Schistosomes: The Snail, the Prawn, and the Dam

It’s a story that has played out in Africa all too often. A river is dammed to control flooding, or for hydroelectric power, or to create a water reservoir in case of drought (or all of the above at once). The dam, of course, creates a large lake and lots of good habitat along the shore for aquatic plants, which in turn support a large snail population.

Schistosome Life Cycle

People come to live near the water’s edge, to use the water and to fish. Generally, human contact with water is high, particularly for children and fishers. If schistosomes are present, along with the right snail species, escalating levels of schistosomiasis soon follow. This is how it works:

• People infected with schistosomes pass the eggs in urine or feces, and if those eggs find their way into fresh water, they hatch.

• The emerging larva seeks out an aquatic snail – only the right snail will do – and penetrates the snail, after which it multiplies asexually in the snail’s tissues.

• Larvae leave the snail host and, if a human enters the water, penetrate human skin, maturing to adult worms that reside in the tiny blood vessels around the bladder or intestine.

• Adult worms produce eggs which are passed in the urine or feces, and so on.

It makes perfect sense that water resource projects can result in rapidly increasing incidence of schistosomiasis because they bring together the two hosts of the parasites – the human and the snail – in circumstances ideal for transmission. We hardly need a more complicated explanation. It seems, however, that there’s at least one other factor that’s been left out: the African river prawn.

River Prawns and Schistosomes

River prawns are not small!
This is Macrobrachium carcinus (molt).
Clinton & Charles Robertson. CC BY 2.0

Fascinatingly, the adult river prawn lives in fresh water, but the young require brackish water – a mixture of river water and sea water – in order to survive. When we dam an African river, we cut off access to the river’s mouth where the water is brackish. Adult prawns cannot migrate downriver; young prawns cannot migrate upriver. The prawn population upstream of the dam is doomed. And what do prawns eat? Lots of things, but one of those things is snails.

So just as the dam provides beautiful habitat for the proliferation of snails, it also wipes out a key predator of those snails. Instead of tasty prawns, people living on the shores of the reservoir get schistosomiasis. That’s a bad deal.

At least one project – Project Crevette - is now rearing the prawns and releasing them into an affected river, effectively reducing the level of schistosomiasis and providing food and marketable prawns for the people in the area. That’s a good deal.

Michael B. New writes that postlarvae (the stage of prawn young that migrate upstream) of at least one species “can climb vertical surfaces and cross land, provided there is abundant moisture available” (Farming Freshwater Prawns: A Manual for the Culture of the Giant River Prawn Macrobrachium rosenbergeii. Food and Agriculture Organization of the United Nations, 2003, pg 10). One wonders how difficult it would be to re-establish the natural life cycle of the river prawn.

Angiostrongylus vasorum - Canine Lungworm Larvae in Snails, Slugs, and Slime

Is this the face of a killer? This slug, Arion rufus, has
been shown to carry Angiostrongylus vasorum larvae.
Image by Guillaume Brocker; CC BY-SA 3.0.

The roundworm Angiostrongylus vasorum goes by many common names: canine lungworm, canine heartworm, French heartworm – all descriptive labels acknowledging the fact that the adult worms reside in the heart and pulmonary artery (the blood vessel that carries blood from the heart to the lungs) of domestic dogs and wild canine species.

Life Cycle of Angiostrongylus vasorum

The life cycle of A. vasorum is complex:

• Adult worms produce eggs that are carried to the lungs by the blood circulation.
• The eggs hatch in the lungs, releasing larvae, which break through into the air space of the lungs, travel up the airways to the throat and are swallowed.
• Passing through the intestine, larvae are deposited in the environment with feces.
• Slugs and snails feeding on dog feces either ingest the larvae or the larvae penetrate the mollusk’s foot and undergo further development in the mollusk’s tissues.
• Infective larvae are released in the slime secretions left behind by snails and slugs, or are ingested with the mollusk when it is eaten by something (frogs, dogs, etc.)
• It is thought that dogs swallow the larvae and become infected when they eat snails and slugs, when they eat frogs or other animals that have eaten infected snails and slugs, or when they eat or lick things that are contaminated by slug or snail slime.
• Larvae migrate to lymph nodes, where they develop further, and then move to the heart and pulmonary artery.

Dogs Eat Snails and Slugs

Dogs eat slugs and snails, and frogs. While many dog owners will readily agree with this statement, it leaves others, like me, scratching our heads.

Would my dog eat a slug or snail? She has never shown the slightest interest in doing so (and there are lots where we live).

Would she eat a frog? Sniff it maybe. Play with it maybe. Step on it by accident; yes, I’ve seen this. Eat it? No.

Would she eat a food item left lying on the ground and possibly crawled over by slugs and snails (or with slugs and snails still on its surface)? Definitely.

And that is one of the reasons why I was particularly interested in this video of the movements of snails in a British garden at night.


I had no idea they traveled so far and so fast.

Risk of Angiostrongylus vasorum Infection

A lot is still unclear about A. vasorum.  We don’t know what the most common route of infection is: snails, slugs, slime, frogs etc. We don’t know how many dogs have the parasite but exhibit no symptoms. We don’t know how many larvae a dog would need to ingest to become ill.

And of course, different species of slugs and snails are likely to differ in the distance they travel and the number of larvae they leave in their wake. Clearly, however, wherever A. vasorum occurs, all dog owners should take the threat seriously. As Eric Morgan et al write, the infection “is associated with coughing, dyspnoea [shortness of breath], exercise intolerance, weight loss, vomiting, abdominal pain, lumbar pain, neurological signs, heart failure, bleeding diathesis [tendency to bleed], and sudden death.”

Sources

 

Barcante, Thales Augusto, et al. "Angiostrongylus vasorum (Baillet, 1866) Kamensky, 1905: emergence of third-stage larvae from infected Biomphalaria glabrata snails." Parasitology Research 91.6 (2003): 471-475.

Fletcher, Damien. "Snails Can Travel at One Metre an Hour and Piggy-back on Others' Slime to Save Energy." Mirror News. Aug 23, 2013.

Morgan, Eric R., et al. "Angiostrongylus vasorum: a real heartbreaker." Trends in Parasitology 21.2 (2005): 49-51.