Sunday, April 13, 2008

Answer to Case 17

From the answers I received, I think I made this harder than I intended to. While not 'classic', this egg is most diagnostic for human hookworm (either Necator americanus or Ancylostoma duodenale, since they cannot be distinguished morphologically). The germinal mass is usually in the 4-8 cell stage (early cleavage stage), although in this example, it is in an advanced stage and it is harder to make out individual cells. Other distinguishing features include a thin hyaline shell which is rounded at both ends and a germinal mass that mostly fills the egg. Eggs measure approximately 60 x 40 micrometers.

Without being given the size, other eggs with a near identical appearances include:
1. Trichostrongylus spp. - usually the germinal mass is in a more advanced stage than hookworm eggs and does not fill the egg. The egg measures approximately 80 x 40 micrometers, and tapers at one end. It has been reported from humans worldwide.

2. Oesophagostomum spp. - These eggs are indistinguishable from human hookworm eggs, although it is common to find an early larva (L1) within the egg. The most common species, O. bifurcum, is found in Africa in regions where Necator americanus is also highly prevalent. Culture is required to definitely distinguish to two.

3. Ternidens deminutus - This rare nematode is primarily found in South Africa. Eggs are also identical to hookworm eggs, but are slightly larger at 85 x 50 micrometers.

4. Strongyloides stercoralis - I mention this one just for completeness sake, since eggs are not usually seen in feces, and light microscopic diagnosis is based on finding of classic larval forms.

Other diagnostic considerations include vegetable matter and decorticated Ascaris eggs. The former usually have rigid rectangular walls, while the latter have a much thicker shell than hookworm eggs.

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