Schistosoma japonicum

Background:

Human schistosomiasis is principally caused by 1 of 6 species of parasitic worms: Schistosoma haematobium, Schistosoma intercalatum, Schistosoma japonicum, Schistosoma malayensis, Schistosoma mansoni, and Schistosoma mekongi. Schistosomes are blood flukes and belong to the class Trematoda. Unlike other trematodes, schistosomes are elongated but become round in adaptation to residing in blood vessels of the genitourinary or gastrointestinal tracts. They require a vertebrate and an intermediate water-dwelling snail host to complete their life cycle. Geographic distribution and maintenance of human infection by schistosomes is dependent on and limited by the presence of a suitable snail host. Clinical features of human schistosomiasis depend on the species, developmental stage, and site of infection in the body. This can be summarized into 3 major syndromes: (1) cercarial dermatitis, (2) acute schistosomiasis or Katayama fever, and (3) chronic fibro-obstructive disease.

Pathophysiology:

Schistosomes are digenetic, ie, mature adult worms reproduce sexually in the definitive human host with asexual reproduction in the larval forms (common to all trematodes or flukes), and are also diecious, ie, adults are sexually distinct, and male and female genitals do not occur within the same individual (a characteristic that separates them from other flukes). The schistosome life cycle differs from that of other trematodes in that humans become infected by the penetration of cercariae through the skin rather than through oral ingestion. Human schistosomes can infect other vertebrates and provide an animal reservoir of infection, although it is of epidemiologic significance only for S japonicum and possibly S mekongi. Snails of the genera Biomphalaria, Bulinus, Neotricula, and Oncomelania are the principle intermediate hosts for S mansoni, S haematobium, S mekongi, and S japonicum, respectively.

Frequency In the US

: Frequency of infection has been estimated to exceed 400,000 persons and is principally because of immigrants from endemic areas. Transmission of the disease cannot occur in the United States because of lack of suitable snail hosts. Epidemics of acute schistosomiasis have been reported in Americans traveling in or returning from endemic areas. Cercarial dermatitis due to avian schistosomes is reported in the Great Lakes region of the United States.

· Internationally:

Human schistosomes currently infect more than 200 million people in 74 countries worldwide in the endemic areas of Africa, the Caribbean, Central America, South America, East Asia, and the Middle East. Prevalence is thought to be increasing. The most severely affected countries in Africa are Angola, Chad, Congo, Egypt, Ghana, Kenya, Madagascar, Malawi, Mali, Mozambique, Nigeria, Senegal, Sudan, Tanzania, Uganda, Zambia, and Zimbabwe. Yemen has the most infected people in the Middle East. Brazil is the most affected country in the Americas, with 25 million people living in endemic areas and an estimated 3 million infected.

S mansoni is found in 54 countries, including the Arabian peninsula (the most heavily infested population is in Yemen), Egypt, Libya, Mauritania, Somalia, Sudan, sub-Saharan Africa, Brazil, the Caribbean (except Antigua, Guadeloupe, Martinique, Montserrat, and St. Lucia), Suriname, and Venezuela.

S haematobium is endemic in 53 countries in the Middle East and most of the African continent, including the islands of Madagascar and Mauritius. The infection is unlikely to be of public health significance in Lebanon, Mauritius, Oman, Syria, Tunisia, and Turkey because transmission is low or nonexistent. A disputed and ill-defined focus exists in India and requires further confirmation.

S japonicum is endemic in China, the Sulawesi province of Indonesia, and the Philippines. China is the most affected country, with an estimated 900,000 people infected. The parasite has been eradicated from Japan since 1982.

S intercalatum has been reported from 10 countries in central and western Africa. S mekongi is confined to Cambodia and Laos where the borders run along the Mekong River. Neotricula snails have been reported in southern China, but no reports of S mekongi exist from these areas. S malayensis infects aboriginal people in a small jungle focus in Malaysia.

Mortality/Morbidity:

· Mortality from human schistosomiasis is related to complications of fibro-occlusive disease secondary to the immune stimulus of schistosome eggs and end organ damage.

· Morbidity of schistosomiasis correlates well with the worm burden as calculated by fecal and urinary egg counts. The disease significantly disrupts the nutritional status and growth from middle childhood to adolescence. Prevalence of infection in endemic communities demonstrates a negative binomial distribution, with most infected individuals having low worm burdens and only a small percentage with heavy infestations. A possible genetic link exists between human leukocyte antigen (HLA) antigens and the occurrence of end-stage liver disease in S japonicum and S mansoni infections.

Sex:

Both sexes are equally susceptible. Because of different local and cultural work and social practices, either sex may be more exposed to infection.

Age

: People of all ages are susceptible. Immune responses are heightened and more intense with secondary and subsequent exposures to schistosome cercariae. Surveys in endemic areas have demonstrated that severity and prevalence of infection takes several years to peak in children and that both decrease with age. Most manifestations of disease occur in the second decade of life with a relation to peak egg output. Evidence points to a partial and acquired immunity to schistosomes that is determined by specific antibodies and eosinophils, and the immunity targets immature adult worms.

History

: Symptoms of schistosomiasis include edema, pruritus, fevers, nausea, vomiting, lymphadenopathy, diarrhea/dysentery, abdominal pain, urticaria, malaise/general fatigue, gastrointestinal bleeding, seizures, myelopathy, cough with or without hemoptysis, palpitations, dyspnea, weight loss, urinary frequency, dysuria, hematuria, and suprapubic/perineal pain. Most infected people are asymptomatic or have only mild nonspecific symptoms. Only 5-10% of infected individuals develop severe clinical symptoms, which are usually associated with heavy infestations. Such symptoms are as follows:

· Edema and pruritus are symptoms of cercarial dermatitis (see Image 1) and are rarely observed in primary exposure.

· High fever or Katayama fever is described in heavy primary infections.

· Nausea is observed in acute schistosomiasis and at the onset of fevers.

· Vomiting may be experienced in acute schistosomiasis.

· Generalized lymphadenopathy is described as part of the Katayama fever syndrome.

· Profuse diarrhea or dysentery is associated with Katayama fever.

· Hematemesis or melena is the usual presenting symptom of chronic schistosomiasis due to S japonicum and S mansoni.

· Abdominal pain may be observed with acute and chronic schistosomiasis secondary to liver and splenic enlargement (see Image 2).

· Urticaria may occur with cercarial dermatitis or may be more extensive, affecting large body areas in acute schistosomiasis.

· Malaise, generalized fatigue, and weakness are sometimes observed with acute schistosomiasis.

· Focal epilepsy is a recognized complication of chronic S japonicum infection in East Asia (see Image 3).

· Spinal cord lesions due to S mansoni or S haematobium can cause a myelopathy.

· A dry cough (with or without hemoptysis) secondary to inflammation in the lungs can be experienced in acute schistosomiasis and is associated with migration of schistosomula through the lungs. Cough is also a symptom of cor pulmonale resulting from chronic pulmonary schistosomiasis. The subsequent pulmonary hypertension may lead to hemoptysis.

· Palpitations may be felt as a complication of cor pulmonale in chronic pulmonary schistosomiasis.

· Shortness of breath may be observed in both acute and chronic pulmonary schistosomiasis as a result of cor pulmonale in chronic cases. Acute pulmonary symptoms of cough and dyspnea may occur 3-6 weeks after exposure and occur without fever.

· Weight loss may be experienced as a consequence of vomiting and profuse diarrhea.

· Urinary frequency is regularly associated with acute S haematobium infection.

· Dysuria is a common feature of acute S haematobium infection.

· Terminal hematuria is also a regular presenting symptom for S haematobium infection or bilharzia.

· Referred suprapubic or perineal pain may be associated with S haematobium infection.

Physical:

· Urticaria: Urticaria may be observed in cases with cercarial dermatitis or acute schistosomiasis with Katayama fever.

· Papular dermatitis: Papular lesions may develop from cercarial dermatitis and remain for 5-7 days and up to 10 days after water exposure. However, this is more often observed with avian schistosomiasis.

· Generalized lymphadenopathy: General enlargement of lymph nodes may be detected in acute schistosomiasis with Katayama fever.

· Abnormal pulmonary sounds: Areas of moist crackles may be heard over both lung fields in acute schistosomiasis.

· Hepatosplenomegaly: Hepatosplenomegaly is often detected in acute and chronic schistosomiasis with S japonicum and S mansoni. Jaundice is a rare clinical finding.

· Purpura: Purpura resulting from thrombocytopenia is observed with hypersplenism.

· Hematuria: Hematuria with S haematobium typically progresses from microscopic to frank bloody urine as the bladder mucosa ulcerates.

· Myelitislike syndrome: A transverse myelitislike syndrome may be observed with S mansoni or S haematobium disease.

· Focal neurologic deficits: Focal neurologic deficits may be found due to cerebral granulomas in cases with seizures and chronic S japonicum infection.

· Right ventricular hypertrophy: Clinical signs of right ventricular hypertrophy are observed in cases with cor pulmonale and chronic schistosomiasis. This includes palpable sternal heave and increased and delayed pulmonary second heart sound.

· Female genital tract disease: Female genital tract disease (schistosomiasis) is defined as the presence of schistosome eggs and/or worms in the upper and/or lower genital tracts. Obvious vulval, vaginal, and/or cervical schistosomal disease has been reported from both endemic and nonendemic areas. Possible consequences include hypogonadism, retarded puberty, infertility (primary and secondary), ectopic and tubal pregnancy, tubal abortion, hemoperitoneum, anemia due to chronic blood loss, metaplasia, miscarriage and preterm delivery, carcinoma, increased risk for sexually transmitted diseases, destruction of the hymen and/or clitoris, and vesicovaginal fistula.

Causes:

· The diagnosis of schistosomiasis should be considered in any individuals who have resided in endemic areas with significant fresh water contact.

· Life cycle of human schistosomes

o Once cercariae have entered the body, they are called schistosomulae, which migrate through the tissues to invade blood vessels. The organisms are transported to the lungs and then to the liver, where they mature into an adult worm within 6 weeks before descending to their final positions in the venous circulation. Adults of S haematobium are mostly found in the venous plexi of the bladder, prostate, and uterus, while S intercalatum, S japonicum, S mansoni, and S mekongi are observed in the portal, inferior, and superior mesenteric veins. Maturity of female worms depends on the presence of a mature male because they form pairs, with the female lying enclosed within a groove formed by the male.

o Adult worms have a mean life span of 5-10 years, with females releasing 300-3000 eggs per day. Eggs are deposited in the terminal venules of the bladder (S haematobium), intestine, and rectum (S intercalatum, S japonicum, S mansoni, S mekongi), where they mature to contain a miracidium over the next 10 days. The miracidium releases proteolytic enzymes, which facilitate larval movement through the tissues into either the genitourinary tract or the gastrointestinal tract. Schistosome eggs contain spines (see Image 4) in positions characteristic of each species. Eggs are passed in human urine and excrement, and motile miracidia are released upon contact with fresh water. Miracidia then actively seek out snail hosts in which they develop first into mother and daughter sporocysts by asexual division and then into cercariae over 4-6 weeks. Cercariae then leave the snail and penetrate the human skin on contact with the assistance of their glandular secretions.

· Granuloma formation

o Not all schistosome eggs are excreted from the body, and up to 50% can embolize to other body areas, leading to a host immune reaction and granuloma formation. Granulomas begin to form with maturation of the miracidium at 6 days and are focal within 2 weeks.

o The most common sites are the liver for S intercalatum, S japonicum, and S mansoni and the bladder for S haematobium. Other areas less commonly affected include the lungs, central nervous system, and kidneys.

· Immune response

o The immune response to schistosomiasis is highly regulated and of the delayed hypersensitivity type, including TH1 and TH2 responses with local cytokine production. Granulomata are notable for the presence of eosinophils and lead to the development of widespread collagen deposition and scar tissue. This causes major lesions of chronic schistosomiasis with blood flow obstruction in affected tissues.

o Human skin appears to be a critical site in which the initial events of the host and parasite interaction occur and where the immune response is commenced. Induction and modulation of granuloma formation is under the control of clones of CD4 and CD8 T cells. Cytokines produced in response to the parasite, such as interleukin 7 in the skin and interferon-gamma in the liver, also seem to influence the development of schistosomal immunity.

· Specific schistosomal parasites

o S mekongi is thought to be the principle human schistosome pathogen of Cambodia and Laos. It has smaller eggs and a different intermediate snail host (Neotricula aperta) than S japonicum. Hepatosplenomegaly, portal hypertension, or both are the main clinical findings in infected persons.

o S intercalatum (localized to West and Central Africa) is of less epidemiologic importance in these regions than S haematobium and S mansoni. Observed symptoms include abdominal pain and bloody diarrhea.

· Cercarial dermatitis is more commonly featured in human infection with avian schistosomes, cercarial demise, and immediate hypersensitivity reactions occurring at skin invasion sites.

· Katayama fever occurs with the onset of egg production. The syndrome includes lymphadenopathy and diarrhea, with symptoms and signs similar to a serum sickness reaction. It is commonly associated with S japonicum infection and is probably due to the larger number of eggs released by this species. It is less common with S mansoni infestation and is rare with S haematobium. Onset of symptoms is 20-40 days after exposure, and temperatures may reach 105°F. Fevers spontaneously subside 2-10 weeks after onset.

· During infection by S haematobium, eggs are deposited in the mucosa and submucosa of the bladder and lower ureters. Granulomas are very cellular and form intraluminal polyploidal lesions that can lead to hydronephrosis. Lesions tend to necrotize, ulcerate, and bleed. With age, the lesions become acellular, fibrose, and calcify and are termed sandy patches. Calcification may lead to bladder deformation, ureteric obstruction, secondary infections, hydronephrosis, chronic pyelonephritis, and renal failure. Carcinoma of the bladder is a long-term sequela of chronic infection.

· In acute schistosomiasis, the enlarged liver and spleen are initially soft from passive venous congestion and granuloma cellular proliferation. In chronic disease, the liver and spleen remain large, nodular, and nontender. Stigmata of liver disease and cirrhosis such as ascites, spider nevi, peripheral edema, testicular atrophy, and feminization are usually absent. However, a small number of patients decompensate and show the above signs of chronic liver disease.

· Focal epilepsy is secondary to a localized brain granuloma or generalized encephalopathy. It constitutes 3% of the complications of chronic disease.

· In transverse myelitis, spinal cord lesions are the result of the retrograde flow of eggs and granuloma formation in either S mansoni or S haematobium infection.

· Pulmonary involvement (ie, cor pulmonale) is most commonly observed with S haematobium infection because ectopic egg deposition can occur more readily from the vesical plexus to the lungs via the inferior vena cava, bypassing the liver. Pulmonary complications with S japonicum and S mansoni are usually only observed after the development of a collateral circulation secondary to severe hepatosplenic disease.

Lab Studies:

· Egg detection

o Definitive diagnosis of schistosomiasis depends on detection of specific schistosome eggs excreted in stool and urine. This occurs from 5-13 weeks after infection and is determined by worm burden.

o Thick smears of feces, nucleopore filtration of urine, and formalin-ether concentration techniques for stool or urine are recommended.

o Collection of urine is usually recommended between noon and 2:00 pm, when excretion of ova is greatest.

o Multiple examinations may be required in light or chronic infections. If infections are active, schistosome eggs contain live and mature miracidia.

o Do not attempt detection of schistosome eggs in feces or urine until the incubation period of the infection passes, usually 3 months since last known fresh water contact.

· Complete blood cell (CBC) count and coagulation studies

o Eosinophilia is prominent in acute schistosomiasis.

o The CBC count may reveal thrombocytopenia, anemia, or prolonged prothrombin times in severe chronic schistosomiasis.

· Liver function tests

o Serum bilirubin and transaminase levels are usually within the reference range or only mildly elevated.

o Hyperglobulinemia may be evident in chronic schistosomiasis.

· Serologic tests

o Most commonly used assays are circumoral precipitation test (COPT), indirect fluorescent antibody test, indirect hemagglutination test, enzyme-linked immunosorbent assay (ELISA), and immunoblot.

o In the past, serologic tests for schistosomiasis were generally considered difficult to interpret because of cross-reactivity with other helminth infections, persistent elevated titers posttreatment, and a prolonged immunologic response of the host. Still, serology remained useful and important in the assessment of returning travelers from endemic areas, people with light infections, or individuals with cerebral and ectopic schistosomiasis in whom no eggs are demonstrable. The sensitivity of assays has improved in the last decade with the use of purified antigens (eg, egg antigen CEF6) rather than crude egg preparations (eg, S mansoni soluble egg antigen [SEA] or whole worms).

o Acute and chronic schistosomiasis may now be differentiated by the level of immunoglobulin G (IgG) and immunoglobulin M (IgM) directed against keyhole limpet hemocyanin (KLH) and immunoglobulin A (IgA) against SEA. Sensitivity of salivary fluids to detect IgG antibodies against SEA is comparable to serum and offers an alternative to blood collection.

o Detection of circulating anodic antigen (CAA) and circulating cathodic antigen (CCA) in serum and urine for the diagnosis of active schistosomal infections appears promising as a complimentary diagnostic tool with fecal/urine microscopy and antibody detection.

o Realize that serodiagnosis remains a more reliable diagnostic method than examination of feces and urine, particularly in the typically light infections acquired by returning travelers, and serodiagnosis should not be undertaken until the incubation period for the parasite passes.

· Hepatitis screen

o In some endemic areas, S japonicum, S mansoni, and viral hepatitis are the most common causes of chronic liver disease. The hepatitis B surface antigen carrier state has been noted to be 4 times higher in patients with schistosomiasis, the significance of which is uncertain. Different explanations have been proposed for the association of S mansoni with hepatitis B and include (1) impaired cell-mediated immunity, which reduces host resistance; (2) low socioeconomic conditions and educational levels, which increase the risk of exposure; and (3) repeated treatments in the past with intravenous or parenteral drugs or blood transfusions.

o Patients with coexisting hepatitis C virus (HCV) and hepatic schistosomiasis have more advanced liver disease, higher HCV titers, predominance of HCV genotype 4, higher histologic activity, and higher frequency of cirrhosis and hepatocellular carcinoma.

· Bacterial and viral culture of feces and urine: Routine cultures of feces and urine exclude common infections of the gastrointestinal and genitourinary tracts.

· Blood cultures for bacteria and viruses: Cultures of blood may reveal Salmonella species or other pathogens that explain persisting fevers.

Imaging Studies:

· Chest radiography: Abnormalities may be detected in both acute and chronic disease and include findings ranging from multiple nodules to diffuse interstitial infiltrates.

· Ultrasonography of the abdomen and pelvis

o Ultrasonographic examination is well established for the staging of schistosomiasis-related solid organ pathology and is especially useful to monitor its status after chemotherapy and/or cessation of exposure to schistosomes. Ultrasonographic changes in acute schistosomiasis are nonspecific and include hepatosplenomegaly and enlarged abdominal/perihilar lymph nodes.

o Ultrasonography for the assessment of urinary schistosomiasis has also been validated with concurrent cystography, pyelography, and computed tomographic (CT) scanning.

· Intravenous pyelography and voiding cystourethrography: These studies may reveal hydronephrosis, bladder calcification, and filling defects.

· Head, chest, abdominal, and spinal CT scanning and/or magnetic resonance imaging (MRI): These are useful in viewing granulomas of the brain, lungs, liver, or spinal cord, which, if present, are demonstrated as ring-enhancing lesions with contrast studies.

Other Tests:

· Skin testing: Crude antigenic extracts of schistosomes and eggs are used in skin testing. The sensitivity and specificity are poor and provide no indication of the intensity of infection.

Procedures:

· Rectal biopsy is useful in cases with light, chronic, or inactive infections. It is also of benefit in assessing the response to chemotherapy.

· Cystoscopy for S haematobium reveals bleeding mucosal points, ulcers, and polyps in acute cases and calcified patches in chronic disease.

· Endoscopy, bronchoscopy, and sigmoidoscopy/colonoscopy

o Eggs incite a granulomatous response in the small intestine and colon, causing inflammation and edema of the mucosa with papular lesions, small hemorrhages, and ulcers.

o A diffuse transmural fibrosis occurs with continued oviposition.

o Esophageal varices are often present in patients with hematemesis and/or melena.

o Bronchoscopy with transbronchial biopsy may be used to detect an eosinophilic pneumonitis if diffuse chest radiography findings are observed.

· Liver biopsy is not usually indicated unless schistosomal eggs are undetected or the diagnosis is unclear.

· Laparoscopy in patients with chronic S japonica may reveal yellowish small speckles sparsely clustered over the liver surface; these correspond to subcapsular calcified ova of S japonica. These areas correspond with abnormalities observed on ultrasonography, CT scanning, and histology.

Histologic Findings: Histology of liver biopsies is unique for schistosomiasis. Hepatic granulomas and thrombophlebitis destroy hepatic radicals of the portal system but are replaced by newly formed thinner blood vessels. This maintains normal blood flow but contributes to portal hypertension. A progressive increase in fibrous tissue eventually surrounds and compresses the hepatic venules. Portal vascular and fibrotic changes lead to a pipestem fibrosis appearance, which differs from other forms of cirrhotic liver disease. Hepatocyte damage and necrosis are rare but may be observed in severe cases of chronic schistosomiasis.

Staging: Identification of severity and staging of schistosomal disease is achieved by a combination of the investigations described above. This includes serology, abdominal and perihilar ultrasonography, body CT scanning, endoscopy, cystoscopy, laparoscopy, and histology.

· Acute schistosomal disease: Changes detected on ultrasonographic studies in acute schistosomiasis (Katayama fever) include focal liver hypoechogenicities that may reflect secondary abscess formation with bacterial superinfection, pleural effusions, and pericardial effusions. Enlarged lymph nodes may reveal an echodense center surrounded by an echopolar halo.

· Mild schistosomiasis: On laparoscopy, the liver surface is mostly smooth, although multiple whitish markings and irregular wide grooves are observed with more advanced disease.

· Chronic schistosomiasis: Ultrasonographic features are characteristic and include echogenic thickening of the walls of portal branches and of the portal vein frequently extending to the gall bladder and ligamenta.

· Moderate schistosomiasis: Ultrasonography reveals areas of high echogenicity, and CT scanning shows network patterns and lineal calcified spots.

· Severe schistosomiasis: Laparoscopy demonstrates a liver surface distorted with blocklike formations of variable size separated by grooved depressions, producing a turtle shell–like appearance. Ultrasonography reveals areas of high echogenicity, and CT scanning shows network patterns and lineal calcified spots.

Medical Care:

· Evaluation for acute symptoms may be performed on an outpatient basis.

· Inpatient assessment is required for gastrointestinal bleeding, bacterial sepsis, and chronic liver disease.

Surgical Care:

· Surgery may be necessary in severe or chronic schistosomiasis.

· Procedures include resection of bladder and colonic polyps, correction of obstructive uropathy, and partial colectomy for gastrointestinal polyposis and fibrosis.

· A distal splenorenal shunt can reverse portal hypertension.

· Cerebral cortical granulomas have been resected after failure of chemotherapy.

Consultations:

· Specialist in infectious diseases

· Gastroenterologist

· General surgeon

· Nephrologist

· Neurologist

· Neurosurgeon

· Urologist

Diet: No special diet is required for acute disease. Individuals with chronic liver disease may benefit from a high-protein, low-salt diet.

Activity: Limit activity for patients with acute gastrointestinal bleeding or severe thrombocytopenia.
Praziquantel (a pyrazinoquinolone) has become the main antischistosomal agent because it is effective against all 5 human pathogens and is well tolerated orally. Other oral compounds available are oxamniquine (a nitroquinolone that is no longer available in the United States) and metrifonate (an organophosphorous cholinesterase inhibitor), but these have limited parasite specificity.

Drug Category: Anthelmintics -- Parasite biochemical pathways are different from the human host, thus toxicity is directed to the parasite, egg, or larvae. Mechanism of action varies within the drug class. Antiparasitic actions may include the following:

1. Inhibition of microtubules causing irreversible block of glucose uptake

2. Tubulin polymerization inhibition

3. Depolarizing neuromuscular blockade

4. Cholinesterase inhibition

5. Increased cell membrane permeability, resulting in intracellular calcium loss

6. Vacuolization of the schistosome tegument

7. Increased cell membrane permeability to chloride ions via chloride channels alteration

Drug Name


Praziquantel (Biltricide) -- Animal studies show that praziquantel induces rapid contraction of schistosomes by a specific effect on the permeability of the cell membrane. The drug further causes vacuolization and disintegration of the schistosomal tegument. The effect is more marked on adult than on young worms. After PO administration, praziquantel is rapidly absorbed (80%). It is subject to a first pass effect and extensive metabolism.

Adult Dose


S haematobium, S intercalatum, S mansoni: 20 mg/kg body weight PO bid on day 1
S japonicum, S mekongi: 20 mg/kg PO tid on day 1

Pediatric Dose


Administer as in adults

Contraindications


Documented hypersensitivity; coexisting ocular cysticercosis because parasite destruction within the eye may cause irreparable damage

Interactions


Hydantoins may reduce serum praziquantel concentrations, possibly leading to treatment failures

Pregnancy


B - Usually safe but benefits must outweigh the risks.

Precautions


Because of more severe liver disease in S japonicum cases and the use of higher doses of praziquantel, plasma concentrations of drug usually are significantly raised; adverse effects usually are more pronounced; increased doses are also recommended in extrahepatic disease because of extensive metabolism of the drug after a PO dose; may cause abdominal discomfort, dizziness, drowsiness, fever, headache, and loose stools; symptoms usually resolve within 48 h



Drug Name


Oxamniquine (Vansil) -- No longer available in the United States. Only active against S mansoni. Effective in disintegrating schistosome tegument to which phagocytes attach and cause death.
Medication is well absorbed and metabolized extensively to inactive metabolites that are in turn excreted in urine. Plasma half-life is approximately 1-2.5 h, and a peak in drug concentrations is usually reached in 1-1.5 h following its PO administration.

Adult Dose


12-15 mg/kg PO once

Pediatric Dose


<30 kg: 20 mg/kg/d PO divided bid for 2-3 d; administer after meals or hs to minimize GI discomfort; increased dosage may be required in endemic areas
>30 kg: Administer as in adults

Contraindications


Documented hypersensitivity

Interactions


None reported

Pregnancy


C - Safety for use during pregnancy has not been established.

Precautions


May cause fever, dizziness, or hallucinations; use caution and closely monitor patients with history of seizures because they may experience epileptiform convulsions; EEG abnormalities may develop in patients with normal pretreatment recordings



Drug Name


Metrifonate (Bilarcil) -- Not available in the United States. An organophosphate derivative with antihelminthic and anticholinesterase activity. It is used as an alternative to praziquantel to treat S haematobium infection. The drug is well-absorbed from the GI tract, with peak levels occurring 1 h after administration. An alternative for S haematobium infections; not effective for S japonicum or S mansoni.

Adult Dose


10 mg/kg/dose PO q2wk for 3 doses; single doses of 10 mg/kg at intervals of 3, 6, and 12 mo have also been used

Pediatric Dose


Administer as in adults

Contraindications


Recent exposure to pesticides or administration of drugs that exacerbate cholinesterase inhibition (ie, neuromuscular blocking agents) because depression of blood cholinesterase is the most serious adverse effect (if exposure to the above agents is necessary, delay for 48 h after administration of metrifonate)

Interactions


Do not administer with drugs that inhibit cholinesterase (eg, neuromuscular blocking agents)

Pregnancy


B - Usually safe but benefits must outweigh the risks.

Precautions


May cause dizziness, headache, weakness, abdominal pain, vomiting, or diarrhea; depresses plasma cholinesterase immediately and erythrocyte cholinesterase gradually

Further Inpatient Care:

· Admit patients with chronic liver disease or if further episodes of gastrointestinal bleeding or bacterial sepsis occur.

Further Outpatient Care:

· Patients who have been treated with antischistosomal chemotherapy should be monitored for treatment effectiveness.

· Stool and urine samples should be continue to be examined for 1 year after therapy.

In/Out Patient Meds:

· Specific antibiotic therapy for secondary bacterial sepsis may be required.

· Steroids are recommended for severe acute schistosomiasis/Katayama fever and transverse myelitis.

Deterrence/Prevention:

· Avoid further fresh water contact in endemic areas if possible.

Complications:

· Possible complications include fibro-occlusive disease secondary to the immune stimulus of schistosome eggs, end organ damage, end-stage liver disease, and others described above (see Clinical).

· Possible complications of female genital schistosomiasis infection include hypogonadism, retarded puberty, infertility (primary and secondary), ectopic and tubal pregnancy, tubal abortion, hemoperitoneum, anemia due to chronic blood loss, metaplasia, miscarriage and preterm delivery, carcinoma, increased risk for sexually transmitted diseases, destruction of the hymen and/or clitoris, and vesicovaginal fistula.

· Persisting bacteremias with Salmonella species are observed in chronic infestations with S haematobium, S japonicum, and S mansoni.

· Immune complexes formed in response to schistosomal infection may be deposited on glomerular capillaries and renal basement membranes, leading to mesangioproliferative glomerulonephritis, nephrosclerosis, and renal failure. The severity of renal disease is related to the worm burden and extent and duration of hepatic fibrosis and collateral circulations.

Prognosis:

· Cure rates for oral medical treatment range from 70-100%, depending on the drug used.

Patient Education:

· Advise patients to avoid further fresh water contact in endemic areas if possible.

Medical/Legal Pitfalls:

· Incorrect diagnosis

o Initially missing the diagnosis of schistosomiasis is certainly possible because of the infrequency of cases, specifically in the Western hemisphere.

o Major complications in this scenario would be a late diagnosis resulting in a greater degree of individual patient morbidity and failure of timely epidemiologic notification of a case.

o In suspicious lesions, a travel history must be obtained and remembered to avoid this pitfall.

· Inappropriate treatment

o Although this scenario is much less likely than incorrect diagnosis, inappropriate treatment of schistosomiasis is a potential issue even if the diagnosis is made correctly.

o Consult an infectious-diseases specialist in all cases of suspected schistosomiasis outside of endemic nations to prevent inappropriate treatment from occurring.

· Reaction to treatment

o Take care to ascertain whether patients with schistosomiasis have ever had any antiparasitic drugs and if these were ever noted to cause problems.

o Failure to ascertain past reactions to treatment with a resultant adverse reaction to prescribed medication is a clear-cut legal pitfall that should be eliminated in practice by following the standards of care and obtaining an appropriate patient history.

Special Concerns:

· Asymptomatic infection

o Potentially serious asymptomatic infections are common in travelers returning to developed countries.

o A detailed fresh water exposure history, symptom history, and physical examination may add little in detecting cases.

o Stool microscopy, schistosomal serology, and the eosinophil count tend to be the best tools for evaluating suspected disease.

· Other parasitic infections

o Patients with schistosomiasis are, by default, at risk for other parasitic infections because areas that are endemic for schistosomiasis are also endemic for other parasites.

o After treatment, patients should be monitored for other symptomatology characteristic of parasitic infections.