Friday, February 17, 2012

Schistosoma haematobium: Worms of the Water


http://www.bio.davidson.edu/people/midorcas/GISclass/GISwebsites/grayson/Schistosomiasis.htm
Introduction: Schistosoma haematobium are trematodes, also known as blood flukes, which infect humans and cause the parasitic disease urinary schistosomiasis. Infection also heightens the risk of contracting urinary bladder and cervical cancer. High distribution of this parasite is found in most of Africa and the Middle East, India, and Turkey. It follows malaria as the third most devastating tropical disease in the world [1]. The parasite transmits itself from water, to the Bulinus snail, to a human definitive host. Transmission to a human host can also be directly through water [2]. Freshwater lakes and streams can usually be attributed to the source of the disease, and geographic spread continues because of water resource engineering issues in developing countries and the migration of infected populations [1].



Symbiont Description: The genus of this trematode is Schistosoma, and the species is haematobium. Adult males are 10 to 5 mm long, and contain deep grooves called gynecophoral canals in which the adult females lie. The males have many small tubercles on their dorsal surfaces and tiny spines on their suckers and inside their gynecophoral canals. Females are typically longer, averaging 16-22 mm in length, and are smoother and more slender than males. Adults are found in Venous plexuses around the urinary bladder. Released eggs cross the wall the bladder causing haematuria, which is the presence of red blood cells in urine, and fibrosis of the bladder [2]. Due to calcification of the bladder, there is increased pressure on the ureters and kidneys, known as hydronephrosis. This can also cause inflammation of the genitals. Research has shown a relationship between S. haematobium infection and the development of urinary bladder cancer [1].




Host Description: S. Haematobium has no vectors. The intermediate hosts are snails, from the genus bulinus. The infective cercariae are free-swimming. Humans are the definitive host [2].




Life Cycle: In fresh water, excreted eggs hatch form free-swimming miracidia if they come into contact with water and penetrate into the Bulinus snail. S. haematobium locate suitable snail hosts by using external stimuli such as light and snail-derived chemicals. The miracidium sheds it epithelium and develops into a mother sporocyst inside the snail. Within two weeks, the mother forms daughter sporocysts. In search of a human host, the cercariae are released back into the water, and use water turbulence and skin-derived chemicals to cycle from the top to the bottom of the water for three days. The cercariae come into contact with a human and enter through the epithelium [1]. The free-swimming infective cercariae burrow into human skin when it comes to contact with contaminated water. The cercariae enter the blood stream of the host, and travel to the liver to mature into adult flukes. The flukes then migrate to the bladder for copulation. The female fluke can lay up to 3,000 eggs per day which migrate to the lumen of the urinary bladder and ureters of the host. Each ovum contains a ciliated larva, known as miracidium, which secretes proteolytic enzymes that help the eggs migrate into the lumen of the bladder. Through urination, the eggs are eliminated from the host into
the water supply [1].

                                http://www.dpd.cdc.gov/dpdx/HTML/Schistosomiasis.htm




Ecology: Endemic in 53 countries in the Middle East and Africa, S. haematobium is largely found in rural areas, but urban urinary schistosomiasis is increasing in many countries. This is due to the man-made reservoirs and irrigation systems, as well as population growth and migration. It has been estimated that over 700 million people in 74 countries are exposed to the risk of schistosomal infection, and about 200 million were estimated to be infected in 2003 [3]. Urinary schistosomiasis is often chronic and can cause pain, secondary infections, kidney damage, and even cancer [2]. Distribution of schistosomiasis depends on variations in snail populations and human-water contact behavior. Prevalence and intensities of infection generally show a peak at the ages of 5-15 years and a decrease in adults. Main human risk groups are school-age children, specific occupational groups (fisherman, irrigation workers, and farmers), and women and other groups using infested water for domestic purposes [3]. S. haematobium causes the disease urinary shistosomiasis, and there has also been a positive correlation between S. haematobium infection and urinary bladder cancer. Although, paratistic infection is not the sole cause, S. haematobium increases the risk of females being diagnosed with cervical cancer and other malignancies of the female genitalia [1].




An example of a cercarial encounter strategy: S. haematobium uses its’ rhythm of cercarial emergence to synchronize its behaviors with the most efficient time frame that favors host encounter. These patterns are linked to the regular alternation of day and night. Cercarial emergence rhythms correspond to light levels in either the savannah or the forest. It is also selected for the best time where cercarial emergence occurs and when human hosts most often visit aquatic environments. For example, cercarial emergence is more likely to be during the middle of the day because that is the most frequent time of the day for villagers to be the water for agriculture and domestic work, swimming, etc [4].

                                                       http://youtu.be/u05x3fSGKC0




1. Laboratory Identification of Parasites of Public Health Concern [Internet]. Atlanta: Centers for
           Disease Control and Prevention; [cited 2012 Feb 13].Available from:
           http://dpd.cdc.gov/dpdx/html/Scistosomiasis.htm



2. Schistosoma haematobium (blood flukes) [Internet]. California: Stanford University; c2004 [cited
           2012 Feb 17]. Available from://www.stanford.edu /class/humanbio103/ParaSites2004/Schisto/
           website.html



3. Dalton P. R., Pole D. 1978. Water-contact patterns in relation to Schistosoma haematobium       
           infection. Bulletin of the World Health Organization. 56(3): 417- 426.

4. Combes C. 2001. Parasitism: The Ecology and Evolution of Intimate Interactions. Chicago:
          University of Chicago Press. 629 p.

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