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

Benzimidazoles

Generic Name Brand Name Safety Level

FenbendazolePanacur,

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

Tetrahydropyrimidines

Generic 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 Lactones

Generic 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 Periods

Anthelmintic Expected Egg Reappearance

Period

Strongyle contaminative

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: 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

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