By Luke D. Peterson, DVM, MS
Fungi are a broad category of microorganisms that typically occur in nature as either a mold or yeast form. Many are environmentally beneficial but there are a handful that poses great risk to both dogs and humans. This two part series will focus on the fungi Blastomyces dermatitidis and Histoplasma capsulatum which are found in the environment in most of the eastern half of the United States. The incidence of either of these diseases is relatively lower than that of other infectious diseases such as Anaplasmosis, Ehrlichiosis, or Lyme disease. However, knowledge of these diseases may help you identify or advocate for testing your hound should you suspect he may have one of these fungal diseases. On a personal note, I live and practice in an area that is considered highly endemic for Blastomycosis with about 1 to 2 of 1000 dogs being affected. This last fall was a bad one, and I have already diagnosed three dogs this year with two others pending test results.
Blastomcyes dermatitidis is the cause of Blastomycosis, more commonly referred to as “Blasto”. It’s one of those diseases that’s fun to say but not fun to have. Blasto is endemic in the Mississippi and Ohio River valleys and the Great Lakes Region, and is commonly found in decomposing wood and leaves, and sandy, acidic river soils. There is no breed predisposition for getting Blasto, but if you read anything on it, most sources will tell you sporting breeds are more at risk. This has more to do with lifestyle than breed. Any dog that spends more time sniffing in areas with decomposing wood or river areas are going to be at higher risk. I have seen ten pound “purse” dogs with Blasto because they lived next to a sandy river basin. Likewise, dogs are not necessarily more susceptible than humans, but are 10 times more likely to get it than humans because dogs have their noses to the ground sucking up more soil spores. Most of us humans don’t stick our noses to the ground for extended periods of time while in the woods.
Blasto grows as a mold in the environment and releases fungal spores during the reproductive phase of growth. These spores are capable of surviving in the environment for years, waiting for the right conditions to grow. Unfortunately, it is these spores which are infective to mammals, especially dogs and people. The spores are inhaled into the lungs and begin to grow as a yeast form. These yeast are then able to spread through the blood stream or lymphatic system to other locations in the body. The time frame from spore inhalation to clinical signs arising is often 8 to 12 weeks in dogs. So, for example, if you travel to a trial in a region that has a lot of Blasto and two to three months later, one of your hounds displays symptoms, there should be a voice in your head saying “it might be Blasto”, and it would be good to tell your veterinarian that you traveled to a Blasto region. Again, because this disease is not as common as others, some veterinarians that do not practice in endemic regions may not be thinking about it as a possibility unless they have knowledge that your dog traveled to a different geographical region.
In dogs, Blasto can go just about anywhere it wants to in the body. It causes symptoms in all of the following organ systems:
Lungs – 65-85% of cases involve the lungs and dogs may have coughing, rapid or labored breathing, purple to blue gums from lack of oxygen
Lymph Nodes – 30-50% have multiple enlarged lymph nodes which are often infected. The nodes usually swell 2-3 times their regular size
Eyes – 20-50% have internal eye infections, retinal lesions, and may have secondary glaucoma due to accumulation of pus within the eye
Skin – 30-50% have skin lesions that start as firm swellings and progress to pus filled lesions that break open and drain
Bone – up to 30% have bone infections which present as lameness and limb pain
Central Nervous System – 3-6% of cases show neurologic symptoms including decreased alertness, circling, seizures, stumbling, uncoordinated walking
Other sites include the kidneys, prostate, testes, nasal passage, and joints.
Diagnosing Blasto varies with what organ system is affected. The type of pneumonia caused by Blasto has a typical appearance on chest x-rays. Often times, if skin or lymph nodes are affected, the fungus can be found under microscopic examination of a sample taken from the affected tissue, although occasionally it is not found due to vast amount of inflammatory cells in the area responding to the infection. Bone is less specific. The lesions seen on x-rays of affected bone often have the same appearance as that of bone cancer. One of the best tests out there is a urine test. Studies have shown that infected dogs urinate proteins from the fungus which can be identified with the test. It can also be used to identify if a dog is clear of the fungus after a course of antifungal drug therapy.
Treatment of Blasto is the bane of every veterinarian and owner because there are only a small number of drugs effective at treating Blasto and they are relatively expensive. The current drug of choice is Itraconazole with Fluconazole a close second, as it is the only one to reach therapeutic levels in the central nervous system for the small percentage of dogs with neurologic signs. The treatment success rate is about 70-85% with these drugs. The sooner treatment is initiated, the more likely treatment will be effective. Here is the kicker on treatment – dogs need to be treated with oral pills every day for a minimum of two months, and some as long as six months, to clear the infection. When you start doing the math on treatment costs, it can be discouraging. Dogs with severe pneumonia often don’t make it through the first week of treatment because as the fungus is dying off, it creates an even greater amount of inflammation and pus accumulation in the lungs leading to respiratory failure. Dogs with severe pneumonia that do survive treatment have about a 30% chance of having a relapse within 6 months following the end of drug treatment.
Prevention of Blasto is almost non-existent. There is no way to readily remove it from the environment, no vaccine (although there is a physician, Dr. Bruce Klein, at the University of Wisconsin-Madison whose research is focused on developing a human vaccine), and no way to predict when or if spores might be present in a given environment. Most exposure occurs in summer and fall as the fungus begins to release spores as conditions dry out. For most of us, this is prime running time. I have yet to be deterred from running because of this disease and nor should you. Just remember that early identification of an infection is the greatest predictor in whether a dog can survive with treatment.
Histoplasmosis is another fungus that can cause multiple forms of disease in dogs. Histoplasma capsulatum can be found in a large portion of the central and eastern United States (see map in figure 1) primarily through the Ohio and Mississippi river valleys with the highest region in southern Illinois, southern Indiana, western Tennessee, western Kentucky, southeastern Missouri, eastern Arkansas, northern Mississippi, and the northwestern corner of Alabama. The fungus resides in soil and is especially associated with areas containing bird or bat droppings. Pointers, Weimeraners, and Brittany Spaniels have an increased risk compared to other breeds. Exposure to the fungus does not always result in disease. Research conducted during post-mortem exams isolated the fungus from 36% (Kentucky) and 44% (Virginia) of otherwise healthy dogs displaying no disease.
The fungus produces spores which are released into the air when the soil is disturbed. Dogs and humans are risk when those spores are inhaled into the lungs. The spores then transform into yeast and spread to lymph nodes and through the blood stream to various organ systems. The yeast form is not directly contagious from one dog to another or from dog to human. The length of time it takes for the disease to progress in severity varies based on the health of the dog in question. But studies have found a more aggressive form which takes 2-4 weeks to become fatal and occurs in about 10% of affected dogs compared to a slower chronic form which takes 2-20 months the other 90% of the time. Disease syndromes most commonly found are pulmonary, mediastinal lymphadenitis (this is infection/inflammation of the mediastinal lymph nodes which are located in the chest cavity), progressive disseminated histoplasmosis.
Pulmonary – most common manifestation in humans and dogs and causes pneumonia, fever, difficulty breathing, coughing, and lethargy. X-ray findings are similar to that of other fungal infections such as Blastomycosis showing an invasion of the yeast and inflammation within the lung tissue.
Mediastinal lymphadenitis – enlarged lymph nodes in the chest cavity impinge on the airways and cause coughing and difficulty breathing. X-rays show enlarged lymph nodes in the chest, often with some mild infiltration into lung tissue.
Progressive Disseminated Histoplasmosis – Diarrhea, intestinal bleeding, anemia, and reduced activity are the primary findings in affected dogs but other tissue such as lungs, liver, spleen, eyes, bone, and skin may be involved as the organism disseminates throughout the body.
Diagnosis for Histoplasmosis is similar to that for other systemic fungal infections and is best done with a urine sample to identify yeast particles. The diagnosis can also be made by identifying the yeast microscopically from tissue samples.
Treatment is again similar to that for other fungal infections with Itraconazole and Fluconazole being the drugs of choice. Drug therapy should be done for a minimum of 4-6 months and often is as long as 8 months. Treatment success is similar for either drug and is about 50%. The pulmonary form is most responsive to treatment with the disseminated form with diarrhea having the least successful outcome.
Prevention of the disease for dog or man is limited to avoiding areas more likely to contain bird and bat droppings. No vaccine is available and likely won’t be. The yeast form of the fungus has a capsule around it making it difficult for vaccine developers to isolate surface molecules to create vaccines against. Like other systemic fungal infections, the incidence rate is very low in the population. No recent studies have been conducted but prior studies found it to be the most common of the endemic systemic fungal infections. Based on my literature review, I estimate the incidence rate anywhere from 5 cases per 100,000 dogs in less endemic regions to as high as 500 cases per 100,000 dogs in highly endemic regions