Here is a question from Christina...
I recently took a stray cat in to the vet for a checkup, vaccinations, etc. The vet discovered that he was "loaded with roundworms" and gave me a topical dewormer to administer when we got home, which I did. It's been about 36 hours and the cat has had 2 bowel movements, but I have yet to see any worms in his feces. Should I be worried? He is eating, drinking, and eliminating normally. How long after the dewormer was applied should I expect to see the worms being expelled?
A great question, Christina. Roundworms are a common parasite, and no surprise to find in a stray. They live within the intestinal tract, and you will rarely see them come out in the stool. If the infestation is bad enough, they will sometimes vomit worms, but that's uncommon. As long as the worms are alive, they will stay within the intestinal tract, and there may not be any obvious signs that they have them. When we examine a fecal sample, we are looking for the worms' eggs, not the worms themselves. The numbers of eggs we find will depend on several factors: the number of worms in the pet, the current state of the reproductive cycle when we get the sample, the amount of fecal material collected, and the method of preparation of the sample prior to microscopic examination. Studies have shown that it's possible to have a "negative" result, even if worms are present. In fact, there can be as high as a 20% chance that we can miss worm eggs even if the worms are present. I mention all of this because quantification of the worm eggs doesn't always correlate with the actual numbers of adult worms.
When the worms die, they may not be released into the feces all at once. It is completely normal for a cat or dog to "poop spaghetti" after a deworming, but this is less common than seeing normal feces. Just because you don't see any worms coming out doesn't mean that it isn't working. If you took each defecation and teased apart the feces, you would probably find the worms. However, I doubt that you would have an interest in doing that, and I wouldn't blame you. I wouldn't do it myself.
Hopefully your vet talked about potential human infections, as it is possible for humans to be infected. Transmission is "fecal-oral", which means that you have to ingest the eggs. This occurs most commonly through touching areas contaminated with infected feces, and then touching your mouth or food without having washed your hands. Simple hygiene will prevent transmission, such as wearing gloves or washing your hands after cleaning the litterbox. Here are some resources from the American Veterinary Medical Association....
Internal Parasites, Larval Migrans
So to answer your question, Christina, this is a perfectly normal situation. You shouldn't expect to see any worms, even if it is working. And it sounds like your cat is not in any health danger. If you want to be certain that the worms are gone, talk to your vet and have the fecal exam repeated in about 3-4 weeks.
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