Blood flukes are parasitic flatworms that affect millions of people globally, causing a disease known as schistosomiasis. These organisms are a significant public health challenge, particularly in tropical and subtropical regions. Understanding these parasites, their life cycle, and the diseases they cause is important for prevention and control efforts.
What Are Blood Flukes?
Blood flukes are parasitic worms belonging to the genus Schistosoma. Unlike other flukes, they inhabit the blood vessels of their human hosts, feeding on blood and plasma. These worms are relatively small, and possess a flat, oval shape. They attach to host organs using specialized suckers.
Blood flukes have distinct male and female sexes, unlike many other flukes that are hermaphroditic. The male worm is larger and holds the female within a specialized groove for reproduction. Several Schistosoma species can infect humans, with Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum being the most common. S. mansoni primarily affects the liver and intestines, S. haematobium targets the bladder and reproductive organs, and S. japonicum can impact the liver, intestines, and brain.
How Do Infections Occur?
Infections with blood flukes begin when humans contact fresh water contaminated with the parasite’s larval forms, called cercariae. These larvae are released from freshwater snails, which serve as intermediate hosts in the fluke’s complex life cycle. Snails become infected when they encounter miracidia, another larval stage that hatches from eggs excreted in human urine or feces into fresh water.
Once released from snails, cercariae swim in the water and are attracted to human skin. They penetrate the skin, shedding their forked tails as they enter the body, transforming into schistosomulae. These schistosomulae then travel through the bloodstream, migrating to the lungs and heart before reaching the liver, where they mature into adult worms. Adult male and female worms pair up and move to specific blood vessels, such as the mesenteric veins around the intestines or the venous plexus of the bladder, depending on the Schistosoma species. Here, the female worms lay thousands of eggs, which are then released into the environment through the host’s feces or urine.
The Diseases They Cause
The disease caused by blood flukes is known as schistosomiasis, also called snail fever or bilharzia. Symptoms arise from the body’s immune reaction to eggs trapped in tissues, not from the adult worms. Many infections are initially asymptomatic, but early signs can include an itchy rash, often called “swimmer’s itch,” appearing within days of skin penetration.
Within one to two months, an acute systemic reaction known as Katayama fever may develop. This acute phase can manifest with symptoms such as fever, chills, cough, muscle aches, headache, and abdominal pain.
Chronic schistosomiasis develops if the infection remains untreated, with symptoms varying based on the species and the location where eggs accumulate. Intestinal schistosomiasis, caused by species like S. mansoni and S. japonicum, can lead to abdominal pain, diarrhea, and blood in the stool. Urinary schistosomiasis, caused by S. haematobium, often presents with blood in the urine and urination problems. Long-term consequences of chronic infection can include severe organ damage such as liver fibrosis, enlargement of the liver and spleen, kidney damage, and an increased risk of bladder cancer. In rare cases, eggs migrating to the brain or spinal cord can cause seizures, paralysis, or inflammation.
Preventing and Treating Infections
Diagnosing blood fluke infections involves examining stool or urine samples under a microscope to identify characteristic eggs. Serological tests, which detect antibodies in the blood, can also be used. Early diagnosis is important for preventing the progression to chronic disease.
The primary medication for treating schistosomiasis is praziquantel, an oral drug that is effective against all major Schistosoma species infecting humans. Praziquantel works by affecting the parasite’s membrane permeability, leading to paralysis and exposing the worms to the host’s immune system. While generally safe and well-tolerated, the timing of treatment is important, as praziquantel is most effective against adult worms. For travelers, treatment is often recommended 6 to 8 weeks after their last exposure to potentially contaminated water to allow worms to mature.
Preventing blood fluke infections involves several strategies. Avoiding contact with contaminated freshwater is important, including refraining from swimming, bathing, or washing clothes in lakes, rivers, or ponds in endemic areas. Safe water practices, such as boiling or filtering water for drinking and bathing, are also important.
Improvements in sanitation infrastructure, including proper disposal of human waste, help prevent water source contamination with eggs. Snail control measures, which target intermediate hosts, are implemented in affected regions to disrupt the parasite’s life cycle. Mass drug administration programs, where praziquantel is given to entire communities in endemic areas, are a widespread public health strategy to reduce infection rates and control morbidity.