Tom Volk's Fungus of the Month for January 2001

This month's fungus is Blastomyces dermatitidis, cause of blastomycosis, also known as North American Blastomycosis, Gilchrist's disease, Chicago disease, or Namekagon River Fever

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Blastomyces dermatitidis yeast formIt's the beginning of the spring semester, and I'll again be teaching medical mycology, the study of fungi that infect humans and other animals. This month's fungus is one of the nasty ones that can cause a severe lung infection in humans. It is also especially common in some areas as a disease of dogs.

Blastomyces dermatitidis is one of the dimorphic fungi, which means it has two forms. At room temperature is takes the mycelial form shown below, forming a single conidium that looks kind of like a lollipop. At 37oC, the mycelium transforms to a large yeast (8-15 um) with characteristic thick cell walls and broad-based buds (shown to the left). In this form, and with the typical lactophenol cotton blue stain, it is known as the

Big Blue Broad Based Budding yeast - Blasto.

Blastomyces mycelial form, with conidiaEach yeast is multinucleate with 2-5 nuclei per cell. The mechanism for the phase transition is unknown, but the reason for the transition is quite clear, as described below. . The fungus can also form a sexual state; the so-called teleomorph form is Ajellomyces dermatitidis, which forms a gymnothecium. The gymnothecium is kind of like a cleistothecium (closed ascocarp) as found in powdery mildews or Aspergillus. The gymnothecium, however, has an outer covering that is more loosely woven, so that ascospores can fall out without having the covering degraded by other microbes.

Blastomycosis starts out as a lung infection caused by inhalation of the conidia. The conidia transform into the yeast form, The transformation into a yeast involves many physiological and biochemical changes, having the effect of "buying some time" for the fungus to become established. The fungus evades the body's immune system by changing its surface antigens. Once established, the infection progresses as an occult, insidious (hidden and sneaky) process or as a chronic, expanding, and eventually systemic infection. The patient may have severe presenting symptoms or may be inapparent infection that resolves spontaneously or disseminates to another location. If symptoms do appear, the incubation period may be long - 21 to 106 days. It starts with mild progressive respiratory infection with a dry cough, some pleuritic pain, hoarseness, and a low grade fever, blastomycosis infection of the nosesymptoms that overlap many other diseases, such as tuberculosis or chronic histoplasmosis. If the primary pulmonary infection does not resolve, severe progressive blastomycosis can result. It can sometimes be fatal. Sometimes the disease spreads to the skin (cutaneous blastomycosis, as shown to the right), the bones (osseous blastomycosis), the urogenital tract, or the central nervous system. The tissue response is variable, but organism shows up as big blue broad based budding yeast. An unequivocal diagnosis cannot be made unless this form is seen. A diagnosis may also involve isolating the fungus and growing it at room temperature and body temperature. Blastomyces grown at two temperaturesTypical cultures are shown below.

Interestingly, there is no evidence of human to human or animal to human transmission of the disease. Why do you think this is the case? The answer is at the end of this page*.

Animal blastomycosis is most common among dogs, especially the hunting breeds. This could be because the dogs have their "noses to the ground," increasing the chance of inhaling the conidia. Because dogs are more susceptible to infection, they can be used (inadvertently) to located hotspots in endemic areas. The association of canine disease with outbreaks of human infection is so close that blastomycosis of the dog is considered a harbinger of human illness. Dogs show the same symptoms as humans except ocular (eye) involvement is common.

The ecological niche of this organism is uncertain due to infrequent isolation in nature. Only about 20 isolations have been made worldwide from nature. The first isolation was in Georgia along the Savannah River where 10 out of 356 samples tested positive. The location in nature is most often determined by the occurrence of disease. There is no skin test as with histoplasmosis which makes it difficult to determine who has been exposed to B. dermatitidis. We knon that Blastomyces generally grows in an acidic pH environment with high organic content --not necessarily in soil, although it seems to be most common along riverbanks. Lab experiments have found it is very sensitive to drying, although conidia release only when wetted. Reported cases of blastomycosis have occurred after rain of foggy weather. One theory states that B. dermatitidis cannot compete with the normal flora and can only grow in restricted environments.

Distribution is worldwide, primarily in North America and Africa. Endemic areas in the U.S. include the midwest, eastern coast, and along the Mississippi and Namekagon Rivers. There have been several localized epidemics of blastomycosis. There have been a total of 12 documented human outbreaks (ranging from 3 - 48 people) in the U.S., 5 of which came from Wisconsin. Out of the 12 outbreaks, only 2 had positive isolations.

Blastomycosis is not a disease that is required to be reported to the Center for Disease Control on a national scale. However, it was a reportable disease from 1986 -1995 in Wisconsin after 2 human outbreaks in 1985. During those years, a total of 670 cases were reported; 26 of them were fatal. The 10 northern counties in Wisconsin contributed 294 cases of the 670. In 1986, Eagle River, Wisconsin had an outbreak where 48 out of 95 people visiting a camp contracted the disease. Out of 47 environmental samples taken from Eagle River, 2 of them tested positive for Blastomyces, primarily from a beaver lodge. The fungus also seems to be common along the Namekogen River in NW Wisconsin. A Native American (Lac Courte Oreilles band of the Ojibwe) died from this 1997. Seven Namekagon canoeists died in 1979. There was another outbreak in northern Minnesota in 1999. Many people who live in this region fear taking their dogs along the river. A severe outbreak occurred several yearsago along the Des Plaines River near Chicago, where many dogs died in a new subdivision built near the river.

So how is this disease treated? Most often is is misdiagnosed, since most physicians receive very little training in mycology in medical school. The disease is most commonly misdiagnosed as tuberculosis or some other bacterial disease, and the doctor prescribes antibacterial antibiotics. The bacteria are killed, and the fungus is left with no competition -- and the infection gets worse. When properly diagnosed, itraconazole (trade name SporanoxTM) can be used on mild cases. More severe cases begin with amphotericin B intravenous treatment for 4-10 days followed by itraconazole treatment for 2-6 months. However, itraconazole is expensive --- it can cost more than US$1000 to treat a hunting breed sized dog with Blastomycosis. The less expensive ketoconazole is also being tested for its effectiveness.

x-ray of a dog infected with Blastomyces*I asked above why you can't get blastomycosis from your dog or from another infected person? Remember that only the conidial form is infectious. You probably figured out that once the disease is established in the body it has transformed to the yeast form, which is not infectious. The human or other animal is a dead end in the life cycle of the fungus. It seems to be quite accidental that the fungus can infect humans or other animals, since it doesn't have to "pass through" the body to complete its life cycle.
I hope you enjoyed learning about this month's fungus. Medical mycology is a fascinating field. Some other medically important fungi that have been fungus of the month include Histoplasma capsulatum, cause of histoplasmosis, Candida albicans, cause of most yeast infections in humans, Epidermophyton floccosum, one of the causes of athlete's foot, and Aspergillus, a deuteromycete useful in industrial mycology-- and also a human pathogen.

Remember that I am NOT a medical doctor. Although I sympathize with you and your infections, please DO NOT email me with medical problems. I recommend you seek out a doctor with training in medical mycology.

Thanks once again to Dr. John Rippon, emeritus professor at the University of Chicago, for the images on this page.

If you have anything to add, or if you have corrections or comments, please write to me at

This page and other pages are © Copyright 2001 by Thomas J. Volk, University of Wisconsin-La Crosse.

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