The culprits come as a pair: freshwater snails harboring parasitic worms. Once released from the snails, the worms can burrow into the skin and deep inside the body of any human daring to enter its waters.
The infection at hand is schistosomiasis -- also known as bilharzia -- a chronic infection caused by parasitic Schistomosa worms that can live inside blood vessels for years on end causing fever, chills and inflammation in their wake.
"Any freshwater which has these snails in them could be the cause of infection," says Alan Fenwick, Professor of Tropical Parasitology at Imperial College London.
The majority of infected waters are found in Africa, particularly the continent's largest lake -- Lake Victoria -- where risk of infection is high.
The challenge in controlling the disease is that people often don't develop symptoms for years, but can continue to transmit the infection.
A perpetual cycle of transmission
Inside the human body, female worms grow into adults and lay eggs that migrate through the body for release in faeces. If released into freshwater -- through defecation in the water -- they hatch and become ready to infect any freshwater snails in their path.
Once inside the snails, the young worms transform into versions of themselves now capable of burrowing back into human skin. On release back into the water they swim ready to, again, infect humans in their vicinity.
It's a perpetual cycle in which the parasites use both snails and humans to their advantage, manipulating both, to ensure the survival of their species.
The eggs of the parasites travel primarily to the intestine when inside humans, for release, but along this journey can become trapped in organs and intestinal lining to cause inflammation. The characteristic symptom of the disease is a swollen abdomen.
"The size of the burden is immense," says Fenwick who is also Director of the Schistosomiasis Control Initiative
(SCI), a charity working to tackle the disease in sub-Saharan Africa.
More than 61 million people were treated for schistosomiasis in 2014, according to the World Health Organization
(WHO), and more than 258 million required preventative treatment . The majority of cases are in Africa and outweigh numbers affected by other diseases in the region.
"There are 25 to 35 million people infected in sub Saharan Africa with HIV [and] ten times that number of people after affected by schistosomiasis," says Fenwick.
The number of deaths caused by the disease are difficult to record and are estimated to be between 20,000 to 200,000 deaths per year, according to WHO.
Why one region dominates
According to WHO, 90% of those requiring treatment for schistosomiasis live in Africa, but most of them live around lake and river regions. The factor helping the disease persist, is poor sanitation.
"The problem that sub Saharan Africa has is a lack of fresh water, safe water, and adequate sanitation," says Fenwick. "People who need to urinate and defecate tend to do so on the open ground, and their excreta can be washed into water where the eggs will then infect snails."
Infections primarily affect young children, but symptoms can take years to appear, making finding and treating those infected a challenge.
"We have to proactively go out and find children who are infected and treat them so that we protect them from an early grave," says Fenwick.
The busy shores of Busaabala
One of the prime hotspots remaining in sub-Saharan Africa, is the district of Wakiso, Uganda located on the Northern shores of Lake Victoria. The root cause here, remains access to sanitation.
"People are still using the lake... they defecate there," says Juma Mpima, Vector control officer with Uganda's Ministry of health who focuses on controlling infections in lakeside districts, like Wakiso.
Mpima regularly visits the shore of Busabaala, which is popular with locals for the fresh fish sold in the market each day. "Many people come, from a distance...[to] get fish and also sell [fish]," says Mpima.
The result is more than 800 people visiting the shore each day -- with only one toilet among them.
"With one facility, and over 800 people to use one facility...people resort to using other waters," he says.
Increasing the number of latrines in lakeside communities is becoming an important part of Uganda's strategy, but remains a challenge.
Progress, but not enough
Schistosomiasis was once worldwide and found commonly in parts of China and the Philippines, but large scale control programs, improved environments and greater sanitation facilities meant infections were controlled.
The same has not been true for populations surrounding the lakes of Africa, particularly Uganda where the highest burden now remains. The region's fishing and island communities rely on these lakes for their livelihood, making them hotspots for the disease.
"In those fishing villages, people have to have contact with the lake for their survival and for their living and some of these lakes have the highest transmissions in the world," says Fenwick.
Control through treatment
The main control strategy to date has been mass treatment using the drug praziquantil.
"We give the treatment to all the people in the area irrespective of whether they're infected or not infected," says Mpima. This strategy is known as mass drug administration and is an approach used to control a range of infectious diseases including hookworm
The idea is to treat everyone who is at risk as the cost of treating them is much lower than diagnosis and as symptoms can take years to emerge, most people don't know they're infected.
"It isn't necessarily 100% effective but it certainly is very effective indeed in that maybe 85 to 95% of the worms will be killed," says Fenwick. His organization delivers treatment to school age children and high-risk adults across sub-Saharan Africa.
Despite annual delivery of drugs, hotspots of the disease remain making reinfection inevitable.
"For people who live on the banks of the Nile and on the banks of these lakes, they can easily get reinfected because they don't have a water supply, they go back swimming, they go back fishing, and there isn't a great deal of immunity," says Fenwick.
The World Health Organization
has set a goal to control morbidity from the disease by 2020 as well as eliminate schistosomiasis as a public health problem by 2025.
Progress has been made to date in other African countries, such as Zanzibar and Burundi, but when it comes to these hotspots, there's a long road ahead.
"In the hotspots at the moment I think that we'll be treating for ten years to come," says Fenwick who stresses the need to improve access to sanitation facilities, such as toilets.
"We're not 100% sure that we'll be able to eliminate," he says. "But we believe that we can certainly get quite close."
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