Schistosomiasis, often known as snail fever, is a parasitic disease caused by flatworms of the genus Schistosoma. This condition poses a significant public health challenge, particularly in tropical and subtropical regions globally, with hundreds of millions of people affected. While the parasites can impact various organ systems, their presence in the human body often leads to substantial damage within the liver.
The Parasite and Its Journey to the Liver
Schistosomiasis is caused by specific species of parasitic flatworms, with Schistosoma mansoni and Schistosoma japonicum being the most common types associated with liver involvement. The life cycle of these parasites begins when eggs, shed in the feces or urine of infected humans, enter fresh water. Once in water, the eggs hatch and release miracidia, which are immature larval forms.
These miracidia then swim and penetrate specific freshwater snails, which serve as intermediate hosts. Inside the snail, the miracidia undergo asexual reproduction, developing into sporocysts and then into thousands of free-swimming cercariae. These cercariae are then released from the snails into the water.
Humans become infected when their skin comes into contact with fresh water containing these cercariae. Upon penetrating the skin, the cercariae shed their tails and transform into schistosomula. These schistosomula then enter the bloodstream and travel through the lungs and heart before reaching the hepatic portal system. In the liver’s blood vessels, the schistosomula mature into adult male and female worms. These adult worms then pair up and migrate to the mesenteric veins surrounding the intestines, where the female worms begin laying eggs.
How Schistosomiasis Affects the Liver
Liver damage in schistosomiasis primarily stems from the host’s immune response to the parasite eggs, not directly from the adult worms themselves. While adult worms reside in the mesenteric veins, many of the eggs they lay are not excreted and instead become trapped in various organs, predominantly the liver and intestines. These trapped eggs, particularly in the small portal venules of the liver, release antigenic substances that trigger a strong inflammatory reaction.
This immune reaction leads to the formation of granulomas, which are small masses of immune cells that surround the eggs. Over time, the continuous inflammation and granuloma formation in the portal spaces cause the development of fibrous tissue, a process known as periportal fibrosis. This type of fibrosis, often described as “Symmers’ fibrosis” or “pipestem fibrosis,” is characterized by wide fibroid strips that extend along the branches of the portal vein within the liver. This fibrotic tissue can obstruct blood flow within the liver’s portal venous system.
The obstruction of blood flow due to periportal fibrosis leads to increased pressure in the portal vein, a condition called portal hypertension. Portal hypertension can manifest in several ways, including splenomegaly, an enlargement of the spleen, and ascites, the accumulation of fluid in the abdominal cavity. It can also lead to the formation of esophageal varices, which are enlarged veins in the esophagus that carry a risk of bleeding. While hepatic function is often preserved until the disease reaches late stages, severe portal fibrosis is a common cause of morbidity and can lead to life-threatening complications like gastrointestinal hemorrhages from varices.
Schistosomiasis can present in acute and chronic forms. Acute schistosomiasis typically occurs 1 to 2 months after initial exposure to contaminated water. This acute phase is a systemic hypersensitivity reaction to the parasite antigens and can involve symptoms such as fever, chills, cough, abdominal pain, diarrhea, and enlargement of the liver and spleen. Some individuals may develop more persistent and severe disease.
Chronic schistosomiasis, resulting from repeated exposure and persistent egg deposition, is characterized by the progressive development of periportal fibrosis and its associated complications. The severity of liver damage can vary significantly, influenced by factors such as infection intensity, duration, and individual genetic predispositions. The presence of granulomas around the parasite eggs is more pronounced in the early stages of the disease, with fibrosis becoming the dominant feature in chronic infections.
Diagnosing and Treating Liver Schistosomiasis
Diagnosing liver schistosomiasis involves a combination of methods to identify the parasite and assess the extent of liver damage. The primary diagnostic approach involves detecting parasite eggs in stool or urine samples. For intestinal and liver schistosomiasis, stool examination is employed to quantify the number of eggs excreted. In cases where eggs are not readily found, biopsy specimens from affected tissues may be examined.
Serological tests, which detect antibodies to schistosomal antigens, are also used, particularly for travelers or individuals from non-endemic areas. These tests can indicate exposure to the parasite, but they may not differentiate between active and past infections, as antibodies can persist in the blood even after successful treatment. Molecular techniques, such as PCR assays, can detect schistosome DNA in stool, urine, or blood to identify active infection.
Imaging techniques play a significant role in assessing liver involvement and its complications. Ultrasound is a widely used and effective tool for diagnosing periportal fibrosis, evaluating the size of the liver and spleen, and detecting signs of portal hypertension. Specific ultrasound patterns, such as the “mosaic pattern” or “bull’s-eye” appearance, can indicate periportal fibrosis. Other imaging modalities like CT scans and MRI can also reveal features of liver fibrosis, splenomegaly, and portal vein abnormalities.
The primary treatment for schistosomiasis is the anti-parasitic drug praziquantel. This medication is highly effective in killing the adult worms, causing severe spasms and paralysis of their muscles, leading to their elimination from the body. It is administered orally. While praziquantel effectively targets the adult worms, inflammation around eggs already trapped in tissues may persist, which is particularly relevant for existing liver fibrosis.
Managing advanced liver disease complications, such as portal hypertension, is also a consideration. This may involve addressing issues like bleeding esophageal varices, a serious consequence of portal hypertension. Established fibrosis and its resulting complications may require additional medical or surgical interventions. Co-infections, such as viral hepatitis, can further complicate the liver’s condition and require separate management.