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Actinomycosis
Actinomycosis is a chronic, suppurative process characterized by a tendency for the inflammatory process to
dissect through soft tissue and to present at the skin surface in the pattern of draining sinuses. Pulmonary or abdominal structures may be primarily affected. Oral lesions, primarily those affecting bone, but with
sinuses draining to the skin surface, may also be a pattern of presentation. The organism, Actinomyces israelii, is filamentous. It is a normal inhabitant of the oral cavity. In its growth in tissue, it forms
colonies; the colonies are associated with a suppurative response. They can be identified in the pus liberated at the skin surface. The sinuses are bordered by organizing, inflamed granulation tissue. Granulomas are
not a significant feature.
The staining qualities of the organisms forming the colonies are variable. The organisms may be hematoxylinophilic but, if dead,
may be eosinophilic. They are Gram positive (if viable), PAS +,
and acid-fast negative. An eosinophilic deposit outlines the colonies; it forms globular projections along the surface of the colonies; it is a variation of the Splendore-Hoeppli phenomena (S11C21P16-1-6).
Nocardiosis
Nocardiosis also is a suppurative process; the organism is filamentous but generally does not form
colonies. In the skin, the infiltrates are poorly organized (S11C21P16-7-9). The disease may present as a
localized infection, in the pattern of a sporotrichoid process, or as a mycetoma. Mycetomas are caused by filamentous bacteria, such as Nocardia, or by true fungi.
The organisms are weakly acid fast and are gram positive. It is difficult to identify organisms on H&E
stained sections.
Protothecosis
The causative agent of protothecosis is an alga. The disease, with few exceptions, is confined to the skin and
soft tissue.
Protothecosis is a granulomatous process; the granulomas commonly are necrotic centrally. The organisms are
found in the cytoplasm of histiocytes and giant cells. Cell walls are thick; cell bodies are basophilic. Endosporulation is a characteristic features (S11C22P17-1 & 2). The organisms are PAS positive. With a Gomori’s silver methenamine stain, they are
argyrophilic. Under polarized light, they also may be birefringent, appearing in the pattern of a maltese cross.
Boyd AS, Langley M, King LE jr: Cutaneous manifestations of Prototheca infections. J Am Acad Dermatol 1995;32:
758-64.
Mucormycosis
Mucormycosis (zygomycosis or phycomycosis) is an opportunistic fungus infection; it is a threat for the
immuno-compromised. If found in the skin, this infection usually is evidence of disseminated disease; rarely, it is primary in the skin. It is commonly associated with hematologic disorders, such as leukemias and
lymphomas and may appear during the treatment of such disorders. Being, in this manner, associated with depressed cell counts in the peripheral blood, the histologic findings are necrotizing, but are usually
relatively free of inflammatory infiltrates. Neutrophils are represented in the infiltrates. The organism does not respect tissue planes; it readily invades vessels. The affected vessels undergo thrombosis. Much of
the tissue damage is related to a compromised blood supply.
The organisms are coarse hyphae that are not uniform in diameter. The cell wall is hematoxylinophilic and the
organisms are easily identified on H&E stained sections (S11C22P17-3-5 & S11C22aP17a-1 & 2). The hyphae branch at wide angles to the parent.
The patterns in a fusarium infection resemble those of mucormycosis. In a fusarium infection, the hyphae tend to
be more uniform in outline (S11C22aP17a-3).
Tunga Penetrans
Tunga penetrans (jigger, sand flea, chigoe) is a burrowing flea. The fertilized flea burrows into the skin to produce itching and
irritation. The affected site may become secondarily infected.
Amebiasis
Cutaneous amebiasis is usually a secondary phenomenon; cutaneous lesions may develop in the anogenital region in
association with amebic colitis. Hepatic or pulmonary abscesses may perforate to involve the skin surface. Open drainage of a hepatic abscess may result in the formation of an infected sinus. Acanthamebiasis often
presents as a meningoencephalitis; the portal of entry is the nasal cavity and the source is polluted water. Amebiasis is seen with increased frequency in the immuno-compromised.
Amebic trophozoites are mononuclear organisms which are rounded in outline; the cytoplasm is pale and often contains phagocytized
red blood cells. The nucleus is small and round with a single karyosome (S11C22P17-6).
In the recognition of the organisms in histologic sections, the character of the nucleus betrays the nature of the cell; the cell,
by its nuclear characteristics, can be identified as being foreign to the host. The tissue response is one of coagulation necrosis. The organisms are found at the interface between viable and necrotic tissue. They
invade blood vessels and, in this manner, can disseminate. At the periphery of the zones of necrosis, the tissue response is non-specific. Secondary infections are common and, in response, the lesions become
suppurative. They may break down to form draining sinuses.
Rosenberg AS, Morgan MB: Disseminated acanthamoebiasis presenting as lobular panniculitis with necrotizing vasculitis in a patient
with AIDS. J Cutan Pathol 2001;28: 307-13.
Filariasis
In the category of nematode infestations, certain thread-like organisms have an affinity for connective tissue,
including the skin; they are grouped as members of the superfamily, Filarioidea.
The adults are associated with an inflammatory response which includes eosinophils and with a fibrosing
reaction. Gravid females contain microfiliariae (S11C22P17-7-8). The microfiliariae are released into the
surrounding tissue (S11C22P17-9) and, for some variants, are present in lymphatics; from the latter location,
they may find their way into the peripheral blood.
Scabies
The reaction to an ectoparasite, as seen in a lesion of scabies, is characterized by a variable response in the epidermis. Usually,
there is acanthosis. Spongiotic and psoriasiform changes are common. The epidermal infiltrates characteristically contain eosinophils. The ectoparasite is found in the keratin layer; the organism will be found in a
rounded defect (tunnel). The reaction in the dermis has vasculitic qualities and often extends along vessels into the deeper portion of the dermis, even into the subcutaneous tissue. Fibrinoid necrosis and
thrombosis may be a feature of the reaction. Lymphocytes, histiocytes, and eosinophils extend from the perivascular spaces into the reticular dermis among collagen bundles (eosinophilic, or eosinophilic-histiocytic
collagenosis). The process is immunostimulatory; connective tissue cells are activated. The reaction in Well’s syndrome is similar; it includes areas of eosinophilic necrosis (i.e., flame figures) and even small
allergic granulomas. In the tunnels in the keratin layer, it may be difficult to find portions of the ectoparasites (S11C22bP17b-1-5).
The markers, when found, include mature organisms, eggs, and excrement.
Buntin DM, et al: Sexually transmitted diseases: Viruses and ectoparasites. J Am Acad Dermatol 1991;25: 527-34.
Wong DE, et al: Seabather’s eruption. J Am Acad Dermatol 1994;30:399-406.
Leishmaniasis
Leishmaniasis is a protozoal infection. The clinical features are related to the subspecies; some subspecies are
prone to produce visceral involvement, particularly affecting cells of the reticulo-endothelial system. Some remain as skin infections and some may be associated with lymphatic involvement.
The amastigotes are found in the skin in the cytoplasm of histiocytes. The infiltrates are dense; they are
composed of lymphoid cells with scattered clusters of histiocytes. The organisms, in tissue, have a cell wall with a clear area between the cell wall and a small nucleus (S11C23P18-1-4 & S11C22aP17a-7). A small kinetoplast complex is also represented.
Koff AB: Treatment of cutaneous leishmaniasis. J Am Acad Dermatol 1994;31: 693-708.
Kubba R, et al: Clinical diagnosis of cutaneous leishmaniasis (oriental sore). J Am Acad Dermatol 1987;16:
1183-9.
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