Our research takes my colleagues and myself to Kenya in East Africa to study the parasite responsible for causing Africa's silent disease. Within the scientific community this disease is called schistosomiasis but it has many other names. In the villages where we work, the most common of these are bilharzia and snail fever or Kichocho (schistosomiasis in Kiswahili). The numbers affected worldwide by schistosomiasis are large – over 700 million people live in at risk areas and approximately 250 million people are infected with 200,000 deaths annually and almost 3 million years of healthy, productive life lost each year. Why is it a silent disease? These numbers make schistosomiasis the world's deadliest 'neglected tropical disease' and second only to malaria for its toll on human health, yet until recently it remained low on the priority list of the world's largest research institutes and grant funding bodies. The real impact of human suffering can be seen on the faces of those who struggle to go about their daily lives because of the long-term effects of disease-related illness. For large numbers of children, this means delayed development and difficulties concentrating and learning at school and an inability to help with household chores. For adults it leads to economic hardship because they are too sick to work. As is often the case, Africa shoulders the greatest burden with over 90 percent of those infected.
The parasites that cause schistosomiasis have a complex life cycle that needs a human host and a freshwater snail. In Kenya, infected snails are present in shallow waters along the shoreline of Lake Victoria as well as slow moving streams and rivers that feed the lake. They can also be found in irrigation ditches, ponds and streams that cover much of the landscape of central and western Kenya. In many villages and farmed areas these rivers and streams are often not only the place where children might play, but also where they collect drinking water, where their mothers wash clothes and bathe their brothers and sisters, where adults bathe and where many will go to urinate and defecate. An individual infected snail has the capacity to release thousands of tiny parasites that cause schistosomiasis into the water and these can survive long enough to swim to and infect any nearby people. Once a person is infected, the parasite can live for years in the veins near the bladder or intestines, laying thousands of eggs that will damage these organs. Some of the eggs are eventually released in their feces or urine and will hatch on contact with fresh water. The hatched parasite will then in turn infect the snails. A lack of fresh drinking water and proper sanitation greatly increases the chances of adults and especially children becoming infected.
We are fortunate to work in collaboration with a great team of local scientists, students and assistants who are associated with Kenya Medical Research Institute (KEMRI) based in Nairobi and Kisumu. While daytime traffic in Nairobi is often slow and noisy, the Institute itself is an oasis of peace and quiet. We work with Dr. Gerald Mkoji and his team and rely heavily not only on their lab and field skills in isolating and maintaining the parasite but in driving us to rural areas and acting as our interpreters in the many different local languages and dialects as well as Swahili. Getting to the rural areas can be an adventure in itself as many roads are heavily potholed and often flooded during the rains. Once we arrive at a collecting area we are usually soon surrounded by local kids and a few amazed infants who are seeing Westerners for the first time and have to be convinced we are not ghosts! When collecting snails we will sweep the vegetation at the side of slow moving streams then pick over our haul using tweezers to collect the species that act as a host for the parasite. We bring them back to the lab and put them in fresh water in the hope that they will shed parasites if they are infected. If we want the parasite stage that infects people we take a different approach. Our KEMRI colleagues explain to adults what we need and, if they are agreeable, provide them with a plastic pot and lid to provide us with a stool sample. As almost everyone knows about the disease they are usually glad to help. Unfortunately, children tend to be more highly infected than adults so we visit schools and with permission from the head teacher and parents, we collect samples from the children too. These samples are taken back to the lab and examined for parasite eggs and then mixed with fresh water to hatch the eggs. When we find any children or adults that are infected we treat them with drugs that the kill the parasite that causes schistosomiasis as well as other worm-based infections common to Africa.
Dr. Pauline Cupit & KEMRI colleagues sorting snails collected in eastern Kenya responsible for transmitting schistosomiasis. Photo credit: Charles Cunningham.
Why do we collect parasites from snails and humans? Unfortunately there is no vaccine to treat schistosomiasis and only one drug available. This drug is called praziquantel and will kill almost all of the parasites that cause schistosomiasis when administered to children and adults. While this of course is good news there are three drawbacks. The first is that the drug cannot cure the disease, as a small number of parasites usually remain unaffected. These parasites are at a stage during human infection when they cannot yet reproduce but as they are unaffected by the drug they will do so in the future. So the drug can help an individual feel better but cannot completely remove infection. The second drawback is that the drug is often only given to children and even then only once per year. So even after reducing the level of infection, children become re-infected quickly through their daily contact with snail infested water. Finally, because praziquantel is the only drug available to treat the disease there is continual concern resistance may develop. As we do not know the mechanism by which praziquantel works its loss due to resistance would make it harder to develop a new generation of better drugs quickly. Our work in Kenya is aimed at both understanding how the drug works, why it fails to kill young parasites as well as monitoring for the emergence of drug resistance.
Working in Kenya is always a humbling experience. It can also be frustrating one minute and greatly enjoyable the next. We have been very fortunate in the colleagues we work with and the people of Kenya who have helped us with our research.
by Dr. Pauline Cupit, UCSD