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 bad-

tasting

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 FDA-certified)

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 broad-

spectrum

Purge Deworming Drugs How they work Classes & Brand Names

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

BenzimidazolesGeneric Name Brand Name Safety Level

Fenbendazole Panacur, Safeguard 100X

Oxfendazole Benzelmin 10X

Oxibendazole Anthelcide EQ 60X

Benzimidazoles Effective control of following

parasites: 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

TetrahydropyrimidinesGeneric Name Brand Name Safety Level

Pyrantel Pamoate Exodus, Rotectin-P, Equi-Cide, PSI’s suspension, Liqui-Care P, Strongid Paste

20X

Pyrantel tartrate Strongid C (2X), Continuex (2X)

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Tetrahydropyrimidines Effective against the following parasites:

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 LactonesGeneric Name Brand Name Safety Level

Ivermectin Zimecterin, Rotation 1, Ivercare, Equell

60X

Moxidectin Quest, Quest Plus, ComboCare

3X - 5X*

Macrocyclic Lactones Effective against the following

parasites: Strongyloides Ascarids** Large Strongyles Small Strongyles (adult only – ivermectin;

all stages - moxidectin) Bots Pinworms

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

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

Ä 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 Autumnwinter 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 PeriodsAnthelmintic Expected Egg

Reappearance Period

Strongyle contaminative

PeriodBenzimadazoles 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:

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

worms1. Small number of resistant worms

present2. Deworm – kills off non-resistant

worms3. 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

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