Blood flukes, scientifically known as Schistosoma, are parasitic flatworms that cause a debilitating disease called schistosomiasis. This infection is also frequently referred to as Bilharzia or “snail fever.” Schistosomiasis is recognized globally as a major neglected tropical disease, affecting hundreds of millions of people who live in tropical and subtropical regions. The consequences range from acute, flu-like symptoms to severe, long-term organ damage.
The Parasite’s Life Cycle and Transmission
The transmission of schistosomiasis begins when the parasite’s eggs are passed into freshwater via the urine or feces of an infected person. These eggs hatch, releasing a larval form called a miracidium, which must find and penetrate a specific species of freshwater snail. The snail serves as the intermediate host where the parasite undergoes asexual reproduction and transforms.
After several weeks inside the snail, thousands of microscopic, fork-tailed larvae, known as cercariae, are shed back into the water. These free-swimming organisms are the stage that is infective to humans. Infection occurs when skin comes into contact with this contaminated fresh water, such as during swimming, bathing, or working.
The cercariae penetrate the skin, shedding their tails in the process, and become schistosomula. These juvenile worms then enter the bloodstream and migrate through the body’s circulatory system, eventually reaching the veins surrounding the liver. Here, they mature into adult male and female worms, which then pair up and migrate to their final destinations, either the veins of the intestine or the bladder, depending on the Schistosoma species involved.
Stages of Schistosomiasis and Manifestations
The symptoms of schistosomiasis are primarily caused by the body’s inflammatory reaction to the thousands of eggs the adult worms produce. Upon initial skin penetration, some individuals may develop a localized, itchy rash called cercarial dermatitis. Weeks later, as the worms begin to lay eggs, a systemic illness may occur, particularly in individuals with their first heavy infection.
This acute stage, sometimes called Katayama fever, typically manifests 14 to 84 days after exposure. It is characterized by a sudden onset of symptoms:
- Fever
- Chills
- Dry cough
- Muscle aches (myalgia)
- Headache
Physical examination often reveals an enlarged liver and spleen (hepatosplenomegaly), along with elevated levels of eosinophils, a type of white blood cell.
If the infection persists and becomes chronic, the body’s reaction to eggs trapped in tissues leads to significant long-term pathology.
Intestinal Schistosomiasis
For intestinal schistosomiasis, caused by species like Schistosoma mansoni and Schistosoma japonicum, trapped eggs in the intestinal wall and liver induce granuloma formation and scarring. This chronic inflammation can eventually lead to liver fibrosis, which may cause portal hypertension. Portal hypertension, an elevated blood pressure in the vein leading to the liver, can result in the enlargement of the spleen and the development of fragile, swollen veins in the esophagus (esophageal varices).
Urogenital Schistosomiasis
For urogenital schistosomiasis, caused by Schistosoma haematobium, eggs lodge in the bladder wall and urinary tract. Symptoms include painful urination (dysuria) and blood in the urine (hematuria). Chronic infection can lead to bladder calcification, scarring of the ureters, and damage to the kidneys. A serious, long-term manifestation is an increased risk of developing squamous cell carcinoma of the bladder. In women, the eggs can cause female genital schistosomiasis, leading to lesions on the cervix, vagina, or vulva, which may increase susceptibility to other infections.
Geographic Distribution and Populations at Risk
Schistosomiasis is geographically restricted because the parasite requires specific freshwater snail species to complete its life cycle. The infection is prevalent across 78 countries, primarily in tropical and subtropical regions. Sub-Saharan Africa bears the greatest burden, accounting for over 90% of individuals requiring treatment globally.
The disease is also found in parts of South America, the Middle East, and Asia. Distribution is intrinsically linked to poverty and lack of access to safe water and adequate sanitation. High-risk populations are those whose daily life or occupation involves frequent contact with contaminated fresh water.
These vulnerable groups include:
- Agricultural workers
- Fishermen
- People who use rivers or lakes for washing clothes, bathing, or recreation
- Children, who are particularly susceptible due to play habits involving wading in infected water
The movement of populations, such as refugees or tourists traveling to endemic areas, can also lead to new exposures.
Methods of Diagnosis and Treatment
Diagnosing schistosomiasis typically involves identifying the parasite’s eggs in a person’s stool or urine sample using a microscope. The sample type depends on the suspected species: S. mansoni and S. japonicum eggs are found in stool, while S. haematobium eggs are primarily found in urine. Since egg excretion can fluctuate, multiple samples may be necessary to confirm light infections.
When eggs are difficult to find, particularly in the early stages of infection or in travelers, serological tests are used to detect antibodies against the parasite in the blood. These tests can indicate exposure but do not confirm a current, active infection because antibodies can persist after the parasites have been cleared. Newer diagnostic tools, such as the detection of parasite antigens in the urine, are also being utilized in field settings.
The medication Praziquantel is the standard and effective treatment against all major Schistosoma species. Praziquantel works by causing severe spasms in the adult worms, paralyzing them and exposing them to the host’s immune system for destruction. Treatment timing is important because Praziquantel is most effective against adult worms and has limited impact on the juvenile stages developing shortly after exposure.
Preventing Schistosome Infection
Preventing schistosomiasis involves breaking the parasite’s life cycle to stop transmission. The most direct individual measure is to avoid swimming, bathing, or wading in fresh water in endemic areas. Water that has been boiled for at least one minute is considered safe for drinking and bathing, as the heat kills the infectious cercariae.
Public health strategies focus on two major interventions: mass drug administration (MDA) and environmental control.
Mass Drug Administration
Preventive chemotherapy programs involve treating entire populations at risk with Praziquantel regularly. This reduces the overall burden of infection and limits the number of eggs shed into the environment. This strategy focuses particularly on school-aged children, who often carry the highest intensity of infection.
Environmental Control
Environmental interventions include improving access to safe drinking water and adequate sanitation facilities to prevent human waste from contaminating natural water sources. Controlling the intermediate host population through the use of molluscicides, chemicals that kill the snails, is another method used in specific local contexts. These combined efforts are necessary to eliminate schistosomiasis as a public health problem.