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Research Note: Heterobilharzia americana

Clinician's Brief (Capsule)


|August 2016

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Heterobilharzia americana, a schistosomal trematode, is endemic in the coastal southern United States; most cases occur in Texas and Louisiana. Definitive hosts are dogs, raccoons, and other mammalian species; lymnaeid freshwater snails are intermediate hosts. The adult parasites reside in mammalian mesenteric veins. They release their eggs, which migrate to the intestines and are excreted in the feces. Once in freshwater, the eggs release ciliated miracidia, which penetrate the mantle of snails. The miracidia mature into cercariae in the snails before being released. Free-swimming cercariae then re-infect mammalian hosts by penetrating intact skin and migrating to the mesenteric veins. Aberrant migration can cause infection of the liver and other organs, which can cause granulomatous inflammation and fibrosis. H americana infections typically cause GI signs (eg, decreased appetite, weight loss, vomiting, diarrhea) and occasionally cause dermatitis or coughing from migration through the skin and lungs. Approximately half of reported cases have hypercalcemia of unknown cause. 

A fecal sodium chloride sedimentation is usually necessary to diagnose this infection. Polymerase chain reaction is also available to test feces, liver, and intestinal biopsies. If samples are available, eggs and parasites can be found on histopathology of affected tissues. In this case report, an H americana diagnosis was made via cytology of fine-needle aspirates of the patient’s liver. The authors concluded that cytology is a useful, accurate tool for diagnosing canine schistosomiasis.


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