Histoplasmosis in dogs: Treatment and prevention

Histoplasmosis is a serious fungal infection in dogs that can be challenging to diagnose and treat. Because histoplasmosis often looks similar to other illnesses that are more common, it can take time to diagnose, and treatment usually requires many months of antifungal medication and symptomatic care.

Dogs are at highest risk of exposure to histoplasmosis when they inhale fungal spores from soil contaminated with bird or bat droppings, so limiting digging and intense sniffing in certain areas reduces risk. Because histoplasmosis is so difficult to diagnose and treat, knowing more about the disease can help pet parents identify when their dog may have been exposed. Some of the things we’ll discuss here includes:

• Symptoms of histoplasmosis infections
• How histoplasmosis is diagnosed and treated
• What dogs are most at risk for histoplasmosis
• How owners can help prevent histoplasmosis in their dogs

Histoplasma capsulatum is a fungus found in soil worldwide. In the US, histoplasmosis is reported most often in central and eastern states, particularly around the Ohio and Mississippi River valleys. Dogs are at highest risk in areas where soil is heavily contaminated with bird or bat droppings. Infection happens when dogs inhale the fungal spores, which turn into yeast in the lungs, which can then spread throughout the body.

Systemic fungal infections, such as histoplasmosis, are often challenging to diagnose because they’re less common than many other conditions with similar signs and usually require specialized testing. Treatment usually involves several months of oral antifungal medication alongside supportive care. If left untreated or if a dog already has a compromised immune system, the disease can become severe.

Because there’s no vaccine or other preventative medication for histoplasmosis in dogs, avoiding risky behaviors in areas where the fungus is commonly found is the best protection, and if your dog shows signs of illness, even mild ones, talking to a veterinarian right away can help improve the outcome for any illness. For nonurgent concerns, a virtual vet appointment is a convenient way to get professional advice based on your dog’s symptoms and history.

Histoplasmosis, often shortened to “histo,” is a systemic fungal infection caused by a soil-dwelling fungus called Histoplasma capsulatum. Histoplasma spores start out as mold in contaminated soil and, once inhaled, transform into yeast that can replicate and spread, causing a systemic infection. A systemic infection is one that affects multiple parts, or systems, of the body, as opposed to being isolated to one small part, such as a patch of skin. Dogs usually become infected with histo when they inhale fungal spores in the environment. Not all dogs who are exposed to or inhale the spores develop the disease.

Clinical signs of histoplasmosis in dogs

Because infection typically happens due to mold spores being inhaled into the lungs, you may see respiratory symptoms in your dog, including:

• Cough
• Increased breathing effort or shortness of breath
• Exercise intolerance
• Nasal discharge

Histoplasmosis can also cause more general signs of illness, such as:

• Lethargy
• Weight loss
• Loss of appetite
• Swollen lymph nodes

The gastrointestinal tract is also often affected, causing clinical signs like:

• Diarrhea, sometimes with blood or mucus
• Vomiting

Less commonly, histo can affect your dog’s eyes, skin, joints, bone marrow, and other parts of the body. In these cases, you might observe:

• Change in vision
• Squinting
• Swollen or red eyes
• Pale gums
• Skin ulcers
• Nodules under the skin

Regardless of which body system is affected, symptoms may take weeks or even months to appear after your dog first inhaled the mold spores.

When to seek emergency care

Although histoplasmosis usually develops slowly, advanced cases can lead to sudden emergencies, such as breathing difficulties, collapse, or seizures. Seek immediate, in-person veterinary care in the case of:

• Difficulty breathing
• Blue gums/tongue/lips
• Collapse or loss of consciousness
• New seizures
• Sudden loss of vision

Because the signs associated with histoplasmosis are shared by many more common conditions, the disease can’t be diagnosed based on your dog’s symptoms alone. The diagnostic process starts with a thorough physical examination and a review of your dog’s medical history to narrow down the potential causes of their symptoms. Based on the physical exam, diagnostic testing is often recommended and may involve:

• Blood tests
• Diagnostic imaging, such as X-rays
• Urinalysis
• Cytology or biopsy of lesions

“A vet may suspect a fungal infection based on certain abnormal findings in blood work or X-rays, but detecting proteins from the Histoplasma organism through antigen testing of a blood or urine sample is necessary to confirm the diagnosis,” explains Dr. Jo Myers, a veterinarian on the Vetster platform. “It’s often more efficient and in the patient’s best interests for the vet to try treating a dog with respiratory or GI symptoms for something more common to see if that works first before taking a diagnostic approach that looks only for specific fungal diseases.” It can take time and money to reach a diagnosis, as there are many conditions that are more common that cause similar symptoms, so specific testing often isn’t pursued unless a dog fails to respond to treatment for more obvious and straightforward causes.

Treatment for histo involves the long-term use of antifungal medications. Eliminating a systemic fungal infection takes a long time, often more than six months, and antifungals may have adverse effects and require periodic blood tests, so veterinarians typically use diagnostic tools to confirm a systemic fungal infection before starting treatment.

Additional symptomatic care may also be recommended, such as oxygen therapy for dogs who are having difficulty breathing and pain relief. Repeated antigen tests throughout treatment are often recommended, and relapse is common. Prognosis varies depending on how severe the condition is and which part(s) of the body is affected. In general, prognosis typically gets worse the more severe the symptoms are and the higher the number of bodily systems affected.

There are several risk factors that make it more likely for certain dogs to be exposed to and to contract histoplasmosis. Knowing what they are can help prevent infection in at-risk dogs.

Geographic and environmental risk factors

While Histoplasma spores can be found worldwide, most infections are reported in the Mississippi, Missouri, and Ohio River valleys. Dogs who live in or have traveled to these geographic areas are at a higher risk of getting histoplasmosis.

The environmental risk of histoplasmosis isn’t limited to one geographic region, however; any area with droppings, such as caves, barns, attics, roosting trees, vacant buildings, or construction areas, can be dangerous if the soil is disturbed.

Dog-specific risk factors

Dogs that dig, play, or hunt in contaminated soil are more likely to be exposed to disturbed fungal spores. Healthy adult dogs are most likely to be able to fight off infection, never showing signs of illness or only developing mild symptoms. Dogs that are very young, are seniors, or have weakened immune systems are most at risk for developing disease after exposure to the spores.

Overall risk is made up of multiple factors: geographic location, soil-disturbing behaviors, exposure to soil enriched with bird or bat droppings, and age and overall health. The highest risk occurs when these factors overlap.

Prevention of histoplasmosis in dogs

Currently, there’s no vaccination or medication to prevent dogs from getting histoplasmosis. The best way to prevent histoplasmosis is to avoid risky behaviors in the geographic regions where most cases of the disease are reported (endemic areas).

If you live in or frequently visit an endemic region, try to keep your dog from digging in or sniffing the soil, especially if it’s contaminated with bat or bird droppings. If your dog shows signs of illness and has been in these areas, it’s important to tell your vet as soon as possible. It won’t prevent illness, but it can help your dog get a diagnosis sooner and give them the best chance of recovery. Histoplasmosis cannot be spread between dogs or from dogs to people.

Histoplasmosis in dogs often begins with subtle, generalized signs of illness. An online vet can help by virtually examining your pet, discussing their symptoms, and talking about their medical and travel history in convenient, at-home appointments. Vets can then make a professional recommendation on what to do next. If your dog is showing emergency symptoms, call an in-person veterinary clinic or emergency hospital right away to get instructions on what to do next.

Can dogs transmit histoplasmosis to humans?

Histoplasmosis is not contagious between dogs and humans. Humans are exposed to histo the same way dogs are: through soil and dust contaminated with the fungus.

What happens if histoplasmosis is left untreated in dogs?

Histoplasmosis is often already widespread in the body by the time it’s recognized. Even when diagnosed early, treatment requires months of antifungal medication and close monitoring by a vet. Some dogs relapse or fail to respond to treatment even under optimal care, and fatalities do occur. Without treatment, the illness progresses and the chances for recovery decrease. In general, the longer therapy is delayed, the harder the infection is to control and the poorer the prognosis. Early evaluation and timely treatment give the best chance of success. It’s important to seek veterinary care any time your dog is showing signs of illness to get an accurate diagnosis and appropriate treatment.

Can I prevent my dog from getting histoplasmosis?

There is no vaccine or other preventative drug for histoplasmosis. The best way to prevent histoplasmosis in dogs is to avoid risky behaviors, such as digging and sniffing in dirt that’s heavily contaminated with bird or bat droppings. This is especially relevant in geographic regions where the disease is endemic, such as the Ohio, Missouri, and Mississippi River valleys.

Disclaimer: This article, its content, and its related references do not constitute veterinary advice and should not be considered a substitute for veterinary care. If your pet is showing any symptoms that could indicate a medical emergency, please seek immediate emergency care.