Schistosomiasis is a parasitic blood fluke disease, chronically infecting 200 million people worldwide. It is endemic in 76 countries, although the greatest burden of disease is in Africa and Madagascar, followed by localised risk areas in Yemen, Iraq, Syria, Brazil, some Caribbean islands and parts of SE Asia.
Chronic schistosomiasis is a major cause of malnutrition, anaemia, poor physical, intellectual and economic development and chronic health problems for local people, particularly in Africa and Yemen. One form, female genitourinary schistosomiasis (FGS), involves lesions on the cervix and other parts of the female genital tract, and is thought to play a major factor in the increased levels of HIV infection found in adolescent girls and young women in endemic areas.
The reason why Africa bears the greatest disease burden is because this is where the four human-specific species of Schistosoma blood flukes co-evolved with humans over 100,000s of years.
Understanding the ecology and life cycle of schistosomiasis:
The life cycle of the human-specific Schistosoma fluke is dependent on spending part of its development within a water snail and part within a human. Understanding and breaking the cycle of transmission is, as ever, the key to prevention:
1. Water snails release the free-swimming, ‘cercaria’ stage of the Schistosoma larvae when the sun is at its highest, i.e. between 10.00 a.m. and 2.00 p.m. These 0.5mm long cercariae have a tail to propel them up through the water towards the surface, where they need to find a human body to infect.
This is the weakest link in the life cycle: Cercariae have only limited stores of energy in their tails and can only survive outside the snail for up to 18 hours at most. That is in ideal conditions, i.e. warm, shallow, slow-moving water. In cool, deep or fast-flowing streams, their energy is used up more quickly and they may only survive a few hours.
2. On finding human skin, the head of the cercaria burrows into the skin and the tail drops off. The head part, now known as a ‘schistosomulum’, incorporates some of the host’s tissue antigen into its outer membrane, so it is not recognised as ‘foreign’ by the human immune system. It can then travel undetected via the skin tissue, lymph and blood up to the liver, where it matures into an adult and seeks out another larva of the opposite sex with which to pair up.
3. The young adult pairs then wrap themselves in a permanent embrace and migrate upstream to lodge themselves in the veins either above the bladder (S. haematobium) or around the intestine (S. mansoni, S. intercalatum & S. guineaensis), where they will mate continuously and release eggs intended to pass out via the urine or stool respectively.
Not all eggs reach the outside: around 50% may go astray and end up in the bladder wall, cervix, liver, spinal cord or brain, where the surrounding granulation reaction can cause cancer or localising neurological symptoms such as transverse myelitis, seizures etc.
4. Those eggs that do reach fresh water, hatch on contact and drop to the bottom to try and find another snail to infect…
Informed prevention advice
The standard travel health advice in Northern countries is: “Don’t go in the water! But if you can’t help yourself...”
As an African, Jim prefers a more informed approach to minimising the risk of bilharzia, based on an understanding of the ecology, supported by the evidence and the experience of South African bilharzia experts. He is also aware of the offence that may be inadvertently caused by foreigners behaving in an over-precious way, when local people have no choice but to draw water, wash and fish for food from a particular body of water, and may have to live with the consequences of infection, without access to serology tests.
Jim would therefore recommend the following practical advice:
Jim has grown up in Africa and explored many waterways in his native continent by canoe. He has swum in Lake Malawi/Lago Niassa on at least 200 occasions, in communities where he knows there is bilharzia, because his team has been treating it. He is, however, selective when and where he goes in the water, and he tests himself every time he comes home: he has never tested positive for schistosomiasis. He must be doing something right!
The risk in Asia
There are also two Asian species of Schistosoma, i.e. S. japonicum and S. mekongi, known to also accidentally infect humans as a ‘dead-end host’. However, these are essentially animal (water buffalo) species, which only occasionally cause disease in local people, usually after prolonged exposure in paddy fields, rivers etc. The risk of infection with these species for travellers is very small.
A much more common risk from fresh water contact in SE Asia is leptospirosis (Weil’s disease), particularly from travellers helping to wash sanctuary elephants in Thailand. We can provide advice and preventive treatment for this.
© Jim Bond, Oct 2017