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Laboratory Diagnosis

The accurate diagnosis of schistosomiasis depends on the identification of the eggs and on the serological tests. Eggs can be recovered from urine or stools. The excretion of eggs occurs with a circadian rhythm and peaks about noon; urine should therefore be collected between 10:00 a.m. and 2:00 p.m. (1000-1400 hours). Quantitative urine filtration is the most reliable method. For stool, cellophane fecal thick smears (Kato) are best. If not readily identified by urine or stool tests, eggs can be reliably found by rectal biopsy; three or four specimens should be taken from the region of the first rectal valve, immediately pressed between two slides, and examined under the microscope. This method takes a few minutes, permits accurate identification of the schistosome species, and obviates the need for time-wasting stool or urine examinations. There is little or no risk attached to it. (Local bleeding may be prevented by direct pressure or, very occasionally, by the application of 10% silver nitrate to the bleeding area. Only very rarely is electrocauterization required.) There are no contraindications except possibly in very young children, very elderly patients, and those on anticoagulant therapy, or who have a bleeding tendency. It has been suggested that rectal biopsy should be avoided during an acute febrile state, but if there is a strong possibility of schistosomiasis, then the establishment of the diagnosis is of more importance than this theoretical contraindication.

Miracidial hatching is often successful, but takes time and experience.

Serological and Skin Tests

The serological diagnosis of schistosomiasis is generally accurate, but can be falsely negative, particularly in patients with longstanding infections. There are numerous tests from which to choose. The most reliable are indirect immunofluorescence (IIF) (a titer of 1:16 is diagnostic) and an enzyme-linked immunosorbent assay (ELISA) ( a titer of 1:32 is diagnostic). A fast ELISA, using purified adult worm microsomal antigen, is excellent and may be the best test. Also available are the circumoral precipitin test (COPT), the cercaria-Hullen reaction (CHR), and total and fractionated soluble adult worm antigens (SWAP). While many of these tests are useful, they should not be used as the sole basis for treatment because avian schistosomes may produce confusing results. These tests are more important as an indication for further attempts to find the ova and this is where rectal biopsy may be particularly useful, because 20% of people infected with schistosomes will not pass ova, making positive identification in the stool very difficult in the early stages. Furthermore, confusion can be caused by finding nonviable eggs of S. mattheei or other nonhuman (avian or cattle) schistosomes, especially in North America and other temperate climates.

Of course, hematuria alone is a valid indication of infection in the right population. Microhematuria can be detected by reagent strips (dip sticks), which will recognize blood and protein. They are nonspecific but particularly useful in children. For more accuracy, a low-intensity infection may require a 24-hour urine collection. The great difficulty in many populations is to distinguish an active from a previous infection, particularly after treatment, and for this situation, the circulating schistosome antigen is probably the most reliable test.

The skin tests are accurate at the time of the first exposure, but do not indicate which cercaria are present. This is a histamine release phenomenon, triggering mast cells. If there is a delayed reaction it usually indicates previous infection, because skin tests remain positive in about 50% of patients after treatment, even when the infection is eradicated. False-positive tests may indicate infection with a nonpathogenic species and false-negative results occur, most frequently in children. There is a specific skin test for S. mansoni , but patients with trichinosis also react strongly to the S. mansoni antigen (the reverse does not happen).

Used together, the skin and serological tests are a very accurate index of either active or previous schistosomal infection. They do not indicate the extent, nor necessarily the activity, but if these tests are negative the likelihood of the current illness being due to schistosomiasis is slight.

Bladder Calcification

There are only a few causes of bladder wall calcification, none of which resemble the eggshell calcification which is seen radiographically in S. haematobium infections. This calcification is due to large numbers of calcified eggs in the bladder wall. However, while indicating that the patient has had schistosomiasis, this does not show whether there is still active infection. The quantity of calcified or other dead eggs found in the urine is related to the number of accompanying embryonated eggs rather than the stage of the disease. But, unknown to the patient (or physician), calcified eggs are discharged in the urine and the number in the bladder wall slowly decreases over the years when there is no reinfection; thus, there may be less calcification in the bladders of elderly people than in their younger neighbors.

Bladder calcification, then, is a strong indication that the patient has, at some time, had schistosomiasis. It provides no further information.

Kidneys

Kidney damage, with marked hydronephrosis, may be the end result of urinary tract outflow obstruction, but the renal parenchyma may be damaged by schistosomiasis even if the lower urinary tract is relatively normal. This can occur in both S. haematobium and S. mansoni infections, and renal biopsy is useful to establish the cause. There is a high frequency of renal disease in hepatosplenic S. mansoni infections, particularly the nephrotic syndrome. An immuno-complex glomerulonephritis is a possible explanation because the many vascular collaterals allow the antigen-immune complexes to bypass the phagocytes in the liver and settle in the kidney or elsewhere. Electron-dense deposits in basement membranes of the glomeruli, together with laminated bodies near the mesangial cells, have been found in S. mansoni infections. The severity of renal damage can be correlated with the load of S. mansoni, but, as so often happens with schistosomiasis, there is geographical variation in both the severity and the rate of progress. A segmental but locally diffuse sclerosing glomerulonephritis occurs in severe cases; occasionally there may be glomerular capillary aneurysms.

Hematological Status

In some parts of the world, in Asia particularly, but also in Egypt and elsewhere, blood loss from hematuria and/or colonic disease can be sufficient to cause anemia. The fluke does not consume much blood but hematuria can cause blood loss equivalent to 37 mg of iron daily, and colonic-rectal loss can account for 26 mg. However, when a patient is anemic this may not be the sole reason and other causes should be sought. Eosinophilia may be high but is not invariable, especially during chronic infections. A routine check of the blood urea is advisable to exclude renal damage.

Equivocal Laboratory Tests

There are occasions when positive identification of the parasite is not possible and yet immediate treatment may be indicated:

1. When there is a positive history of exposure, together with a strong clinical indication.

2. When more than one of the following are also present; eosinophilia, a history of hematuria, or of the Katayama syndrome or of freshwater swimmer's itch (saltwater swimmer's itch, also known as seabather's eruption, is not related to schistosomiasis but to larvae of the species phylum Cnidaria). Equally important is a positive radiographic examination or ultrasound scan.

3. When a positive serology is found and yet parasites may not be identified, as in the invasive stage (the Katayama syndrome) or in the early stages of a light or subclinical infection. Rarely, this can also occur when there has been a predominantly unisexual infection or when infection occurs at an ectopic site.

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Copyright: Palmer and Reeder