Heterobilharzia americana, the dog schistosome, is a parasitic flatworm that has raccoons as main final hosts, but can also infect armadillos, bobcats, coyotes, nutrias, opossums, rabbits, wolves and occasionally also dogs.

It is endemic to the southeastern United States, in regions with tropical and subtropical climate. In some regions up to 70% of the raccoons can be infected. Infection rates of dogs in endemic regions are not known.

Heterobilharzia americana is related to the blood flukes of the genus Schistosoma, the causative agent of schistosomiasis, a human and veterinary parasite that affects close to 200 million people in Africa, Asia and South America.

The disease caused by Heterobilharzia americana in dogs is called canine schistosomiasis or heterobilharziosis.

This parasite does not affect cattle, sheep, goats, horses, swine or poultry.

Are animals infected with Heterobilharzia americana contagious for humans?

• NO. If dogs or other animals are infected with Heterobilharzia americana, they are not contagious for humans, neither through contact, nor when consuming meat, milk or blood of contaminated animals, nor through the feces. Human schistosomiasis is cause by other parasite species.
• However, aquatic immature stages of Heterobilharzia americana (cercariae, see life cycle below) can penetrate the human skin and cause a skin inflammation (dermatitis) known as swimmer's itch, lake itch or cercarial dermatitis, which is also caused by immature stages of other species of the genus Schistosoma and is a benign dermatitis: it will spontaneously recede about a week after infection.

Final location of Heterobilharzia americana

Predilection sites of Heterobilharzia americana are the mesenteric veins.

Anatomy of Heterobilharzia americana

As other blood flukes Heterobilharzia americana does not resemble most other fluke species. Instead of being flat and with an oval shape, they look very much like "normal" roundworms. Also unlike most other flukes, Heterobilharzia americana is not hermaphroditic but bisexual, and males and females show a different form (sexual dimorphism). Males are about 10 to 20 mm long and 1 to 2 mm thick, whereas females are significantly longer and thinner.

The oral and ventral suckers are rather small. Each male has a special structure along its body, gynacophoric canal, where at least one but sometimes more than one adult female resides permanently. As in other flukes the digestive system of Schistosoma is blind, i.e. it has no exit but ends in a blind branch, the cecum.

The eggs have an oval shape, (~90x70 micrometers) are operculated, contain a fully developed miracidium, and don't have the typical spine of Schistosoma eggs.

Life cycle and biology of Heterobilharzia americana

Heterobilharzia americana has an indirect life cycle with freshwater snails as the intermediate hosts, mainly of the genus Lymnaea (e.g. Lymnaea cubensis).

The adult females lay eggs in the mesenteric veins. The eggs produce enzymes that digest the wall of the mesenteric veins and of the intestine which allows them to get into the host's gut to be shed with the feces. Once outside and in contact with water the eggs release small swimming larvae, the miracidia, which find a suitable snail and penetrate into its body.

Inside the snail miracidia develop further through two generations of sporocysts to asexually produce dozens of cercariae. Mature infective cercariae leave the snail and swim to find a suitable final host, typically a swimming or wading raccoon or another carnivore.

Once they find a host, infective cercariae actively penetrate through the skin of the host. Inside the host's body they get into a blood vessel and migrate through the lungs to the liver vessels while they mature. Once they reach the mesenteric vein they complete development to male and female adults, mate and start producing eggs.

The prepatent period (time between infection and first eggs being shed with the host's feces) is about 10 weeks.

Harm caused by Heterobilharzia americana, symptoms and diagnosis

Infection of dogs with Heterobilharzia americana is not very frequent. Harm to infected dogs depends on the number of parasites. Cercariae penetrating the skin can cause dermatitis with appearance of pustules and itching. Cercariae in the lungs can cause coughing. The passage of eggs through gut wall can cause major damage leading to enteritis (inflammation of the intestine) with bloody or mucous diarrhea, anorexia (loss of appetite), weight loss, fatigue, and even fatalities in extreme cases. Eggs failing to reach the gut's lumen can reach other organs and cause granulomas, especially in the liver, which can end in cirrhosis. Anemia has also been described.

Eggs detected in the sediment of fecal samples confirm diagnosis. However, egg shedding is intermitent, i.e. false negatives are possible.

Prevention and control of Heterobilharzia americana infections

In endemic regions it is advisable to prevent dogs from swimming or wading in potentially contaminated waters, although this may be quite difficult to achieve in rural regions or with hunting dogs.

Chemical control of Heterobilharzia americana flukes is unusual and there are almost no pet anthelmintics with a claim against Heterobilharzia americana flukes. There are reports that praziquantel (at the usual therapeutic dose against tapeworms) and fenbendazole (40 mg/kg, PO, sid for 10 days), or a combination of both are effective against Heterobilharzia americana, but not always.

Chemical control of the snails with molluscicides (i.e. snail killers) such as copper sulphate, sodium pentachlorophenate, niclosamide, etc. is never an option for controlling Heterobilharzia americana flukes in the environment. It would never be economic and is anyway hopeless and useless, because it is virtually impossible to treat every place where the snails can survive, and they reproduce extremely quickly. In addition, the use of such molluscicides is not approved in most countries for obvious environmental reasons.

There are so far no vaccines against Heterobilharzia americana. To learn more about vaccines against parasites of livestock and pets click here.

Biological control of Heterobilharzia americana (i.e. using their natural enemies) is so far not feasible.

You may be interested in an article in this site on medicinal plants against external and internal parasites.

Resistance of Heterobilharzia americana to anthelmintics

So far there are no reports on resistance of Heterobilharzia americana to anthelmintics.

This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that it is not due to resistance. Either the product was used incorrectly or it was unsuited for the control of Heterobilharzia americana. Incorrect use is the most frequent reason for failure of antiparasitic drugs.