Hello super-fly and parasite enthusiasts. Time for blog post 2, which is coming to you from the Mwanza region of Tanzania, bordering the banks of Lake Victoria. My colleague and I are here to collect specimens of the blood fluke Schistosoma from infected humans and snails.
Infection with the blood fluke Schistosoma causes a disease called Schistosomiasis (aka Bilharzia). Over 200 million people worldwide are infected with over 700 million people living at risk of infection. Over 80% of infected people live in sub-Saharan Africa. It is a disease of low socio-economic status, affecting the poorest communities and most neglected, vulnerable people. Infants and children are especially prone to infection due to their less developed immune system.
Children in a school in Niger, West Africa, queuing to be tested for schistosomiasis. The little boy at the front is showing the swollen liver and spleen symptom. A result of being infected with schistosomiasis.
There are two forms of the disease, depending on the species of the infecting schistosome worm:
- diarrhoea, bloody stool
- anaemia, stunted growth
- enlarged liver and spleen
- severe damage to the liver leading to liver fibrosis
- blood in urine, painful urination
- anaemia, stunted growth
- damage to the genitals, kidneys and bladder
- bladder cancer
- increased risk of sexually transmitted diseases like HIV
Urine samples from children infected with Schistosoma haematobium, the urogenital form of schistosomiasis. The red colour indicates blood seeping out with the urine due to the damage done by the schistosome eggs.
To help fight schistosomiasis we need to understand the complex life cycle of Schistosoma, which involves a vertebrate (in this case human) host, a snail host and transmission via water contact.
The blood fluke life cycle
Lets start with a worm pair living inside a little boy in sub-Saharan Africa. The worm pair resides in the blood system of the little boy, either around the intestinal tract or around the urinary tract depending on the species of Schistosoma.
The worm pair releases schistosome eggs into the blood system. The eggs pierce through the wall of the intestinal/urinary tract and exit the boy when he defecates or urinates. They reach fresh water and hatch out into a larval stage called miracidia. These infect a specific aquatic snail species and reproduce asexually, creating thousands of clonal larval stages called cercaraie.
Cercaraie leave the snail to locate and infect a human by piercing through exposed skin in the water. They travel to the liver via the blood system and there they mature into adult worms, ready to reproduce and continue the life cycle (see diagram below).
The life cycle of blood flukes, Schistosoma, involving a vertebrate (e.g. human) host and an aquatic snail host. Transmission is through contact with infested freshwater. The yellow circles are the stages and specimens we collect when doing fieldwork. Credit: Aidan Emery.
A schistosome worm pair. The fat male carries the thinner female worm folded in a little groove where he feeds and shelters her whilst she produces eggs. The worm pair lives inside the veins of animals.
The schistosome species I work on (Schistosoma mansoni) causes intestinal schistosomiasis. It lives in the vein blood system of the liver and intestinal tract of humans. The adult worms themselves don’t cause much harm but it is the eggs they produce that cause the disease, by:
- Piercing the barrier between the blood system and the intestinal wall = bloody diarrhoea and painful cramps.
- The eggs that don’t make it out get trapped in organ tissues, causing the immune system to overreact and damage the surrounding human tissues.
- More worm pairs = more eggs = more damage to the organs and the host. This is what causes the chronic and more severe aspects of the disease such as kidney failure, bladder cancer and liver fibrosis in adulthood.
Thankfully there is an effective oral drug called Praziquantel that kills the adult worms. However it cannot prevent children from becoming infected. So in areas where there is no clean piped water or a sewage system, the local water bodies such as Lake Victoria are the only sources of water for the local population. People have no choice but to fish, wash, bathe and collect water from these schistosome-infested waters and therefore are reinfected quickly.
Treatment needs to be repeated regularly to avoid heavy worm numbers and high egg outputs. Regular treatment means controlling the disease but does not mean eliminating it. For this we need more research to develop better tools to fight the disease. There is also a major the worry that the parasite will become resistant to the drug. If the parasite develops resistance and the drug stops working there is currently no alternative treatment.
Clean water is not available so local water-bodies are used, such as this irrigation canal in Niger, West Africa.
We are researching the impact of human treatments on the parasite population. This will reveal how the parasite is responding to ongoing treatment programmes, if the drug is working effectively and if there are any warning signs regarding drug resistance.
That’s it for now. Coming up, a visit to the schools to collect stool samples from infected children. Disgusting work but someone’s got to do it.