Schistosoma hematobium

(urinary blood fluke)

Dr. Hala Al Daghistani

Schistosoma haematobiumis the only blood fluke that infects the urinary tract, causing Urinary Schistosomiasis, and is the leading cause of bladder cancer

  • Adult Schistosoma haematobium is monoecious (male and female sex organs in one body), but with distinct male and female bodies.
  • The male forms the flatworm part, measuring 10–18mm in length and 1mm in width. It bears oral and ventral suckers towards its anterior end. Its leaf-like flat body is curled up from both sides to form a channel or groove called Gynaecophoric canal in which the female is wrapped up.

Infection and transmission

  • Transmission occurs when people suffering from schistosomiasis contaminate freshwater sources with their excreta containing parasite eggs, which hatch in water.
  • People become infected when larval forms of the parasite – released by freshwater snails – penetrate the skin during contact with infested water.
  • In the body, the larvae develop into adult schistosomes. Adult worms live in the blood vessels where the females release eggs. Some of the eggs are passed out of the body in the feces or urine to continue the parasite’s lifecycle. Others become trapped in body tissues, causing immune reactions and progressive damage to organs.

Epidemiology

Schistosomiasis is prevalent in tropical and subtropical areas, especially in poor communities without access to safe drinking water and adequate sanitation. It is estimated that at least 91.5% of those requiring treatment for Schistosomiasis live in Africa.

Life cycle of S. haematobium

  • S. haematobium completes it life cycle in humans, as Definitive hosts, and freshwater snails, as Intermediate hosts
  • Unlike other Schistosomes that release eggs in the intestine, it releases its eggs in the urinary tract and excrete along with the urine.
  • In freshwater, the eggs hatch within 15 minutes into the larvae called miracidia. Miracidia are covered with hair-like cilia with which actively swims searching for Snails.
  • Unless they infect a snail within 24–28 hours, their food (glycogen) reserve runs out and die. Species of snail belonging to the genus Bulinus, can harbour the miracidia.
  • The miracidia simply move through the soft skin of the snail and move to the liver. Inside the snail, they transform into Sporocysts and undergo active cell division after two weeks.
  • Each sporocyst forms new larvae called Cercariae. One mother sporocyst produces half a million cercariae. After a month, the sporocysts rupture and cercariae are liberated.
  • When human comes in contact with an infested water, the cercariae attach themselves on the skin using their suckers.
  • After proper orientation, they start penetrating the skin by secreting proteolytic enzymes that widen the skin pores (hair follicles). This process takes about 3–5 minutes and produces itching
  • When they enter the blood vessels, they are known as Schisotomulae. They enter the systemic systemto reach the heart and then the liver, and along the way many are killed by the immune cells. Survivors enter the liver within 24 hours. From the liver they enter the portal vein to reach different parts of the body.

Schistosomulae of S. haematobium reach the visceralvessels. After living inside small venules in the submucosa and wall of the bladder, they migrate to the perivesical venous plexus (a group of veins at the lower portion of the bladder) to attain full maturation.

  • Sexual maturation is attained after 4–6 weeks of initial infection. A female generally lays 500-1,000 eggs in a day and the eggs can be released into the bladder.
  • The embryonated eggs penetrate the bladder mucosa using proteolytic enzymes, aided by their terminal spines and by the contraction of the bladder.
  • Eggs often fail to penetrate the bladder mucosa and remain trapped in the bladder wall; it is these which produce the lesions by releasing their antigens and provoking Granuloma formation.
  • Granulomata in turn form tubercles, nodules or masses that often ulcerate. This is the condition behind the pathological lesions found in the bladder wall, ureter and renal; and also tumor, both benign and malignant.

Granuloma formation.

Infections are characterized by pronounced acute inflammation, squamous metaplasia, blood and reactive epithelial changes. Granulomas and multinucleated giant cells may be seen. The eggs induce a granulomatous host immune response which is indicated by lymphocytes (which mainly produce T-helper-2 cytokines such as interleukins 4, 5, and 13), eosinophils, and, also activated macrophages. This granuloma formation induces chronic inflammation.

Egg of S. haematobium. Note the pointed spine on the left tip.

Schistomiasis can be divided into three phases:

(1)The migratory phase lasting from penetration to maturity

(2)The acute phase which occurs when the schistosomes begin producing eggs

(3)The chronic phase which occurs mainly in endemic areas.

Pathology and Pathogenesis

  • The most significant pathology is associated with the schistosome eggs, not the adult worms.
  • Female schistosomes can lay hundreds or thousands of eggs per day within the venous system. When eggs are released, many are swept back into the circulation and accumulate in the urinary bladder (S haematobium), and pass out with the urine
  • Under serious infection, urinary tract can be blocked leading to obstructive uropathy (hydroureter and hydronephrosis), which can be further complicated by bacterial infection and kidney failure. In the most severe condition, chronic bladder ulcers and bladder carcinoma develop.

  • However, there is urinary tract involvement: urethral pain, increased urinary frequency, dysuria, hematuria, and bladder obstruction leading to secondary bacterial infections.In endemic regions, haematuria is so widespread that it is thought a natural sign of puberty for boys, and is confused with menses in girls.
  • In travelers to endemic countries, clinical findings of acute schistosomiasis include an itchy rash (swimmer’s itch) that occurs within an hour after cercariae penetrate the skin, followed by headache, chills, fever, diarrhea, and eosinophilia.
  • In women, urogenital schistosomiasis may present with genital lesions, vaginal bleeding, pain during sexual intercourse, and nodules in the vulva.
  • In men, urogenital schistosomiasis can induce pathology of the seminal vesicles, prostate, and other organs. This disease may also have other long-term irreversible consequences, including infertility.

Diagnosis

Schistosomiasis is diagnosed through the detection of parasite eggs in urine specimens. Antibodies and/or antigens detected in blood or urine samples are also indications of infection.

For urogenital schistosomiasis, a filtration technique using nylon, paper or polycarbonate filters is the standard diagnostic technique. Children with S. haematobium almost always have blood in their urine which can be detected by chemical reagent strips.

For people living in non-endemic or low-transmission areas, serological and immunological tests may be useful in showing exposure to infection and the need for thorough examination, treatment and follow-up.

Prevention and control

The control of schistosomiasis is based on

  • large-scale treatment of at-risk population groups
  • access to safe water
  • improved sanitation
  • hygiene education
  • snail control.

Groups targeted for treatment are:

  • School-aged children in endemic areas.
  • Adults considered to be at risk in endemic areas, and people with occupations involving contact with infested water, such as fishermen, farmers, irrigation workers, and women whose domestic tasks bring them in contact with infested water.
  • Entire communities living in highly endemic areas.