Scedosporium apiospermum is a widespread filamentous fungus and a serious opportunistic pathogen capable of causing life-threatening infections. This organism is an asexual form belonging to a complex of species that includes its historical sexual counterpart, Pseudallescheria boydii. Although the nomenclature has evolved, S. apiospermum is the primary agent of disease, particularly in vulnerable populations. Its medical significance lies in its ability to resist many common antifungal medications, making infections difficult to manage.
Environmental Habitat and Routes of Exposure
Scedosporium apiospermum is a saprophyte, meaning it lives on dead or decaying organic matter, and is commonly found globally in the natural environment. The fungus is particularly abundant in soil, sewage, and polluted water, thriving in nutrient-rich, poorly aerated habitats such as mud from eutrophic ponds or agricultural manure. Studies show that the organism’s presence is often enhanced in areas with high human activity and environmental contamination, including urban playgrounds, wastewater treatment plants, and garden soil.
Human exposure typically occurs through two primary mechanisms. The most common route is the inhalation of airborne spores, or conidia, aerosolized from contaminated sources like disturbed soil or decaying debris. Once inhaled, the spores can colonize the respiratory tract, particularly in individuals with pre-existing lung conditions.
The second major route is direct inoculation, which happens when the skin or soft tissue is breached and comes into contact with contaminated material. Severe trauma, especially incidents involving near-drowning in polluted water, represents a significant risk for direct inoculation into the lungs or sinuses.
The Range of Infections Caused by the Fungus
Infections caused by S. apiospermum range from localized infections to highly invasive, disseminated disease. Localized infections often occur following traumatic inoculation and may present as mycetoma, a chronic, destructive infection of the skin and subcutaneous tissue, typically found on the limbs. Other localized presentations include keratitis (an infection of the cornea often linked to ocular trauma), soft tissue infections, osteomyelitis, and arthritis.
The most concerning clinical threat involves systemic and deep-seated infections, collectively referred to as pseudallescheriasis. The fungus has a propensity to infect the lungs, causing pulmonary scedosporiosis, which can manifest as a fungal ball (mycetoma) in pre-existing lung cavities or as invasive pneumonia. From the lungs, the organism can spread through the bloodstream, leading to disseminated infection that affects multiple distant organs.
A particularly severe manifestation is infection of the central nervous system (CNS), which typically presents as brain abscesses and is associated with a high mortality rate. CNS infection is a recognized complication of disseminated disease in immunocompromised patients, but it also occurs in otherwise healthy individuals following near-drowning incidents where the fungus may travel directly from the sinuses or lungs to the brain. The fungus can invade almost any tissue and organ, including the sinuses, eyes, heart (endocarditis), and bones.
Populations at Highest Risk
The risk of developing a severe S. apiospermum infection is strongly influenced by the host’s immune status and specific exposure events. Immunocompromised individuals, whose weakened immune defenses allow the opportunistic fungus to establish a foothold and disseminate, are highly susceptible. This includes patients undergoing chemotherapy, those with advanced HIV infection, and individuals receiving immunosuppressive drugs after solid organ or hematopoietic stem cell transplantation. For solid organ transplant recipients, Scedosporium species are a significant cause of non-Aspergillus mold infections.
Patients with cystic fibrosis (CF) are also highly susceptible, often experiencing chronic colonization of their airways by Scedosporium species. This colonization can contribute to the progressive deterioration of lung function and poses a significant risk for invasive, disseminated infection should the patient require a lung transplant and subsequent intensive immunosuppression. Scedosporium is considered the second most common filamentous fungus colonizing the airways of CF patients.
Beyond immunosuppression, severe trauma is a high-risk factor, regardless of the patient’s immune status. Victims of near-drowning in contaminated water are particularly vulnerable because the physical trauma and water exposure directly inoculate the organism into the respiratory tract and sinuses. These individuals can develop aggressive infections, including disseminated disease and brain abscesses, despite having a healthy immune system.
Identification and Treatment Strategies
Diagnosis of scedosporiosis is often challenging because the clinical and radiological signs can resemble other common fungal infections like aspergillosis. Identification relies on culturing the fungus from tissue or fluid samples obtained from the infected site. Molecular techniques, such as polymerase chain reaction (PCR) and sequencing, are increasingly utilized to provide rapid and accurate species identification, which is important for guiding therapy.
Treatment is complicated by the organism’s intrinsic resistance to many standard antifungal medications, including amphotericin B. Voriconazole, a triazole antifungal, is generally considered the first-line treatment due to its superior in vitro activity against S. apiospermum. However, even with voriconazole monotherapy, treatment failures and relapses are common, particularly in cases of disseminated or CNS infection.
In many severe or localized cases, a combination of medical and surgical intervention is necessary. Surgical debridement is often performed to remove infected tissue, especially in cases of mycetoma, osteomyelitis, or brain abscesses. For immunocompromised patients, reducing the level of immunosuppression when medically feasible allows the host’s own immune system to contribute to the clearance of the infection.