Prototheca are ubiquitous, colorless, saprophytic algae. Species of this organism are found in grass, trees, garbage dumps, soil, raw and treated sewage, animal wastes, tap water, fresh water streams, swimming pools, and stagnant water. They have also been found colonizing the human skin, fingernails, respiratory tract, and digestive system. Prototheca is classified among the lower algae, the Chlorophyceae, and belong to the same family as the green algae Chlorella. These organisms require carbon in order to thrive, and obtain this nutrient through consumption of organic substrates.

Prototheca species are oval or spherical single-cell organisms ranging from 3 to 30 µm in diameter. These organisms do not have glucosamine, a specific fungal cell wall component, or muramic acid, a specific component found in bacterial cell walls. Under microscopic analysis, Prototheca species resemble a fungal organism, and can therefore, be mistaken when attempting to identify these algae. They are distinguished from other algae, such as Chlorella, by their lack of chloroplasts and the presence of a two-layered, instead of three-layered, cell wall on electron microscopy.

Pathogenic Prototheca

Two species, P. wickerhamii and P. zopfii, are known to cause infection, though the majority of human infections are due to P. wickerhamii. Although such infections are infrequent, they can manifest themselves clinically as cutaneous lesions, olecranon bursitis, and, even more rarely, as disseminated or systemic infections. In patients with algaemia, Prototheca spp. are often associated with bacteria, viruses, or yeasts which cause co-infections. Disseminated protothecosis is currently rare but, due to the algae's ubiquitous nature, increasing use of immunosuppressive therapy, and increasing incidence of hematologic malignancy, Prototheca spp. may emerge as opportunistic pathogens2.

Cutaneous protothecosis
Credit: PubMed Central

Protothecosis in Domestic Animals

In cattle, P. zopfii is an important cause of environmental mastitis. Inflammation of the udder is mild in comparison with bacterial mastitis, but it is invasive and chronic. The etiological agent causes pyogranulomas in the mammary gland and the regional lymph nodes. Milk production in the affected quarter diminishes, and small clots may be found in the milk. In dogs and cats protothecosis is caused mainly by P. zopfii, causing cutaneous infections or a systemic form affecting many organs and causing massive invasion of the bloodstream. The severity of the disease varies according to the organs affected. Cats mainly develop cutaneous infection of the face and nose. Protothecosis caused by P. wickerhamii was recently reported in goats causing rhinitis and dermatitis of the face and pinna.4,5.


  1. Human Protothecosis
  2. Fatal Algaemia in Patient with Chronic Lymphocytic Leukemia
  3. Unusual Fungal and Pseudofungal Infections of Humans
  4. Camboim EKA, Neves PB, JĂșnior FG, Medeiros JM, Riet-Correa F Pesq Vet Bras 2010 Jan; 30 (1): 94-101. Protothecosis
  5. Zoonoses and Communicable Diseases Common to Man and Animals. Third Edition, Volume I, Bacterioses and Mycoses. Pan American Health Organization