Pathogenic Fungi

Phylum Ascomycota
Phylum Ascomycota is the largest fungal group, containing marine, fresh water, and terrestrial species.  They are recognized as cup fungi in which their fruiting bodies contain ascospores which are involved in sexual reproduction.

Histoplasma capsulatum
H. capsulatum is the infectious agent of histoplasmosis, a fungal infection of the lungs.  While it is a disease that occurs world wide, infections are particularly common in the areas surrounding the meeting of the Ohio River into the Mississippi.  Among these areas, over 75% of individuals have been found to contain the antibodies for histoplasmosis.  Because the fungal conidia of H. capsulatum travel through the air, transmission of the fungus occurs when the conidia are inhaled.  However, an infection will only progress if the pH and moisture levels are favorable to the fungus.  Such environments include areas that contain a large amount of bird or bat droppings.  This is because bird droppings are a good source of energy for the fungus where as bats serve as reservoirs for H. capsulatum, and therefore, people who come in contact with these kinds of environments are more susceptible to getting an infection.  Outside of the body, at lower temperatures the fungal body is a filamentous structure. Upon entering a human host where the temperature increases, the cells transform into a single celled, yeast-like morphology.  These cells are then capable of invading host macrophages, where they reproduce.  In most cases, a histoplasmosis infection is minor, although, in an immunocompromised individual, the infection can become quite severe by entering the circulatory system where it can produce lesions within many organs.

Anamorphs are a group that differ from other Ascomycetes in that over time that have diverged from other taxons within the phylum and secondarily lost the capability to reproduce sexually.

Tinea corporis

T. corporis is the causative agent of the fungal skin infection, ringworm of the body, in humans.  On the skin, the fungus inhabits the epidermis where it feeds and grows on the keratin produced by the host cells. Transmission of this skin infection, is through direct or indirect contact.  People can acquire an infection by coming in contact with contaminated clothes, hair brushes, on the tiles of pools and shower surfaces, or through a feline reservoir.  Because T. corporis prefers a warm and wet environment, maintaining good hygiene and keeping skin dry can decrease the risk of obtaining such an infection even if contact with the fungus has been made.  The initial symptoms of an infection include a rash made up of clustered red bumps that itches significantly.  As the infection progesses, the rash will become circular it is shape and produce a red ring.  The skin of the outer portion of the ring usually becomes rather dry and peels.  A ringworm infection can easily be treated with an over-the-counter cream, however complications including secondary bacterial infections, cellulitus, and pyoderma, occasionally can occur.

Sporothrix schenkii  
S. schenkii is a species of fungi that, upon entering the subcutaneous tissues through an opening in the skin, causes sporotrichosis.  Sporotrichosis causes a chronic fungal skin infection to ensue in the subcutaneous layer of the person infected.  Because S. schenkii naturally inhabits the soil where is acts as a saprobe, feeding on decaying vegetation, it is able to penetrate into the deeper layers of the skin, where as most fungal skin infections can only colonize the surface cutaneous layers.  When a human infection does occur, the onset of symptoms is very slow.  At the site of entry, a small ulcer which is diagnostic to this infection takes shape.  The ulcer can take up to three months after the initial infection to develop.  Because this this species of fungus is thermally dimorphic, entering human tissues causes the fungus to morph into a yeast form where they live within the body as single cells.  Colonization of the subcutaneous tissues allows for easy access to the circulatory system.  Here they invade the lymphatic system, producing lesions, however, fatal infections are rare.  People with AIDs who obtain an infection can develop disseminating sporotrichosis in which numerous visceral organs of the body are affected.  Sporotrichosis can be treated by orally taking a potassium iodine solution.

Coccidioides immitis
C. immitis is a dimorphic, soil dwelling fungus that is found in the Southwestern United States as well as in Central and South America.  In the soil it appears as a filamentous fungi where it produces spores contained within an arthroconidia structure.  Because the arthroconidia use wind to disperse their spores, humans that inhale the arthroconidia can develop coccidioidomycosis, a respiratory infection caused by C. immitis.  Once inside the human body, the arthroconidia develops in the an encapsulated spherule structure, containing endospores.  The endospores are released and in turn form more spherules.  While this respiratory infection is common in endemic areas, infections rarely become severe, with the extent of the symptoms including chest pain, fever, cough, and weight loss.  However, on the rare occasion, the acute respiratory infection can progress into a disseminating form, similar to the pathology of tuberculosis.