Every time you go to see your veterinarian, he or she wants to "do a fecal" or check a stool sample for intestinal parasites. Exactly what happens to that stool sample when the technician disappears into the laboratory area of the clinic with it?

Though we commonly refer to the test as a "fecal", the full name is "fecal flotation". Adult intestinal worms live inside the body and pass their eggs in a pet's stool to propagate themselves to other victims. This test looks for those eggs.

HOW IT WORKS

Approximately 1/2 teaspoon of stool is mixed with a solution of a sugar or salt that is very thick. In chemical terms, the solution has a high specific gravity. In practical terms, the heavy solids that are undigested food sink and most worms' eggs, being lighter than the solution, will float to the top. The test is allowed to process for about 10 minutes, then a small sample of the solution from the top of the test container is applied to a glass slide and examined under a microscope for the presence of eggs from hookworms, roundworms and whipworms. Other parasite eggs may also be seen less commonly, such as those of tapeworms and lungworms.

So far our discussion has centered on methods of finding evidence of worms in the stool, but there are other parasites commonly found in cats' and dogs' intestinal tracts that are of medical significance.

Protozoan parasites may show up on a flotation, but are best seen on a test called a direct smear. Examples include coccidia, giardia and amoeba. These parasites may be too heavy to float in a flotation test.

The advantage that a fecal flotation has over a direct smear for eggs of worms is that it effectively tests a volume of stool 10,000 times larger than a direct smear. That increases the chances of finding eggs of worms that produce small numbers of eggs, such as whipworms.

Clients sometimes comment that, "I never see any worms in his stool". There are two reasons that you usually won't. One, the worms are inside the body, not often passed in the stool. Two, hookworms and whipworms are microscopic, and only roundworms and tapeworms can be seen with the naked eye. Eggs of all of these worms are also microscopic, and that is why a microscope is required to read the flotation test.

It is also important to remember that we are not only testing for intestinal worms, we are also looking for other parasites in the body, such as protozoa and lungworms.

Tapeworms present a unique challenge to diagnosis. The eggs of these worms are passed in packets, which you may see in the stool, looking like grains of rice that move about. Inside each of these packets are hundreds of eggs, but they often will not show up on a flotation because the packet is sealed. Doctors train pet owners to spot these segments in the stool because they are part of a whole worm still inside the intestine.