Most people probably wouldn’t believe it if they heard that snails kill around 200,000 people every year, making them the second deadliest creature on the planet after mosquitos.
When you think of the world’s most dangerous animals, you might picture lions, snakes, and sharks. But one of the deadliest creatures on this planet can’t even bite you… yes, that’s true -Kind of.
One thing that many people are aware of is that Mosquitos are the deadliest animals. Because they transmit the deadly single-celled parasite which causes Malaria. It is often said that Malaria has killed half of all the people who have ever lived.
But it is no longer true today it was most likely true for most of mankind’s history. Even today there are an estimated 200 million new malaria cases every year and between 300.000 to 1.000.000 malaria-related deaths.
This means Malaria is still the most socioeconomically devastating parasitic disease. So the next question you must be asking yourself is how snails come second? Well, similar to mosquitoes snails also transmit a deadly parasite: A parasitic flatworm which causes Schistosomiasis, which is the second most deadly parasitic disease in the world.
This is, however, far less know, which is probably why you are here. So what exactly is Schistosomiasis? The disease is the result of an infection with what is commonly called blood flukes.
These are water-bound parasites that you can find in all kinds of freshwater habitats like lakes, swamps, and rivers, rice fields, puddles or roadside ditches.
They begin their life as tiny larvae free-floating in water. Almost immediately after hatching they start actively looking for their intermediate host (freshwater snails) which they need to develop further.
Once they find one they penetrate the soft body of the snail and begin to reproduce asexually. They do this multiple times during the 1-2 months inside the snails and as a result their numbers quickly grow exponentially.
A process that started with a single larva will end up producing thousands of offspring. These so-called fork-tail cercariae then break out of the snail into the water and start looking for their final host for which they are now specifically adapted for.
They have around 48 hours to do so before they die. There are dozens of different species of blood flukes, each specialized on a specific set of vertebrate hosts – birds, mammals and reptiles all fall victim to these parasites – and of course humans.
There are 5 species that are responsible for the major forms of human Schistosomiasis. Once a cercaria comes into contact with its host the larva penetrates the skin, sheds its tail and transforms into the juvenile form.
It then starts to bore through the flesh until it reaches the blood vessels. Here it will spend the next couple of weeks migrating through the circulatory system via the heart into the liver at which point it will have matured into an adult male or female worm around 1-2cm in length.
Next, the worms begin to join together in pairs to reproduce (this time sexually). For that, the more slender female is held permanently in a groove in the front of the male’s body.
The two worms usually remain jointed together like this for life although it can happen that a female divorces a male to look for a more genetically distinct mate.
From the Liver, the pair eventually migrates to their final destination, either in the veins of the walls of the intestine or the bladder – depending on the species.
They will remain here for the rest of their lives feeding on the red blood cells and the dissolved nutrients inside the blood, hence the name blood flukes, while the female continuously produces eggs.
Although all this sounds like something straight out of a horror movie, the worms themselves are essentially harmless. What causes the problem is the eggs they produce. A single female lays multiple hundred eggs per day for on average 5 but sometimes as long as 20 years.
Only about 50% of these eggs penetrate the walls of the veins make their way into the bladder or intestines and are released with the feces or the urine to start the cycle again. The rest remains embedded in the body.
Transported around by the bloodstream they will cause a host of long-term chronic problems in many organs. Which brings us to the disease itself: Schistosomiasis can have a wide range of symptoms depending on the species of worm or the individual.
Common symptoms are abdominal pain, diarrhea, and blood in stool or urine, the accumulation of fluid in the peritoneal cavity and hypertension of the abdominal blood vessels, long-term kidney liver and bladder damage as well as an increased risk of bladder cancer and HIV infection, particularly in women.
In children who are most commonly affected by the disease, it can also cause anaemia and impact body development and the ability to learn.
In most cases, these symptoms impair and disable rather than kill but with over 78 countries affected and nearly 800 million people that live in areas where infections with blood flukes can occur, the raw number of deaths even if they are comparatively rare is obviously still extraordinarily high.
It is estimated that currently, over 230 million people are infected and in need of medical treatment. It affects mostly those who are unable to avoid contact with water, either because of their profession (agriculture, fishing) or
because of a lack of a reliable source of safe water for drinking, washing, and bathing.
Children between ages 10 and 15 are due to their careless nature and low level of resistance most heavily infected.