Schistosomiasis is also called Bilharziasis or Bilharzia or Snail fever; Schistosomiasis is a disease caused by a parasitic worm called Schistosoma which is a blood fluke. Schistosomiasis is a serious parasitic disease that is regarded as second in the scale of its impact on human populations only to malaria; the disease is one of the most important causes of morbidity in the tropics.

Schistosomiasis Epidemiology

Schistosomiasis affects mainly children and young adults within the age range of 10 to 20 years. According to WHO in 2004, more than 200 million people are affected worldwide with Schistosomiasis especially in the tropics and subtropical regions. Schistosomiasis is endemic in 74 tropical and subtropical countries of Africa, South America and Asia.

Schistosoma hematobium occurs predominantly in Africa, the Near East, and questionable occurrence in India; Schistosoma mansoni is predominant in Africa, the Caribbean and the north-eastern parts of South America; Schistosoma japonicum is predominant in Southeast Asia and the western Pacific especially in China, Indonesia and the Philippines; Schistosoma intercalatum occurs focally in central and western Africa while Schistosoma mekongi occurs in Laos and Cambodia.

Schistosomiasis Transmission

Human Schistosomiasis is transmitted when cercariae released by infected freshwater snails penetrate the skin. Human infections occurs when there is contact with standing or slow-moving freshwater containing the cercariae. The freshwater snail serves as the intermediate host.

Schistosomiasis life cycle: Schistosoma Life cycle showing the variation in the different species of schistosoma

Schistosomiasis Life cycle

1. The schistosoma eggs are the diagnostic stage of Schistosomiasis . the eggs are released by adult worms
2. Released eggs then hatch into immature ciliated larvae that are called Miracidia
3. Miracidia then penetrate freshwater snails tissue
4. Miracidia develop into Sporocysts in snail after successive generations
5. Sporocysts develop to mature larvae called Cercariae in a favorable environment. The cercariae are the infective stage of Schistosomiasis. The released cercariae swim freely in water until the find the human host
6. Cercariae then penetrate the skin of humans and lose their tails to become Schistosomutae. Schistosomutae are then taken to other organs of the body when they enter the circulation. Those that enter the superior mesenteric artery pass into the portal circulation and reach the liver and mature into adults; S. mansoni and S. japonicum adults migrate against the portal flow to reside in the mesenteric venules while S. haematobium adults reach the bladder veins through the venous plexus between the rectum and the bladder.
7. Matured adults of schistosoma then migrate to the mesenteric venules of the rectum and bowels for S. japonicum, S. mekongi, S. intercalatum and S. Mansoni while the adult worms of S. haematobium migrate to the venous plexus of the bladder.
8. Adult worms then lay eggs that are released in feces for all species of schistosoma or are released in Urine for Schistosoma haematobium and Schistosoma japonicum from here they hatch and the cycle continues.

Adult schistosomes in the venous system are able to survive and release eggs for many years because they are not attacked by the immune system due to the fact that schistosomes use molecular mimicry (incorporation of host antigens onto their surface) in order to deceive the host’s immune system that the schistosomes are not foreign proteins. However, cercariae (mature larvae) and schistosoma eggs can trigger the immune system and cause the systemic illness seen with Schistosomiasis.

Schistosoma egg: The eggs can be easily distinguished by their spines. Schistosoma hematobium egg has terminal spine; Schistosoma mansoni egg has large lateral spine

Types of Schistosoma Species (Types of Schistosomes)

There are different schistosoma species that cause disease in man:

1. Schistosoma haematobium: this affects the urogenital tract directly, that is, it causes urinary Schistosomiasis.
2. Schistosoma mansoni: this indirectly affects the genitourinary tract by causing glomerulonephritis by immune complexes
3. Schistosoma japonicum
4. Schistosoma mekongi
5. Schistosoma intercalatum

Schistosoma Species with associated species of Snails as intermediate host

1. Schistosoma japonicum has Onchomelania species of snails as intermediate host where asexual regeneration takes place
2. Schistosoma haematobium has Bulinus species of snails as intermediate host where asexual reproduction takes place
3. Schistosoma mansoni has Biomphalaria species of snails as intermediate host where asexual regeneration takes place

Schistosomiasis Pathophysiology (Pathogenesis of Bilharzia)

Most of the symptoms and signs of Schistosomiasis arise due to the presence of schistosoma eggs in the liver, spleen, or wall of the intestines or bladder. Depending on where the schistosoma ova are found, the symptoms that follow are according to the organ affected. Eggs in the liver can induce granulomas leading to liver fibrosis, hepatomegaly and portal hypertension. The hepatocytes are usually undamaged and liver function tests remain normal. Liver fibrosis leads to portal hypertension which in turn causes splenomegaly.

Schistosoma mansoni eggs in the colon damage the wall of the distal colon (inferior mesenteric venules), whereas Schistosoma japonicum eggs destroy the walls of both the small and large intestines (superior and inferior mesenteric venules). The destruction is mainly because of the digestion of tissue by proteolytic enzymes produced by the eggs and also due to the host inflammatory response that forms granulomas in the venules. The eggs of S. haematobium in the wall of the bladder induce granulomas and fibrosis, which may cause Bladder cancer (carcinoma of the bladder).

Schistosomes can stay for as high as 40 years in the human body because they have evolved a remarkable process for evading the host defences by coating their surfaces with host antigens, thereby limiting the ability of the immune system to recognize them as foreign bodies.

Types of Schistosomiasis (Different Forms of Bilharzia)

1. Urinary Schistosomiasis(urinary bilharziasis)
2. Intestinal Schistosomiasis (intestinal bilharzia)
3. Cerebral Schistosomiasis (Cerebral bilharzia) this is relatively frequent in 2 to 4% of Schistosoma japonicum infections
4. Hepatic Schistosomiasis (Hepatosplenic Schistosomiasis)
5. Pulmonary Schistosomiasis (observed mainly in severe Schistosoma mansoni infections)

The different types of Schistosomiasis are classified based on the localization of the lesions.

Schistosomiasis Symptoms and Signs

1. Anemia
2. Malnutrition
3. Fever
4. Abdominal pain
5. Fluid buildup (Edema)
6. Liver fibrosis
7. Hepatomegaly (enlargement of the liver)
8. Splenomegaly (enlargement of the spleen)
9. Portal hypertension
10. Body itching
11. Dermatitis
12. Diarrhoea
13. Llymphadenopathy
14. Gastrointestinal haemorrhage

Clinical Stages of Schistosomiasis

1. Acute stage
2. Chronic stage

Acute stage of Schistosomiasis

Most patients with Schistosomiasis may not show symptoms (they are said to be asymptomatic); but there may be symptoms in chronic infections. The acute stage starts after skin penetration by cercariae and produces symptoms such as body itching and dermatitis. These symptoms are then followed 2 to 3 weeks later by fever, chills, diarrhea, lymphadenopathy, and hepatosplenomegaly. The eosinophilia seen at this stage is due to the immune response to the migrating larvae. The acute stage stage usually resolves spontaneously. The associated symptoms in the acute stage of Schistosomiasis is known as Katayama syndrome or Katayama Fever.

Chronic stage of Schistosomiasis

The chronic stage of Schistosomiasis is more severe and can cause significant morbidity and mortality. In patients with S. mansoni or S. japonicum infection, gastrointestinal hemorrhage, hepatomegaly, and massive splenomegaly can develop. The main cause of death in this stage is exsanguination from upper gastrointestinal bleeding due to ruptured esophageal varices. Patients infected with S. haematobium have hematuria as the main early symptom.

Swimmer’s itch is often experienced by swimmers in some lakes in the United States of America and this itch causes pruritic papules which are due to immune reaction to the presence cercariae of schistosomes that do not cause disease in humans. The pruritic papules appear within minutes to hours after exposure, suggesting that this is an immediate (IgE-mediated) hypersensitivity. These nonhuman schistosomes are unable to replicate in humans and do not cause disseminated disease.

Schistosomiasis Diagnosis

Diagnosis of Schistosomiasis is made by detection of Schistosoma eggs in stool or urine; it is easy to identify schistosoma eggs because of the large lateral spine of Schistosoma mansoni and the rudimentary spine of Schistosoma japonicum while Schistosoma haematobium has a large terminal spine. Schistosomiasis can also be diagnosed by serology through identification of specific antibodies against schistosomes in the serum.

Schistosomiasis Treatment

The drug of choice for the treatment of Schistosomiasis is Praziquantel. Another drug that could be used for treatment of Schistosomiasis caused by schistosoma mansoni is Oxamniquine. Treatment of Schistosomiasis with praziquantel may increase symptoms as the dead of these schistosomes evokes a vigorous immune response.

Schistosomiasis Complications

1. Urinary obstruction in the form of urethral strictures
2. Urinary bladder cancer
3. Childhood Hematuria
4. Portal hypertension
5. Hepatocellular carcinoma (Liver cancer)
6. Acute kidney injury

Schistosomiasis Prevention

1. Prevention of Schistosomiasis involves proper disposal of human waste
2. Eradication of the snail host when possible
3. Avoid Swimming in areas of endemic infection