Internal Parasites, Part II Control
INAG 120 – Equine
Health Management
November 16, 2011
Deworming Protocols
 Rotational Purge Deworming:
 Goal is to keep the load of eggs and
larvae to a minimum
 Done every 8-12 weeks via oral paste
 Kills adult and/or larval stages of
worms inside the horse before they
start producing eggs
Drawbacks of Purge Deworming
 If other horses in same area not
dewormed when needed, environmental
reduction won’t work
 Some parasite eggs can live as long as 30
years in the soil!
 Immature worms  mature worms =
migration through intestinesand other
tissues = damage!
 Success depends on timing
 Many purge dewormers don’t kill bots
Myths: Vets must tube-worm for it
to be effective
 Only way to guarantee that all the
drug is given
 Old dewormers were irritating or badtasting
Myth: Toxicity will build over time
in my horse
 Organophosphates (used in 60’s and
70’s) did cause problems
 Major concern with modern
dewormers:
 Ascarid impaction in foals
Myth: Dewormers aren’t safe for
use in broodmares
 If drug label says it’s safe for mares:
 Manufacturer must TEST it for FDA
approval
 Requires 2 years of demonstrated efficacy
 The Pfizer Babies of CSU
 Praziquantel study
in France
 Quest?
Myth: Diatomaceous Earth is just
as effective as chemical dewormers
 Herbs and other compounds are not
required to be tested (not FDAcertified)
 No scientific evidence supports use as
dewormer
Myth: Rotational deworming will
prevent development of resistance
 Rotational deworming started about
40 years ago
 Reason was NOT resistance
 Earlier dewormers were not broadspectrum
Purge Deworming Drugs
 How they work
 Classes & Brand Names
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Benzimidazoles (Panacur Pack)
Tetrahydropyrimidines (Pyrantel Pamoate)
Heterocyclic compounds (Piperazine)
Macrocyclic lactones (Ivermectin/Moxidectin)
Isoquinoline-Pyrozines (Praziquantel)
 Safety and efficacy
Dewormer Products
 All must be approved by FDA
 Rigorous testing required
 Must be proven safe and effective
 Must remove at least 90% of target
parasites
 Most are broad-spectrum
 Don’t require refrigeration but can be
damaged by excessive heat
How do they work?
 Nematocides
 Death by starvation
 Death by paralysis
 Worms can’t store energy
 Must eat continuously
 Most parasites will die within 24 hours if
eating process is interrupted
 Paralysis blocks ability to stay in gut
Benzimidazoles
 Interfere with metabolism on a cellular
level
 Bind to a particular structure, thereby blocking
energy metabolism
 Because of mechanism of action, can also
kill eggs
 Available in granules, paste and suspension
 More effective when given several days in a
row
Panacur Powerpac
 5 Days of Panacur
(Fenbendazole) at 2x dosage
 Kills everything
 Good for new horses coming into
your herd as treatment
 Good for all horses moving to new
area where there were no horses in
the past
Pancur Powerpac
Panacur Powerpac
Benzimidazoles
Generic
Name
Brand Name
Safety Level
Fenbendazole
Panacur,
Safeguard
100X
Oxfendazole
Benzelmin
10X
Oxibendazole
Anthelcide EQ
60X
Benzimidazoles
 Effective control of following
parasites:
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Strongyloides (except benzelmin)
Ascarids
Large strongyles
Pinworms
NOT Bots
NOT Small Strongyles
 Exception is Panacur PowerPak
Tetrahydropyrimidines
 Say that 5 times fast…
 Mimic activity of acetylcholine (a
neurotransmitter that causes muscle
contraction)
 With tetrahydropyrimidines, contraction is
permanent  rigid paralysis of parasite
 Fastest activity of any deworming product
 Only affect adult parasites (not larval stages)
 Parasites bounce back quickly
 Paste, suspension, and pelleted forms
Tetrahydropyrimidines
Generic
Name
Brand Name
Safety Level
Pyrantel Pamoate
Exodus, Rotectin-P,
Equi-Cide, PSI’s
suspension, LiquiCare P, Strongid
Paste
20X
Pyrantel tartrate
Strongid C (2X),
Continuex (2X)
--
Tetrahydropyrimidines
 Effective against the following parasites:
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Ascarids
Large Strongyles
Small Strongyles
Pinworms
NOT effective for Bots
NOT effective for Strongyloides
Effective for tapeworms when given at twice the
normal dosage
Heterocyclic Compounds
 Only one used in horses (Piperazine)
 Depolarizes muscle membranes –
resistant to acetylcholine
 Worms become paralyzed
 Limited to adult parasites
 Available only in powder and liquid
(stomach tube), pelleted
 Brand name: Piperazine (1X safety
factor!)
Macrocyclic Lactones
 Act on parasite’s nerve and muscle
cells
 Normal transmission of stimuli
disturbed
 Flaccid paralysis  inability to feed or
swallow nutrients
 Most potent killers! But, slow to act
 Ability to kill external parasites
 Lice, mites, ticks
Macrocyclic Lactones
Generic
Name
Ivermectin
Moxidectin
Brand Name
Zimecterin,
Rotation 1,
Ivercare,
Equell
Quest, Quest
Plus,
ComboCare
Safety Level
60X
3X - 5X*
Macrocyclic Lactones
 Effective against the following
parasites:
Strongyloides
Ascarids**
Large Strongyles
Small Strongyles (adult only –
ivermectin; all stages - moxidectin)
 Bots
 Pinworms
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Isoquinoline-Pyrazines
New kid on the block
NO ACTIVITY AGAINST NEMATODES!
Effective only against tapeworms
Disrupts worm’s outer layer  worm
can’t maintain fluid balance
 Generic name: Praziquantel
 Brand names: Equimax, Quest
Plus, ComboCare Gel, and
Zimectrin Gold
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Purge Deworming Sample Adult
Horse Program (Maryland):
 February: Deworm with Ivermectin +
Praziquantel to kill bots and Tapeworms
 April: Deworm with Moxidectin to kill
encysted strongyles
 August: Deworm with Ivermectin or
Ivermectin + Praziquantel to kill bots
and tapeworms
 October: Deworm with Oxibendazole
 December: Deworm with Ivermectin to
kill bots
Problems with rotational
deworming as we know it
 Reasons for deworming often not
known
 Drug chosen may not be effective
against parasite present in horse
 Don’t discriminate between horses in
different parts of the country (i.e.
Florida vs. New York)
 Horses vary widely in susceptibility to
parasites
How should we deworm, then?
 Target parasites
 Tapeworm – once a year, during spring or
autumn (more often for known problems)
 Bots – must enter host prior to winter, deworm
late autumn/early winter
 Large strongyles
 Most horse owners have unknowingly
eradicated large strongyles already
 Treat all horses at intervals of 6 months for 18
months
 Small strongyles present greatest problem today
Targeting small strongyles
 Objective of control
 NOT to “kill worms”
 Prevent contamination of environment
with eggs
 Kill female worms before they reproduce
 Environmental factors
 All horses pass strongyle eggs at a
predictable time post-treatment
 Infectivity of eggs is dependent upon
environmental factors
Targeting small strongyles…
 Environmental factors…
 Northern states:
 Hatching and development during spring,
summer and autumn
 Autumnwinter favorable for persistence!
 Southern states:
 Hatching and development during autumn
and spring
 Summer: development and survival poor;
winter ok
Targeting small strongyles…
 Host factors
 Individual horses differ!
 Routine deworming may be unnecessary for
some horses in a herd
 Categorize horses
 Perform quantitative fecal examinations
 Anthelmintic Issues
 FECRT (Fecal Egg count Reduction Testing)
 Know expected egg reappearance periods for
the different compounds
Determining Strongyle
Contaminative Potential
 Requires fecal egg counts!
 If horses haven’t been dewormed recently:
 20-30% = high egg counts
 30-50% = low egg counts
 Less than 150 eggs per gram = Low
Contaminators
 Greater than 500 EPG = High
Contaminators
 Examine fecal samples 4 weeks after
expiration of egg reappearance period
Expected Egg Reappearance
Periods
Anthelmintic
Expected Egg
Strongyle
Reappearance contaminative
Period
Period
Benzimadazoles 4 weeks
8 weeks
Pyrantels
4 weeks
8 weeks
Ivermectin
8 weeks
12 weeks
Moxidectin
12 weeks
16 weeks
Sample Schedule for Deworming
 October – Moxidectin and Praziquantel for ALL horses
 November, December, January, February –
NOTHING (too cold)
 March – FEC, identify contaminators
 Ivermectin to all horses
 April – NOTHING
 May – FECRT
 Strongid to moderate and high contaminators from
March
 Recheck fecal in 10-14 days
 June – Strongid to high contaminators
 July, August – NOTHING (too hot)
 September – Strongid to moderate and high
contaminators
Slow Rotation
 A recommendation by some
parasitologists
 Rotate annually:
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Moxidectin – year one
Strongid – year two
Ivermectin – year three
Panacur – year four
Deworming Protocols
 Daily Dewormer:
 Prevents damage done by immature
worms migrating through internal
organs
 Few worms will survive to maturity 
few to no worm eggs in manure 
decreased likelihood of reinfestation of
environment
Drawbacks of Daily Deworming
 Active ingredient does not kill bots
 You will have to give an ivermectin twice
per year!
 Light infestation may lead to natural
immunity, dailies may prevent that
 Not “natural” care even though no
studies have shown any toxic effects
 Accumulation over time?
Daily Dewormer Schedule
 Spring Thaw: Ivermectin or Moxidectin to
kill bots
 Spring – early summer: daily dewormer
 June 1: if tapeworms are a problem,
double-dose of pyrantel pamoate or
praziquantel
 June 2 – killing frost: daily dewormer
 Killing frost day: Ivermectin or moxidectin
to kill bots
 Day after – spring thaw: Daily dewormer
Anthelmintic Resistance
 Drug resistance = ability of worms in
a population to survive a treatment
that once was effective against the
same population
 Same drug
 Same dose
 Same parasite
 VERY common
Resistance
 High mutation rate among some
worms
1. Small number of resistant worms
present
2. Deworm – kills off non-resistant
worms
3. Resistant worms survive and
reproduce, population grows
Does resistance exist in horse
populations?
 Small strongyles = most problematic
internal parasites in horses
 Wide range of symptoms
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Rough hair coat
Poor growth
Suboptimal performance
Life-threatening chronic diarrhea, colic, and
severe weight loss
 Most effective control = deworming
medications
 Some small strongyles are resistant to
dewormers!
Does resistance exist in horse
populations?
 Resistance to Panacur on 90% of the farms tested
 20% of farms - resistance to Strongid
 No evidence of resistance to Ivermectin on any farm
 Few farms were tested
 Ivermectin resistance may exist elsewhere
 Ivermectin-resistant parasites have been found in
sheep and goats
 Continue using ivermectin, moxidectin, and even
pyrantel – check for resistance in your herd
 No benefit to rotating dewormers with each treatment
 Slow rotation recommended: one class per year
Management to control
Parasites
 Pasture Management!
 Remove feces from
congregation areas
 Drag pastures
regularly to break up
manure
 Do not overstock
pasture!
 Rotational grazing
 Biological vacuum cleaners
 Compost manure before spreading