Agriculture Reference
In-Depth Information
same answer as the question “How often do I need to wash my car?” When
necessary!
Lambs that are run on pasture with ewes are more prone to problems
than mature animals, so you may need to worm them separately and more
frequently than the other animals. If you use pharmaceutical dewormers for
lambs, do so at 2½ to 3 months of age, and be sure to adhere to the number
of withdrawal days noted on the label before slaughter.
Worm resistance.
Resistance is a process of selection rather than one of
development. Contrary to popular opinion, worms, insects, and bacteria can-
not alter their genetic makeup and become resistant to a particular drug or
insecticide simply because they have been exposed to it — for example, in
the manner that your skin develops calluses and thus resistance to blisters
by exposure to a shovel handle. Instead, individuals with higher-than-average
resistance to something are more likely to survive and pass on that trait to
their offspring.
Resistance increases with the frequency of treatment because we keep
killing the susceptible worms and leaving the resistant ones to regenerate
the population. Resistance can also develop from improper use of a drug, so
always follow instructions to the letter. This is part of the reason why you
should never give too much or too little of any kind of medication. By not fol-
lowing the recommendations, you may allow a marginally resistant worm to
survive and propagate offspring with greater natural resistance when it might
have been susceptible to the full dose.
However, every type of parasite does not develop resistance to a particu-
lar dewormer any more than every species of bacterium develops resistance
to a particular antibiotic. Some species do and some species don't. Do not
assume that just because you use a dewormer the parasites are developing
resistance to it. In fact, the old recommendation to change wormers often to
avoid developing resistance is wrong — evidence suggests that this practice
may be the fastest way to cause the worm population to become resistant
to everything. In fact, the current recommendation from veterinary parasi-
tologists — those on the frontline of this problem — is that you use the same
dewormer until you see resistance beginning to develop. When you do change
dewormers, make sure to switch the class of dewormer because resistance
usually develops along chemical class lines, not brand names. Read the fi ne
print for the generic name or chemical class. If you can't fi gure it out, ask your
veterinarian or county Extension agent for help.
