Stool Samples and Heartworm Tests

There are two topics I want to cover tonight — neither one in great depth, but more to give you an idea about what to expect when you bring your pet in for a routine visit. The staff spends quite a bit of time clearing up confusion on these topics, which tells me we’re not being particularly clear about what we’re doing. I hope the post helps clear up the confusion.

Stool Samples

When you make an appointment for a routine healthy examination (with vaccines or bloodwork), the reception staff probably reminded you to bring in a stool sample. We use the stool sample to check for intestinal parasites (primarily worms, but some other protozoan critters, too). Stool samples do not check for heartworm!

We mix the stool sample up with a very dense solution of zinc sulfate, then transfer it through a screen into a large test tube. The solution fills the tube right up to the very top. We then lay a very thin square of glass atop the tube (a cover slip). It’s held in place by the surface tension of the liquid.

Then, we spin the sample tube in the centrifuge. What this does is cause the worm eggs and some parasites to float up to the top of the tube. Centrifugation increases the success rate of detecting worms from a particular patient.

When we lift off that glass square, the eggs are pulled along with some of the solution. The square is placed on a microscope slide and then visually inspected for eggs or organisms.

We are readily able to find hookworm, roundworm, and whipworm. Some of the protozoans we see commonly are giardia (diarrhea!) and coccidia. Tapeworm eggs are generally heavier than the solution, so they sink to the bottom and are rarely collected.

Sometimes, we have to look at the bacterial population in a stool sample. In this case, only a very tiny amount of the sample is mixed with plain saline (0.9% NaCl) and observed under the microscope. This is called a direct fecal. We usually do this for patients having diarrhea or some other GI problem.

Heartworm Testing

Heartworm is a parasite that can affect many different mammals. Dogs and cats are both at risk in Michigan, no matter how little or how much time they spend outside. ( Indoor-only cats get heartworm just as often as outdoor cats! I’ll save the many other details of heartworm disease in dogs and cats for another post. )

Heartworm is transmitted to pets by the bite of a mosquito. The mosquito has heartworm larvae (babies) in its body. When it bites a pet, the larva are transferred to the pet. The larva go through some changes as they migrate from the bite to the heart and the major blood vessels going to the lungs. It can take 5-7 months for the worms to develop into full adults in the pet’s body.

We take a blood sample to look for heartworm. The heartworm test does not tell us anything about intestinal worms! We usually send samples out to the lab for testing, but in special cases we can run a test right at the hospital.

When we test for heartworm in dogs, we’re looking for signs of adult female worms. A special chemistry test is done on blood to look for a part of the female worm’s outer coat that is released into the bloodstream. Male worms will NOT cause a test to be positive. For dogs, the test is very sensitive. It’s also very accurate — there are very few false positives or false negatives. The only catch with dogs is that if the dog has been bitten in the past few days to 7 months, the test might be negative on that day, but positive later. This happens because the larva have not yet developed into adults that can be picked up by the test.

Cats have special rules, as usual. 🙂 They handle heartworm very differently than dogs. There are usually very few worms (1-3 in general), so there is a much higher possibility that there are all males, or not enough females to make a standard test turn positive. Because of the complexity of testing cats, I’ll save that for another post. Suffice to say for now that it requires more than one test and even then, you can’t always be sure.

So, just to summarize:

Poop is used to look for gastrointestinal parasites — intestinal worms and protozoans.
Blood is used to test for heartworm disease.

If you’re interested in learning more about intestinal worms — including which are transmissible to people and how that works — please check out the Companion Animal Parasite Council pages. The CDC has a ton of great information for pet owners. It’s well worth checking out.

If you’re interested in the fine details of heartworm disease, check out the American Heartworm Society web page. All of our guidelines for testing, treating, and preventing heartworm come from the AHS. They’re the leading authority on heartworm. Don’t miss the prevalence maps! Here’s a video to get you started, too.

Later this week, I’ll explain all of the different options for prevention.
IF YOU HAVEN’T STARTED YOUR PET ON HEARTWORM PREVENTION YET, YOU’RE LATE! Call us right away to set up an appointment so we can get your pet protected! Dogs will require a test before starting prevention. Cats only need to be current on their examinations to get prevention. Heartworm disease isn’t something you want to mess around with.


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Filed under infectious diseases, laboratory, preventive care

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