Blood flukes are parasitic flatworms that infect humans. These organisms belong to the class Trematoda and feed on blood and plasma. Blood fluke infections, known as schistosomiasis, pose a substantial global health challenge, impacting millions worldwide. It is considered a significant parasitic illness, sometimes compared in prevalence to malaria.
The Main Human Blood Flukes
The main blood flukes that infect humans belong to the genus Schistosoma. Several species cause human infections. Schistosoma mansoni is found in parts of Africa, the Middle East, Brazil, Venezuela, Suriname, and certain Caribbean islands, typically affecting the liver and intestines.
Schistosoma haematobium is prevalent across Africa and parts of the Middle East. This parasite primarily targets the bladder and reproductive organs, causing urinary schistosomiasis. Schistosoma japonicum is distributed in East Asia, including China, the Philippines, and Indonesia, and also affects the liver and intestines, with potential impacts on the brain. Other less common human-infecting species, such as S. mekongi and S. intercalatum, have more restricted geographical ranges in Southeast Asia and parts of Africa.
How Humans Become Infected
Humans acquire blood fluke infections through direct contact with fresh water contaminated with the parasite’s larval forms. The life cycle of Schistosoma involves freshwater snails as intermediate hosts. Infected humans release parasite eggs into fresh water through urine or feces, depending on the Schistosoma species.
Once in the water, these eggs hatch, releasing free-swimming larvae called miracidia. The miracidia then seek out and penetrate specific types of freshwater snails. Inside the snail, the parasites undergo several developmental stages before transforming into the infective larval stage known as cercariae. These cercariae are released from the snails back into the fresh water. When humans come into contact with this contaminated water, the cercariae actively penetrate the skin, initiating the infection.
Signs and Health Impacts
Upon initial infection, some individuals may develop a localized rash or itchy skin, often referred to as “swimmer’s itch,” within hours to days of cercarial penetration. Weeks to months after exposure, an acute systemic reaction known as Katayama fever can occur. This acute phase can manifest with symptoms such as fever, chills, headache, fatigue, muscle aches, cough, and abdominal pain, sometimes accompanied by bloody diarrhea or blood in the urine.
If left untreated, blood fluke infection can progress to a chronic phase, causing long-term health problems due to the body’s immune response to trapped parasite eggs in various tissues. Depending on the Schistosoma species, symptoms vary. S. mansoni and S. japonicum lead to intestinal issues like abdominal pain, bloody diarrhea, and potentially liver damage. S. haematobium infections are associated with urinary problems such as blood in the urine (hematuria), bladder inflammation, kidney damage, or an increased risk of bladder cancer. Rarely, eggs can travel to the brain or spinal cord, leading to neurological symptoms like seizures or paralysis.
Identifying and Treating the Infection
Diagnosing blood fluke infections primarily involves identifying parasite eggs in stool or urine samples. For suspected S. mansoni or S. japonicum infections, stool examination is performed. For S. haematobium, microscopic examination of urine is the standard approach to detect eggs. Since egg excretion can be intermittent or in small quantities, repeated examinations or concentration procedures may be necessary to enhance detection. Blood tests that detect antibodies against the parasite can also aid in diagnosis.
The main treatment for all major Schistosoma species is praziquantel, an oral medication. This drug is effective against adult worms, causing paralysis and making them susceptible to the host’s immune system. A single oral dose of praziquantel is often used. Cure rates are high after a single treatment, although a second course may be given if symptoms persist or if egg excretion continues.
Avoiding Infection
Preventing blood fluke infection largely involves avoiding contact with contaminated freshwater in regions where the parasites are common. It is advisable to refrain from swimming, wading, or bathing in rivers, lakes, or ponds in endemic areas. Ocean water and properly chlorinated swimming pools are considered safe.
For water used for bathing or washing, boiling it can kill the parasites. Alternatively, allowing water to sit in a storage tank for one to two days can also make it safer. Public health efforts in affected regions also contribute to prevention through measures like improving sanitation infrastructure to prevent human waste from contaminating water sources. Additionally, mass drug administration programs, where praziquantel is given to at-risk communities, and snail control measures aimed at reducing intermediate host populations, are employed to limit transmission.