Why Snails Kill 200 000 People Every Year

Why Snails Kill 200 000 People Every Year


Most people probably wouldn’t believe it
if they heard that snails kill around 200,000 people every year, making them the second
deadliest animal on the planet. But it is true – sort of. One thing that many people are aware of is
that Mosquitos are the deadliest animals. That is of course 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. And while this is probably 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. What has that to do with snails? 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. Many parasites need to go through a complex
lifecycles, with morphological distinct generations and one or more intermediate hosts before
they can infect their final hosts and these worms are no exception. They begin their life as tiny larvae free
floating in water. Almost immediately after hatching they start
actively looking for their intermediate host (fresh water 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 it 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 cause the problem are the eggs they produce. A single female lays multiple hundred eggs
per days 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 faeces or the urine to start the cycle again. The rest remains embedded in the body. Transported around by the blood stream 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, diarrhoea,
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 (the latter particularly in women). In children which are most commonly affected
by the disease it can also cause anaemia and impact the 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. The disease is only endemic to subtropical
and tropical regions and is mainly a rural problem. 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 age 10 and 15 are due to
their careless nature and low level of resistance most heavily infected. Impacted regions are South America, South
Asia, the Middle East, the Caribbean Islands and most notably Africa which has around 90%
of all known Schistosomiasis cases. The impact this disease can have not just
on the people but also on the economy of entire countries is serious. In areas that are most severely affected like
north-eastern Brazil and much of sub-Saharan Africa the working ability of large portions
of the population is severely reduced as a result of the weakness and lethargy causes
by the disease. That’s why, in the past few decades, health
organisations have begun to work relentlessly on limiting the impact of Schistosomiasis,
but the process is difficult and complex and while successful in some regions on a global
scale it was relatively unsuccessful thus far. That’s why the disease now ranks second
on the list of the 18 neglected tropical diseases issued by the world health organization. The biggest issue is the lack of an available
vaccine or any other reliable form of immunization. Merely the infection itself can be treated
through the use of drugs. Normally Praziquantel is used, which is effective
against all forms of Schistosomiasis, kills the parasites and even helps to reverse some
of the damage done by the infection. But while this treatment has helped to overcome
many of the long term problems of the disease and thereby reduced its mortality rate it
doesn’t prevent reinfections. That means that the ultimate goal – the
elimination of Schistosomiasis – is likely not reachable through medical means alone
– at least not at the moment. Instead it needs an integrated approach – So
a multi-component approach that targets the entire life cycle of the parasite: for instance:
preventive chemotherapy using praziquantel, improved detection and treatment of sick people;
improvement of sanitary facilities for safe and acceptable disposal of human excreta;
provision of safe drinking-water; reduction of contact with contaminated water; health
education; and snail control. But such an approach is of course very difficult
to realize and requires a long-term well-structured control program. What makes matters worse is that each region
is unique. The areas affected differ in part significantly
both socially, geographically, and economically and in terms of their flora and fauna. Each control measure must therefore be applied
and adapted specifically for each area to be effective, which explains why the elimination
of Schistosomiasis is such a challenge. Even in China where the disease has been actively
battled for more than 60 years now it still couldn’t be eradicated completely. But China’s example also demonstrates what
is possible through a long-term well-coordinated effort. Since their program was initiated in 1956
the number of detected cases of Schistosomiasis has dropped from more than 12 million to under
100,000 One country that could successfully eradicate
the disease is Japan. Here Schistosomiasis was endemic as far back
as the 16 hundreds. The southern, sub-tropical regions of the
country were most affected. After the cause of the disease was finally
identified in 1904, Japan as one of the first countries to do so, began to implement a series
of control measures to combat the disease. To kill the eggs, human faecal matter was
stored for 2 weeks or more before using as fertiliser. Handpicking, snail pesticides and other chemicals
where used to reduce the snail population and to kill the cercariae and susceptible
cows were replaced with horses as the animal of burden for agriculture, which were more
resistant to infection. To further control the snail population wetlands
were drained, river bottoms were dredged and ditches around the rice fields were cemented
destroying much of the snail’s habitats. All this was undertaken with government support
and performed by the affected communities. And it worked. In 1970 after 50 years of continuous effort
the number of infections had been reduced from around 10000 to a few hundred and no
new cases have been reported since 1977. In 1996 Schistosomiasis was then officially
declared eradicated. As a consequence of the program the freshwater
snail responsible for disease in Japan was largely eradicated with it. But a small population of them survived. They are now living a life free from the parasite
as well.

100 thoughts on “Why Snails Kill 200 000 People Every Year

  1. Because so many people commented about this under my last video I thought I make a video about it. Hope you like it.

  2. Well, this tells us HOW they kill so many people. But it doesn't tell us WHY, which is the title. Why do you do this, snails?? 🙂

  3. Amazing and terrifying at Same time. Hows does something like this come about by random chance. Sir Francis Crick didn't think so.

  4. Also dont shit where you eat. Also Bring up water to shore and boil it then take a bath or put in container and let sit in hot sun for day or two. Simple

  5. With unfettered immigration and open borders this will soon come to a neighborhood near you. It was nice while it lasted.

  6. I knew someone who was killed by a snail. He tried to run away, but it was just too fast for him. Oh wait, I'm mixing up snail with that other thing…. what's it called…. convertible.

  7. "Jointed" gibt es nicht. Es heisst joined. Auch "these ones" ist schlechtes English. These oder this one, aber bitte nicht these ones.

  8. This channel is incredible! Everything from the amazingly well-done animation to the way you present the information just oozes quality. Really looking forward to seeing more videos from you! And thanks for providing your sources as well.

  9. Some French restaurant just tried to kill me with a plate of snails! They tried to hide them under butter and garlic. So, I learned from the Maitre'd that guzzling down 3-4 bottles of his most expensive French wine would save me. Phew!!!

  10. Now I remind all my baths in rivers, lakes, seas… I remind every time, when I touched a snail… I'm pretty scared!
    5:31 Oof, I'm not endangered. Nice.

  11. It is so ridiculous that DDT has been outlawed when mosquitoes kill over 1,000,000 people every year from malaria. Remove the band and save people. Yes it is possible that a few 100 people may die of cancer but it's worth it.

  12. Well, if bloodflukes are animals… wouldn’t that make bloodflukes the second deadliest animals in the world? Because, you know, they’re worms?

  13. No other video will do justice to the dangers of Schistosomiasis other than this channel. A new subscriber here. Thanks for the sources. It is very important.

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