The main diagnostic test is identification of eggs in the patient's stool by microscopy, although this can sometimes be tricky since the eggs are indistinguishable from those of H. heterophyes, and the endemic areas of the two parasites have a large degree of overlap.
One characteristic that may be useful when distinguishing an M. yokogawai infection from an H. heterophyes infection is the ability of M. yokogawai adults to burrow into the intestinal mucosa, resulting in the signs listed on the "Syndrome" page. H. heterophyes is not known to invade the intestinal mucosa. Other than this difference, the life cycle and associated disease symptoms of the two parasites are very similar. Therefore, if a patient presents eggs in the stool and shows extra-intestinal signs, it is more likely that he or she is infected with M. yokogawai than H. heterophyes.
Alternatively, adult flukes may be identified after they are evacuated from the body. However, this can only be done after antihelminthic therapy is administered, because the drugs work to kill the adults and knock them loose from their strong grasp onto the intestinal mucosa. DNA analysis of heterophyids and scanning electron microscope observation of the topography of adult flukes may help in clarifying identification problems with adult heterophyids found in humans.
Another possibility is to identify the adult flukes at autopsy. Clearly, at this point it is too late to do anything to help the individual patient; however, this may be useful for public health reasons, such as to identify an outbreak or to get a picture of prevalence and incidence.
Recently, the specificity and sensitivity of serologic tests for all flukes have progressed to being helpful in diagnosis with a certain degree of confidence. The extent to which this will be useful for the diagnosis of metagonimiasis is still uncertain.
A Comprehensive Diagnosis
To make a sound diagnosis, one must:
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