Lake Victoria is the largest lake by area, and one of the biggest freshwater lakes in the world (second only to Lake Superior in North America). But this body of water has a grim secret – the lake is also one of the most lethal in the world.

The 70,000-square kilometers (43495 miles) lake is a vital resource for millions of people, and it straddles three East African countries: Kenya, Tanzania, and Uganda. It is the chief reservoir of the Nile and is home to more than 80 islands.

This Lake’s erratic weather conditions and a mix of poor communication results in around 5,000 deaths a year.

But the Lake also harbors a deadly animal, and it is not the typical predator that you may think of. It is a specific species of snail. This small snail is a host to schistosomiasis, also known as bilharzia. This causes Lake Victoria to be one of the most well-known hotspots of schistosomiasis in the world.

What is schistosomiasis?

Schistosomiasis is a parasitic disease caused by a small parasitic worm (fluke) that is classed as a neglected tropical disease and has a very complex lifecycle, which involves humans and freshwater snails.  

It was first discovered by Theodor Bilharz (hence the name bilharzia) in the 1850s but has been around for a long time, and infection evidence has even been found in Ancient Egyptian mummies.

People can get the parasite by taking a refreshing dip in infected freshwater. There are five main types of schistosomes that are responsible for the disease. They all have the same basic lifecycle and general pathways, but they cause slightly different diseases.

In Lake Victoria, the main parasite is Schistosoma Mansoni. When there was a screening of schoolchildren in Mwanza, Tanzania, who live on the shores of the lake, almost 25 percent or more had this parasite in a single stool sample.

What is the lifecycle of Schistosomes?

When an infected person passes contaminated urine or poop into freshwater, the eggs hatch into larvae (little baby Schistosoma, or the more technical name is miracidia). These larvae swim through the water and will penetrate and infect any unsuspecting freshwater snails. When the larvae have had ample time to develop and multiply asexually in the snail, then they will be released back into the water body in a form called cercaria.

These cercariae have a tail and head and will burrow their way back out of the snail into the surrounding water. They will swim around (up to three days) looking for their next host – an unsuspecting human. They will ‘worm’ their way into the body, losing the tail in the process and dive into the bloodstream.

Once they have migrated into the bloodstream, they go on a little magical tour around the body. Stopping in the lung and eventually ending up in the liver. If there are male and female schistosomes then the female will wedge their body into a canal of the male (called the gynecophoral canal), in a spooning-like position. Then they will travel as this conjoined pairing to the final destination, this final destination depends on which species of parasite it is. Some migrate around the intestines and others to the bladder.

The female will then lay a lotttttt of eggs, up to 1,000 eggs a day. These eggs will then either bury through the intestine where you poop them out. Or if the adults are hanging around the bladder, then the eggs will burrow into the bladder and be released through the urine. Then if the infected person goes into freshwater and poops or wees into it with the right snails, then this cycle can start all over again.  

What are the symptoms of schistosomiasis?

The symptoms of this disease are often from the body’s reaction to the eggs, not the worms themselves. The eggs often lodge into the intestine or bladder and can result in inflammation and scarring. Children that are infected repeatedly can develop malnutrition, learning difficulties, anemia, and after years of infection – organ damage.

However, there are sometimes signs after the initial dip into the water. When cercariae penetrate the skin, people can get a rash, sometimes called a "swimmers itch", which will often go away over time.

But within 1-2 months of infection people can sometimes display "Katayama fever"; chills, cough, fever, and muscle aches can occur, which is often from the hypersensitivity reaction that the body mounts up against the worms swimming around the body.

If no treatment has been administrated, then the schistosomiasis can persist for years. The chronic symptoms of this can include enlarged liver, abdominal pain, and problems passing urine and blood in the stool or urine. If adults have a chronic infection with Schistosoma in the urinary infection, then this can increase the risk of bladder cancer.  

The schistosomiasis that is hanging around the bladder can also affect other organs in the pelvis, and can cause genitourinary schistosomiasis. This can lead to bloody semen in some people, in others it can cause cervical lesions. This type of schistosomiasis is also associated with increased HIV infection and transmission.

Sometimes the eggs can cross the blood-brain barrier and cause neuroschistosomiasis. This can actually cause problems like seizures and other neurodegenerative diseases.  

What is the historical and recent burden of schistosomiasis?

In the past, the burden was so great that it was thought that men normally menstruated as they were seeing blood in the urine. In 1798, when Napoleon’s troops landed in Egypt they called it the “land of the menstruating men”.

In fact, this observation was also seen by Herodotus in Ancient Egypt and they originally thought that it was a disease that could enter the body through the penis. This era also saw the use of primitive condoms that was thought to be related to schistosomiasis, however, this is a controversial thought.

Currently, it is estimated that nearly 800 million people are at risk of this infection, and over 200 million people are infected worldwide. Children often have the highest burden and are often the most affected.

Adults who have high burdens are normally people who are exposed to freshwater sources for a living. In 2016, it was estimated that the global burden of the disease was around 1.7-4.5 million disability-adjusted life years (DALYs), however, the actual figure may be more or less than this – it is difficult to figure out.

How do you get rid of schistosomiasis? 

There is only one treatment approved for the use and that is praziquantel. This is a one-dose medication that can be administered to large groups of people, but the problem is this medication is only effective in adult worms and not for any other stages of the worms. One problem with this medication is that it is slowly becoming drug-resistant.

Other ways that are being trialed is getting rid of the snails themselves and a public health campaign to try and limit people infecting the snails.

The World Health Organization is trying to eliminate schistosomiasis as a public health problem in all the endemic countries.

And there you have it. The very complicated story of schistosomiasis, and why you should be careful if you want to take a plunge in Lake Victoria.