November 25, 2013

Guest: Dr. Rob Rew
“Parasite Resistance in Beef Cattle” 
Learn ways to overcome parasite resistance in cattle in order to reduce production losses.

(Dr. Dan) Hi there, folks. Welcome to DocTalk. I’m Dr. Dan Thomson here
from Kansas State University and today we have a great show for you and I’m
excited about it because we’re gonna talk about parasite resistance to the
different products that we have to try to take care of parasites and we’re
gonna talk about some of the reasons why. I recently had an opportunity to
sit down with Dr. Rob Rew, who is a boarded parasitologist and known
internationally for discussing such topics in food animals and today’s
discussion with Dr. Rew will be fantastic. I’m glad you joined us. Stay

(Dr. Dan) Hi there, folks. Dr. Dan here and thanks for joining me this
morning on DocTalk. Recently, I had an opportunity to sit down with one of
the world’s leading parasitologists, Dr. Rob Rew from Rew Search
Incorporated. Dr. Rew was born and raised on a farm. In his twenty some
years on the farm and then his experience as a veterinarian lead him to
understanding how we control parasites in sheep and cattle. Today, we’re
gonna discuss anthelmintic resistance and why we have resistance and some
of the things that contribute to it. (Male) What I’m really trying to do is
give you some realistic understanding of what anthelmintic resistance in
cattle in internal parasites is. First of all, we have to define the fact
that resistance starts out with a population of parasites that are set to
the drug at the dose rate to the stage additionally. When resistance
begins, we begin to see a selection of some pre-existing individuals in
that population that are susceptible that were susceptible to the anti-
parasitic that are no longer susceptible. The resistance may be based on
altered active binding site or where the drug binds. It could be on some
sync protein that’s sucking up the drug that comes into the parasite. Could
be some sort of active, detoxification system or it could be some
behavioral event. We talk about resistance, reduced efficacy and we tend to
focus on the products that are used, but the reality is, we need to focus on
a whole lot of factors, which may be much more important to the product.
First of all, we’ve got the parasite, then we’ve got the product, then
we’ve got the host’s genetic system or the immunity that we just talked about
and we’ve got stressors on the host, then environmental issues
get involved and then we’ve got management issues that are so important in
efficacy and then we’ve got evaluation techniques, so I’m gonna walk
through those quickly. The parasite, first of all, we have five stages of
each parasite. We have eggs. We have L1’s, L2’s, L3’s, L’4, inhibited L4’s
and adults. Each one of those stages would have a different efficacy
pattern. The L3’s are quite susceptible. The adults are reasonably
susceptible to drug treatment, the L4’s, a little worse. The inhibited L4’s
are really hard to get, so when we talk about efficacy, what are we talking
about? On what stage and how much percentage are we getting of this stage
verses that stage, when we dose an animal. Different species have different
responses to anthelmintics. The lungworms are very, very
susceptible to macrocytic galactodes. Copperia, pretty hard to get really
for efficacy. Eighty percent efficacy gets copperia is really about what
you can expect. The amandiers, you don’t hardly touch them with a
macrocytic galactodes. On the other hand, let’s take levasole, they don’t
even touch inhibited L4’s at all. In fact, they even have difficulty with
non-inhibited L4’s and you may get if you’re treating with a Tramisol or a
Levasole, you’ll only get about thirty percent efficacy against deeply
inhibited L4’s in cattle. (Dr. Dan) Hey, when we come back, we’ll continue
our discussion with Dr. Rew about anthelmintic resistance, some of the
things that contribute to it. Stay tuned.

(Dr. Dan) Hi there and welcome back to DocTalk. I’m Dr. Dan Thomson. We’re
gonna continue our conversation that I had with Dr. Rob Rew about
anthelmintic resistance and parasite resistance in cattle and sheep. In the
previous segment, we talked about the parasites and how they contribute to
resistance. In this segment, we’re gonna talk about how the products
themselves contribute to it. (Male) Okay. Now we’ve got these molecules.
The Levasole in that class of compounds are very active against adults. A
little less active against L4’s. Don’t touch inhibited to speak of. The
mode of action is very rapid. It’s on the nervous system. It’s a very water
soluble compound. It’s in the body, out of the body, twelve hours of
efficacy, okay. If we expect to have more efficacy, longer period of time,
we’re doing a disservice to classic water soluble compounds. They’re less
water soluble, so they get L4 as well. They get inhibited L4’s
between nine and thirty percent, depending on how deep the inhibition is.
In the southern U.S. in the southern climate, you’ll get thirty percent. In
the northern U.S. in the summertime, you get a hundred percent. So you have
to be aware of that as a limitation of that class of compounds and you get
adults and you get some egg activity too. Their mode of action is
microbial inhibition, thirty-six hours of efficacy. Now the macrocytic
galactodes are very insoluble in water, very fat soluble molecules or big
thicks. The activity gets L3’s, L4’s, inhibited L4’s, adults and the mode
of action on the nervous system, not to say part of the nervous system is
levasole, but they have from one to five weeks of activity, depending on
the parasite and the conditions of the challenge. Also, we’ve got carrier
issues in our products. Another product, if you treat an animal that has a
full abomasum or it’s full of food, you reduce the amount of drug that’s
absorbed by thirty percent. If you fast it for twelve to twenty-four hours,
you get a boost of thirty percent more product in the animal. Injectable’s,
we have to be real careful when we talk about injectables because are we
going sub-q? Are we going I.M.? Are we going I.V.? I.V. can be very toxic.
If you’re trying to do a subcutaneous product and you hit I.M., it could be
very damaging to the muscle and get hung up in the muscle. These are issues
we have to be aware of. Pour-on’s, oh my goodness, pour-on’s, dirt on the
hair, long hair, cold weather, licking, we’ve got all kinds of ways we can
change the amount of product available to that animal and we may call that
resistance or lack of efficacy. In fact, it’s just simply a physical barrier
issue. Oxidative-free excipients for products with unstable oxidative
molecules. This is one of our problems for macrocytic galactodes. If you
have oxygen in that bottle, there is a bond between position twenty-two and
twenty-three on that molecule that cleaves very easily. Now we can have
oxygen because we don’t have real, pure recipients. We could have oxygen
because we use part of the bottle, but bounce it or drive it in our truck
for a while. We can have oxygen for a lot of reasons, but it will
inactivate the molecule. (Dr. Dan) Stay tuned for more of our discussion
with Dr. Rew on anthelmintic resistance and parasites and this time, we’re
gonna talk about how the host can contribute to the problem.

(Dr. Dan) Hi there, folks. Welcome back to DocTalk. Dr. Dan Thomson here.
we’re gonna continue our discussion that I had with Dr. Rob Rew about
anthelmintic resistance and this time we’re gonna talk about the impact of
the host, whether it’s the sex, the age or the breed of the animal and
we’ll talk about some of the physiological states or stressors that may
lend itself to parasite resistance of anthelmintics. (Male) Okay, what
about the hosts? My goodness, they’re gonna have an impact on the efficacy
because really what we talk about efficacy, we’re talking about the product
interacting with the immune system to really get full efficacy. Now, if
we’re looking at different species, of course, goats, their immune system
for internal parasites are useless. In fact, we should never actually graze
a goat. They are born browsers. They are not geared to be good animals to
graze and pick up larvae and kill those larvae with the immune system. They
also have another issue and that is the fact that because they are born
browsers, they have very, very active metabolic insides in their liver and
they’ll chew up drugs. You’ve got to we made this mistake for years and years.
I’ve been in this business for forty years and we’ve been doing it for
forty years. Here we get a sheep registration and we put it in goats, okay,
let’s say it’s at a hundred in sheep. It needs to be a hundred and fifty or
two hundred. It goes to get the same effect and yet, we put goat on the
label. Cattle, you know, they have the best immune system of those three
species. Also, there’s sex issues. Male animals, especially intact males,
have the worst immune systems in terms of parasite control and then there’s
steers. There’s cows. There’s yearlings and breeds. Dairy breeds are the
worst though English is up to something with an ear and a hump and age,
calves are very susceptible or have very weak immune systems, yearlings, a
little better and adults are a very strong immune system. Okay, what about
other stressors in the system because we’re talking immune information.
We’re talking about transport information. We’re talking about metabolism
information. We have all kinds of stressors that affect those things,
calving. A cow will go from an eight count of five to an eight count of
thirty, which drops a calf. She didn’t get more parasites, but her immune
system dropped and allowed the parasites in her to shed eggs. Viral
infections, we have the same immune system working with viruses that work
against the parasites, internal parasites. If we have a viral infection,
we’re fighting for the energy. Steroid implants, heat stress, cold stress,
the cold stress can be on the animal or it could be on the product.
It could be poor absorption. Heavy parasite infections, we
cannot control parasites under very high challenge the way we can under a
lower challenge and adequate nutrition. I mean that’s the driving force
from the immune system. That’s the driving force for the growth. That’s
gonna impact the affect. (Dr. Dan) Hey, when we come back after the break,
we’ll continue the discussion with Dr. Rew and we’ll talk about how we’re
gonna look and find the efficiency of the anthelmintic problems, some of
the issues with managements and why it’s so difficult to spot parasite
resistance right here in the United States.

(Dr. Dan) Hi there and welcome back to DocTalk. We’re gonna continue our
discussion with Dr. Rob Rew on anthelmintic resistance of parasites in food
animals such as beef cattle and sheep. We’re gonna talk about some of the
management strategies that can be incorporated and we’ll talk about some of
the things on handling the products and storing them that can cause
problems as well. Finally, we’ll talk about why it’s so difficult to find
anthelmintic resistance right here in the United States when it’s right
underneath our nose. (Male) Okay, management, here we go. Now probably the
number one thing is we don’t weigh our animals before we dose them, so we
don’t dose to the right amount. Either you’re under dosing or over-dosing
or we don’t know what we’re doing and we say, This product didn’t work.
Well maybe I under-dosed that animal by thirty percent. Why wouldn’t it
work? Of course it’s not gonna work in the way it should. Stocking
densities, I mean, they’re gonna get heavy stocking densities. The animals
are gonna get infected more rapidly. You may look at a fecal egg count
reduction that comes up very quickly after the treatment because they’re
re-infected. Rotational grazing, that’s an excellent way of doing things,
just put on older hosts before you to kind of suck up the parasites before
you put on younger hosts. The refusia, that’s something that’s come up
really big in this FDA Report recently. It’s basically talking about having
a susceptible population of parasites put on the pasture so that they would
kind of drought out the effect of the resistant parasites on the pasture.
To me, that’s a red herring and I’ll get back to that in the last line.
Proper nutrition, without proper nutrition, we don’t get good efficacy and
we don’t get good growth. Animals missing the chute, we all know about
that, but that’s probably pretty minor. Good records, oh my, if we want to
really evaluate our parasite control programs, we have to keep good records
of how they’re growing or what the calving rates are or what the calving
weights are or whatever it is that we’re gonna use as a parameter to evaluate
did we get a good parasite control this year? Last year, was it better? Was
it worse? What did we change in the meantime? We have to have that
information and that is so key, record keeping is really key because we’ll
talk about my recommendations for how to look for whether the product
worked or not. FDA guidelines, they’re recently talked about harrowing
pastures. That’s a good idea if you have hot, dry weather and you have no
animals on the pasture. Don’t do it if you have animals still on the
pasture and you have moist weather. You’re just gonna get yourself in a
worse place and they don’t put that in those guidelines. Evaluation
techniques. Now in sheep and goats, we have this fabaceae technique which
because it is a blood sucker, the eyes turn white when an animal is really
sick and you can know which animals to treat and which animals you don’t
have to treat. That’s wonderful. We don’t have that in cattle. We have
fecal egg count reduction. Fecal egg count reduction is not a quantitative
test. It is a qualitative test. The parasites are there or they’re not
there. That’s all it’s telling you because you can’t measure L4’s. You
can’t measure L3’s. You can’t measure inhibited L4’s. You can’t measure
probably two-thirds of your parasites in the animal with the fecal egg
count. Weight gains, that’s what I use as my primary measurement of
parasite control, end of story. Body conditioning score, raise calving
weights, yeah, it’s good. They’re helpful, but there’s so many other
variables in developing those particular scores that it’s very, very hard.
Now here’s the problem, we have no idea what the extent of anthelmintic
resistance in cattle in the U.S. is. I have been working this field for
forty years and the truth of the matter is, I have three documented cases
of cattle parasite resistant properties, internal parasite resistant
properties and I can tell you where they are, but I have no…There is
nothing else in the literature that tells me what our problem is in the
United States. We need a treatment and timing and an efficacy for these
products really done well. Now my, I mean, we always hope and we always look
forward to new technologies that are coming along. That are gonna help us
with some of these things, but in the meantime, what we need is to be able
to make sure we understand what resistance is, what causes it and we need
to understand how to quantitatively evaluate it in cattle in this country.
(Dr. Dan) Hey there. Thanks for joining us today on DocTalk. I sure hope
you enjoyed the show with some great information from Dr. Rew and our hat
goes off to him for sharing that with us today. Remember, always work with
your local veterinarian and if you want to know more about what we do at
the College of Veterinary Medicine or on DocTalk, you can find us at or at Thanks for watching DocTalk today.
I’m Dr. Dan Thomson here from the College of Veterinary Medicine at Kansas
State University and I’ll see you down the road.

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