BLASTOMYCOSIS
Blastomycosis is a fungal infection that is found pre- dominantly in North America. This disease is also known as North American blastomycosis or Gilchrist’s disease. However, because it has also been reported in Central and South America, the preferred name of this disease is blastomycosis. It is endemic in the areas of the United States and Canada that border the Great Lakes, the Saint Lawrence Riverway, and the Mississippi River Valley. Most cases have been reported from Wisconsin and Ontario. The infection is common in other mammals such as dogs. Most cases are isolated and sporadic in nature; however, outbreaks of the infection have occurred in which many people who came into contact with the same environmental source were infected.
Clinical Findings: The organism is first inhaled into the lungs, where it quickly reverts
to its yeast state. Most infections are controlled by the local immune
response, and minimal to no symptoms occur. The disease most frequently stays
localized within the pulmonary system. It can, however, spread to any other
organ system in an immunosuppressed host. After the conidia (spores) are
inhaled, the most frequent symptoms are coughing, fever, pleurisy, weight loss,
malaise, arthralgias, and hemoptysis. The symptoms may initially mimic those of
an influenza infection. Approximately half of the patients with symptomatic
disease have only pulmonary findings; the other half have both pulmonary and
other organ system findings.
Cutaneous findings are nonspecific and have been
classified as verrucous or ulcerative. The verrucous lesions can range from
small papules and plaques to large nodules with sinus tract formation. The
central face and nose are common locations of involvement. Ulcerated lesions
can occur anywhere and are associated with underlying abscess formation and
drainage. The skin lesions can mimic those of skin cancers, and biopsy is
required to make the appropriate diagnosis.
Histology: Biopsies of
blastomycosis show pseudoepitheliomatous hyperplasia of the epidermis. Within
the dermis is a granulomatous infiltrate of predominantly noncaseating
granulomas. Neutrophils are prominent. The yeast can be appreciated on routine
hematoxylin and eosin staining. They appear as oval cells with a thick,
refractory wall. Often, broad-based budding is noted. This form of solitary
broad-based budding is specific for Blastomyces dermatitidis. Other
special stains can be used to better highlight the fungus, including the
periodic acid–Schiff and silver stains.
The best means of diagnosing this fungal infection is
by culture on Sabouraud’s media. The mold begins to grow quickly and forms
white to gray, waxy colonies. Special DNA probes can be used to quickly
identify the fungus growing in the medium.
Pathogenesis: Blastomycosis
is directly caused by infection with the dimorphic fungus, B. dermatitidis.
This organism inhabits soil and vegetation in its mold or mycelial form. When
the environment that contains the fungus is disrupted, the spores of this
fungus may gain entry into a human (or other mammal) by direct inoculation or
by inhalation. Once the fungus has entered the human body, the increase in
temperature causes it to convert to its yeast form. The yeast form is not
contagious, and the human acts as a host for reproduction but is unable to
transmit the disease to any other human. The normal host is able to contain the
inhaled spores within alveolar macrophages and granulomas in the lung, but the yeast form of the fungus is much more
resistant to killing by natural host responses. If the host is
immunocompromised, the fungus may disseminate to other organs, particularly the
cutaneous surface. Dissemination occurs via vascular spread of the yeast
organisms.
Treatment: Prompt
treatment with amphotericin B is the therapy of choice for those with
disseminated or severe
disease or any evidence of immunosuppression. Milder cases can be treated with
prolonged courses of the azole antifungal agents; amphotericin B is used if the
disease fails to respond to this treatment. Fluconazole and itraconazole are
the two antifungal agents most frequently used, although other options are
available. Before antifungal therapy was available, more than 80% of cases were fatal.
