WHEN I dispense a worm treatment in my clinic, clients often ask the question, "how long does this last?"
This seems a fair enough thing to ask, but it reveals a misunderstanding of how worm treatment drugs work.
I always just answer with the recommended date of the next dosage, for example, repeat in three months.
But the wording of the question suggests that people might think that the worm tablet offers some sort of "forward protection" like a vaccine immunisation or that it has some sort of sustained release effect.
Leaving aside the new 12-month heartworm treatment, which is in fact a slow release form of anthelmintic drug which is on-board for a year, most regular worm treatments are metabolised in the body and gone within a day or so, having hopefully killed any worms that might have been present.
The timing of the next dose is determined more by factors such as the life cycle of the parasite in question and your pet's habits and environmental exposure to possible new infection from worm eggs in the soil or faeces of other animals or mosquito bites in the case of heartworm.
The drugs available today are highly effective and have a wide safety margin.
The same drugs that we use in our pets are used to control worm infections in humans which are a serious cause of disease in developing, especially tropical countries.
The considerable market for veterinary anthelmintics in first world countries results in these drugs being available for saving human lives in Africa!
"Worms are bad, kill them all with drug X" is an easy "sell" for pharmaceutical companies.
While worm infections can no doubt cause serious disease, there is a school of thought among immunologists, that the elimination of intestinal parasites which have co-evolved with mammals for millions of years might be associated with the increased incidence of allergic, inflammatory and auto-immune diseases in developed countries.
It's called the Hygiene Hypothesis but that's another story.
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