Heterobilharzia: Liver and Intestinal Parasite in Dogs

The parasite heterobilharzia americana enters the definitive host, the dog, through the skin. There seems to be no breed discrimination, although dogs who enjoy water will be more at risk. Reports have been made as far north as Kansas but H. americana appears to be largely confined to the Southeastern United States, concentrated mostly in Louisiana and the Atlantic Gulf Coast. Raccoons are widely known as a definitive host for heterobilharzia americana but other animals such as white-tailed deer, minks, beavers, bobcats, mountain lions, coyotes, and opossums have been known to serve as hosts as well.

Heterobilharzia americana is a clinically underdiagnosed parasite. It is contracted by a dog that goes wading or swimming in freshwater where infected snails (the intermediate host) are present. Heterobilharzia is frightening, debilitating, and often fatal. It had been considered a rare parasite but recent studies prove it to be more widespread than it was originally thought.

Clinical symptoms vary widely and unfortunately can be easily mistaken for another affliction. When dogs serve as a definitive host for heterobilharzia americana they often have symptoms similar to that of a gastrointestinal disturbance but multiple organs can be affected. Many clinical symptoms include but are not necessarily limited to:

• Chronic diarrhea (most often bloody)
• Excessive drooling
• Dermatitis (swimmer’s itch seen on short haired dogs or humans)
• Cough
• Fever
• Anorexia
• Anemia
• Lethargy
• Dehydration
• Hypocalcemia
• Weight loss
• Increased thirst
• Collapse

Types

Heterobilharzia americana has to go through several stages before it reaches its definitive host, can complete its life cycle, and reproduce.

• Egg - The egg is located in the fecal matter of the definitive host
• Miracidium - Is the swimming stage that penetrates the snail (intermediate host)
• Sporocyst - Mother and daughter asexually replicate inside the snail
• Cercaria - The stage that emerges from the snail
• Adult - Is found in multiple organs of the definitive host

Heterobilharzia is contracted through the dog’s skin in fresh water where infected snails (the intermediate host) are present. Avoidance of standing water, shallow pools, marshes, swamps, bayous or mudflats will reduce the risk. It is not contracted directly from dog to dog although the infection of one dog will increase the risk of nearby dogs if fresh water is present, because the infected dog will shed eggs in his feces. If the eggs are able to make contact with freshwater they hatch and swim to find and infect a freshwater snail. Once time is spent in the intermediate host a new stage emerges from the snail and again uses fresh water as transport to its definitive host.

Heterobilharzia americana is not easily diagnosed. The dog sheds eggs in his fecal matter intermittently so an initial fecal test may come back clean. The eggs of heterobilharzia americana also do not float in a standard fecal test. Diarrhea and other clinical symptoms can easily be mistaken for another affliction. Direct fecal saline smears can help to definitively diagnose heterobilharzia americana. Ultrasound and abdominal x-rays may show changes in organs or the intestinal walls specific to an H. americana infection. The ELISA test and the polymerase chain reaction assay can confirm the presence of H. americana.

As with all diagnoses, an accurate history of your dog is of utmost importance. A history of time spent in mudflats, for example, or a rescue dog from Louisiana or Texas may help your veterinarian come to an accurate conclusion.

Due to the difficulty in diagnosing Heterobilharzia americana, there is often extensive damage done to the intestinal tract and possibly the organs of the definitive host. In these cases, the response to treatment may be ineffective. 

Methods of treatment include high doses of deworming medication (praziquantel or fenbendazole) for ten days, sometimes repeated in three weeks. In many instances, the use of this medication results in complete resolution.

Heterobilharzia is difficult to diagnose. Often a dog suffers with no real explanation for some time. Once finally diagnosed, current treatments may be ineffective if extensive damage has been done to the intestinal wall and multiple organs have been affected. Euthanasia is a dismal  decision that many dog owners often reach because the dog has already suffered so much due to a lengthy diagnosis and the hope of full recovery appears to be slim. 

Pets who have been treated with deworming medication with positive results may need to return to the clinic for additional testing, in order to determine if the parasite has indeed been eradicated.