First off, there are some apples vs. oranges arguments being made
above. There is a correlation being spoken of between overimmunization and
autoimmune disorders, then an assumption is made about lack of immunity and
possible autoimmune consequences.
If you are worried about overimmuniztion, do you propose to stop
annual vaccination of your horses?
Secondly, there is not 100% protection from daily pyrantel
administration. It is more on the order of 85-90%. So there is a low level
of exposure to allow the development of an "immunity" (for lack of a better
term). Having seen on post mortem the damage done to young horses by the
migration of GI parasites through the liver and lungs, I will continue to
recommend daily pyrantel use in babies. (I have seen a decrease in
respiratory infections in foals/weanlings on same.)
There have been ranches using daily wormer for well over 10 years
now, with no apparent buildup of resistance. Given the nature of evolution,
someday, somewhere, resistance will develop, I'm sure, but you can say that
about any wormer. There is evidence of some parasites of small ruminants
develping ivermectin resistance. (Interestingly, these animals are dewormed
far less often than horses, in general.)
There are good arguments for and against daily administration of
pyrantel. Personally, having limited turnout area, as well as limited time
to pick paddocks, I use it. But please, let's not use pseudoscientific
arguments to condemn it.
--CMNewell, DVM