Epizootic lymphangitis is a chronic granulomatous disease of the skin, lymph vessels, and lymph nodes of the limbs and neck of Equidae caused by the dimorphic fungus Histoplasma farciminosum. The disease is seen in Asian and Mediterranean areas but is unknown in the USA. The fungus forms mycelia in nature and yeast forms in tissues and has a saprophytic phase in soil. Infection probably is acquired by wound infection or transmission by bloodsucking insects.
The disease is characterized by freely movable cutaneous nodules, which originate from infected superficial lymph vessels and nodes and 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 the 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 clinical features 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 globose or oval bodies (3–4 μm) with a central basophilic body surrounded by an unstained zone. The organism closely resembles H capsulatum. 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.