In protothecosis, the skin is most commonly involved, resulting from primary inoculation through a wound or abrasion. The infection is usually localized to the site of inoculation; however, in immunocompromised individuals, it can become widespread.
Prototheca is an achlorophyllic mutant of the green alga Chlorella. The organism is ubiquitous in the environment, particularly in aqueous locales. Infection usually occurs as a result of inoculation into or beneath the skin with exposure to contaminated water or tree slime. Traumatic inoculation with no water exposure has also been reported.  Person-to-person transmission does not occur. However, Prototheca has been cultured from under the fingernails and other cutaneous sites in healthy individuals.
While healthy individuals can become infected, the organism has low virulence. Protothecosis infections are more commonly described in patients who are immunosuppressed. Use of immunosuppressive drugs also increases the risk of protothecosis infections.  Of all possible immunosuppressants, glucocorticoids, whether topically applied, taken orally, or locally injected, are the most specifically associated with the onset of protothecosis.  In healthy individuals, the infection is localized and curable, but cases of disseminated disease in individuals who are severely immunocompromised can be fatal. Cases of disseminated disease have involved the blood, the peritoneum, the GI tract, the liver, and the meninges.
Protothecosis is a rare infection, with approximately 129 cases (87 involving the skin and nails)  reported since 1964. Most cases in the United States are from the Southeast, though cases from virtually all geographic regions have been reported.
Protothecosis is a rare infection, but it is seen worldwide, with cases reported on every continent except Antarctica. 
No racial predilection is noted.
No sexual predilection is evident.
Protothecosis typically affects those older than 30 years or elderly persons, although pediatric cases have been reported.
Patients with localized disease have an excellent prognosis and can expect cure. The prognosis of patients with severe disease and immunosuppression is poor.
In immunocompetent individuals, the infection usually remains confined to the skin at the site of inoculation. Olecranon bursitis can develop from protothecosis. Rarely, tenosynovitis can occur secondary to protothecosis and has been reported following injection of a sclerosing agent for varicose vein treatment. 
Rare cases of systemic infection occur almost exclusively in patients who are severely immunocompromised, as in patients receiving chemotherapy, or immunosuppressed patients, such as those on infliximab. Involvement of the meninges has been reported in a few cases of patients with AIDS.
Patients who have contracted protothecosis on one occasion should avoid bathing or swimming in lakes, streams, and ponds.
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