Schistosomiasis

Synopsis

CAPC Recommends

• Consider a diagnosis of canine schistosomiasis in dogs presenting for signs of persistent diarrhea (+/- blood), vomiting, hepatopathy, hypercalcemia, and weight loss.
• Treatment of canine schistosomiasis is not always effective. A combination of praziquantel and fenbendazole has been used successfully to treat infections.
• To prevent infection, dogs should not be allowed to swim or wade in contaminated freshwater sources.

Overview of Life Cycle

• Raccoons are the natural definitive host for Heterobilharzia americana. Dogs can also serve as a definitive host.
• Dogs acquire infections when cercariae of H. americana penetrate the skin while dogs are swimming or wading in contaminated freshwater sources.
• Cercariae then migrate through the lungs to the liver where they mature to sexually dimorphic adults. 
• Adult H. americana travel through the portal veins to the mesenteric veins where they mate and deposit eggs.
• Once deposited, the eggs produce proteolytic enzymes which allow for penetration through the mesenteric veins and intestinal mucosa to enter the canine intestine.
• Eggs, containing a fully formed miracidium, are shed within feces of the canine host.
• If the shed H. americana eggs are exposed to fresh water, the ciliated miracidia hatch and swim to find and infect a lymnaeid snail.
• Once inside the snail, H. americana sporocysts and daughter sporocysts develop.  Free swimming cercariae develop within the daughter sporocysts and emerge from the snail to find a suitable host to complete its lifecycle.

Stages

• Egg:  a non-operculated, thin shelled ova measuring 87x70µm. 
• Heterobilharzia americana eggs contain a fully formed miracidium when shed in feces. 
• Miracidium: a ciliated, pear-shaped motile stage that penetrates the snail.
• Sporocyst and daughter sporocyst: asexually replicating stage inside the snail which leads to the development of cercariae.
• Cercaria: a motile stage that emerges from the snail and is infectious to the definitive host.
• Adult: Measuring 9-17mm in length, H. americana adults have separate sexes but live in close association with one another.  The female fluke resides in a gynaecophoral groove along the larger male worm after mating. Heterobilharzia americana are considered polygamous, and multiple females can be found within the gynaecophoral groove of one male.

Adults of Heterobilharzia americana

Disease

• The migration of H. americana eggs directly across the intestinal wall to the lumen induces severe granulomatous inflammation.
• Clinical signs in infected dogs may include diarrhea (which may be blood-tinged), vomiting, hyporexia to anorexia, weight loss, lethargy, and polyuria/polydipsia. However, there are increasing reports of infected dogs that are asymptomatic.
• Laboratory findings may include hypoalbuminemia, hyperglobulinemia, hypercalcemia, azotemia, anemia, eosinophilia, lymphopenia, thrombocytopenia, and elevated liver enzymes.
• Debilitation associated with severe lesions in some chronic infections may result in death or euthanasia.

Prevalence

• The majority of cases of canine schistosomiasis have been described in Texas and Louisiana. However, cases have also been diagnosed in dogs throughout much of the southeastern United States. Infections are also described from dogs as far north as Indiana, Utah, and along the Colorado River in California.
• While prevalence of H. americana infection in domestic dogs is not well known, prevalence in raccoons has been reported between 1-71% with the highest reported rates in Florida and Louisiana.

Host Associations and Transmission Between Hosts

• Heterobilharzia americana is transmitted by direct skin penetration of cercariae from an aquatic environment and is not directly transmitted between dogs.
• However, infected dogs and raccoons continue to contaminate the water with eggs, creating an ongoing source of infection to other animals in the same locale.

Prepatent Period and Environmental Factors

• The prepatent period of H. americana, from the time of skin penetration to when eggs are first detected in the feces of canines, is about 10 weeks. 
• Eggs of H. americana must be shed into fresh water where a lymnaeid snail is found to develop to cercariae and infect a definitive host.

Site of Infection and Pathogenesis

• Heterobilharzia americana adults are present in the mesenteric and hepatic veins of infected dogs.
• The eggs produced by each adult pair migrate directly through the wall of the intestine to exit the feces of the infected dog.
• The eggs, which induce a pronounced inflammatory response, may also be carried in the circulation throughout the body, resulting in the development of disseminated visceral granulomata.

Diagnosis

• Sedimentation with saline is the recommended way of detecting H. americana eggs in feces. If exposed to fresh water, the miracidium will hatch, making identification difficult. Examining a direct saline smear of feces may also allow identification of H. americana eggs in samples from infected animals.
• A highly sensitive fecal PCR is available for the detection of H. americana DNA. The reported sensitivity of the assay is 1-2 eggs per gram of feces.
• An antigen capture ELISA has also been used to diagnose H. americana infections. 
• Infections may also be diagnosed by histopathology.
• Abdominal radiographs and ultrasonography may detect thickening of intestinal walls and enlargement of liver, spleen, and lymph nodes associated with reaction to H. americana eggs.
• Given eggs cannot be found on routine fecal flotations, the diagnosis may be delayed leading to increased tissue damage and clinical signs.

An egg of Heterobilharzia americana recovered from a dog on fecal sedimentation

Treatment

• Treatment is often unrewarding. Severe lesions, including extensive fibrosis and mineralization, may already be present when clinical signs develop.
• While treatment is not always effective, a combination of high dose praziquantel (25 mg/kg by mouth twice or three times daily for 2-3 days) and fenbendazole (40 mg/kg by mouth once daily for 10 days) may result in resolution of clinical signs in some infected dogs.
• A combination of low dose praziquantel (5 mg/kg by mouth three times daily for 2 days) and fenbendazole (24 mg/kg by mouth once daily for 7 days) may be effective in treating asymptomatic dogs.

Control and Prevention

• To avoid Heterobilharzia americana infection, dogs should not swim or wade in contaminated freshwater canals, marshes, ponds, lakes, or rivers.
• Routine prophylactic administration of praziquantel or fenbendazole to eliminate infections early, prior to development of disease, has been proposed but has not been evaluated experimentally.

Public Health Considerations

• Heterobilharzia americana cercariae from snails have been incriminated in causing dermatitis in humans (“swimmer’s itch”) following skin penetration.
• Infected dogs do not pose a direct zoonotic risk to people, but their presence in a given area may increase the level of cercariae and thus contribute to schistosome-associated dermatitis.
• Cercariae of a number of other schistosomes of wildlife also cause dermatitis in people.

Selected References

• Baniya A, Goldy CJ, Ardpairin J, Achi P, Chang YW, Adrianza RC, Vitta A, Dillman AR. 2024. Canine Schistosomiasis in the West Coast: Heterobilharzia americana in two natural intermediate hosts found in the Colorado River, California. Pathogens. 13:245.
• Corapi WV, Ajithdoss DK, Snowden KF, Spaulding KA. 2011. Multi-organ involvement of Heterobilharzia americana infection in a dog presented for systemic mineralization. Journal of Veterinary Diagnostic Investigation. 23:4. 826-831.
• Cridge H, Lupiano H, Nipper JD, Mackin AJ, Steiner JM. 2021. Efficacy of a low-dose praziquantel and fenbendazole protocol in the treatment of asymptomatic schistosomiasis in dogs. Journal of Veterinary Internal Medicine. 35. 1368-1375.
• Flowers JR, Hammberg B, Wood SL, Malarkey DE, van Dam GJ, Levy MG, McLawhorn LD. 2002. Heterobilharzia americana infection in a dog. JAVMA 220:2. 193-196.
• Graham AM, Davenport A, Moshnikova VS, Gilmour LJ, Fabiani M, Bishop MA, Cook AK. 2021. Heterobilharzia americana infection in dogs: a retrospective study of 60 cases (2010-2019). Journal of Veterinary Internal Medicine. 35. 1361-1367.
• Johnson EM. 2010. Canine schistosomiasis in North America: an underdiagnosed disease with an expanding distribution. Compendium. March 2010. E1-E4.
• Loker ES, Dolginow SZ, Pape S, Topper CD, Alda P, Pointier JP, Ebbs ET, Sanchez MC, Verocai GG, DeJong RJ, Brant SV, Laidemitt MR. 2021. An outbreak of canine schistosomiasis in Utah: acquisition of a new snail host (Galba humilis) by Heterobilharzia americana, a pathogenic parasite on the move. One Health 13.
• Ruth J. 2010. Heterobilharzia americana infection and glomerulonephritis in a dog.  Journal of the American Animal Hospital Association. 46. 203-208.