Schistosomiasis
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 fresh water sources.
Cercariae then migrate through the lungs and to the liver where they mature to sexually dimporphic adults. Unlike most trematodes, H. americana are not hermaphroditic.
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 when mature, emerge from the snail to find a canine host to complete its lifecycle.
Stages
Egg: an unoperculated, thin shelled ova measuring 87x70 µm. 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 a 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 include diarrhea (which may be blood-tinged), vomiting, anorexia, weight loss, lethargy, and polyuria/polydipsia.
Laboratory finding include hypoalbuminemia, hypoglobulinemia, hypercalcemia, azotemia, anemia, and eosinophilia.
Debilitation associated with severe lesions in some chronic infections may result in fatalities.
Prevalence
The majority of cases of canine schistosomiasis are in Texas and Louisiana. However, cases have been diagnosed in dogs throughout much of the southeastern United States, and infections are also described from dogs as far north as Kansas.
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 in order 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 infected dog in the feces.
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.
Both antigen capture ELISA and polymerase chain reaction assay have been used to diagnose H. americana infections.
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.
An egg of Heterobilharzia americana recovered from a dog on fecal sedimentation
Treatment
Treatment is often unrewarding. Severe lesions, including extensive fibrosis, may already be present when clinical signs develop. Special requirements for fecal sedimentation and examination with saline to identify eggs can delay diagnosis.
While treatment is not always effective, a combination of praziquantel (10mg/kg PO tid for 2 days) and fenbendazole (24mg/kg PO sid for 7 days) may result in resolution of clinical signs in some infected dogs.
Control and Prevention
- To avoid Heterobilharzia americana infection, dogs should not have contact with water through such activities as swimming in canals or ponds.
- 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
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.
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.
Johnson EM. 2010. Canine schistosomiasis in North America: an underdiagnosed disease with an expanding distribution. Compendium. March 2010. E1-E4.
Ruth J. 2010. Heterobilharzia americana infection and glomerulonephritis in a dog. Journal of the American Animal Hospital Association. 46. 203-208
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 fresh water sources.
Cercariae then migrate through the lungs and to the liver where they mature to sexually dimporphic adults. Unlike most trematodes, H. americana are not hermaphroditic.
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 when mature, emerge from the snail to find a canine host to complete its lifecycle.
Stages
Egg: an unoperculated, thin shelled ova measuring 87x70 µm. 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 a 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 include diarrhea (which may be blood-tinged), vomiting, anorexia, weight loss, lethargy, and polyuria/polydipsia.
Laboratory finding include hypoalbuminemia, hypoglobulinemia, hypercalcemia, azotemia, anemia, and eosinophilia.
Debilitation associated with severe lesions in some chronic infections may result in fatalities.
Prevalence
The majority of cases of canine schistosomiasis are in Texas and Louisiana. However, cases have been diagnosed in dogs throughout much of the southeastern United States, and infections are also described from dogs as far north as Kansas.
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 in order 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 infected dog in the feces.
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.
Both antigen capture ELISA and polymerase chain reaction assay have been used to diagnose H. americana infections.
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.
An egg of Heterobilharzia americana recovered from a dog on fecal sedimentation
Treatment
Treatment is often unrewarding. Severe lesions, including extensive fibrosis, may already be present when clinical signs develop. Special requirements for fecal sedimentation and examination with saline to identify eggs can delay diagnosis.
While treatment is not always effective, a combination of praziquantel (10mg/kg PO tid for 2 days) and fenbendazole (24mg/kg PO sid for 7 days) may result in resolution of clinical signs in some infected dogs.
Control and Prevention
- To avoid Heterobilharzia americana infection, dogs should not have contact with water through such activities as swimming in canals or ponds.
- 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
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.
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.
Johnson EM. 2010. Canine schistosomiasis in North America: an underdiagnosed disease with an expanding distribution. Compendium. March 2010. E1-E4.
Ruth J. 2010. Heterobilharzia americana infection and glomerulonephritis in a dog. Journal of the American Animal Hospital Association. 46. 203-208