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In parasitology, while examining the stool sample, one important thing you do is test whether the eggs float on water or sink. We never did that since our coursework limits itself to microscopic examination, but we still are expected to remember examples of eggs under each category.

So my question is, what makes flotation such an important criterion? And how is it done? How do you know if the eggs are floating or not? Does it depend on from where you take the sample (as in, add the stool sample to water and then either take from top or bottom)?

Another interesting thing we are expected to remember is that the fertilized eggs of Ascaris float while unfertilized ones sink. What's the basic funda behind all this? What's in fertilization that changes the buoyancy?

Polisetty
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  • Are you sure you've got the test right? From a quick search online it sounds like the test for fertilised *Ascaris* eggs is done with zinc sulphide solution rather than water, so the density is different. Furthermore, it sounds like with zinc sulphide it's the unfertilised eggs which sink, not the fertilised ones (http://www.med-chem.com/para-site.php?url=org/ascalumb). – arboviral Jun 23 '16 at 08:59
  • @arboviral sorry! Ya, its the other way round. Edited :) – Polisetty Jun 23 '16 at 16:03

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Okay, I found the last part of your question the most interesting and can't give a full answer to that, but I just noticed I can answer the rest so here goes.

How it works (the simple version) It works on the principles of buoyancy. The eggs and oocysts are less dense (lighter) than the fecal flotation solution and, consequently, they float on the very surface of it. The feces themselves are more dense (heavier) than the fecal float solution and, as a result, they sink to the bottom of the solution, out of the way. Commonly used flotation solutions include sodium nitrate, Sheather's sugar solution, zinc sulfate solution, sodium chloride solution and potassium iodide solution.

The eggs of some important parasites (for example. Schistosoma species and Fasciola hepatica) are more readily diagnosed by fecal sedimentation (sediment) tests because their eggs tend to sink in flotation media.

What makes it so important? Some parasite eggs float and some don't. Faecal flotation is a simple, rapid and helpful diagnostic that narrows down the agent causing the clinical signs in the animal.

How it's done Use a "Fecalyser", a commonly used test kit for faecal floats. The best stool samples to get are those that are fresh - straight out of the animal within the last 30 minutes where possible. Detailed walkthrough of the protocol here and video tutorial here.

Why is there a difference in the buoyancy of fertilised versus unfertilised eggs? No idea, sorry. Similar effects are seen in some insect eggs - Stomoxys eggs which sink mostly hatch, while floaters don't hatch. The fact that Anopheles eggs have floats and Culex lay egg rafts implies to me that their viable eggs would otherwise tend to sink too (this could be to do with oxygenation levels being higher at the surface, but I'm speculating here). During embryonation, a serosal cuticle forms which makes the eggs highly desiccation-resistant; non-viable eggs dry out. However, according to some sources online it sounds like for parasite eggs it's the unfertilised eggs which sink, not the fertilised ones.

arboviral
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  • Thanks! Now I get it! This method can be used only for eggs that float, eggs that don't, simply sink along with the debris and are not picked up on the slide/cover slip, so we just can't see them! (At least using this method) I had thought that a sample is taken from both the meniscus and the sunken part, but I guess only the meniscus is used. Thanks :) – Polisetty Jun 23 '16 at 20:03
  • Glad to help! There is something called a sedimentation test or sediment test for eggs that sink; I've added a couple of lines to the answer mentioning them. – arboviral Jun 24 '16 at 10:45
  • @Polisetty Any chance you could accept this if you're happy with it? :) – arboviral Jun 24 '16 at 13:08
  • Waiting for the why part.. – Polisetty Jun 24 '16 at 14:23
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    I'm at a meeting of (mostly) parasitologists this week, so I'm trying to get an answer to this for you... – arboviral Jun 28 '16 at 06:25
  • Whoa! That'd be great!! :D – Polisetty Jun 28 '16 at 14:32
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    Nobody knew. Although someone did point out that potentially a more interesting question is why Ascaris would lay *any* unfertilized eggs... – arboviral Jul 05 '16 at 13:10
  • Whoa! That's an altogether new and amazing question! Sometimes it amazes me on how we fail to question such stuff, which is right in front of our eyes! – Polisetty Jul 06 '16 at 18:06
  • Agreed! This has turned out to be a particularly interesting question, by the way - thanks for asking it. – arboviral Jul 07 '16 at 08:12