AMEBIASIS AND COMPARISON OF MICROSCOPY: DISCUSSION

24 Dec
2009

misdiagnosis

Since microscopic examination cannot distinguish with certainty between E. dispar and E. histolytica parasites, the amoebal infection detected with the microscope wrongly overestimated the number of people infected with E. histolytica. Even if the microscope test is positive for E. histolytica, it is still highly likely (1-PPV=95%) that the patient does not have E. histolytica infection. However, when microscopic examination revealed negative results, then E. histolytica infection is very unlikely.

The ELISA technique is considered an ideal gold standard with which microscopy technique is compared. Although the ELISA technique cannot perfectly distinguish between E. histolytica and E. dispar, it has excellent sensitivity, specificity, PPV, and NPV On the other hand, microscopic examination gives more false positives, and has low sensitivity (50%) and exceedingly low PPV (3.6%). canadian cialis

The observed prevalence of 0.8% and 7.4% for E. histolytica and E. dispar, respectively, for amebiasis in Kilimanjaro confirms the postulated idea by Clark that the pathogenic amoeba (E. histolytica) is only about 10% of all amoebal infections. Although metronidazole is effective in treating E. histolytica infection, it has side effects and is expensive. In addition, there is a possibility of developing resistance to the drug through widespread and unnecessary use. The use of metronidazole is intended only for elimination of tissue-invasive organisms and is not effective against intestinal lumen infections. Metronidazole was given for the treatment of nonpathogenic E. dispar, which also delayed the treatment for the actual cause of illness. The cost of this unnecessary treatment was calculated in one of the Kibosho’s hospitals for passive cases. The total number of E. dispar infected patients was 3,539 over the period of three years (1,416 patients per year), and the cost of metronidazole (twice a day for seven days) in U.S. dollars was $16.19 per patient. Therefore, the cost of treating E. dispar infection at Kibosho hospital was $22,918.60 per year, which was considered very expensive in the country where the majority of people earn less than $1 U.S. per day. The question is, would it be cost-effective to switch to the ELISA test? The answer is not clear. However, it will reduce unnecessary treatment. The experts on amebiasis supported development and use of alternative diagnostic methods for both clinical and epidemiological studies—most of which have been rendered absolute by differentiation of the two similar species. The expert committee further recommended that, ideally, E. histolytica should be specifically identified and treated.

The cases reported for amebiasis are usually a mixture of pathogenic E. histolytica and nonpathogenic E. dispar, resulting from reliance of conventional microscopic diagnostic methods. Simple, inexpensive diagnostic tests for distinguishing E. histolytica infections from those with E. dispar are needed to reduce unnecessary drug prescription and to allow for collection of accurate prevalence and incidence data. The prevalence of E. histolytica infection is often quoted at 10% of the world population or 500 million infections. This is obviously misleading, if 90% of these infections are due to E. dispar.

Why is E. dispar important despite the fact that it does not cause disease? Part of the answer lies in the realm of diagnosis. In endemic areas, E. dispar is by far the more prevalent species by a ratio of perhaps as much as 10: 1. In Europe and North America, where invasive amebiasis is rare, almost all infections previously ascribed to E. histolytica were in fact due to E. dispar. The significance of this is that in most cases, there is no need for antiparasitic agents to be administered. Indeed, the new WHO recommendation specifically states that drug treatment is not recommended unless E. histolytica is unequivocally shown to be present or if there is strong reason to suspect that the patient may be carrying E. histolytica and not E. dispar.
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