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Cryptococcosis Causes and Risk Factors
The major causes of cryptococcosis are infection by C. neoformans and C. gattii. These two species have several types (serotypes) of different capsular polysaccharides that cause the majority of human cryptococcosis (A, D, and AD for C. neoformans; B and C for C. gattii). The three strains of C. gattii that are most frequently found causing cryptococcosis are designated VGlla, VGllb, and VGllc.
Cryptococcus spp. was thought to occur only as a yeast form until 1976 when Dr. Kyung Joo Kwon-Chung described the mycelial form (taking the form of branching, threadlike structures) of C. neoformans (termed Filobasidiella neoformans). C. gattii also has a mycelial form. The polysaccharide capsule covering the yeast forms renders Cryptococcus species resistant to human and animal immune defenses. The fungi do not usually cause major problems during the initial infection but slowly multiply. The organ damage begins to occur when the slowly replicating fungi develop fungal masses (termed cryptococcomas) that start to compress or distort the involved organ (usually the lung or brain) and its vasculature. Some of these fungi can separate from a lung fungal mass or infected pulmonary nodule and then be carried by cells or swept into the bloodstream to then lodge and grow in other organs, especially the brain. Many patients may show no symptoms related to the lung infection during this process. Because there is little or no inflammatory response to these organisms, symptoms do not develop in humans and animals until late in the disease process when the fungal mass begins to alter the organ where it is located. This is why some individuals show brain changes as their first symptoms of having cryptococcosis.
Risk factors for cryptococcosis caused by C. neoformans are inhalation of fungi that are associated with various bird droppings or guano, especially from pigeons. People who are immunocompromised, especially those with HIV/AIDS, are the most susceptible people to acquire infection. Risk factors for cryptococcosis caused by C. gattii are different from C. neoformans. In general, C. gattii infections were mainly associated with tropical or semitropical climates around the world and most frequently associated with inhalation of plant propagules, especially those from eucalyptus, red river gum, and forest red gum trees. However, C. gattii seems capable of inhabiting other areas; in 1999, there were cases of C. gattii noted in animals (cats, dogs, ferrets, marine animals) and a few individuals in Vancouver Island, Canada. In 2006, an outbreak of cases (over 100) occurred with at least six deaths attributed to C. gattii on Vancouver Island. Since that time, other cases have been found in Washington and Oregon in the U.S. Currently, C. gattii has been sporadically isolated from plant debris, dust and air samples, and usually near stands of trees or logging areas in the Pacific Northwest. Some authors suggest C. gattii was imported with semitropical vegetation, inadvertently released into the environment, and has begun to adapt and grow in the northwest states. Consequently, a risk factor occurs when people encounter airborne propagules or "dust," especially around logging operations and sawmills. Other investigators suggest that C. gattii has been around for a long time and only recently we have developed tests that distinguish C. gattii from C. neoformans and that many infections previously attributed to C. neoformans were actually caused by C. gattii. Others speculate that more virulent strains of C. gattii have evolved and are now being noticed by the medical community.
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