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Prospect for Schistosome Vaccine - Introduction

 

Introduction

Schistosomiasis, also known as bilharzia (bill-HAR-zi-a), is a disease caused by parasitic worms. Infection with Schistosoma mansoni, S. haematobium, and S. japonicum causes illness in humans. According to the center for disease control (CDC), although schistosomiasis is not found in the United States, 200 million people are infected worldwide. Especially the developing countries are hit the hardest by this parasite.

History of Discovery

Schistosomiasis has been recognized since the time of the Egyptian pharaohs.  Theodor Bilharz, a young German pathologist, from whom the disease took its original name, Bilharziasis, eventually discovered the worms responsible for the disease in 1851. In 1910, Sir Armand Ruffer described eggs of this helminth in both Egyptian and Chinese mummies. 

Theodore Maximillian Bilbarz is the discoverer of human schistosomeTheodore Maximillian
Bilharz
is the discoverer of human schistosome
(http://www.path.cam.ac.uk/~schisto/History/History.html)

 

Synonyms: Schistosomiasis, Swimmer’s Itch, Katayama Fever, Snail Fever, Bilhazaria
Classification and Taxonomy: Kingdom: Animalia, Phylum: Platyhelminthes, Class:Trematoda, Subclass: Digenea, Order: Strigeiformes, Family:Schistosomatidae, Genus: Schistosoma, Species: haematobium

Currently there are about 4 flat worms that are though to cause schistosomiasis:  (Source: WHO). Schistosoma mansoni causes intestinal schistosomiasis and is prevalent in 52 countries and territories of Africa, Caribbean, the Eastern Mediterranean and South America. Schistosoma japonicum/Schistosoma mekongi cause intestinal schistosomiasis and are prevalent in 7 African countries and the Pacific region. Schistosoma intercalatum is found in ten African countries. Schistosoma haematobium causes urinary schistosomiasis and affects 54 countries in Africa and the Eastern Mediterranean. 

 

Clinical Presentation/Diagnostic tests

After about 24 hrs after an individual is infected, there is a “swimmer’s itch”. Within days after becoming infected, you may develop a rash, fever, chills, cough, and muscle aches can begin within 1-2 months of infection. Most people have no symptoms at this early phase of infection. Eggs travel to the liver or pass into the intestine or bladder. Rarely, eggs are found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation. For people who are repeatedly infected for many years, the parasite can damage the liver, intestines, lungs, and bladder. Symptoms of schistosomiasis are caused by the body's reaction to the eggs produced by worms, not by the worms themselves. The diagnosis is also done by looking at your stool or blood sample. (CDC)
Current Treatment

  • Praziquantel - effective in the treatment of all forms of schistosomiasis, with virtually no side effects
  • Oxamniquine - used exclusively to treat intestinal schistosomiasis in Africa and South America
  • Metrifonate - effective for the treatment of urinary schistosomiasis

(http://www.tulane.edu/~dmsander/WWW/224/Schisto.html)

 

Transmission/Vector/Reservoir/Epidemiology

Transmission of schistosomiasis is through contacts with a pathogenic form of a snail called cercarie. In agricultural areas or where irrigation is common, human feces enters the ponds or freshwater carrying parasitic eggs called miracidium. This parasite enters snails and develops into highly complex pathogenic form of parasite called cercariae. The cercariae penetrate through the skin and enter the blood vessels where they continue to develop further. No reservoir is known but zoonotic animals exist while they don’t play crucial roles. The infective cercariae are free-swimming and they don’t have vectors but there are different species of intermediate host. Schistosomiasis is reported in about 74 countries and the World Health Organization (WHO), claims that about 80% of those who are infected are in sub-Saharan Africa.

 

Morphology

TDR Image Library
                                        
   Schistosoma haematobium: focal lesions caused by eggs penetrating intestinal wall (left)  (Source: TDR)
     Cercaria of schistosome developing in intermediate host the snail (right)
       (http://www.klinikum.uni-heidelberg.de/index.php?id=7060)
   

Schistosoma haematobium:cercaria (left)
Schistosoma haematobium:miracidium (right)
Source: TDR/WHO


Schistosome worms: the male worm carries the female in the “gynecophoric canal
(http://www.klinikum.uni-heidelberg.de/index.php?id=7060)

LIFE CYLELife cycle of schistosome

SOURCE: CDC

The free swimming cercariae enters the snail where it develops and enters the human skin, then the eggs are release after the worms go though intricate biological development and the eggs are released into the fresh water where they again enter the snail and develop further.