Prototheca is a genus of single-celled organisms classified as achlorophyllous algae. This means they lack chlorophyll, cannot photosynthesize, and obtain nutrients from their environment. While Prototheca species are ubiquitous in nature, they are a rare cause of infection in both humans and animals, a condition known as protothecosis.
Environmental Sources and Transmission
Prototheca microorganisms are commonly found across various natural and man-made environments globally. These algae thrive in moist areas with abundant organic matter, including soil, fresh and salt water, sewage, and animal waste. They can also be isolated from tree slime and various food items. Prototheca species can survive in sewage effluents even after chlorination, allowing them to return to the environment.
Transmission to humans and animals occurs through direct contact with a contaminated source. This usually happens when the algae enter the body via a break in the skin, such as a cut, scrape, or other traumatic wound. Traumatic inoculation is the most common mode of infection. While ingestion is a possible route of exposure, it is less commonly associated with active disease compared to skin inoculation.
Human Protothecosis
Human protothecosis can manifest in several distinct clinical forms. The most frequent presentation is a localized cutaneous (skin) infection, which may appear as papules, nodules, ulcers, or crusted erythematous lesions. These skin infections can occur in individuals with healthy immune systems, often following a traumatic injury that introduces the algae into the skin.
Another localized form is olecranon bursitis, an infection affecting the bursa located at the tip of the elbow. This condition tends to occur in immunocompetent individuals and is linked to repeated elbow trauma. Symptoms typically develop gradually over several weeks following the initial inoculation.
A less common but more severe manifestation is disseminated, or systemic, protothecosis, where the infection spreads throughout the body. This widespread form occurs almost exclusively in individuals with compromised immune systems, such as those with cancer, organ transplants, long-term steroid therapy, HIV/AIDS, or diabetes mellitus.
Disseminated infections can present with diverse symptoms, including fever, abdominal pain, diarrhea, headache, and widespread skin lesions. Commonly affected organs include the skin, blood, gut, liver, lungs, peritoneum, and brain. The species Prototheca wickerhamii is most frequently identified in human cases, though Prototheca zopfii (now P. bovis or P. ciferrii) has also been implicated.
Protothecosis in Animals
Prototheca infections in animals present with varying clinical signs depending on the host species. In cattle, bovine mastitis is the primary and most impactful manifestation of protothecosis. This udder infection is persistent and challenging to treat. It is predominantly caused by Prototheca bovis (formerly P. zopfii genotype 2) and Prototheca ciferrii (formerly P. zopfii genotype 1). Infected udders may produce watery, white to yellow milk with pus or flakes, and edema and induration. The infection is often linked to teat trauma from mechanical milking and environmental contamination.
In dogs, protothecosis typically presents as a severe, disseminated disease that is often fatal, with clinical signs including chronic, intermittent hemorrhagic diarrhea and progressive weight loss. Ocular lesions, including blindness or retinal detachment, are common. Neurological problems, such as ataxia, seizures, lethargy, and deafness, may also develop due to central nervous system spread. Other affected organs include the skin, kidneys, liver, spleen, and lymph nodes. Both Prototheca bovis and Prototheca wickerhamii are known to cause disease in dogs.
Medical Diagnosis and Treatment
Diagnosis of protothecosis is typically made by obtaining a tissue sample, such as a biopsy, from the affected area. Pathologists examine these samples under a microscope to identify the organism’s unique cellular structures, often appearing as spherical cells (sporangia) containing internal spores. Prototheca can also be grown in a laboratory culture from clinical specimens. On culture media, it forms smooth, moist, white to cream-colored colonies that resemble yeast.
Treatment strategies for protothecosis vary depending on the infection’s extent and location. For localized cutaneous lesions, surgical removal of the infected tissue is often the most effective approach. Systemic or disseminated infections present a greater challenge, frequently involving antifungal medications like amphotericin B or itraconazole.
These drugs are effective because Prototheca’s cell wall contains ergosterols, a component targeted by these agents. Amphotericin B demonstrates stronger activity. Treatment courses can be prolonged, and achieving a complete cure, particularly in disseminated cases, can be difficult.