Every year, millions of people are infected
with diseases you’ve probably never heard of. There’s tons of media coverage about things
like malaria, Zika, and HIV — all of which pose serious threats to public health. But there are many more diseases out there
that are just as deadly -- yet not as infamous. They’re called neglected tropical diseases,
or NTDs, because historically they haven’t been paid as much attention as other diseases,
and they mainly affect people in tropical areas. But these days, global healthcare initiatives
are paying attention to them -- working to find ways to prevent and treat these diseases. The deadliest NTD — caused by the second-deadliest
parasite in the world, after malaria’s — is schistosomiasis, also known as snail fever. It’s caused by a parasitic worm, called
schistosoma, that’s transmitted through water that’s been contaminated with the
parasite’s eggs. The parasites use certain kinds of snails
in the water as hosts, to grow and multiply. Eventually, the parasites’ larvae are released
into the water, and that’s when anyone who comes in contact with the water is at risk,
because the larvae can burrow through the skin. Infected people often don’t have symptoms
for the first month or two, though they can sometimes end up with a rash or other symptoms
like fever, aches, and a cough. Symptoms are much more common later on, once
the larvae have grown into adults and laid eggs. That’s when the infection is considered
chronic. But the symptoms of chronic schistosomiasis
aren’t caused by the full-grown worms — the worms can actually incorporate proteins from
the host to trick the immune system into ignoring them. It’s the eggs that cause an incredibly strong
immune response, which can strike almost anywhere: the intestines, the lungs, the bloodstream,
even the brain. So schistosomiasis can cause all kinds of
symptoms, from diarrhea to wheezing to seizures. More than 200 million people have schistosomiasis
right now, and about 200,000 people die from it every year. The main way to treat it is to prevent it:
if people are regularly given a drug called praziquantel, even if they don’t have the
disease, they’re less likely to develop a severe infection later on. But the best way to protect people in the
long term is to make sure communities have access to sanitation and safe drinking water,
to stop the parasite from spreading. Another neglected tropical disease is Chagas
disease. It’s most prevalent in Central and South
America, where about 8 million people have the infection, and about 10,000 die from it
every year. Chagas is caused by a parasite called Trypanosoma
cruzi, a single-celled protozoan. The parasite spreads through the feces of
infected insects, often known as kissing bugs. Usually, the parasite enters the body through
mucous membranes — like in the eyes and mouth — or through broken skin, when someone
scratches a kissing bug bite. The best time to treat the disease is just
a few days after infection, but most people don’t show any symptoms for weeks. And even in those who do, the symptoms are
pretty mild and nonspecific — like fever, a rash, and diarrhea. And then something frightening happens: After
this initial phase, Chagas infection usually goes dormant -- for years. After as much as a decade or two, about 30%
of infected people then develop chronic, life-threatening symptoms — including swelling of the heart,
or the intestinal tract. So, prevention and early detection are very
important here. But as with other NTDs, it often affects people
who don’t have easy access to healthcare. Fortunately, efforts are underway to help
those at risk, mainly with the help of insecticides. If you kill the bugs, they can’t spread
the disease. Then there’s African trypanosomiasis, otherwise
known as African sleeping sickness, which affects parts of sub-Saharan Africa. Like Chagas, this disease is caused by parasites
carried by insects — in this case, by the bite of the tsetse fly. In the first phase of sleeping sickness, the
parasite reaches the peripheral nervous system — the nerves that aren’t part of the brain
or spinal cord. Here, symptoms include things like fever,
joint pain, and itching, which can be caused by lots of different things. So — again like Chagas — sleeping sickness
is hard to diagnose at first. But symptoms become more severe when the parasite
invades the central nervous system — the brain and spinal cord. Then, it can cause convulsions, confusion,
and the symptom that gives the disease its name: uncontrollable sleepiness. If left untreated, sleeping sickness can be
fatal. But it is treatable with combined medications,
even in the second stage. And initiatives to treat sleeping sickness
have become more and more successful in recent decades. In 1995, there were an estimated 300,000 new
cases of African sleeping sickness. But since then, health organizations have
been working on testing people for the disease and getting treatment to those who need it. They’ve also been using insecticides to
kill tsetse flies. By 2014, there were fewer than 15,000 new
cases of sleeping sickness. There are lots of other NTDs out there as
well. But they aren’t being neglected anymore,
either, and that’s making a big difference. Thanks for watching this episode of SciShow,
which was brought to you and sponsored by Bill Gates. Visit GatesNotes.com to learn more about neglected
tropical diseases and how people around the world are working to fight them -- like the
Uniting to Combat NTDs Coalition, which recently set the world record for the most medication
donated in 24 hours, with more than 200 million doses donated to prevent and treat NTDs.
