Blastomyces
Blastomyces is a pathogenic, dimorphic fungus primarily found in soil, known for causing a respiratory infection called blastomycosis. This fungus exists in two forms: a mold in the environment and a yeast within the host's body, the latter being more virulent. Infection typically occurs through inhalation of airborne spores (conidia), leading to lung infections that can sometimes disseminate to other organ systems. Blastomycosis is most commonly reported in specific regions of North America and Africa, especially near the Mississippi and Ohio rivers, with incidence rates reaching up to 40 cases per 100,000 people in endemic areas.
While many individuals may remain asymptomatic or experience mild symptoms, those with weakened immune systems are at a higher risk for severe disease and complications. Treatment options include antifungal medications, with amphotericin B being a historical standard for severe cases, though it comes with significant side effects. Other, less toxic alternatives like itraconazole and fluconazole are also utilized, particularly for mild to moderate cases. Early diagnosis and appropriate treatment are crucial, as delays can lead to chronic illness that may resemble more serious conditions like tuberculosis. Overall, understanding the characteristics and risks associated with Blastomyces is essential for effective management and prevention of this infection.
Blastomyces
- TRANSMISSION ROUTE: Inhalation
Definition
Blastomyces is a pathogenic, dimorphic, endemic soil fungus that causes a respiratory infection known as blastomycosis.
Natural Habitat and Features
Blastomyces is a soil fungus. Like other fungi, it can reproduce both sexually and asexually. The size, shape, and arrangement of cells are often very different between the sexual and asexual morphological forms of the same fungus. The sexual form of any fungus is called the teleomorph, in contrast to the asexual morphological form, called the anamorph. Technically, Blastomyces refers to the anamorph stage of the fungus only. Ajellomyces dermatitidis is the teleomorph stage and is the official name of the fungus. Blastomyces is one of only a few dimorphic fungi that can cause serious disease in humans. Infection by Blastomyces is called blastomycosis and has been recognized since the nineteenth century.
While most fungi are always either molds or yeastlike cells, dimorphic fungi can grow as either yeastlike cells or as filamentous forms (molds). Molds grow by branching and longitudinal extension (adding cells to the end of filament), while yeasts grow by budding or binary cell division. In soil or the environment, Blastomyces grow as filamentous molds. These molds produce conidia (asexual spores), which can become airborne and cause infection through inhalation. The conidia, after inhalation, lodge in the lungs, where infection begins. Blastomyces is thermally dimorphic, that is, the increased temperature of the host’s body causes it to convert to yeastlike cells after infection.
Most dimorphic fungi (such as Blastomyces, Coccidioides, Paracoccidioides, and Histoplasma) are also considered endemic fungi because each of them is found primarily in a particular region of the world. For Blastomyces, the vast majority of cases are seen in certain regions of North America and Africa, although cases are seen worldwide. In the United States, areas endemic for Blastomyces are the central and southeast states, especially along the Mississippi and Ohio rivers and the upper Great Lakes region. In highly endemic areas, mean annual incidence rates can be as high as 40 per 100,000 persons. The exact ecological niche of Blastomyces is still unknown. Studies using molecular techniques to detect Blastomyces in endemic areas have generally failed to find it.
Blastomycosis is a common infection among dogs in endemic areas. Infection in dogs may serve as a warning of potential human outbreaks. Blastomycosis is also reported in other animals.
Blastomyces is infectious only in the filamentous mold form that grows in soil. Therefore, transmission occurs only through environmental sources; human-to-human or animal-to-human cases have not been reported.
Pathogenicity and Clinical Significance
Blastomycosis begins as a lung infection after inhalation of the Blastomyces conidia. After lodging in the lungs, the conidia germinate into the yeast form of the fungus. This change provides Blastomyces a distinct advantage for infection, as yeastlike cells are much more resistant to phagocytosis than are filamentous mold forms. Virulence factors are also produced in the yeast form, but not the mold form of Blastomyces. The infection can then disseminate through the blood and lymphatics to other organs. The inflammatory immune response results in macrophage recruitment and, ultimately, granuloma formation.
In most immunocompetent people, the innate immune system (especially the action of alveolar macrophages) provides a natural resistance to infection with Blastomyces. Studies of epidemic exposures indicate that about 50 percent of people exposed to Blastomyces develop no symptoms (are asymptomatic).
Symptoms of blastomycosis are usually mild or nonexistent. If symptoms do appear, they include a dry cough, chest pain, hoarseness, and a low-grade fever. The symptoms may take several months to appear after the initial inhalation of the conidia. Because blastomycosis symptoms are similar to those of many other diseases, such as tuberculosis, acute pneumonia, histoplasmosis, and even influenza, diagnosis is often delayed or missed. Pneumonia is the most frequent symptom of blastomycosis, and, except in rare cases of skin infection, the lungs are the site of first infection. Infection of skin, bone, prostate, and the central nervous system are also seen in blastomycosis.
If the primary pulmonary infection does not resolve, severe progressive (chronic) blastomycosis can result. Chronic illness may resemble tuberculosis or lung cancer, with symptoms of low-grade fever, a productive cough, night sweats, and weight loss. It can sometimes be fatal. Blastomycosis can disseminate and spread infection to the skin, bones, urogenital tract, prostate, and the central nervous system. Dissemination outside the respiratory tract occurs more often in those who are immunocompromised and in persons with chronic pulmonary illness.
Reactivation of blastomycosis may occur after a pulmonary infection. In these cases, although the initial infection is resolved, live Blastomyces remains inside the host and are capable of causing secondary infection. Reactivation at extrapulmonary sites is very rare.
Immunocompromised persons are at much higher risk for developing severe Blastomyces infections. Prognosis is also much worse for immunocompromised persons with blastomycosis. Among people with human immunodeficiency virus infection or with acquired immunodeficiency syndrome, mortality rates for blastomycosis were estimated at 37 percent in 2019. According to the CDC in 2024, mortality rates for general cases of blastomycosis usually range from 8 to 10 percent but rose to 17 percent in 2021, most likely because patients were also affected by the COVID-19 global pandemic.
There are no known practical measures for the prevention of blastomycosis. Minimizing morbidity and mortality from blastomycosis depends primarily on early recognition and appropriate treatment of the disease.
Drug Susceptibility
The in vitro drug susceptibility of Blastomyces, like many fungi, correlates poorly with treatment efficacy. That is, antifungal agents that may work well in the laboratory do not always work well in clinical cases. The primary antifungal drugs for treating Blastomyces infection are amphotericin B, itraconazole, flucanazole, ketoconazole. Amphotericin B has historically been the most effective antifungal for severe blastomycosis (especially in children). However, while effective, amphotericin B has many deleterious side effects and must be administered intravenously. Liposomal formulations of amphotericin B have with significantly less toxicity, and anecdotal reports suggest they may be effective against Blastomyces.
The azole class of antifungals is an equally effective and less toxic alternative to amphotericin B for treating mild-to-moderate blastomycosis. Azoles used to treat blastomycosis include itraconazole, flucanazole, and ketoconazole. While these oral drugs are less toxic than amphotericin B, they are often less effective at treating blastomycosis with central nervous system involvement. Because they most often resolve spontaneously, many cases of mild blastomycosis are treated by close supervision, rather than by antifungal therapy.
Bibliography
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