Dewormers are divided
into three classes based
on their mode of
action/chemical
structure. Because of
the chemical
similarities and common
mode of action, all
drugs in a class are
similar in many
respects, but some
members of a class may
have unique
characteristics as
compared to other
members of that class.
The first dewormers
included phenothiazine
which is no longer
available. The next
generation of dewormers
were the benzimadoles.
This class is also
referred to as white
drenches. They include
Thibendazole (TBZ) fenbendazole
(Panacur, Safeguard),
oxfenbendazole (Synanthic)
and albendazole (Valbazen).
Since they are the
oldest class of
dewormers in use, the
worms have had a longer
period of time to
develop resistance to
them. Thibendazole is
essentially useless and
is no longer marketed.
In more humid areas of
the US where dewormers
have been used quite a
bit, the only one that
may be effective is
Valbazen whereas in
other parts of the US
where there has been
less dewormer used, all
members of this class
may be effective. All
three drugs kill
roundworms (such as
Barberpole worm, Black
Scour worm and Brown
Stomach worm),
lungworms, worm eggs and
tapeworms. In general,
they have poor efficacy
against arrested L4
larvae which may be
improved by using a
higher dose. In
addition, Valbazen also
kills adult liver
flukes, but has poor
efficacy against the
juvenile and immature
stages.
The second class of
dewormers are the
imidazothiazoles (cell
depolarizers). They
have also been around
quite a while, and may
still be effective in
many herds, a likely a
consequence of not
having been used as much
in goats due to the
narrow margin of
safety. Levamisole (Tramisole,
Levasole and Prohibit)
are water soluble
powders that often
require mixing, or may
come in an injectable
form. Morantel Tartrate
(Rumatel, Positive
Pellet Dewormer) is
another member of this
class which may be mixed
with feed. Rumatel
must be mixed and used
as per instructions on
the bag because there
are no provisions for
“off label” use of feed
additives. This class
is basically only
effective against
roundworms, arrested
roundworms and
lungworms. Levamisole
is typically given to
goats at 1.5 times the
sheep dose. However, at
twice the sheep dose,
toxicity can be
observed; the animal
walks like he is drunk,
he may salivate and
slobber, his eyes may
tear like he is crying,
and he may defecate and
urinate. If he is only
given a mild overdose,
he will usually recover
on his own in one to
three hours. Therefore,
it is important to have
a good idea of what your
animals weigh before
selecting a dose. Also,
it would be good to take
a break from deworming
to watch animals to see
how they are behaving 30
minutes after being
dewormed. Since other
classes of dewormers are
quite safe (animals can
usually tolerate at
least several normal
dose), it is easy to get
into the habit that if
in doubt whether an
animals has swallowed
the dewormer, give him
another dose. However,
don’t do that with this
class of drugs because a
second dose may be toxic
and possibly fatal.
When worms become
resistant to Rumatel,
Levamisole will still
work for a time. Since
the gene for resistance
to this drug in the
worms is a recessive,
drug resistance will
likely be slower to
develop as compared to
other dewormers.
The third class of
dewormers is the
macrocyclic lactones (avermectins/milbemycins.
Common members of this
group include ivermectin
(Ivomec) dormectrin (Dectomax), eprinomectin (Eprinex)
and moxidectin (Cydectin).
This is the most
recently developed class
of dewormers, and are
effective against
roundworms, arrested
roundworms, lungworms
and sucking lice.
However, this class of
dewormers are fat
soluble and therefore
drug withdrawal periods
can be long because the
drug is trapped in the
fat of the animal.
Cydectin is the most
recent member of this
class and appears to be
the most potent,
probably due to its long
persistence in the body,
but resistance is
rapidly developing to
this drug because it has
been used so frequently.
Since only two dewormers
are approved for use in
goats, use of all the
other dewormers is
“off-label” which is
described in boxed
text. Goats require at
least 1.5 times the
sheep dose of dewormer
due to having a
proportionately larger
liver to metabolize the
dewormer and due to a
faster rate of passage
through the digestive
tract. Be careful on
the dose with the
Tramisol/Levasol/Prohibit
in that too much can be
toxic as discussed
above. The greater dose
of dewormer in goats
also requires a longer
drug withdrawal period
than for sheep. There is
a list of withdrawal
periods for giving the
sheep dose to goats at
http://www2.luresext.edu/goats/library/field/dawson02b.html.
The withdrawal period
will need to be
lengthened when using a
goat dose. There is a dewormer dose chart for
goats (goat dose) based
on bodyweight at
http://goatconnection.com/articles/publish/article_168.shtml.
Administer dewormers
over the back of the
tongue so that they go
to the rumen. There are
extensions for syringes
to facilitate correct
placement of the
dewormer. This will
provide a prolonged
residence time in the
digestive trace and be
more effective at
killing worms than if
the animal sucks the
dewormer in the front of
their mouth where most
of it may go into the
true stomach (abomasum).
Pour-on dewormers appear
to work poorly in goats
and should not be used.
Injection of dewormers
can promote dewormer
resistance. If dewormer
is given in the feed or
in blocks, there is
often a problem of
non-uniform dosing of
animals, feed hogs get a
higher dose of dewormer
and timid and wormy ones
who aren’t able to fend
as well for themselves
get little dewormer.
There are several
strategies for
increasing the efficacy
of dewormers. These
require more labor and
may only be practical in
special cases. Animals
can be dewormed with a
full dose of the
Benzimidole classes of
dewormers twice, 12
hours apart. This keeps
a high level of the drug
in the blood for a
longer period of time,
increasing the drug’s
effectiveness. Another
trick is to fast
overnight, deworm the
next morning and keep
off feed all day. This
slows the passage of
digesta and dewormer and
increases its
effectiveness. This
should not be done with
late pregnant does or
ewes because of the risk
of pregnancy toxemia.
There are several
different strategies for
using dewormers that
have been used in the
past and there are
disadvantages to each
one.1) One strategy is
to deworm the herd when
animals die or show
symptoms of bottle jaw.
The disadvantage is that
animals are very wormy
and the pasture has
become heavily
contaminated. There has
already been a
significant loss in
animal production and
health. It is much
better to deworm animals
before infection gets
bad. 2) We can deworm
at stategic times when
worms are most likely to
be a problem such as
prekidding and weaning.
This can work very
well. However, if
refugia is low at this
time, we can be
promoting dewormer
resistance. This also
neglects year to year
differences in weather
which is a major factor
in determining the level
of worms. 3) We
may deworm animals when
we are working them or
some other convenient
time. Deworming
animals that don’t need
to be dewormed will
increase resistance to
the dewormer. 4) We may deworm when weather
conditions are favorable
for worms. An old
recommendation was to
deworm 2 weeks after you
have a month with more
than 2 inches of rain
and the mean temperature
is over 60oF.
While this is a good
idea, the pasture
infectivity may not be
as high as we guess it
to be and the animals
may be withstanding the
worm burden well. 5) We
may deworm at regular
intervals such as 30 or
45 days, to make sure
that worms do not cause
us a problem. This
practice is effective at
controlling worms in the
short term, but is
unsurpassed as the
quickest way to develop
dewormer resistant
worms. The best
strategy for deworming
when the Barberpole worm
is the dominant worm is
to determine
individually if an
animal needs dewormed
using the FAMACHA
program. This will
reduce the development
of dewormer resistance,
control pasture
contamination and
prevent lost production
due to worms. However,
if the Bankrupt worm or
Brown stomach worm are
causing our worm
problems, we must use
fecal egg counts of the
herd to determine when
animals need dewormed
and deworm the whole
herd on this basis.
Fortunately, in the US,
these worms do not have
as much dewormer
resistance as the
Barberpole worm.
Dewormer resistance has
followed the use of
dewormers. Where a
given dewormer is used
heavily over a period of
time, resistance to
that dewormer has
developed. Resistance
to the benzimidazole
class of dewormers was
documented in the
1960's. Resistance to
levamisole was
documented in the
1980's. Resistance to
ivermectin was
documented in the 1990's
(was a good useful
dewormer for a while).
Moxidectin resistance
was first documented in
the 2000's. In most of
the southeastern US,
there is documented
resistance in sheep and
goats to multiple
dewormers. Several
factors have contributed
to the development of
dewormer resistance.
Veterinarians have
pursued the eradication
of worms instead of
controlling them (deworming
when the level of worms
is high enough to impact
the health or
productivity of
animals), producers
found it easier to give
a dewormer regularly (by
the calendar rather than
to evaluate whether
there was a need for
deworming (and dewormers
were cheap!). In
addition producers did
not take the time to
manage pastures to
reduce parasitism.
Pharmaceutical companies
have encouraged heavy
use of dewormers, driven
by the profit motive.
And, the worms were
illiterate; they didn’t
read the pharmaceutical
company ads to know
which dewormers should
kill them. The dewormer
resistance on a given
farm is unique to that
farm due to the history
of dewormers that the
animals have been
exposed to. However, if
animals are traded in a
local area, the dewormer
resistance will be
similar to other farms
that the animals were
traded with due to the
sheep/goats carrying
their worms and dewormer
resistance of those
worms to the next farm
with them. Most of the
information, whether
from academia or drug
ads was about using
dewormers and not about
using management to
suppress worms. We are
now suffering from the
consequences of that
misguided approach to
parasite control.
How do worms become
resistant to dewormers?
Worms have a very
genetically diverse
genome. Therefore, one
worm in millions of
worms will have some
genetic characteristic
that enables him to be
resistant to a specific
dewormer. The potential
for dewormer resistance
exists in at least a few
worms before a dewormer
is ever used. Repeated
use of that dewormer
will gradually kill all
the non-resistant worms
in a specific
population, leaving only
resistant worms (Figure
1). Over time, the
level of resistant genes
(and resistant worms)
will increase in your
worm population.
Basically, resistance is
permanent although there
are a few rare
exceptions. When worms
in your sheep/goats
become resistant to a
dewormer, you can forget
about ever using that
dewormer again in your
management program.
Also, resistant worms
are spread by animal
movement. You may have
not have worms that was
resistant to a dewormer,
but when you buy an
animal, you get their
worms for free, but
worse, you get the
dewormer resistance that
may come with them.
That is the reason for
recommending that all
new animals be dewormed
with two classes of
dewormer (theory being
that most worms will be
resistant to only one
class of dewormer) and a
fecal sample of the
sheep/goat needs to be
checked a week or two
later to verify the
absence of eggs (Before
putting them in the
pasture with the rest of
your animals!). Sheep
and goats that originate
in more humid areas, and
higher value animals are
more likely to have
resistant worms due to
greater exposure to
dewormers.
figure 1
Dewormer resistance can
be detected by fecal egg
count reduction test.
That is taking a fecal
egg count from a sample
of animals, preferably
6-10 hd, deworming with
whatever product and
then taking another
fecal egg count 7-14
days later. If the
dewormer was effective,
the second fecal egg
count should have only 1
or 2% of eggs of the
first fecal egg count
(If the dewormer is
highly effective, the
fecal egg count will be
0). The first sign of
dewormer resistance is
when the second fecal
egg count has more that
5% of the eggs of the
first fecal egg count.
This means that there is
significant resistance
to the dewormer. When
the resistance level is
this high, it will
progress rapidly to
where the dewormer kills
less than 50% of the
worms. When a dewormer
only kills 50% or less
of the worms, there may
be some improvement in
the appearance of the
animal for a short time
when they are dewormed,
but they will need
dewormed sooner than
expected or they may
die Often the great
danger is that we think
deworming took care of
our worm problem and we
are surprised by animal
deaths and they are
often attributed to
other causes since
animals were just
dewormed. Keep a check
on eyes and watch for
bottle jaw. Dewormer
resistance can also be
determined using a kit
called the Drenchrite
test. The test is
currently being offered
by Dr. Ray Kaplan of the
Department of Infectious
Diseases of the
University of Georgia
Veterinary School
http://www.scsrpc.org/SCSRPC/Files/Files/D'Rite
Scsrpc11‑05.pdf
. One sobering fact of deworming resistance is
that when our animals
become resistant to all
classes of dewormers, we
are either going to have
to do very intensive
management to suppress
worms or go out of the
goat business. There is
a recorded case of the
worms being cleaned out
of sheep with multiple
doses of different
dewormers and then
dewormer susceptible
worms were used to
reinfect the sheep to
replace the resistant
worms. Also, there is a
new class of dewormer
which is in the FDA
approval process and
will likely take 5 more
years to get approval.
It is very potent
against worms that are
resistant to our present
three classes of
dewormers but worms can
still develop resistance
to it in time.
Dewormer resistance may
be slowed down by 1) not
importing resistant worms
as described above. 2)
Reduce deworming and 3)
Increase refugia.
Refugia is defined as
the proportion of the
worm population that was
not exposed to the
dewormer. This includes
eggs and larvae on
pasture at time of
treatment and the worms
in animals that were not
dewormed (reason for
selective treatment with
FAMACHA). The
importance of refugia
can be illustrated by a
case in Australia where
it was hot and dry for 3
months and animals were
dewormed in the dry
period. Deworming was
very effective in that
no eggs or larvae
survived on pasture and
only a few worms
survived the deworming
of the animal. However
100% of those few
surviving worms were
resistant. It took
little time for the worm
population to build
(remember the Barberpole
worm can lay 5,000-6,000
eggs per day and every
4-5 weeks there is a new
generation of worms
producing eggs), but
100% of the worms were
resistant because there
were no susceptible
worms surviving on
pasture for them to mate
with. Worms in refugia
are more susceptible to
the dewormer and dilute
out the dewormer
resistant genes of the
worms that survived
deworming. So refugia
is very important in
reducing the rate of
development of dewormer
resistance. Note, that
we do not prevent the
development of dewormer
resistance, but it may
take two or three times
longer for the worms to
become resistant to the
dewormer, enabling us to
use the dewormer for a
longer period of time.
There are three
management practices
that promote the
development of dewormer
resistance: 1) Frequent deworming (more than 3
times per year) 2) Underdosing the
dewormer (increases
proportion of worms with
genes for dewormer
resistance) 3)reducing refugia. Ways that we
commonly reduce refugia
are treating all animals
at the same time,
treating all animals and
moving to another
pasture that has few
worm eggs or infective
larvae, and treating
when few larvae are on
the pasture such as
during a severe draught.
Selective treatment is
very effective at
increasing refugia and
slowing the development
of dewormer resistance.
Figure 2 shows the fecal
egg counts in a herd.
There is one bar for
each animal’s fecal egg
count. The height of
the bar represents the
number of eggs in a gram
of feces. One thing
that jumps out is that a
few animals have much
higher fecal egg counts
than others. In fact,
the 6 animals with the
highest fecal egg counts
have 50% of the total
fecal egg count of the
herd and the top 12 of
36 total animals have
80% of the total fecal
egg count of the herd.
These top 12 animals
have fecal egg counts
that are at a level that
reduces productivity and
the animals with higher
fecal egg counts are
heading towards death.
The 24 animals with
lower fecal egg counts
do not need to be dewormed because they
are able to be
productive with a low
level of worms. A low
level of worms is normal
and beneficial in that
it keeps the immune
system functioning
against worms and
provides refugia. If we
deworm only these top 12
animals, we restore them
to health and the result
is observed in figure
3. Note that the graph
has been magnified 10
times (look at the
numbers on the left side
of the graph and compare
it to figure 3). After
dewoming, the animals
that were not dewormed
contribute 19 times more
eggs to the pasture (refugia)
than the animals that
were dewormed.
Therefore, the resistant
worms that arise from
eggs produced by
dewormed animals will
have many worms from
eggs produced by animals
that did not get
dewormed. In this way,
genes (for dewormer
resistance) from the
dewormed animals will be
diluted and the
resultant dewormer
resistance will be
increased only
slightly. This is why
FAMACHA is so important
to help slow the
development of dewormer
resistance. It is the
only practical way that
we can selectively
deworm our animals and
maintain a high level of
refugia which slows the
development of dewormer
resistance.
figure 2
figure 3
Because dewormer
resistance can put a
producer out of
business, especially in
the more humid areas of
the southeastern US, a
producer needs to
develop a plan to reduce
the rate at which
dewormer resistance
develops. Management
practices which suppress
worms are going to have
to be the foundation of
the control program and
dewormers are going to
have to be used
sparingly and
intelligently and only
when needed (selective
deworming). These
principles were
discussed in the second
and third article in
this series. Only
through this strategy,
can small ruminant
production remain a
viable farm enterprise
in the southeastern US
in the long term.
Unfortunately, dewormer
resistance is
inevitable, but how soon
it develops it depends
on your management.
Using FAMACHA will slow
the development of
dewormer resistance as
compared to conventional
deworming management.
However, as mentioned
already, the quickest
way to get resistant
worms is to buy them.
Of course no one
purposely buys resistant
worms, but that
expensive good looking
animal that you just
bought brings his worms
along and they may be
very resistant to
dewormers. Expensive
animals tend to be
dewormed excessively
since the owners do not
want to risk worms
killing them. This is
especially true if the
animal comes from a
humid area. Therefore,
expensive animals are
more likely to have
resistant worms. Since
buying resistant worms
is the quickest way to
get dewormer resistant
worms, it is wise to
make sure that those
animals that you bring
into your herd do not
bring resistant worms
in. This is done by
deworming them two
classes of dewormer
(such as Valbazen and
Prohibit; full goat dose
of each at the same
time) and then a week
later, do a fecal egg
count on the animal
which hopefully will be
zero or close to zero.
If not, you may have to
resort to a 3-way
combination of Valbazen,
Prohibit and Cydectin,
all given at the goat
dose given at the same
time. If the fecal egg
count is not zero in a
week, you don’t want
these worms nor the
animal on your farm.
Highly resistant worms
in one animal can become
highly resistant worms
in other animals in your
herd in a short period
of time. How are you
going to control those
worms? Sometimes,
unfortunately, the
solution is to go out of
business although there
are some solutions as
discussed below.
When you have resistance
to all dewormers i.e.
none work, there are a
few techniques which
often require intensive
management that can
enable a producer to
stay in the business for
several more years.
Hopefully, there will be
another class of
dewormers available
within 5 years that will
provide help. One can
use a short duration (5
days on a pasture), long
rest period (40+days)
grazing program so that
sheep/goats pick up few
worms and therefore do
not need to be dewormed.
One may be able to use
alternative dewormers (sericea
lespedeza, copper oxide
wire particles etc.) as
discussed in the next
article in this series.
However, one is going to
have to be good at
parasite management to
stay in the business.
It is better to use good
management to prevent
your animals from
getting into this
difficult problem. Using
a conventional dewormer
at a higher dose
(Prohibit should not be
given at more than 1.5
times sheep dose due to
toxicity), giving
combinations of
dewormers or giving
dewormers (or
combinations) for 2 or 3
days in a row are
techniques that can kill
most resistant worms.
You can select your
animals for genetic
resistance to worms.
All this makes good
parasite management
program to reduce
dewormer resistance
sound better all the
time.
In conclusion, next time
you deworm, do a fecal
egg count to determine
if your dewormer is
working. If not, switch
to another dewormer and
check to see if it is working. Use a proper
dose of the drug and
know the drug
withdrawal. Use FAMACHA to slow down
the development of dewormer resistance. Do
not buy dewormer
resistance and worm all
incoming animals with
two dewormers. Use management to prevent
worms which reduces how
often you deworm.
Deworm only those
animals needing it based
on FAMACHA. Cull
animals requiring the most deworming.
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