Understanding Negative Results When You've Seen WormsUpdated 20 days ago
“My pet’s stool had worms/segments in it recently. Why was their stool test negative?”
We understand that it can be confusing to get a negative, or none seen, result when you may have seen physical evidence of a worm or worm segment in your pet’s stool! There are several possible reasons your pet’s stool may have tested negative when you were expecting a different result and knowing why can help determine best next steps for treatment.
First, it’s important to note that our laboratory does make a note for our lab team when you tell us you have seen evidence of worms or segments. While we inspect each and every sample visually for evidence of parasites, samples that get flagged as having seen worms or segments are given an extra once-over by our technicians both during the setup process and when we review the sample microscopically to make sure we don't miss anything. Fun fact; although this may vary, whole worms are often roundworms but may also be hookworms or whipworms. Segments, or proglottids, are most often tapeworms.
When we do see evidence of whole worms or segments, we will always note this in your pet’s results. However, we may not see evidence of the physical worm you saw. There are several reasons why this may happen.
What you saw was not a worm or parasite. Sometimes called “pseudo parasites,” occasionally random debris in the stool may appear to the untrained eye to be a worm or segment but is actually something else! Cloth, fibers, plastic, and string may be the culprits, but people have been confused by mucus and crayons before as well. We have also seen maggots, ticks and seed pods in stool – all of which could be mistaken for worms or other internal parasites.
Your pet was treated for parasites prior to sample submission. If you noticed evidence of worms and have either gotten treatment at your veterinarian or your pet was given certain medications since then (for example, certain types of heartworm prevention), it is unlikely we would see evidence of parasites afterwards as these medications may be effective within 1 – 2 hours of being given.
The worm or segment has disintegrated. Over time, worms and segments may disintegrate and leave little to no evidence they were there. This is especially true for feces that have been exposed to the elements for a period of time prior to collection, or experienced transportation delays.
The worm or segment noted in a previous stool sample was not present in the sample submitted. Even though you may have seen worms or segments in a stool recently from your pet, that doesn’t mean there will be worms or segments present in EVERY stool your pet passes. Unfortunately, worms and their eggs are not consistently present in uniform numbers throughout the stool a pet produces. Our laboratory methods are designed to recover and detect the eggs, or ova, of parasites; and even those may only shed intermittently. This is a big reason why stool testing is recommended twice yearly for all adult pets, and 4 times during the first year of life for younger pets.2
Follow up Question. Wouldn’t you see the eggs even if the worm wasn’t present? Not always. Some parasites, especially tapeworms, have heavier eggs that may be difficult to recover in routine stool testing methods. This means they don’t “float” as easily and have a lower recovery rate than other types of worms. Additionally, eggs can be intermittently shed throughout stool as well; some sections of stool may be loaded with ova, while other sections may have none at all, which means detection of eggs isn’t always a sure thing even when infection is present.(1)
So, What Now?
We understand that you still have concerns about your pet having an infection. If you receive a negative test result and are sure you see evidence of worms or parasites, you should consult with your veterinarian. In most cases, being treated without an infection is better than not treating when an infection is present, and as we now understand, a negative test result doesn’t always mean there are no parasites. Often, it’s helpful to have photos of what you saw to help understand the type of worm or segment that may have been seen and get the most effective treatment.
Citations:
Adolph C, Barnett S, Beall M, et al. Diagnostic strategies to reveal covert infections with intestinal helminths in dogs. Vet Parasitol. 2017;247:108- 121. doi: 10.1016/j.vetpar.2017.10.002 11 https://pubmed.ncbi.nlm.nih.gov/29080756/
Additional Questions? Email [email protected] or call/text us at 833-738-8378.