Gastrodiscus aegyptiacus is a fluke species that has horses, swine and related wild animals (zebras, wharthogs, etc.) as final hosts. It is found mainly in Africa as well as in India and other Asiatic countries. Little is know about the prevalence and incidence of these flukes. In a study in Egypt it was found that 22% of 156 donkeys and 15% of 40 horses investigated shed eggs of this parasite. Gastrodiscus hominis is a related species that infects humans.
These flukes do not affect cattle, sheep, goats, pigs, dogs or cats.
Are animals infected with Gastrodiscus aegyptiacus contagious for humans?
- NO: There is no evidence that these flukes can infect humans.
Final location of Gastrodiscus aegyptiacus
Predilection sites of Gastrodiscus aegyptiacus are the small intestine and the colon.
Anatomy of Gastrodiscus aegyptiacus
Adult Gastrodiscus aegyptiacus are about 1..5 cm long and 0.5-0.7 cm broad. They have the flat body and the oval shape typical for most flukes, but their body is divided in a small, conical anterior part and a wider posterior part with numerous papillae arranged in transverse rows.
As other flukes they have no external signs of segmentation. The mouth ends in the pharynx, a muscular tube that allows sucking. The digestive system is blind (i.e. without anus: the only opening is the mouth) and not linear, as in most animals, but branched, ending in several blind ducts (called coeca). As most flukes, Gastrodiscus are simultaneous hermaphrodites, i.e. they have both male and female reproductive organs.
The eggs have an oval shape and are rather large (~110x180 micrometers), thin shelled and have a brownish color.
Gastrodiscus aegyptiacus has an indirect life cycle with freshwater snails as intermediate hosts, mainly of the genera Bulinus and Cleopatra.
The eggs shed by adult flukes are expelled with the feces. Once outside the host, the larvae called miracidia hatch out of the eggs in a few days. These larvae can survive for weeks off a host provided there is enough humidity. They die quickly in a dry environment. Miracidia can swim and penetrate actively into the snails where they develop successively to sporocysts, rediae and cercariae, the usual larval stages of most fluke species.
Mature cercariae leave the snail, attach to the vegetation and produce cysts, the so-called metacercariae, which are infective for the final host. Horses become infected when grazing on contaminated pastures.
Once a final host ingests metacercariae while grazing, they migrate to their predilection sites.
Harm caused by Gastrodiscus aegyptiacus infections, symptoms and diagnosis
Most infections with Gastrodiscus aegyptiacus are benign and show no clinical signs. But in case of heavy infestations severe health problems with diarrhea, blood loss (anemia), swellings, weight loss, and poor condition can happen.
Diagnosis is done by detection of eggs or immature flukes in the feces or by identification of the adult flukes after necropsy. A history of grazing in marshy pastures together with such clinical symptoms may suggest infection with Gastrodiscus aegyptiacus.
Prevention and control of Gastrodiscus aegyptiacus
The most important preventative measure is to keep horses or swine away from swamps or marshy pastures or zones that are regularly flooded. Horses should be offered fresh uncontaminated water at drinking points.
Measures to reduce the snail population can also help. Since the snails that act as intermediate hosts are aquatic, effective drainage or any measure that keeps the pastures dry will reduce the snail population.
Most flukicides are approved only for ruminants (cattle, sheep, goats) and not for horses or swine, simply because they are seldom a problem for equine or porcine hosts. They may be used on horses or swine under the supervision of a veterinary doctor, e.g. triclabendazole (at ~10 mg/kg bw), closantel (at ~10 mg/kg bw), clorsulon (at ~7 mg/kg bw), and nitroxinil (at ~15 mg/kg bw).
Neither macrocyclic lactones (e.g. ivermectin, moxidectin), nor levamisole, nor most other benzimidazoles (fenbendazole, mebendazole, oxfendazole, etc.) are effective against Gastrodiscus aegyptiacus.
Chemical control of the snails with molluscicides (i.e. snail killers) such as copper sulphate, sodium pentachlorophenate, niclosamide, etc. can make sense for very specific purposes, e.g. for treating places where horses congregates (water holes, feeding areas, salt licks, shade trees, etc) to keep them free of snails. However, trying to eradicate snails from a property is hopeless and useless. It is virtually impossible to treat every place where they can survive and they reproduce extremely quickly. Cleaned pastures would become re-infested very fast. In addition it would be also very harmful for the environment. In fact, such molluscicides (mainly niclosamide) are approved only in a few countries as an aid in the prevention of human schistosomiasis (also called bilharziosis or snail fever).
There are so far no vaccines against Gastrodiscus aegyptiacus. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Gastrodiscus aegyptiacus (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.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations or regulations for Gastrodiscus aegyptiacus control.
Resistance of Gastrodiscus aegyptiacus to flukicides
So far there are no reports on resistance of Gastrodiscus aegyptiacus to flukicides.
This means that if a flukicide fails to achieve the expected efficacy, chance is very high that it was not due to resistance, but either the product was unsuited for the control of Gastrodiscus aegyptiacus or it was used incorrectly. Incorrect use is the most common cause of product failure.