Epizootic lymphangitis is a chronic granulomatous disease of the skin, lymph vessels, and lymph nodes of the limbs and neck of equids due to the dimorphic fungus Histoplasma capsulatum var farciminosum. The fungus forms mycelia in nature and yeast forms in tissues and has a saprophytic phase in soil.
Epizootic lymphangitis occurs in Africa, Asia, the Middle East and the Mediterranean regions, and is most common in Ethiopia. It is unknown in the US. Infection probably is acquired by direct contact of wounds with exudates from other infected animals, but fomites (including insects) may also play a role. The disease is also known as pseudofarcy. The causative organism is not zoonotic.
Histoplasma capsulatum var farciminosum is closely related to the organism that causes histoplasmosis.
Clinical Findings and Lesions in Epizootic Lymphangitis in Animals
Epizootic lymphangitis is characterized by freely movable cutaneous nodules, predominantly on the legs, neck, chest, and face, that originate from infected superficial lymph vessels and nodes. The nodules tend to ulcerate and undergo alternating periods of discharge and closure. Affected lymph nodes are enlarged and hard. The skin covering the nodules may become thick, indurated, and fused to the underlying tissues.
Lesions also may be present in joints, lungs, conjunctiva, cornea, nasal mucosa, and other organs. The nodules are pyogranulomas with a thick, fibrous capsule and contain thick, creamy exudate and the causative organisms. The disease may spontaneously resolve in some patients; however, most affected animals become debilitated and anorectic as disease progresses.
Diagnosis of Epizootic Lymphangitis in Animals
Characteristic clinical signs in equids and visualization of the organism
Tiny encapsulated ovoid yeasts with narrow-based budding free or within macrophages
The clinical features of epizootic lymphangitis are highly suggestive. Diagnosis can be confirmed by microscopic examination of exudates and biopsy specimens.
The yeast forms of the organisms distend the cytoplasm of macrophages and appear in H&E sections as round or oval bodies (2–5 mcm) with a central basophilic body surrounded by an unstained area. They may demonstrate narrow-based budding and may appear in clusters, particularly inside macrophages. The organism closely resembles H capsulatum var capsulatum, and clinical differentiation may be impossible without specific culture or molecular methods.
Culture of H capsulatum var farciminosum is often unrewarding and is biohazardous. Serologic testing is available with serum agglutination titers of 1:80 or higher reported to be positive. Positive titers may be reflective of past exposure, with specificity for current infection being low. A histofarcin intradermal hypersensitivity test may be available in endemic areas.
Treatment of Epizootic Lymphangitis in Animals
Amphotericin B or azoles
Some cases may respond to iodide treatment and possibly surgical excision
No completely satisfactory treatment for epizootic lymphangitis is known. Surgical excision of lesions combined with antifungal drugs (amphotericin B or azoles) could be used; however, economic limitations in most endemic areas limit treatment to iodides (IV sodium iodide and/or oral potassium iodide).
For More Information
Epizootic lymphangitis. American Association of Equine Practitioners. March 2021. https://aaep.org/document/epizootic-lymphangitis
