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Histoplasmosis in Dogs

What Is Histoplasmosis in Dogs?

Histoplasmosis in dogs is a fungal infection that is more frequently observed in cats but can also affect canines. It is particularly prevalent in dogs under 4 years of age, with certain breeds like the Brittany, Pointer, Weimaraner, and other working dogs being predisposed to the infection. The causative agent, Histoplasma capsulatum, is found worldwide but is especially prevalent in the major river valleys of North and South America.

The infection typically occurs when dogs inhale the fungal organisms, often present in the droppings of birds or bats. There’s also a theoretical possibility of transmission through ingesting the fungus by consuming bird and bat feces or rotting wood, although this has not been definitively documented.

Symptoms

Symptoms of histoplasmosis typically manifest in dogs approximately 2-3 weeks following infection and can vary considerably depending on the organs affected by the fungus.

Common First Symptoms

Common initial symptoms of histoplasmosis in dogs often revolve around gastrointestinal issues due to the frequent spread of the fungus, H. capsulatum, to the gastrointestinal tract. These symptoms typically include:

  • Difficulty or straining during bowel movements
  • Presence of diarrhea accompanied by small amounts of blood or mucus

Additionally, similar to dogs developing certain forms of metastatic cancer, other symptoms may initially be nonspecific, such as:

  • Vomiting
  • Loss of appetite
  • Weight loss
  • Lethargy
  • Fever

Symptoms of More Serious Infection

As the infection advances, dogs may exhibit symptoms specific to the affected organs. Those with a significant fungal presence in their lungs may display:

  • Coughing
  • Rapid breathing
  • Nasal discharge
  • Jaundice (yellowing of the skin) if the liver is involved
  • Swollen lymph nodes

Less commonly, the infection can spread to the eyes, bones, and skin, leading to symptoms such as:

  • Blindness
  • Limping
  • Swollen joints
  • Presence of unusual skin lesions, wounds, or oozing nodules

Causes

Histoplasmosis in dogs is primarily caused by the fungus Histoplasma capsulatum, also referred to as H. capsulatum. The classic source of infection is the ingestion of fungal spores found in bat feces (guano), although transmission can also occur through the consumption of bird droppings, rotting wood, and other sources. Once ingested, these spores infect a dog’s intestines.

While the gastrointestinal tract is the most common site of spread in dogs, other organs are frequently affected as well, including the lymph nodes, spleen, liver, eyes, joints, bones, and skin. Additionally, dogs can inhale the spores, leading to their introduction into the lungs. Once inside the lungs, the fungus proliferates. While the infection may remain localized in the respiratory tract, it often disseminates to other organs, mimicking the behavior of metastatic cancer.

Diagnosis

Diagnosing histoplasmosis in dogs can be challenging due to the vague nature of symptoms and the relative rarity of the infection in many regions of the United States. When faced with a sick animal exhibiting no apparent underlying cause, veterinarians typically conduct baseline diagnostics.

Diagnostic Testing (Blood/Urine)

Diagnostic testing involving blood and urine analysis is a common procedure aimed at assessing organ function, electrolyte levels, and cell counts. While these tests may not always provide a definitive diagnosis, they are valuable in narrowing down potential causes.

In cases of histoplasmosis in dogs, abnormalities such as anemia (low red blood cells), reduced platelet counts, decreased protein levels, elevated calcium levels, increased blood sugar, and heightened liver values may be observed in these tests. However, it’s important to note that these results are not exclusive to histoplasmosis and can be indicative of other diseases as well.

X-Rays and Ultrasounds

Diagnostic imaging techniques such as chest X-rays or abdominal ultrasounds can reveal the presence of nodules or masses in the lungs or other organs, resembling metastatic cancer.

Biopsies and Needle Aspirates

Histoplasmosis in dogs may result in abnormalities in the appearance or size of organs. Biopsies or needle aspirates, which involve extracting cells using a needle, are common diagnostic methods employed by veterinarians to diagnose histoplasmosis.

Direct Testing for Histoplasmosis

In cases where a veterinarian suspects histoplasmosis, direct testing for the disease may be conducted. Blood tests for histoplasmosis are highly reliable but are typically reserved for situations where other tests indicate a potential presence of the disease. This approach is adopted because it’s not feasible to test every sick animal for every infectious disease on every occasion.

Treatment

The primary treatment for histoplasmosis in dogs involves administering long-term antifungal medication to eliminate the H. capsulatum organisms. The objective is to continue treatment until the dog remains symptom-free for at least one month, with many dogs requiring over six months of antifungal therapy. Additionally, other medications or therapies may be necessary to manage symptoms and secondary effects of the disease, such as antibiotics for pneumonia or skin infections. Providing fluids to address dehydration or utilizing feeding tubes for dogs that are reluctant to eat can also aid in maintaining the dog’s comfort until the antifungal medications effectively resolve the infection.

Living and Management

After diagnosing histoplasmosis in your dog, your veterinarian will conduct multiple physical examinations to assess the progress of treatment and to watch for any new symptoms or complications that may arise. Regular blood and urine tests will be performed to ensure that the medications are effectively combating the infection without causing harm to your dog’s organs.

Once your dog has remained symptom-free for at least one month following a negative test result for H. capsulatum, they should undergo another round of testing. A negative result on this final test before discontinuing medications will confirm the complete clearance of the infection and reduce the risk of recurrence. Additionally, your dog should undergo testing again 3-6 months later to detect any potential relapse.

Histoplasmosis in Dogs FAQs

Can humans contract histoplasmosis from dogs?

Although direct transmission from pets to humans has not been documented, H. capsulatum can infect humans independently.

Is histoplasmosis contagious among dogs?

No, histoplasmosis is not transmissible from one dog to another.

Is it safe for dogs to consume bird or bat feces?

Aside from the risk of histoplasmosis, ingesting bird droppings can expose dogs to harmful substances like uric acid and bacteria such as salmonella, leading to vomiting, diarrhea, and other health issues. It’s best to prevent dogs from accessing areas where such feces are present, like bird coops and areas with decaying wood.

Can bat feces make dogs ill?

Yes, bat feces can carry fungal spores that infect dogs with H. capsulatum. It’s advisable to prevent dogs from being in proximity to caves and other locations where bats reside.

How prevalent is histoplasmosis in dogs?

While histoplasmosis is the most common systemic fungal disease in dogs, it remains relatively uncommon overall. Its occurrence is more frequent in the Midwest and South regions of the United States compared to other areas.

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