Parasites are organisms which depend on another organism (host) for nutrition and for survival. They exist either as a unicellular organism, as round or flat worms, as hookworms or as flukes such as the blood flukes, liver flukes and the intestinal flukes.

Schistosomes, sometimes referred to as "bilharziasis ", are the blood flukes. These are parasitic organisms which spread and infest the blood vessels of their host. Adult schistosomes exist as separate sexes but are attached to each other. There are three known schistosomes, each organism is distinguished from each other by the characteristic of its ova, the specific blood vessel that it clings to and the diseases associated to it. In general, the symptoms of Schistosomiasis is abdominal distention, jaundice may be noted, and mild to severe abdominal pain. Blood tests would primarily reveal Eosinophilia. Schistosomiasis can be associated with serious morbidity and mortality. Chronic complications are generally seen in those with a high parasite load, which usually occurs in individuals who live in endemic areas and have recurrent exposure.

The Schistosoma mansoni dwells in the Inferior Mesenteric Vein which is the major venous drainage of the large intestine. It causes the so called intestinal schistosomiasis. Its ovum has a distinctively large lateral spine. The intermediate host of Schistosoma mansoni is the Biomphalaria snail. Schistosomiasis due to this parasite is manifested as portal hypertension which is a presinusoidal, noncirrhotic portal hypertension, rectal polyps and bloody mucoid stools due to granulomatous formation in the site and damages the vein. In some cases of schistosomiasis caused by mansoni, "pipe-stem fibrosis" of the liver is noted with mild jaundice.

Schistosoma haematobium is the urinary schistosoma. It dwells on the large veins in the Urinary bladder. Its ovum has a terminal spine and can be seen in urinalysis. Schistosoma haematobium is associated with Urinary Bladder Carcinoma. Symptoms are bloody urination, painful urination and unexplained weight loss.

Schistosoma japonicum is the culprit for the most number of cases of schistosomiasis in the Philippines. it is a habitat of the Superior Mesenteric Vein of the small intestine. Its ovum has a very small lateral spine. The intermediate host of this parasite is the Oncomalania quadrasii snail usually found in the Philippine provinces of Leyte, Samar and Cagayan Valley and in other Asian countries. Another important disease associated with this parasite is the Katayama Fever, manifested as a high grade fever and abdominal pain, severity includes hypotension and shock. It is due to an overwhelming production of schistosoma eggs at a rate more than 3000 eggs per day which cause traumatic reactions to the body.

Schistosomiasis is diagnosed by a detailed clinical history and a complete physical examination. Routine blood or urine tests may show nonspecific abnormalities. The diagnosis of schistosomiasis can be confirmed by microscopy with egg identification, by serology, or by consistent radiological findings in the appropriate clinical scenario. A routine full blood count may show eosinophilia in infected patients. The degree of eosinophilia relates to the stage, intensity, and duration of infection, as well as to genetic determinants of the host. Most studies suggest between one- and two-thirds of infected patients have a peripheral eosinophilia, which is frequently most marked early in the course of infection. Eosinophilia is also a frequent finding in patients with Katayama fever, a hypersensitivity reaction seen with acute infection. Abdominal ultrasound can also be done. It is said that prevention is better than cure. Schistosomiasis is prevented by wearing long boots and protective vests for those working in rice fields or fresh water swamps in the areas where Schistosomiasis is prevalent. Another way of prevention is avoid eating raw snail delicacies in areas known to have cases of schistosomiasis. The drug of choice for this disease is Praziquantel. It induces ultrastructural changes in the teguments of adult worms, resulting in increased permeability to calcium ions. Calcium ions accumulate in the parasite cytosol, leading to muscular contractions and ultimate paralysis of adult worms. After damaging the tegument membrane, it also exposes parasite antigens to host immune responses. These effects lead to dislodgement of worms from their intestinal sites and subsequent expulsion by peristalsis. In some countries like the Philippines, Praziquantel is provided free by the City health centers to those who are diagnosed with schistosomiasis.

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