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Compendium March 2012 (Vol 34, No 3)

Clinical Snapshot: A Quarter Horse Mare With Acute Colic

by Adam Stern, DVM, CMI-IV, CFC, DACVP

    Case Presentation

    A 17-year-old Quarter horse mare presented with acute colic to the Oklahoma State University Boren Veterinary Medical Teaching Hospital. On physical examination, the horse had tachypnea and tachycardia and was extremely painful. A nasogastric tube was placed, but no reflux was observed. Because of financial reasons, the client chose to have the horse euthanized.

    During necropsy, impaction of the left dorsal colon was diagnosed. In addition, more than 100 firm, well-demarcated, 3- to 7-mm diameter, white nodules were found throughout the hepatic parenchyma and subcapsular region of the liver (FIGURE AFIGURE B) . The nodules were markedly firm to hard and extremely difficult to cut. More than 100 similar nodules were also found throughout the pulmonary parenchyma, and a single nodule was found within the spleen.

    1. What is the differential diagnosis for these lesions?

    2. What is the significance of these lesions?

    Answers and Explanations

    1. The differential diagnosis for hepatic, pulmonary, and splenic granulomas includes diseases caused by bacteria (Mycobacterium bovis, Mycobacterium avium), fungi (Blastomycosis dermatitis, Coccidioides immitis, Aspergillus spp), and parasites (Heterobilharzia americana).1–3 Histologically, the nodules within the liver, lungs, and spleen were consistent with fibrosing granulomas. The central regions of the granulomas were composed of mineralized material and surrounded by a thick layer of fibrous connective tissue  (FIGURE C) . A rim of inflammatory cells (i.e., eosinophils, lymphocytes, and macrophages) lined the periphery of the fibrous connective tissue  (FIGURE D) . On examination of acid-fast stains, no organisms were identified. The results of an aerobic bacterial culture were negative for pathogenic bacteria.

    Based on the absence of acid-fast–positive organisms within the granulomas (impression smears of the liver and histologic sections of the liver and the lungs), the absence of fungi and yeast, and negative bacterial culture results, the granulomas were suspected to be associated with a chronic parasite infection. Although this diagnosis was circumstantial, the gross and histologic findings were consistent with chronic schistosome infections that have been reported in the literature.1,2 In one report,2 11 horses with gross and histologic lesions similar to those reported here were diagnosed with presumptive chronic schistosomiasis of undetermined origin, and broken schistosome eggs were identified within the granulomas of one horse. In horses, schistosomiasis occurs most commonly in Asia and Africa. Species reported to infect horses include Schistosoma japonicum, Schistosoma bovis, Schistosoma indicum, and Schistosoma mattheei. H. americana is considered the primary schistosome species in North America; this organism commonly infects birds and carnivores and uncommonly infects horses.2

    2. The lesions were considered to be an incidental finding because this horse did not have clinical signs of liver disease. An equine case of severe parasitic granulomatous hepatitis due to an unidentified schistosome has been reported.2 Schistosomes are transmitted by freshwater snails that have been invaded by miracidia (infectious larvae) that develop into cercariae (infectious free-swimming larvae) by asexual reproduction within the snails. Cercariae swim through freshwater (irrigation canals and other slow-moving bodies of water) and penetrate the skin of a definitive host (e.g., horse, human) through the use of proteolytic enzymes.1 After entering the host’s skin, cercariae transform into schistosomula and migrate to the lungs and, subsequently, the liver, where they mature. Male and female schistosomes migrate to the mesenteric veins and mate, and females produce hundreds of eggs per day.1 Depending on the species of schistosome, the host excretes eggs via feces or urine. Eggs can become entrapped in the intestines, liver, lungs, spleen, and urinary bladder, resulting in granuloma formation, as in this case. Trematode eggs are destroyed by inflammatory cells and, therefore, are not found within the granulomas.

    Dr. Stern is now affiliated with the University of Illinois.

    1. McAdam AJ, Sharpe AH. Infectious disease. In: Pathologic Basis of Disease. 8th ed. Philadelphia: Saunders; 2010:393-395.

    2. Buergelt CD, Greiner EC. Fibrosing granulomas in the equine liver and peritoneum: a retrospective morphologic study. J Vet Diagn Invest 1995;7:102-107.

    3. Pappagianis D, Higgins J. Coccidiomycosis. In: Equine Infectious Diseases. St. Louis: Saunders; 2007:396-403.

    References »

    NEXT: Equine Laparoscopy: Equipment and Basic Principles


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