Schistosoma haematobium (blood flukes)
Schistosomiasis, also known as bilharzia, is a parasitic disease caused by trematodes from the genus Schistosoma. There are four main species that infect humans. S. mansoni, S. japonicum, and S. mekongi all cause intestinal schistosomiasis. S. haematobium causes urinary schistosomiasis.
Urinary schistosomiasis is often chronic and can cause pain, secondary infections, kidney damage, and even cancer. It has been infecting humans for at least 4000 years and had its own specific hieroglyph in ancient Egyptian. In the time before treatments were widely available, it was still so prevalent in Egypt that boys were traditionally expected to go through a “male menarche”—sometime during adolescence, it was normal for them to urinate blood. S. haematobium infections continue to be a significant public health problem in much of Africa and the Middle East, second only to malaria among parasitic diseases. Continuing efforts and new strategies are needed to reduce the burden of schistosomiasis haematobium infection on the human family.
Schistosomiasis haematobium is endemic to over 50 countries in Africa and the Middle East. It is also occasionally seen in Western Asia. The World Health Organization (WHO) considers it a significant public health problem in much of Africa.
WHO estimates that worldwide, 180 million people live in endemic areas and 90 million are infected with the parasites. Most of these live in Sub-Saharan Africa. Roughly 70 million persons suffer from schistosomal hematuria (blood in the urine), 18 million from associated bladder wall pathology, and 10 million from hydronephrosis (an accumulation of urine in the kidney due to obstruction of the ureter). It is estimated that 150,000 people die each year from resultant renal failure and an unknown but significant number from bladder and other genitourinary cancers. The overall mortality rate is estimated to be at least 2 per 1,000 infected patients per year.
In many places, there is a higher incidence of infection in young boys and women. This occurs because of increased contact with water compared to other population groups in cultures where women typically fetch water for household use and young boys often play in or near water. In some regions where men are primarily freshwater fishermen or farmers using irrigation, they have higher rates of schistosomiasis. These differential rates of transmission depend on cultural practices. It is important to consider them when planning treatment, prevention, and control strategies.
History of Discovery
1950 BC: Egyptian pharaohs wrote of urinary bladder disturbances that probably were schistosomiasis haematobium. The hieroglyph used to denote the disease was a dripping penis.
1200 BC: Schistosome ova have been found in mummies dated from around this time
1851: Theodor Bilharz, a German pathologist, discovered the parasite while working at Kasr El-Eini Hospital in Cairo (bilharzia is named after him)
1915: Lieper, an English scientist, discovered the intermediate snail host
Classification and Taxonomy
Transmission occurs in stagnant or slow-moving fresh water where infected bulinus snails live. Transmission rates to populations that have frequent exposure to water (e.g. fishermen, farmers working in irrigation canals, women fetching water for home use, children who swim regularly) are especially high. Anthropogenic creation of new snail habitat through building dams or irrigation canals may increase rates of transmission to nearby human populations.
None is known, although there have been reports of zoonotic disease in vervet monkeys, baboons, chimpanzees, guenons, sheep, pigs, and Nile rats. These animal hosts are not thought to play a significant role in the perpetuation of transmission of schistosomiasis.
S. Haematobium has no vector; the infective cercariae are free-swimming. There are intermediate hosts, however, snails from the genus bulinus.
Adult males are 10 to 15 mm long. They have deep grooves called gynecophoral canals in which adult females typically lie. Males have many small nodules (tubercles) on their dorsal surfaces and many tiny spines on their suckers and inside their gynecophoral canals. .Females are longer (16-22 mm), smoother, and more slender. Both sexes have two suckers, one anterior and one ventral, which are used to grip venule walls.
Eggs, which can be found in the urine of infected hosts, are 110-170 um long by 40 to 70 mm wide. They are elongated with a distinctive terminal spine and look like microscopic American footballs with a spike on one end. The shells of the eggs are clear and contain miracidia.
Clinical Presentation in Humans
Early: Hives or an itchy papular skin rash (“swimmer’s itch”) and local swelling often starts about 24 hrs after the initial infection and lasts for about 4 days. At one to two months, the infected individual may present with fever, hepatitis, enlarged liver, spleen, and lymph nodes, and eosinophilia. Pulmonary symptoms are possible but rare. This illness lasts for one to three weeks. Not everyone manifests these early-stage signs and symptoms.
Later, gradual onset: After months or years, the infected individual may experience painful or difficult urination (dysuria), blood in urine (hematuria), urethral obstruction, kidney damage from obstruction of urine (obstructive nephropathy), no urination (dysuria), and/or elephantiasis of penis. 50-70% of people with long term infection have some kind of symptomatic urinary tract finding upon examination.
Earlier complications (moths-years): Chronic bacterial urinary tract infections are a frequent complication of the urinary tract dysfunction caused by the parasite. The bladder may also develop tubercles, polyps, ulcers, sandy patches, cystitis cystica, and/or leukoplakia that are visible upon endoscopic examination.
Late complication (years): Bladder cancer (squamous cell carcinoma) is associated with long-term urinary schistosomiasis, but it incidence is not known.
~24 hours to rash
~1-2 months to systemic symptoms
~3-6 months to 1+ year to for urinary symptoms
years for bladder cancer
The most common way to diagnose S. haematobium infection is by identification of ova in urine or in biopsies of the bladder, rectum, or vaginal wall. Urinalysis may also reveal blood in the urine. Infected people often have anemia, high eosinophil levels, and/or low platelets in their blood. Antibody tests are also diagnostic, although they are rarely done.
Management and Therapy
Urinary schistosomiasis usually responds well to drug therapy. Praziquantel (20 mg/kg taken orally 3 times over the course of 1 day) or metrifonate (10mg/kg 1x week every other week, with a total of 3 doses) are the drugs of choice. Corticosteroids may also be given with acute infection. While drug treatment is effective for killing parasites already in the body, it does not prevent new infections. Patients should be encouraged to develop prevention strategies as well as to have repeat treatments if necessary.
Long-term infection, with resulting inflammation, chronic fibrosis, calcification, and tissue damage, often leads to irreversible changes in the urinary tract. Corrective surgery may be needed in cases of obstruction or where strictures have formed. Kidney transplants have been effective in those with S. haematobium-related renal failure. Bladder cancer may be treated with chemotherapy, radiation, and/or surgery.
Public Health and Prevention Strategies
Education: Teaching people who live in endemic areas how to avoid contact with fresh water containing snails allows them to make choices that will help prevent infections. This is particularly important for those who frequently come into contact with water, such as women fetching water for household use, fishermen, or children who play in water. It is also important that there be a safe source of water available; education about transmission can’t do much good unless people have the means to put it into practice.
Molluscides: These chemicals are used to kill snails. The human health and ecosystem consequences of applying these poisons needs to be taken into consideration before they are used.
Biologic control of snails: Snail pathogens and snail predators may be introduced to reduce snail populations. It is important to consider the consequences to the ecosystem of the introduction of a new species.
Environmental modification to reduce snail habitat: Water projects can be designed to minimize or eliminate snail habitat.
Sanitation practices: Providing clean water for cooking, drinking, and bathing gives people an alternative to making contact with water that harbors cercariae. Providing modern sewage systems gives an alternative to urinating in places where eggs can be released to fresh water containing snails.
A topical drug: Niclosamide 1% lotion on skin before going in water reduces the chance of infection.
Vaccination: There is currently no vaccine available to prevent S. haematobium infection. Animal trials have been carried out with mixed results.
The Cambridge University Schistosomiasis Research Group Web Site
The WHO Tropical Disease Research (TDR) Schistosomiasis Site
The Schistosomiasis Control Initiative
The CDC Schistosomiasis page
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Created by Bliss Temple (blisstempleATsbcglobal.net)
for Human Biology 103