Schistosomiasis is an acute and chronic parasitic disease caused by blood flukes (trematode worms) of the genus Schistosoma. Estimates show that at least 206.5 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.
- 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 206.5 million people required preventive treatment for schistosomiasis in 2016, out of which more than 88 million people were reported to have been treated.
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 92% of those requiring treatment for schistosomiasis live in Africa.
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.
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.
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.
Schistosomiasis increases risk of infection with HIV, especially for women and is also associated with a higher HIV viral load
Infection with schistosome parasitic worms has an important role in HIV transmission, especially for women, and may accelerate HIV disease progression, according to research published in PLOS Neglected Tropical Diseases. Women with schistosomiasis had a three-fold increased risk in becoming infected with HIV, compared to women who did not carry the worms. Moreover, HIV viral load after infection was higher among schistosome-infected individuals, increasing both their potential infectiousness to sex partners and the risk of HIV disease progression.
“Our study suggests that interactions exist between HIV-1 and schistosomiasis that may play a critical role in HIV-1 transmission and disease progression in African countries,” comment the investigators.
Schistosomiasis is a disease caused by a tropical parasitic worm infection. Over 90% of infections occur in Africa. The worms live in small veins in host’s pelvis and gut and daily lay hundreds of eggs that migrate to the urogenital and gastrointestinal mucosa where they cause inflammation and physical breaches in the mucosa.
A team of investigators postulated that pre-existing schistosome infection would increase a patient’s risk of acquiring HIV, with the risk especially high for women. They also hypothesised that individuals with active schistosome infection at the time of HIV acquisition would have impaired immune systems, resulting in higher HIV viral loads.
To test these hypotheses, the researchers designed a nested case-controlled study involving adults living in rural Tanzania. In 2007, 2010 and 2013 participants were tested for HIV.
The investigators identified 73 individuals who became infected with HIV during follow-up. These people were matched with 265 controls who remained HIV negative. Dried blood spots from the cases and controls were tested for the presence of schistosome infection to see if it increased the risk of acquiring HIV.
The investigators also compared the viral loads of HIV seroconverters according to schistosome-infection status.
In women, 44% of those who acquired HIV had schistosome infection at the time of their seroconversion compared to 30% of the HIV-uninfected controls. After taking into account potential confounders, the investigators showed that women with schistosome infection had an almost three-fold increase in the risk of becoming infected with HIV compared to women who remained HIV negative (OR = 2.8; 95% CI, 1.2-6.6, p = 0.019).
Just under a third (29%) of men who seroconverted for HIV had schistosomiasis at the time they became infected with HIV, compared to a schistosomiasis prevalence of 38% among men who remained HIV negative. As such, the schistosome infection did not increase the risk of HIV acquisition for men.
“We found that schistosomiasis increases the odds of HIV-1 acquisition in women but not men,” comment the researchers. “This strong gender effect may be due to differential effects of schistosome eggs in the genital mucosa of women versus men.”
Among people who did become infected with HIV, median viral load was approximately 25,000 copies/ml among those with schistosomiasis compared to 5000 copies/ml among individuals who did not carry the parasitic infection, a significant difference (p = 0.017). Viral load differences by schistosomiasis-infection status were more pronounced in women.
“We found that schistosomiasis at the time of HIV-1 infection led to a 0.7 log10 increase in viral load at the time of HIV-1 seroconversion,” write the authors. “A sustained 0.7 log10 HIV-1 load increase equates with an approximate doubling of infectivity among HIV-1-schistosome co-infected individuals and would be expected to accelerate time to symptomatic AIDS or death by 2-3 years.”
The authors conclude that schistosomiasis increases HIV incidence among women and raises viral load at the time of seroconversion. They note that praziquantel is a cheap and safe treatment for schistosomiasis and call for studies to determine the effectiveness of mass therapy with this drug to decrease HIV transmission and slow HIV disease progression.