Anthellmintics are drugs used against parasites the either kill the egg laying adults
or kill the larvae before they become capable of laying the harmful eggs. While
that sounds like it cures all of the problems with the parasites, the reality is
the parasites have developed a resistance to almost all of the known drugs that are
used against the parasites. Research after research has stated that the resistance
is coming from the drugs being overused and/ under dosing of a drug when they were
used. This resistance parasites have to these drugs makes the fight against them
extremely hard because it takes away one of the key tools breeders use to have for
helping control the parasites.
Classes of Drugs
Benzimidazoles – (the white drenches). While these are effective against many types
of internal parasites, it is low against the Haemonchus parasite. There are several
drugs in this family. They are:
A big problem with the Benzimidazole type of drugs is if parasites become resistant
to any one of these drugs, they are also resistant to the others even if you have
not been using them.
When giving benzimidazoles by mouth it is better to hold the animals off feed for
12 to 24 hours before treatment (don't remove water, just food). The drugs will not
pass so quickly through the GI tract and active levels will be maintained in the
The Benzimidazoles (Safeguard®, Panacur®, Valbazen®, Synanthic®), also called "white
dewormers" are broad spectrum and safe to use. They are effective against tapeworms.
Albendazole is effective against adult liver flukes, but should not be used in pregnant
or lactating females
The two drugs in this class are ivermectin and moxidectin.
The Macrolytic lactones or "avermectins" (Ivomec®, Dectomax®, Quest®, Cydectin®)
are the newest family of drugs. They are broad spectrum and have a wide margin of
safety. They are also effective against external (biting) parasites, including nose
bots. Moxidectin is a persistent-activity dewormer that continues to kill worms after
it is administered.
The main drug in this family is levamisole and has shown to be very effective against
the round worm. There has been less parasite resistance shown against levamisole
but that could change with breeders starting to over use it as they have all of the
Levamisole (Tramisol®), also called a "clear dewormer" is broad spectrum and effective
against arrested larvae. However, it has a narrower margin of safety, especially
in the injectable form. Pyrantel (Strongid®) is only effective against adult worms.
Moratel (Rumatel®) is an oral feed additive and is only effective against adult worms.
“The most important aspect of using dewormers is to conserve their effectiveness.
This can be achieved by using them as little as possible and ony when infection levels
dictate that intervention is necessary. The old concepts of treat all animals when
a few show signs or all animals at regular intervals (shorter than every 3-4 months)
is no longer warranted because it promotes dewormer resistance.” Louisiana State
Research and the goat experts use to recommend deworming the entire herd and then
moving them to a new, clean pasture. That is no longer the thinking that is coming
from the recent research. The problem the “deworm and move to a clean pasture” strategy
has is related to the breeding of the parasites inside the stomach. That is the only
place breeding occurs that result in the eggs. If a goat only has the most resistant
parasites in their system, the breeding can only produce similar strong, resistant
parasites. However, if some parasites are the less resistant ones that breed with
the stronger, resistant parasites, the product will be parasites that are not as
resistant as the most resistant ones.
Recent recommendations are to deworm only the animals that really need it and leave
them in the same pasture for a few days before rotating them. This helps keep some
of the less resistant parasites in the system to help minimize the number of “super
parasites” in your herd.
Goats generally metabolize drugs faster than other animals such as cattle. This
means the drugs may go thru the goats system faster than other animals. Since most
parasite drugs are actually developed and approved for animals other than goats,
the directions and doses may not be best for goats. A larger amount of the drug may
have to be given to be effective and instead of giving an injection or pouring it
on, it generally need to be given to goats as a drench.
Another recommendation for breeders to consider is to dry lot the herd for a day
or two after they have been dewormed to minimize the number of eggs and larvae that
survived the deworming are not being dropped in the new pasture.
Per-Parturient Rise (PPR)
A phenomena called the “per-parturient rise” (PPR) in fecal egg output. This occurs
at or around kidding time and extends through most of the nursing period. Because
the kidding and nursing are stressful conditions, the dam’s immune system is compromised.
Furthermore, nutrients are partitioned preferentially to support mammary and fetal
development and then lactation, which also decrease the animals’ ability to generate
an effective immune response to worm infection. This allows the existing worms to
increase the number of eggs laid, thus increasing the number of eggs deposited in
Do not deworm and move to clean pasture (no animal grazing for at least 3 months)
as those worms that survive deworming are probably resistant and then the new pasture
will become more highly contaminated with eggs/larvae of resistant worms
Since no dewormer is 100 percent effective 100 percent of the time, worms that survive
a dose of dewormer are resistant to that dewormer. Frequent deworming increases the
rate resistance develops. Each time animals are dewormed, the susceptible worms are
killed. The strong ones survive and lead to a population of very resistant worms.
Underdosing causes larger numbers of stronger worms to survive. The weakest, most
susceptible worms are killed. But because of the weak dose, more of the stronger
worms will be able to survive and reproduce, creating a population of stronger worms.
Once an animal has been treated, only resistant worms remain. If the animals are
moved to a clean pasture they deposit only resistant worms on the pasture. There
are no susceptible worms to dilute the worm population. Treating all animals regardless
of need ignores the importance of refugia and will lead, in time, to a population
of worms unkillable by dewormers.
Anthelmintics should not be used indiscriminately. Frequent deworming is costly.
It accelerates the development of anthelmintic-resistant worms and leads to a false
sense of security, which may result in unnecessary production losses and animal deaths.
The routine use of anthelmintics is prohibited under the new National Organic Standards.
Another concept that has also been reported to have some success In improving effectiveness
is to take animals off feed for 24 hours before administering the dewormer. This
will reduce rumen motility and the dewormer will pass through the gut slower and
have more contact time with the target worms.