Chủ Nhật, 13 tháng 10, 2013

Schistosomiasis – Symptoms, Causes, Treatment, Prevention, Life Cycle

Schistosomiasis – Symptoms, Causes, Treatment, Prevention, Life Cycle

What is Schistosomiasis?


This is an infection cause by a parasite and is obtained thru contact with water that is contaminated. In its infective phase it is referred to as a cercariae and it freely swims in open areas of water.








When the parasites come in contact with a human being, this parasite will burrow thru the skin where it matures in another stage referred to as schistosomula. It will then travel to the liver and lungs where it grows into the adult variety of the parasite.


This adult worm will then travel to its favored body part, dependent on its species. Locales in the human body include the rectum, bladder, liver, intestines, portal and venous blood systems, lungs and spleen.


Schistosomiasis isn’t normally seen in the United States but is widespread in subtropical and tropical areas around the world.


Schistosomiasis Symptoms


Some individuals can have skin irritation that is minor in the area where the cercariae first enters the skin, most individuals do not have any symptoms or signs until the eggs develop which take approximately 1 to 2 months after penetration into the skin. That is when the cough, chills, fever as well as muscle aches can begin. Most individuals have no symptoms or signs at this early phase of the infection. But there are a few individuals who develop acute schistosomiasis or Katayama fever during the 1 to 2 month period and these symptoms are similar to those of serum sickness and include the following:



  • Fever

  • Pain in the abdomen in the liver or spleen area

  • Cough

  • Fatigue

  • Diarrhea that is bloody or blood in the stools

  • Body aches

  • Headache

  • Rash


Most of the individuals who contract chronic schistosomiasis have symptoms that develop months or years after the first exposure to the parasite. The below list of symptoms are associated with chronic schistosomiasis. Individuals normally develop most of these symptoms:



  • Abdominal pain

  • Abdominal swelling known as ascites

  • Blood in the urine as well as pain when urinating

  • Bloody diarrhea or blood in the stools

  • Coughing and shortness of breath

  • Weakness

  • Chest pain and palpitations

  • Seizures

  • Paralysis

  • Mental status changes


Schistosomiasis Causes


This disease is caused by the parasites of the genus Schistosoma including:



  • S. mansoni

  • S. mekongi

  • S. intercalatum

  • S. hematobium

  • S. japonicum


The acute as well as chronic symptoms of this disease are caused mainly by the egg migration thru the tissue as well as the human immunity system response to these eggs. Chronic symptoms are due to eggs which have not shed from the body.


The presumed diagnosis of schistosomiasis is based on the individual’s history as well as the physical exam of the individual. It is very important to know if the individual has lived or visited areas of the world where this disease is endemic – especially if the individual has had skin exposure to streams or freshwater lakes in tropical areas. If the individual has a history of being in those areas and has the indications and symptoms cited above, a presumptive diagnosis can be made. But since the symptoms resemble those of serum sickness as well as other diseases, better diagnostic tests are normally required. Thick fecal smears as well as concentrated urine tests are usually used to determine if any eggs are found. If they are, the individual can then be definitively diagnosed with schistosomiasis.
Most eggs of each species are differently shaped so it is possible to determine which species is infecting the individual. Definitive diagnosis of species can be made by the examination of samples of biopsy as the eggs are able to be seen in the tissue that is infected.



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Blood tests and more recently polymerase chain reaction or PCR tests can aid in confirming the diagnosis, but results that are positive might only be an indication of past exposure. These tests are usually not positive until the individual has been infected for approximately 6 to 8 weeks due to the fact that it takes time for the eggs to develop and for the human immunity response to be stimulated. These PCR tests are available from the US Centers of Disease Control and Prevention.


There are many other procedures and tests that can be done to establish the diagnosis, especially if no eggs are discovered in the urine or feces, which is normally the status with chronic schistosomiasis. Cystoscopy, colonoscopy, endoscopy as well as biopsy of tissues from the liver are all used to obtain tissue samples. Additionally, chest x-rays, ultrasound, MRI, CT as well as echocardiograms can be used to determine how extensive the infection is in various systems of the organs. Many physicians will also do added blood tests such as CBC, liver function tests, renal function tests in order to determine if these organs are damaged from the parasites.


Complications that are related to this disease are believed to happen because of long-term exposure to the eggs which are highly antigenic. Some complications that can develop normally occur in individuals who are harboring many eggs as well as parasites, particularly when these parasites and eggs have traveled to other organs. Generally, complications normally will involve the central nervous system, gastrointestinal tract, cardiopulmonary system as well as urinary tracts along with the spleen and liver. Major complications that can occur include:



  • Hypertension

  • Bacterial infections

  • Seizures

  • Organ damage or destruction

  • Urinary obstruction

  • Death


Schistosomiasis Treatment


Currently, the treatment for this disease is the drug ispraziquantel brand name Biltricide, but it is only effective against the adult worms and does not kill the immature worms or bother the eggs.


Treatment with Biltricide is fairly simple and the dosage is based on the individual’s weight with 2 doses given each day. The drug causes a rapid disintegration of the worms which in turn allows the immune system to attack the parasite. This response of the immune system can cause very localized reactions that can increase the symptoms. Corticosteroids are used to reduce these reactions to the medication. Because of this response the use of Bilricide is limited.


Ocular schistosomiasis should never be treated with this medication; other organs with abundant parasitic infections can not function properly and require supportive care until the hyperimmune response gets better after the administration of the drug. Other medications including metrifonate, oxamniquine, trioxolanes as well as artemisinins have been used but with limited success. Currently new drugs are being developed.


Schistosomiasis Prevention


In order to prevent this disease, individuals need to avoid all contact of human skin with any sources of freshwater where schistosomiases as well as the snails that are part of the life cycle are endemic. But, this is not likely to happen in most developing areas of the world. Information of efforts to eliminate or decrease the snails from some freshwater sources using snail bait known as molluscicides have shown a decrease in the number of individuals infected, but this method often needs treatments to be repeated and some efforts have simply been halted causing very limited success.


Also, individuals who have been treated with all disease symptoms having been relieved can very easily be re-infected if exposed to the cercariae as the human immune response to this disease often is not believed to prevent re-infection. There is no vaccine for Schistosoma spp. commercially available but current research is continuing and it is believe that there will be a vaccine available in several years.


Life Cycle


This disease in humans is a part of the very complicated life cycle of the parasites involved. Humans enter the freshwater areas which contain the snails that grow Schistosoma sporocysts that then develop into free-swimming cercariae. The cercariae may attach to as well as penetrate the human skin, travel to the blood vessels and thru lung blood capillaries reach the vesicular or portal blood systems. During this migration or traveling, the cercariae change and develop from schistosomula into female and male parasitic adult worms. These worms have incorporated human proteins into their surface structures, so that most humans can produce little or no immunity response to the parasites. After the parasite mating happens in the vesicular or portal blood system, the production of eggs occurs. In contrast to the adult parasites, the eggs stimulate a very strong immunity response by the majority of humans. Some eggs travel thru the bladder or bowel tissue and are shed in urine or feces, while other eggs are swept into the portal blood and become lodged in other tissue sites. Eggs which are shed in feces or urine can reach maturity in freshwater and complete their life cycle by infecting snails that are susceptible. Additionally some adult worms can travel to other organs for instance, liver or eyes. This life cycle is complicated further by S. japonicum species that can also infect wild as well as domesticated animals, which can then become another host system. S. hematobium is the species that normally infects bladder tissues in humans, while the other species normally infect the tissues of the bowel.













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