Dewormer roulette — finding a winning strategy By Steve Hart

Dewormers have been our solution to worms since after World War II when we learned that chemicals had selective toxicity and some were effective against worms. Be- fore that, all we had was management and a few by gosh and by golly chemicals that sort of worked sometimes.

Steve Hart

Thiabendazole, the first broad spectrum dewormer, was wonderful for a while (did stain wool) and was replaced by Pana- cur® (or Safeguard®), an improved version of the molecule. We forgot about management of worms and it was easy to give the de- wormer regularly and not

have worm problems. It worked for awhile — until mother nature fought back with the development of dewormer (i.e. anthelmintic) resistance, which meant that the dewormer didn’t kill the worm any longer.

How do we know that we have dewormer resistance?

The main signs are that animals need

dewormed again 2-4 weeks after they were dewormed, or that animals don’t seem to bounce back after being dewormed or some animals die even though they were de- wormed. Another symptom is that FAMA- CHA scores do not improve within two weeks after deworming.

The scientific method for determining dewormer resistance is to do fecal egg counts on 6-10 animals, deworm them and then do fecal egg counts again 7-14 days later be- cause some worms stopped laying eggs for several days.

Then a fecal egg count reduction can be calculated on each animal and then average these. A fecal egg count reduction is the per- centage of eggs that disappeared after de- worming, which represents the percentage of worms that are killed by the dewormer. If the fecal egg count reduction is greater than 95%, then there is no dewormer resistance and you are very fortunate. If it is less than 95% it means that there is some dewormer resistance.

The dewormer may still be doing an ac- ceptable job at 80% fecal egg count reduc- tion. However, when fecal egg count reduction gets below 50%, the dewormer is

not helping very much and something else needs to be done.

This procedure is too demanding for

many producers. A very practical applied method would be to take stool samples from several animals to the vet 7-14 days after de- worming and if all animals have fecal egg counts that average around 500 eggs per gram or more, you likely have significant de- wormer resistance.

How do you get dewormer resistance? Worms have a loose genome — this means that there is more genetic variability between two worms than there is between man and monkey. This means that a few worms in a million will have a genetic dif- ference that affects how the dewormer works and the dewormer no longer affects that worm.

The Australians and New Zealanders have identified these mutations that result in resistance to various dewormers and know what chromosome they are carried on. The worm has two sets of chromosomes. If the worm has two genes (one on each chromo- some) for dewormer resistance, the de- wormer will not kill that worm. However, in a worm population, there are more worms with only one gene for de- wormer resistance than two genes. The rec- ommended dewormer dose will kill these worms. But if we underdose these worms, they survive and speed up development of de- wormer resistance. (See American Consor- tium website at end of article for correct dose). The ranch manager is the biggest factor in the development of dewormer resistance. He can hurry it along by killing off the non- resistant worms by repeated deworming of all the animals. This leaves only the resistant worms to produce future generations of worms.

However, the manager can virtually stop the development of dewormer resistance by selective deworming — that is, deworm- ing only those animals that need it. This can be done by using the 5-point

check (FAMACHA score, bottle jaw, diar- rhea, body condition and hair coat; see refer- ence on five point check at end of article). A minimum of 15% of the animals should not be dewormed to have sufficient refugia (pool of dewormer-susceptible genes in worms). These worms will mate with the 100%-resis- tant worms that survived in dewormed ani- mals, diluting dewormer resistance in the next generation of worms.

6 Goat Rancher | October 2020

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