Characteristics and importance of the genus Prototheca in human and veterinary medicine

Prototheca spp. are strange algae, assigned to the genus Prototheca, family Chlorelaceae. They are ubiquitous in nature, living predominantly in aqueous locales containing decomposing plant material. Prototheca spp. were isolated from skin scarificates, sputum and feces of humans in absence of infection as well as in a variety of domestic and some wild animals. Prototheca spp. are unicellular organisms, oval or spheric in shape. They differ from bacteria and fungi in size, shape and reproductive characteristics. Of the five known species of the genus, only P. wickerhamii and P. zopfii are considered pathogenic, and they are the only known plant causative agents of human and animal infections. Over the past 25 years medical references reported more than 100 cases of human protothecoses, mostly induced by P. wickerhamii and rarely by P. zopfii. A half of the reports on human protothecoses relates to localized cutaneous infections and oleocranon bursitis. The rarest and most severe form of the infection is disseminated or systemic protothecosis, described in patients with durable course of primary disease or immune disfunction. In veterinary medicine, Prototheca zopfii and rarely also P. wickerhamii are reported as causative agents of cutaneous protothecosis in dogs and cats, systemic protothecosis in dogs and mastitis in dairy cows. Protothecal infections are diagnosed by histopathology examination or, more exactly, by isolation of the agent, although the organism cannot be distinguished from the yeasts by its cultural characteristics. Final diagnosis is made by the carbon-hydrate assimilation test. Protothecal infections are easily missed in routine practice. Pharmacological protocol for therapy of this rare infection has not been developed yet either in human or in veterinary medicine. Several antifungal agents are applied for treatment; however, the effects are variable. Where possible, surgical excision is treatment of choice. Prognosis is promising in patients with localized infection, and healing is often achieved. Prognosis is less predictable, mostly bad, in patients with other diseases and in immunocompromized patients. Sensitivity of Prototheca spp. in vitro does not necessarily correlate with its efficacy in vivo.