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,
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
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®)
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
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
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
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.
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.