Fecal Flotation techniques

 

The principle of fecal flotation is based on the fact that there are differences in the specific gravity of parasite eggs, cysts, and larva and that of fecal debris. The feces is mixed in a solution of about 1.2 specific gravity. Many ova are less than 1.2 specific gravity and will float to the top of the solution where they are collected. Most fecal debris is greater than 1.2 specific gravity and will sink to the bottom of the tube. This test concentrates the ova for microscopic view but remember it is only qualitative. Parasite ova numbers found on this test are no indication of the actual worm burden the animal carries, since some worm species are more prolific ova producers than others. Some ova do not float because they are large and heavy, or dense, and will sink with the fecal debris. Ova that may be missed with this technique are Physaloptera (stomach worm), many flukes, and some tapeworms.

 

  Fecal flotation diagram

 

Fecal flotation solutions vary with their specific gravity depending on their formulation but most are used in the range of 1.2 to 1.25 (actually 1.18 to 1.3 to be exact). Although it is possible to prepare solutions with a higher specific gravity so that all ova types float, there is osmotic distortion which makes it more difficult to identify the organisms and fecal debris will then float also. Specific gravity should be verified with a hydrometer even if you are using a kit formulation.

 

  The hydrometer is used to verify specific gravity of a solution

 

 

Most fecal flotations are either some kind of salt or sugar solution and include Sheather’s sugar solution, zinc sulfate, sodium nitrate, sodiuim chloride, and magnesium sulfate. Sheather's sugar solution does not crystallize or distort ova although there is a sticky residue to deal with, and it is considered more useful in centrifugation techniques, especially for the recovery of Cryptosporidium oocysts. Zinc sulfate also tends not to distort ova and it is superior for flotation of protozoal cysts, especially Giardia, as well as larvae. Sodium nitrate, sodium chloride, and magnesium sulfate effectively float common ova and protozoal cysts, but cause distortion of some cysts. Out of the group of sodium nitrate, sodium chloride, and magnesium sulfate, sodium nitrate is probably the most popular. Since there is a difference in the advantages of each type of solution, you can see that in order to be thorough, you should have more than one kind of solution available for testing.

 

Here is a list showing the specific gravity of commonly recommended fecal flotation solutions

 

Sodium nitrate (Fecasol®)  1.2-1.25

 

Sodium nitrate saturated  1.3

 

33% Zinc sulfate  1.18-1.2

 

Sodium chloride saturated   1.2

 

Magnesium sulfate saturated  1.3

 

Sheather's sugar solution  1.25

 

 

The technique presented here is referred to as simple fecal flotation or standing fecal flotation.

 

  Obtain about one to two gram of feces. One gram of feces is about ½ square inch. You will need more for herbivores because of the high roughage content which can dilute the sample

 

  Mix the fecal sample with the flotation solution

 

    Strain or screen the fecal debris into another cup and squeeze out the excess fluid.

 

      Pour the filtered preparation into a tube.

 

     Add fecal flotation solution to the tope of the tube so that it bulges slightly- creating a reverse meniscus.

 

  Place a cover slip on top of the tube

 

  Let stand for 15 minutes. The fecal debris will sink to the bottom of the tube.

 

  Carefully remove cover slip with the adherent drop of fecal flotation solution that will also contain any ova, oocysts, or cysts that floated.

 

  Place cover slip on a microscopic slide. There should be no chunky debris or air bubbles on the slide.

 

Examine the entire cover slip area at 10X lens objective, using the battlement method. Switch to 40X lens objective as needed. Remember to change your plane of focus slightly as you view each field, since not all diagnostic stages of parasites will be in the same plane of focus. Some will be closer to the cover slip than others.

 

Commercial devices for fecal flotation

 

There are commercial devices available that are commonly used instead of the previous technique. These devices have a smaller volume and are limited in the amount of feces that can be used in a single test. But they are convenient and disposable, and easy to use. All of them have a built in screening device to reduce flotation of fecal debris, while still allowing ova, oocysts, and cysts to float.

 

       The Fecalyzer®. There is an  insert that has space at the bottom for fecal debris and a screening basket that allows flotation of ova.

 

        The Ovatector®. The component parts are assembled. The insert is a circular screen that retains fecal debris in the bottom and allows flotation of ova.

 

       The Ovassay®. The insert has space at the bottom for fecal debris and a basket screen to allow ova to float.

 

  All of the commercial devices use fecal flotation solution mixed with the feces. Some kits recommend and provide a certain flotation solution with the device. The basic procedure is the same as the simple flotation procedure, except that it is performed in the device instead of in a tube. Incubation is 15 minutes.

 

  A common error in set up occurs if the insert strainer is not placed in securely, as shown here. The solution and ova will leak out the sides instead of floating to the top, so that collection of the ova will be decreased.

 

Reporting and interpreting results

 

This test is qualitative and the numbers of ova seen are not necessarily an indication of the severity of worm burden.

 

Positive results are reported as to the type of ova, oocysts, cysts, or larva seen. Ideally genus and species is fabulous information to have but it is not always possible. Most treatment protocols can be devised even with only general information as to the ova type. You should be as specific as you can, but usually terms such as roundworm, hookworm, whipworm, capillarid, trichurid, strongyle, taenid, etc are acceptable to many veterinarians in practice.

 

False negative results are possible:

  • The parasite may be in the prepatent period
  • The specimen is old and the organisms have deteriorated or hatched out
  • The ova or oocyst may have been damaged by the procedure-if incubation was too long or if the specific gravity of the solution was too high
  • The parasite may be suppressed in reproduction
  • Ova may be only sporadically produced and the specimen was tested on a less productive day
  • Diarrhea may dilute the concentration of ova
  • Mucus may interfere with the flotation if it is not dispersed in the solution during the procedure

 

Therefore you should report negative findings as “no ova (or oocysts) seen” or “NOS” as it is sometimes abbreviated. Other abbreviations commonly used are NPS (no parasites seen) or NVP (no visible parasites). If you tell the owners the fecal test was negative, they will assume that the pet is free of parasites, which may not be true.

 

If occult parasitism is suspected, the veterinarian may elect to treat anyway even if findings are NOS. Sequential fecal flotations, usually at least 3, may be needed over several days time to verify the findings. More than one type of flotation solution may be needed also. Other tests should also be considered in the work up, such as fecal smear techniques, acid fast stain preparations, ELISA testing, and fecal sedimentation to rule out organisms that may not be detected by fecal flotation. 

 

Reference:

 

Zajac AM and Conboy GA: Veterinary Clinical Parasitology 7th edition, Blackwell Publishing, Ames Iowa, 2006, pp. 4-6

 

Acknowledgement: Many thanks to Bonne Webster, LVT and Terri Champney, DVM for help with photography in this unit.