Protothecosis is a rare infectious disease caused by a type of alga, unlike more common infections from bacteria, viruses, or fungi. It can affect both humans and animals, though it is particularly uncommon in people.
The Algal Pathogen and Its Habitat
The organism responsible for protothecosis belongs to the genus Prototheca, a group of microscopic algae. Unlike most algae, Prototheca species are achlorophyllic, meaning they do not possess chlorophyll and cannot perform photosynthesis. This unique characteristic often leads to their mistaken identification as fungi due to their appearance and growth patterns in laboratory settings.
These colorless algae are widespread in various natural environments. They commonly inhabit soil, particularly areas rich in decaying organic matter such as leaves and wood. Prototheca can also be found in sewage, on the surfaces of trees, and within contaminated water sources like ponds, streams, and some public water systems. Their presence in these diverse habitats highlights their adaptability and widespread distribution.
Human Infection and Symptoms
Human protothecosis typically begins when the algae enter the body through a break in the skin. This can occur when skin wounds, cuts, or abrasions come into contact with environmental sources contaminated with Prototheca species. The infection is not contagious and does not spread directly from person to person.
Individuals with a weakened immune system are more susceptible, although cases in otherwise healthy people have been documented. The most frequently observed form is cutaneous protothecosis. This localized skin infection often manifests as persistent lesions, which may appear as firm plaques, raised nodules, or slow-healing ulcers.
Another localized form is olecranon bursitis, an infection affecting the bursa located over the elbow joint. This condition results in pain, swelling, and inflammation of the bursa. The rarest and most severe manifestation is disseminated, or systemic, protothecosis, where the infection spreads beyond the initial site to internal organs. This form is almost exclusively observed in patients with severely compromised immune systems and can involve multiple organ systems.
Protothecosis in Animals
Beyond human cases, this alga also affects various animal species, with domesticated animals being the most commonly reported. In dogs, protothecosis is often a severe and disseminated illness. Symptoms in affected dogs include chronic bloody diarrhea, sudden blindness, and various neurological signs. Bone lesions may also develop, contributing to lameness or discomfort.
In cattle, Prototheca species cause bovine mastitis, an inflammatory infection of the udder. This form of mastitis can be challenging to treat, leading to economic losses for dairy farmers. While rare, protothecosis has also been reported in cats, typically presenting with similar skin or systemic manifestations.
Diagnosis and Medical Management
Diagnosing protothecosis typically involves a tissue sample (biopsy) from the affected area. This sample is then examined under a microscope, where the characteristic algal cells can often be identified. To confirm the diagnosis and identify the Prototheca species, the tissue is also cultured in a laboratory.
Treatment approaches for protothecosis depend on the form and severity of the infection. For localized or cutaneous infections, medical management often involves surgical removal (excision) of the lesion. This may be combined with antifungal medications, as certain antifungal drugs are effective against Prototheca.
Systemic or disseminated protothecosis requires aggressive and long-term systemic antifungal therapy. Medications are administered orally or intravenously to target the widespread infection. The general outlook for individuals with localized skin infections and healthy immune systems is typically favorable following appropriate treatment. However, the prognosis is more serious and guarded for systemic infections, especially in immunocompromised patients, and for infections occurring in animals, particularly dogs with disseminated disease.