Guide to Preventing Parasites.pdf - Royal Canin Canada

There is another type of study that provides certain information about Toxocara in an
indirect manner. These studies involve administering macrocyclic lactones to pregnant
and nursing bitches in order to observe the protective effect of this program on their
puppies. In the last trimester of pregnancy and during nursing, tissue larvae reactivate and
find their way to the fetuses and puppies by crossing the placenta or as a result of being
excreted in the colostrum and milk. Thus, the protective effect of macrocyclic lactones
can be evaluated by comparing the parasite load in treated puppies with that in control
puppies. Such studies have been conducted with doramectin, ivermectin and selamectin,
with convincing results (Payne and Ridley, 1999; Payne-Johnson et al., 2000; Schneider
et al., 1996). The residual effect and the effect of periodic treatments on this protection
conferred to puppies has not been evaluated separately, but this is a useful approach in
the context of a prevention program.
5.5 The effect against the different stages
It is relatively easy to determine the efficacy of a given substance against the adult stage,
and the techniques for doing this are well-known. For the larval stages, one must bear in
mind that there are two forms of larvae, migrating and hypobiotic. It is important to
carefully distinguish between “inhibited, quiescent, hypobiotic, or encysted larvae” and
“developing larvae”. The former are often encysted in cells or tissues, their metabolism is
at a minimum, and they exhibit little activity. The latter dwell in tissues or even in the
lumen of organs (liver, blood vessels, intestines, or respiratory tract), have a very high
level of metabolism, and are active. It is easy to imagine that drugs will not necessarily
have the same efficacy against these two different types of larvae, given their location,
their different levels of metabolism, their different surface antigens, and so on.
Previously, this matter was important with regard to drugs that exhibited little intestinal
absorption. Nowadays, most of the drugs used (macrocyclic lactones, benzimidazoles)
pass into the bloodstream and can thus exert their effect on the larval forms. However,
only fenbendazole administered at 150 mg/kg/day for three days has demonstrated a high
level of efficacy against Toxocara canis larvae encysted in tissues other than the central
nervous system.
The supply of migrating larvae probably continues, even after all the sources of infection
have been eliminated. The reactivation of inhibited larvae seems to be a continuous
phenomenon with Toxocara and Ancylostoma. This would explain the reactivation of
larvae in the last trimester of gestation, for example. Using a drug that has no effect
against larvae leaves in place parasites that will start laying eggs relatively early, that is,
before the end of the known prepatency period of that particular species. On the other
hand, using a drug that exerts an effect against migrating larvae will prevent the parasites
from laying eggs for a period of time equal to the prepatency period, but no longer.
Lastly, using a drug that exerts a residual effect lasting 18 days against a given species of
parasite may increase, by an additional 18 days, the interval between treatments for this
parasite. Therefore, the main advantage of using a drug with a residual effect is to enable
us to increase the intervals between treatments. On the other hand, the use of these drugs
can promote resistance, at least in theory, mainly as a result of the exposure of parasites
to the decreasing doses at the end of the between-treatment interval. However, the dose
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