Signalment:  
Gross Description:  
Histopathologic Description:
Morphologic Diagnosis:  
Liver: hepatitis, chronic, portal, nonsuppurative due to Heterobilharzia americana.
Small intestine: Enteritis, chronic, nonsuppurative, transmural, due to Heterobilharzia americana.
Lab Results:  
Condition:  
Contributor Comment:  
In addition to the lesions in the tissues submitted, there were disseminated granulomas surrounding eggs in the pancreas, lung, lymph node, and kidney; the small intestine and liver were most severely affected. The trematode is in the family Shistosomatidae; these parasites are of relatively low importance in North America. In addition to this trematode in dogs, there is a dermatitis of humans (i.e., "swimmer's itch") that is caused by a cercariae of wild waterfowl (Trichobilharzia, Austrobilharzia, and Bilharziella).(1)
The miracidium hatches very soon after the egg comes in contact with water, and then enters a freshwater snail (Lymnaea cubensis), in which cercariae develop in daughter sporocysts. Activity by these snails is dependent upon ambient temperatures between ranges of 10°C-29°C(4). If the ambient temperatures are within this range, development in the snail is completed within 30 days. Upon emergence from the snail, the cercariae penetrate the skin of the host (i.e., dog, rabbit, raccoon, bobcat, or nutria), and migrate by way of the lungs to the liver. After a period of development in the liver, mature males and females make their way to the mesenteric vein and mate. These adults do not reproduce in mammalian hosts, and but may live there for 4-10 years, producing thousands of eggs during that time.(6) Eggs laid in the terminal branches of the mesenteric veins passively work through the bowel wall via a mechanism of proteolytic enzymes that are emitted through an ultramicroscopic pore in the eggshell. They then progress to the gut lumen and escape in the feces. If feces are deposited in water, the process starts over again. The eggs evoke a granulomatous reaction that eventually prevents their egress, and favors their carriage to other organs with consequent production of widely disseminated granulomas.
It is of interest that fluke infestations, which are most often associated as problems in animals in a wet climate, can occur in a "high desert" climate and geographical environment such as the American Southwest. In the area involved, there is both flood irrigation and excessive water runoff situations during a summer wet or "monsoon" season. During these times, the necessary elements for this infection (as well as fascioliasis in cattle) are present, especially along the river bottoms or "bosque" zones. Of further interest is the involvement of the lymnaeid snail that is "double dipping" in trematode infections in several species with two different trematode parasites.Â
JPC Diagnosis:  
1. Liver: Hepatitis, portal, granulomatous, diffuse, moderate, with numerous trematode eggs and nodular hemosiderosis.Â
2. Small intestine: Enteritis, granulomatous, multifocal, moderate, with numerous mucosal and submucosal trematode eggs and intravascular adult schistosomes.
Conference Comment:  
Diarrhea is a typical clinical finding attributed to rupture of enteric mucosal vessels and a type I hypersensitivity reaction that results in enteric ganglionitis. Small granulomas, called pseudotubercles, form in deeper tissues in chronic disease when endophebitis precludes escape of the eggs into the intestinal lumen, as discussed by the contributor. Pseudotubercles are initially primarily eosinophilic and later progress to traditional granulomas. With chronicity, degenerate eggs often mineralize or become coated with Splendore-Hoeppli material. Adult schistosomes elicit eosinophilic endophlebitis, intimal proliferation, and thrombosis in mesenteric and portal veins, as demonstrated in this case. Adults feed on erythrocytes and regurgitate hematin pigment, which was also evident in this case(5). Due to slide variation, not all sections of small intestine featured adult schistosomes in mesenteric vessels.Â
References:
2. Fabrick C, Bugbee A, Fosgate G. Clinical features and outcome of Heterobilharzia americana infection in dogs. J Vet Intern Med. 2010 Jan-Feb;24(1): 140-4.
3. Flowers JR, et al. Heterobilharzia americana infection in a dog. J Am Vet Med Assoc. 2002 Jan 15;220(2): 193-6, 183.
4. Goff WL, Ronald WC: Certain aspects of the biology and life cycle of Heterobilhazia americana in east central Texas. ln: Am J Vet Res, vol. 42, no. 10, pp. 1775-1781 (1981).
5. Maxie MG, Robinson WF. Cardiovascular System. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol 3, 5th ed. Philadelphia, PA:Elsevier Saunders; 2007:95-7.
6. Slaughter JC, Billups LH, Acor GK: Canine heterobilharziasis. In: Compendium Small Animals, vol. 10, no. 5, pp. 606-61 1 (May 1988).