Heterobilharzia americana
Holly Frisby, DVM
Veterinary Services Department, Drs. Foster & Smith, Inc.
Heterobilharzia americana is most commonly found in areas of mud flats of the Mississippi River and swampland in Louisiana. It infests dogs and raccoons.
Even though H. americana is called the 'blood fluke', the signs of infestation are related to the intestinal system. It also causes skin lesions at the site where the parasite enters the body. The disease caused by H. americana is called heterobilharziasis, canine schistosomiasis, or water dermatitis. A related parasite in man causes a disease known as 'swimmer's itch' or 'clam-digger's itch'.
H. americana is related to the important disease-causing flukes in man called 'schistosomes'. These are common in Africa, South America and the Far East.
How is H. americana transmitted and how does it affect all these body systems?
The life cycle of H. americana is indirect - it involves a snail. This is what happens. The egg of H. americana is passed in the feces of the dog and hatches almost immediately in water. The immature form swims around until it finds a snail.
The immature form enters the snail where it matures into an infective form. These forms leave their snail host and swim in search of a dog or raccoon. But, they only leave the snail early in the morning. So, in this case the early dog gets the fluke - sorry, just had to include that. These forms only have one day in which to find a host before they die.
Upon entering a dog, the infective forms enter the veins in the skin and are carried to the heart and lungs. By several routes, they migrate to the veins of the intestine and liver where they mature. A bulkier male worm mates with the very slender and petite female. She then moves into smaller and smaller veins and lays her eggs and then leaves. The contractions of the vessels and intestine actually push the eggs through the wall of the vessel, the wall of the intestine, and into the intestine's lumen. The eggs are then passed in the feces.
What are signs of disease caused by this extensive migration?
Inflammation occurs at the site where the infective form enters. Pustules form and there may be itching.
An intense inflammation occurs in the intestine as the eggs pass through it. Diarrhea, with mucous and often blood, results. Dogs lose their appetite and may become emaciated. The wall of the intestine may become very thickened.
Cirrhosis of the liver can occur if the eggs are laid in its small veins and then enter the liver itself.
How is infection with H. americana diagnosed?
Diagnosis is made by finding the eggs in the feces. Flotation methods are not effective for concentrating these eggs. Best results are obtained by smearing a small amount of feces and water on a microscope slide and examining it.
How is a H. americana infection treated and prevented?
Fenbendazole (Panacur) at a dose of 18 mg/lb for 10 days or praziquantel (Droncit) have been effective treatments.
The logical approach to prevention is keeping dogs out of the water. With hunting dogs, this is impossible. Keeping dogs out of the water in the morning when the infective forms emerge from the snails is not effective since these forms can live 24 hours - there will always be some present.
