Source:boredomtherapy.com / who.int/mediacentre/factsheets.com
When you think of the world’s most dangerous animals, chances are you picture creatures like lions, snakes, and sharks. And why wouldn’t you? All three are known for being exceedingly dangerous to humans, and people do lose their lives to these animals in regular numbers every year.
But, what if you found out one of the planet’s deadliest creatures actually lived in bodies of fresh water and could fit in the palm of your hand? It’d be a difficult thing to believe, but it’s completely true.
Believe it or not, one of the deadliest animals on Earth is the simple freshwater snail. These miniature mollusks might appear harmless, but once you learn about the brain-controlling “zombie” parasite many of them carry, you may never take a dip in a lake again!
The freshwater snail is considered to be one of the world’s deadliest creatures. These tiny mollusks can be found in lakes and ponds all over the world, and many of them carry a deadly transmittable parasite that kills over 200,000 people a year. That’s more than any predatory mammal!
- Schistosomiasis is an acute and chronic disease caused by parasitic worms.
- People are infected during routine agricultural, domestic, occupational, and recreational activities, which expose them to infested water.
- Lack of hygiene and certain play habits of school-aged children such as swimming or fishing in infested water make them especially vulnerable to infection.
- Schistosomiasis control focuses on reducing disease through periodic, large-scale population treatment with praziquantel; a more comprehensive approach including potable water, adequate sanitation, and snail control would also reduce transmission.
- Estimates show that at least 206.4 million people required preventive treatment for schistosomiasis in 2016, out of which more than 89 million people were reported to have been treated.
Schistosomiasis is an acute and chronic parasitic disease caused by blood flukes (trematode worms) of the genus Schistosoma. Estimates show that at least 206.4 million people required preventive treatment in 2016. Preventive treatment, which should be repeated over a number of years, will reduce and prevent morbidity. Schistosomiasis transmission has been reported from 78 countries. However, preventive chemotherapy for schistosomiasis, where people and communities are targeted for large-scale treatment, is only required in 52 endemic countries with moderate-to-high transmission.
Infection and transmission
People become infected when larval forms of the parasite – released by freshwater snails – penetrate the skin during contact with infested water.
Transmission occurs when people suffering from schistosomiasis contaminate freshwater sources with their excreta containing parasite eggs, which hatch in water.
In the body, the larvae develop into adult schistosomes. Adult worms live in the blood vessels where the females release eggs. Some of the eggs are passed out of the body in the faeces or urine to continue the parasite’s lifecycle. Others become trapped in body tissues, causing immune reactions and progressive damage to organs.
Schistosomiasis is prevalent in tropical and subtropical areas, especially in poor communities without access to safe drinking water and adequate sanitation. It is estimated that at least 91.4% of those requiring treatment for schistosomiasis live in Africa.
There are 2 major forms of schistosomiasis – intestinal and urogenital – caused by 5 main species of blood fluke.
Table: Parasite species and geographical distribution of schistosomiasis
|Intestinal schistosomiasis||Schistosoma mansoni||Africa, the Middle East, the Caribbean, Brazil, Venezuela and Suriname|
|Schistosoma japonicum||China, Indonesia, the Philippines|
|Schistosoma mekongi||Several districts of Cambodia and the Lao People’s Democratic Republic|
|Schistosoma guineensis and related S. intercalatum||Rain forest areas of central Africa|
|Urogenital schistosomiasis||Schistosoma haematobium||Africa, the Middle East, Corsica (France)|
Schistosomiasis mostly affects poor and rural communities, particularly agricultural and fishing populations. Women doing domestic chores in infested water, such as washing clothes, are also at risk and can develop female genital schistosomiasis. Inadequate hygiene and contact with infected water make children especially vulnerable to infection.
Migration to urban areas and population movements are introducing the disease to new areas. Increasing population size and the corresponding needs for power and water often result in development schemes, and environmental modifications facilitate transmission.
With the rise in eco-tourism and travel “off the beaten track”, increasing numbers of tourists are contracting schistosomiasis. At times, tourists present severe acute infection and unusual problems including paralysis.
Urogenital schistosomiasis is also considered to be a risk factor for HIV infection, especially in women.
Symptoms of schistosomiasis are caused by the body’s reaction to the worms’ eggs.
Intestinal schistosomiasis can result in abdominal pain, diarrhoea, and blood in the stool. Liver enlargement is common in advanced cases, and is frequently associated with an accumulation of fluid in the peritoneal cavity and hypertension of the abdominal blood vessels. In such cases there may also be enlargement of the spleen.
The classic sign of urogenital schistosomiasis is haematuria (blood in urine). Fibrosis of the bladder and ureter, and kidney damage are sometimes diagnosed in advanced cases. Bladder cancer is another possible complication in the later stages. In women, urogenital schistosomiasis may present with genital lesions, vaginal bleeding, pain during sexual intercourse, and nodules in the vulva. In men, urogenital schistosomiasis can induce pathology of the seminal vesicles, prostate, and other organs. This disease may also have other long-term irreversible consequences, including infertility.
The economic and health effects of schistosomiasis are considerable and the disease disables more than it kills. In children, schistosomiasis can cause anaemia, stunting and a reduced ability to learn, although the effects are usually reversible with treatment. Chronic schistosomiasis may affect people’s ability to work and in some cases can result in death. The number of deaths due to schistosomiasis is difficult to estimate because of hidden pathologies such as liver and kidney failure, bladder cancer and ectopic pregnancies due to female genital schistosomiasis.
The death estimates due to schistosomiasis need to be re-assessed, as it varies between 24 067 (1) and 200 000 (2) globally per year. In 2000, WHO estimated the annual death rate at 200 000 globally. This should have decreased considerably due to the impact of a scale-up in large-scale preventive chemotherapy campaigns over the past decade
- Current estimated total number of individuals with morbidity and mortality due to Schistosomiasis Haematobium and S. Mansoni infection in Sub-Saharan Africa
Schistosomiasis is diagnosed through the detection of parasite eggs in stool or urine specimens. Antibodies and/or antigens detected in blood or urine samples are also indications of infection.
For urogenital schistosomiasis, a filtration technique using nylon, paper or polycarbonate filters is the standard diagnostic technique. Children with S. haematobium almost always have microscopic blood in their urine which can be detected by chemical reagent strips.
The eggs of intestinal schistosomiasis can be detected in faecal specimens through a technique using methylene blue-stained cellophane soaked in glycerine or glass slides, known as the Kato-Katz technique.
For people living in non-endemic or low-transmission areas, serological and immunological tests may be useful in showing exposure to infection and the need for thorough examination, treatment and follow-up.
Prevention and control
The control of schistosomiasis is based on large-scale treatment of at-risk population groups, access to safe water, improved sanitation, hygiene education, and snail control.
The WHO strategy for schistosomiasis control focuses on reducing disease through periodic, targeted treatment with praziquantel through the large-scale treatment (preventive chemotherapy) of affected populations. It involves regular treatment of all at-risk groups. In a few countries, where there is low transmission, the interruption of the transmission of the disease should be aimed for.
Groups targeted for treatment are:
- School-aged children in endemic areas.
- Adults considered to be at risk in endemic areas, and people with occupations involving contact with infested water, such as fishermen, farmers, irrigation workers, and women whose domestic tasks bring them in contact with infested water.
- Entire communities living in highly endemic areas.
The frequency of treatment is determined by the prevalence of infection in school-age children. In high-transmission areas, treatment may have to be repeated every year for a number of years. Monitoring is essential to determine the impact of control interventions.
The aim is to reduce disease morbidity and transmission: periodic treatment of at-risk populations will cure mild symptoms and prevent infected people from developing severe, late-stage chronic disease. However, a major limitation to schistosomiasis control has been the limited availability of praziquantel. Data for 2016 show that 35.6% of people requiring treatment were reached globally, with a proportion of 53.7% of school-aged children requiring preventive chemotherapy for schistosomiasis being treated.
Praziquantel is the recommended treatment against all forms of schistosomiasis. It is effective, safe, and low-cost. Even though re-infection may occur after treatment, the risk of developing severe disease is diminished and even reversed when treatment is initiated and repeated in childhood.
Schistosomiasis control has been successfully implemented over the past 40 years in several countries, including Brazil, Cambodia, China, Egypt, Mauritius, Islamic Republic of Iran, Oman, Jordan and Saudi Arabia. There is evidence that schistosomiasis transmission was interrupted in Morocco. In Burkina Faso, Ghana, Niger, Rwanda, Sierra Leone, the United Republic of Tanzania, and Yemen, it has been possible to scale-up schistosomiasis treatment to the national level and have an impact on the disease in a few years. An assessment of the status of transmission is being made in several countries.
Over the past 10 years, there has been scale-up of treatment campaigns in a number of sub-Saharan countries, where most of those at risk live.
WHO’s work on schistosomiasis is part of an integrated approach to the control of neglected tropical diseases. Although medically diverse, neglected tropical diseases share features that allow them to persist in conditions of poverty, where they cluster and frequently overlap.
WHO coordinates the strategy of preventive chemotherapy in consultation with collaborating centres and partners from academic and research institutions, the private sector, nongovernmental organizations, international development agencies, and other United Nations organizations. WHO develops technical guidelines and tools for use by national control programmes.
Working with partners and the private sector, WHO has advocated for increased access to praziquantel and resources for implementation. A significant amount of praziquantel, to treat more than 100 million children of the school age per year, has been pledged by the private sector and development partners.
(1) Global Health Estimates 2015: Deaths by Cause, Age, Sex, by Country and by Region, 2000-2015.
Geneva, World Health Organization; 2016.
(2) Prevention and control of schistosomiasis and soil-transmitted helminthiasis
Geneva, World Health Organization: 2002.