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Protothecosis is an infection caused by
achlorophyllous algae of the genus Prototheca. Although they do not
contain chloroplasts, these saprophytic algae are believed to be
related to green algae of the genus Chlorella.
Causative organism:
Three species of protothecae are recognized, of which two, P.
wickerhamii and P. zopfii, are known to cause disease.
Almost all authenticated cases of human
protothecosis have been caused by P. wickerhamii.
Geographic distribution:
Human infections are cosmopolitan in distribution, but most have
occurred in the United States.
Source of infection:
The source of infection is often not apparent but can be related to
penetrating injury in some cases.
Clinical forms:
Two clinically distinct forms of protothecosis are recognized:
cutaneous infection and olecranon bursitis.
Cutaneous
protothecosis,
which occurs preferentially
in debilitated or compromised patients, present as spreading
papulonodular or verrucous lesions, usually involving the distal
extremities or head. Infection may extend into the subcutaneous tissue
and rarely spreads to regional lymph nodes.
Olecranon bursitis,
which occurs in otherwise healthy hosts, presents as a subcutaneous
nodule adjacent to the elbow. Bursectomy is the treatment of choice.
Chemotherapy alone has not
effectively eradicated localized infection in most cases.
The single reported case of
disseminated human
protothecosis
occurred in a patient with transient depression of specific
cell-mediated immunity to Prototheca. He recovered after therapy with
transfer factor and amphotericin B.
Life cycle
and features of the two species:
The protothecae are found in tissue
sections in the form of endosporulating sporangia.
Their asexual reproductive cycle in
tissue is similar to that of the endosporulating fungi.
Small, uninucleate, immature sporangia
undergo nuclear division followed or accompanied by progressive
cytoplasmic cleavage to produce mature sporangia that contain
sporangiospores.
Characteristically, the sporangiospores
are polygonal or wedge-shaped, fill the parent cell, and may be
radially arranged around a central sporangiospore, producing the
distinctive “morula” form.
The sporangia of the two pathogenic
protothecae differ in size but are otherwise similar in morphology.
Sporangia of the small form, P.
wickerhamii, measures 2 to 12 micrometer in diameter, where as those
of P. zopfii measures 10 to 25 micrometer in diameter.
Morula forms are uncommon in infections
caused by P. zopfii.
Endosporutating cells of P. zopfii are
oval, and their larger nuclei are more conspicuous than those of P.
wickerhamii.
The cell walls of both the sporangia
and the sporangiospores are stained with the special stain for fungi.
With hematoxylin and eosin, these cells
are hyaline, but their contents may be eosinophilic or basophilic.
The two species are more reliably
distinguished from one another in tissue sections by direct
immunoflourescence and in culture by their patterns of carbohydrate
assimilation.
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. (Dr Tsutsumi)
Pathological features:
Cutaneous lesions often show
hyperkeratosis, parakeratosis, and acanthosis, and they may be
ulcerated.
Algal cells are abundant in the dermis
and may also be found in the epidermis and keratin layer as a result
of transepidermal elimination.
An inflammatory reaction, when present,
may be granulomatous or may consist of a mixture of acute and chronic
inflammatory cells.
Infection of the olecranon bursa
produces necrotizing granulomatous inflammation.
The bursal lining consists of a
stellate zone of necrotic debris, neutrophils, and fibrin that is
surrounded by palisaded epitheloid histiocytes and multinucleated
giant cells.
The adjacent soft tissue contains
granulation tissue, acute and chronic inflammatory cells, and small
granulomas.
Prototheca cells are difficult to find
in these lesions, which can be misinterpreted as rheumatoid nodules if
special stains are not used to detect the algae.
Differential diagnosis:
Endosporulating fungi such
as
Coccidioides immitis
and
Rhinosporidium seeberi
are distinguished from the protothecae in tissue sections on the basis
of their size and distinctive morphology.
Green algae of the genus Chlorella
cause cutaneous and systemic infections in animals, but human green
algal infection has been recognized only recently.
In tissue sections, the cells of
Chlorella, 6 to 14 micrometer in diameter, appear similar to those of
P. zopfii.
However, infections caused by the two
algae can be differentiated by other criteria.
The prothecae can be distinguished from
each other and from Chlorella in tissue sections by direct
immunoflourescence.
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