lymph nodes
• Not wanting to exercise or play
• Limping
• Endophthalmitis (swelling of the internal coating of the eye)
• Chorioretinitis (swelling of the retina and pigmented vascular coating of the eye)

 Intestinal Histoplasmosis

• Chronic diarrhea
• Loss of appetite
• Weight loss
• Fever
• General tiredness 
• Blood in stool
• Difficulty defecating
• Swelling of the liver, spleen, and lymph nodes
• Ulcerated throat
• Moist sores
• Eye infections
• Joint pain

  Types

• Pulmonary Histoplasmosis starts in the airway when your dog breathes in the spores; pulmonary histoplasmosis does not spread any further or infect any area besides the lungs and respiratory system
• Disseminated Histoplasmosis is usually ingested rather than inhaled; therefore, it travels through the entire GI tract, liver, spleen, intestines, and lymph nodes; the infection can also spread to the bones, joints, eyes, and brain

Histoplasmosis (both pulmonary and disseminated) is caused by your dog ingesting spores from the Histoplasma capsulatum fungus. This fungus is most often found in dirt that is contaminated with bird or bat feces and your dog can ingest these spores from digging in the dirt in these contaminated areas.

Be sure to tell your veterinarian where your dog has been in the past few days, including anywhere he was likely to be where he could have been infected by the Histoplasma capsulatum fungus spores. After a complete physical examination, your veterinarian will need to run some tests to rule out any other illness or disease. These tests will include a complete blood count, blood chemistry panel, fungal swab, and urinalysis. Additional tests to enable further investigation will include:

• Stool sample
• Digital radiographs (x-rays)
• Fine needle cytology (aspirating fluid from lymph nodes to test for infection)

If the veterinarian still finds these tests inconclusive but suspicious, he will start your dog on treatment but get a biopsy of a lymph node and send it to a pathologist for a definitive diagnosis.

The treatment for histoplasmosis is the same for both pulmonary and disseminated forms. If the infection is mild and has not spread, the veterinarian will start your dog on an oral antifungal medication twice a day for about six months.

For infections that are severe and have spread to many parts of the body, your dog will be hospitalized and given IV azole medication (i.e. Itraconazole, ketoconazole, fluconazole) and an IV antifungal medication (i.e. amphotericin B) with oxygen and fluid therapy.

These medications can all cause intestinal upset and are tolerated better if given with a meal.

The chances of recovery depend on how severe the infection is and whether your dog has pulmonary or disseminated histoplasmosis. If the infection is mild to moderate and has been caught and treated early, your dog has an excellent chance for a full recovery. If your dog has severe disseminated histoplasmosis, it may require a lengthy stay in the hospital for around the clock treatment and medication. No matter what, be sure to follow the veterinarian’s instructions and return for your follow-up visits.