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Heterobilharzia americana
Heterobilharzia americana is a species of blood flukes that infects raccoons in the United States. Canines and other mammals may be an accidental host for this parasite if exposed to the cercariae in fresh water and may develop schistosomiasis. This species is a member of the Schistosomatidae family and has a similar life cycle using miracidium and cercariae to enter hosts. While this parasite cannot infect humans, it may be fatal to dogs, if the infection is not caught early.

Intermediate host
Eggs are expelled in the feces of the definitive host. Eggs exposed to freshwater hatch within a few hours and release miracidium. The miracidium actively search for an intermediate host (freshwater snail) and can survive up to 20 hours. Miracidium penetrate the soft tissue of the snail and begin asexual reproduction. The miracidum transforms into a mother sporocyst and begins to produce daughter sporocysts. Daughter sporocysts leave the intermediate host around 25 days after exposure.

Definitive Host
After leaving the intermediate host, daughter sporocysts are now termed cercariae. The cercariae are now free-living organisms in search of a vertebral host. Cercariae typically penetrate the skin of racoons and other wild mammals but may enter domestic pets. The cercariae then travel through the blood stream and begin to develop into adults. Young adults can be seen in the lungs within 5 days of infection. Mature adults reside in the mesenteric and intrahepatic portal veins and can be identified 7 days after exposure to cercariae. Mature adults may release eggs which migrate to the lumen of the intestines and are excreted in the feces. This starts the life cycle for the next generation.

Egg
The eggs are oval shaped, measuring 87 um long and 70um wide. The have a thin shelled membrane lacking spines or hooks. Within the eggs are the developing miracidium.

Miracidium
The miracidium is found within the soft tissue of freshwater snails. It is widest in the anterior quarter, measuring 115um in length and 47um wide. The anterior apical papilla has apparent spines and the internal structures are clearly evident with staining.

Mother Sporocyst
The mother sporocyst still contains germinal elements of the miracidium as well as the excretory system. This is where asexual reproduction occurs.

Daughter Sporocyst
Daughter sporocysts are initially seen within the mother sporocysts. They become elongated and convoluted before being excreted from the intermediate host.

Cercariae
The cercariae are elongated, oval in shape and show spines on the mouth, tail, and ventral suckers.

Adult
The adults show no signs of an oral sucker and are found in the liver where all growth and development takes place.

Diagnosis
Clinical signs of an infection of H. americana include mucoid/ bloody diarrhea, ascites, hypercalcemia, weight loss, and vomiting. Lesions in the liver, intestinal submucosa, pancreas, and lungs may be present. Confirmation of H. americana infections includes fecal examination, necropsy, and PCR confirmation of rRNA subunit.

Treatment and Prevention
Praziquantel and fenbendazole are medications used to treat H. americana infections. Praziquantel is used specifically to treat hypercalcemia. The overall prognosis of this infection is good when caught early. To prevent domestic canines from becoming infected it is important to limit contact with bodies of freshwater suspected of harboring H. americana

Epidemiology
This trematode species is found in the Southeastern United States including the gulf coast Southern Atlantic states. Infection is limited by the geographical distribution of the intermediate host.