Guide to Preventing Parasites.pdf - Royal Canin Canada

equired to destroy parasites varies according to the species, and increasing the intervals
between treatments is limited by the parasite species that is the least sensitive to the drug.
Thus, for many drugs, the effect against fleas does not seem to persist beyond one month,
which limits the length of the interval for more-sensitive parasite species, for which
longer intervals can be used. For instance, ivermectin destroys third- and fourth-stage
Dirofilaria larvae at a dose of 6 μg/kg and first-stage larvae at a dose of 50 μg/kg, but has
little of no effect on adult parasites, even at a dose of 200 μg/kg.
5.6 Resistance to anthelmintics
Since antiparasitic drugs are being used more and more commonly and on a wide-scale
basis, many people wonder if we might be creating resistance problems. However, very
few publications have reported such a problem. A first case was reported in a 9-week-old
puppy with a hookworm load that was high enough to be life-threatening. The puppy was
treated twice in three weeks with pyrantel pamoate. The fecal examination performed a
few days after the second treatment still showed considerable egg excretion (Jackson et
al., 1987). It is known that anthelmintics are less effective in highly parasitized animals.
This is probably due more to the fact that these drugs are poorly absorbed than to a
resistance phenomenon. The other two cases reported concern greyhounds, animals that
need to be dewormed against Toxocara and Ancylostoma every two to three weeks for
years (Ridley et al., 1994). In one case, pyrantel pamoate had zero efficacy in two
puppies and 81.6% efficacy in two others. In the other case, the mean efficacy of this
drug was 83.84% in five dogs, which, overall, is acceptable and statistically
nonsignificant. Recently, in a clinical trial performed in Australia, the pyrantel pamoate
efficacy was only 25.7% against Ancylostoma caninum experimentally infected dogs
(Kopp et al., 2006).
There is very likely no basis, at least in the short term, for fearing resistance in parasites
that affect our pets. There are several reasons for this. Resistance usually develops in
animals kept in a relatively closed environment, such as horses, poultry and sheep. It
develops in herds kept on the same pasture or in the same pen and is generally limited to
this environment. And, resistance develops in parasites with relatively short generation
times, such as coccidia, trichostrongyles, and cyathostomes.
Dogs and cats are seldom kept in such conditions. Drugs are used on a wide-scale basis
but often for preventive purposes only, when there are no parasites. Therefore, selection
pressure on parasites is negligible. This is especially true for heartworms and fleas. If
ever the parasite produces resistant offspring, they will first have to contend with
environmental conditions such as freezing in the winter and dry weather and sunshine in
the summer, which will kill most of them. The few survivors ingested by a suitable host
will be targeted by the animal’s immune system, which may encyst them, with the result
that most of them will not produce any offspring. Another factor that can come into play
is the wide variety of drugs used to destroy parasites: fenbendazole, macrocyclic lactones
(ivermectin, milbemycin, moxidectin, selamectin), nitroscanate, pyrantel pamoate, and
piperazine. Rotating drugs in a given region is therefore already being done. Lastly, since
animals in a given region constitute a highly differentiated group, if only because of
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