Intestinal parasites are prevalent and important disease agents in companion animals and are potential causes of zoonotic disease in pet owners.1,2 These parasites can be detected by examining fecal samples with direct smear, sedimentation, flotation, and fecal ELISA techniques. Fecal flotation is the procedure used most commonly in veterinary practice. However, failure to use best-practice techniques, such as centrifugation, when conducting fecal flotation procedures can result in failure to detect parasite stages in fecal samples.2-4
In this article, we review the basics of fecal flotation techniques and describe step-by-step procedures for conducting accurate and effective centrifugal flotation procedures.
HOW FLOTATION WORKS
Fecal flotation procedures are used to separate parasites in all life stages (e.g. eggs, oocysts, sporocysts, cysts, larvae) from other objects and debris based on their different densities. Density is the weight of a parasite or other object per unit volume. It is usually expressed as specific gravity, which is the ratio of a parasite's density to the density of water (the density of tap water is 1.0). A parasite with a density of 1.10 is 1.10 times denser than tap water.
Successful fecal flotation is based on the principle that when a fecal sample is placed in a sugar or salt solution, parasites (and other objects) less dense than the flotation solution will move to the top of the solution and parasites more dense than the solution will eventually settle to the bottom. Buoyancy, the upward force a solution exerts on an object with less density than the solution, forces parasites with less density upward, while gravity and the viscosity (thickness) of the solution retard upward movement. If the buoyant force exceeds the forces of gravity and viscosity, the parasites eventually move to the surface (see the boxed text titled "Finding the right solution").
Finding the right solution
WHY CHOOSE CENTRIFUGAL FECAL FLOTATION?
Centrifugation forces the heavier objects to the bottom of the container sooner than the force of gravity alone does. Removing larger, heavier debris from the surface of the preparation will aid in the flotation of parasites and prevent the debris from obscuring your view of them. Also, in viscous solutions such as sugar, the downward force created by centrifugation is more likely to overcome the force of viscosity and lead to greater parasite recovery.
Therefore, successful recovery of parasites depends on a parasite's density and the density and viscosity of the flotation solution and how these factors interact and affect the time it takes for a parasite to float to the solution's surface. All these factors determine how long we centrifuge the samples or allow flotation preparations to stand before we examine the coverslip for parasites.
Veterinarians or veterinary technicians are sometimes reluctant to perform centrifugal fecal flotation procedures. Here are some myths about fecal flotation: