Agriculture Reference
In-Depth Information
HOW RESISTANCE DEVELOPS:
AN EXAMPLE
If we used a fi ctitious dewormer called XYZ, it would kill all the
worms susceptible to XYZ. The few individuals that were naturally
tolerant of XYZ would not die, and their offspring would carry the
resistance factor. If we continued to use XYZ to worm the sheep,
the percentage of XYZ-resistant offspring in the population would
increase with each generation until the majority of the population
was XYZ resistant. It would be like having a drug that prevented
white sheep from conceiving, but black sheep were naturally resis-
tant to it. If we fed that drug to a fl ock with both black sheep and
white sheep, sooner or later most of the fl ock would be black — not
because the black sheep developed resistance by their exposure to
the drug, but because they were selected from the population.
The more effective a dewormer is on all the different species (called broad
spectrum), the less chance of selection for resistant strains. With a highly
effective drug, the worm numbers become so depleted that they lack the
genetic variability required for selection for resistance in a short time.
To determine whether the worm is becoming resistant to the drug you
are using, you must do an egg count. If egg counts are done just before and
then 1 week after administration of the correct dose of a dewormer and the
decrease in the number of eggs is less than 80 percent, the presence of anthel-
mintic-resistant parasites must be strongly suspected.
To avoid introducing resistant strains when bringing new sheep into your
fl ock, treat all incoming sheep, preferably before you bring them to your farm.
Depending on your locale, you may benefi t from treating with two different
dewormers. Ask your veterinarian. After treatment, the animals should be
penned in a dry, grass-free area for 24 hours to avoid contamination of pasture
with viable nematode eggs that did not pass out of the sheep when the worms
were killed.
Targeted worming.
In most sheep-rearing areas, only approximately
5 percent of the worm population survives on the pasture during the win-
ter months. Thus, the remainder of the spring's worm population is in the
sheep in the form of hypobiotic (arrested) larvae that are encysted in the
tissues. Once the sheep gain access to the pasture during favorable weather
