26 November 2009
Deworming Programs
What is your program?
Deworming is a tough topic to tackle because there are so many theories and so many “old
wives tales” that confuse the issue.
Deworming is an individual horse and an individual herd treatment. The “program” followed
depends on your horse as an individual and the entire herds needs.
Choosing your “program” is a tough decision and should be an informed one.
Firstly, let us look at what these worms cause; secondly, let us look at the culprits involved - you
got to know what you are up against & thirdly, the ammunition available to us to fight these
worms and when we need to use it.
We all treat for worms and know that there’s some kind of reason for all the fuss but what can
you see on your horse that will show you that it has worms
The signs of parasitism are:
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Distended abdomen
Dull hair coat
Dull attitude
Slow or stunted growth
Trouble maintaining weight
Recurrent colic
Diarrhoea
Acute colic
Impactions
Weight loss
Ischaemia of the gut
All these symptoms can be caused by internal parasites and all these conditions are avoidable
with the correct deworming program.
Now that we know what they can do, who are these culprits that cause all this trouble
Here are a few of our top culprits:
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Large Red Worms (Strongyles)
These worms used to be of great concern but modern day dewormers have proven very
effective in eliminating them.
The life cycle of these worms involves migration of larvae from the bowel into the
abdomen, before returning back to the bowel as adult worms. Strongylus vulgaris,
migrates specifically to the arteries supplying the bowel and can cause obstruction to
the blood flow, resulting in a serious form of colic sometimes necessitating surgery.
·
Small Red Worms (Cyathastomes)
These are the main culprits today causing low grade colic and even acute colic due to
the damage done to the bowel. These worms have a relatively short life cycle and have
developed resistance to modern dewormers. This problem of resistance is compounded
by the tendency of these worms to encyst (hibernate) in the lining of the bowel. When a
horse swallows cyathastome larvae from the pasture, the larvae develop within the
lining of the bowel and then re-emerge as adults. However a portion of these larvae
encyst in the lining and may remain there for years. When in the hibernating stage these
worms are very resistant to most dewormers. The larvae then re-emerge, often at the
end of winter and this re-emergence may involve large numbers at the same time,
resulting in enormous damage to the bowel wall, which may even prove to be fatal.
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Tapeworms (Cestodes)
Tape worms can cause colic and are a tricky culprit as their eggs are not usually detected
in faecal samples and they are not sensitive to our Macrocyclic lactones.
·
Roundworms (Ascarids)
This worm’s life cycle includes larval migration through the lungs to the intestine and
can therefore result in coughing. The worms usually affect foals and young horses and
can cause impaction colics in these youngsters.
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Pinworm (Oxyuris)
Causes irritation to the horse around the anus.
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Bots (Gastrophilus)
These are not worms but larvae of the Gastrophilus fly. The fly lays eggs on the horse’s
legs and when the horse grooms itself it ingests these eggs. In the stomach, the eggs
develop into larvae and are passed in the dung as pupae.
Now that we know what we up against, let’s see what ammunition (dewormers) we have to
throw at these worms. There are different classes of dewormer and they work against
different species of worm and at different stages of the life cycle
Classes of Dewormers:
1) Macrocyclic Lactones
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Ivermectin
Abamectin
Moxidectin
2) Pyrantel
3) Benzimidazoles
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Fenbendazole
4) Praziquantel
Now which ammunition can we use for which worm…
I think now is a good time to mention the issue of resistance…. This is when our ammunition
(dewormers) is not effective against certain worms anymore. The reason for this is the
improper and over use of dewormers. More is not always better. The key to avoiding this
problem is proper deworming at the appropriate time.
Firstly, always use the correct amount of dewormer for your horse. This is usually determined
by weight and it is important to know if you are giving the correct amount. Most dewormers
come with a clever dispenser that you can adjust to dispense for a certain weight but how much
does your horse actually weigh? There are tapes available that one can measure the girth size
and gauge the weight that way. On average a “normal” sized thoroughbred horse that is racing
fit probably weighs about 500kgs.
Secondly do not overuse dewormers. Dosing every 3 weeks in a low risk area where the
paddock load is low and the through fare of horses is at a minimum is unnecessary and will lead
to the development of resistant worms.
Thirdly remember that it is important to alternate your dewormer. This does not mean you can
simply use a product with a different “name”, you have to check that they have different active
ingredients. Below are some of the products available in South Africa. Note the Trade names
are different but some of the active ingredients are the same.
Dewormers available in South Africa:
Trade name
Active ingredients
Pegaforte
Ivermectin & Praziquantel
Pegamax
Abamectin & Praziquantel
Pegasol
Pyrantel Pamoate
Bimectin
Ivermectin
EquimaxNF
Ivermectin & Praziquantel
Panacur
Fenbendazole
Antezole
Pyrantel pamoate
Promectin
Abamectin & Praziquantel
Equest
Moxidectin
Solution
Ivermectin
Note that for example, Pegaforte and Equimax NF have exactly the same active ingredients so
alternating between these two dewormers could lead to the buildup of resistance.
There are factors other than purely the culprit (worm) and ammunition (dewormer) to
consider, these include:
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Age
- foals and young horses are more susceptible to certain parasites
Location
- certain parasites are more common in certain areas or climates
Season
- certain parasites are only active during certain seasons
Travel
- horses who travel may be exposed to infected horses
Pasture load - more horses grazing in a given area increases the parasite load
Pasture friends - other animals may carry parasites that can infect your horse
With all these factors to take into account, one can see why the deworming “program” is
different for each horse. Before we get to the programs we need to consider the treatment of
the “main culprit” (small red worms/Cyathostomes)
Small red worms (Cyathstomes)
As we have seen from their life cycle, these worms have developed resistance to most of our
dewormers, they encyst (hibernate) for long periods in the gut and have a mass migration from
the gut wall leading to severe damage to the gut lining. These worms pose a major problem for
your horse and could be the cause of recurrent low grade colic, weight loss and being unthrifty
along with most of the other signs of parasitism.
How do we rid our precious horses of this sneaky parasite? The only treatment proven to work
on the encysted (hibernating) is a 5 day course of Fenbendazole (Panacur) followed by a dose of
a macrocyclic lactone on day 6.
If one is not using faecal egg counts then these worms are usually treated for seasonally just
before and just after winter (May and October)
Now taking into account all the culprits, the ammo and the other factors, let’s see if we can
make it simple. At the end of the day, you need to choose what works for you. There are some
deworming “programs”
Deworming “programs”
Selective deworming
The gold standard for deworming and deciding when to deworm is doing regular faecal egg
counts. This method ensures that horses do not get dewormed unnecessarily.
This is a very simple procedure and will help you to decide when to deworm your horse only
when it has a high parasite burden and prevent overuse of dewormers which is leading to
resistance.
To perform a faecal egg count:
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Collect fresh faeces
Wear a glove
Seal in a plastic bag
Keep the sample chilled
Faecal egg counts are performed at the state laboratories. , The cost is minimal and the results
take about a day to be known. Horses are only treated if the parasite burden is high.
Factors to keep in mind when using this “program” is that worms do not shed eggs all the time
and one could get a low egg count when there is actually a high parasite burden inside the
horse. Tape worm eggs are not routinely seen on faecal egg count so remember to treat for
these worms at least once a year.
If your horse is in a high density paddock or is traveling around often, it is advisable to perform
these counts regularly.
Broad spectrum deworming
This method is deworming every 4 months with a Macrocyclic Lactone based product that also
containsPyrantel or Praziquantal. If using this method remember to treat for the encysted
Cyathostomes as they are resistant to these dewormers.
Rotational deworming
2way Rotational deworming
This program alternates every 3 monthly between a broad spectrum dewormer (macrocylic
lactone and Praziquantal) and a specialized dewormer (Pyrantel or Fenbendazole).
3 way rotational
This program alternates every 2 monthly between a broad spectrum dewormer (macrocylic
lactone and Praziquantal) and a specialized dewormer (Pyrantel) and a Benzimidazole
(Fenbendazole)
We all know that worms are mostly transferred through manure so managing the manure will
go to great lengths to assist in managing the worm burden.
Management - ways to decrease the worm burden:
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Clean
- remove and dispose of manure at least twice per week.
Rake pastures
- break up manure piles to expose eggs and larvae to the sun.
Rotate pastures
- move horses between pastures to naturally break parasite life
cycles.
Group horses
- group horses by age to maximize deworming schedules.
Reduce pasture load - fewer horses per paddock reduces fecal contamination.
Use elevated feeders - lift roughage off of the ground
In conclusion, there is no single correct answer and your deworming program should change
according to your horse’s age, circumstances and the environment where it lives. Please consult
your veterinarian about the best deworming program for your horse. We are happy to assist in
any way we can.