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Preventative Medicine in the Older Horse. Frank M. Andrews, DVM, MS Diplomate ACVIM Professor of LA Medicine The University of Tennessee, CVM. Vaccination and Deworming. Activities of the Older Horses. Equestrian Sports Reproduction Broodmare, Stallion Pleasure Horses.

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Preventative Medicine in the Older Horse

Frank M. Andrews, DVM, MS

Diplomate ACVIM

Professor of LA Medicine

The University of Tennessee, CVM

Vaccination and Deworming

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Activities of the Older Horses

  • Equestrian Sports

  • Reproduction

    • Broodmare, Stallion

  • Pleasure Horses

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Demographics of the older horse

  • Increase in geriatric horses presented to veterinary hospitals (4 to 6 fold, up 55%)

  • NAHMS*-7.5% of horse population is ≥ 20 yrs

    • 75% between 20-29 years (old horse)

    • 25% ≥ 30 years (very old horse)

  • Average age = 25 years (71 human years)

    • 1 horse year  2.85 human years

*NAHMS=National Animal Health Monitoring System, 1988, USDA

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Demographics (as of January 1, 1999)

  • Equine population in US: 5.32 million

    • ~400,000 old horses and ponies in the US

  • Tennessee 3rd in horse inventory behind Texas and California, 190,000 (7th in value of horses sold)

    • 14,250 old horses in Tennessee

      • 10,688 old horses

      • 3,572 very old horses

  • Economic impact: $189.3 million ($2,711/head)

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Changes with Advancing Age

  • Decline in body condition

  • Decreased muscle tone

  • Decline in immune system

  • Increased susceptibility to infections

    • Decreased antibody production

    • Decreased immune response to vaccines

    • Vaccine failure rate ~50%

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Variation in Vaccine Response

  • Genetic variation with age

  • Health status

    • Parasitism

    • Equine Cushings Disease

    • Dental disease

    • Arthritis

  • External stress factors

    • Weather

    • Nutrition

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Vaccination – General

  • No vaccine offers absolute protection

  • Lessen the effects of disease

    • Shorten duration and signs of disease

  • Good vaccine will stimulate immunity while minimizing secondary reactions

    • Swelling at site of injection

    • Injection site abscesses

    • Anaphylactic reaction

    • Fever and illness with loss of appetite

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Vaccine Older Horses and Ponies

  • Relative risk of exposure

    • What diseases will your horse be exposed to?

    • Pastured horse vs. Stabled horse

  • Use attenuated vaccine products

    • Inactivated and killed vaccines

    • Likely to be safer in older horses

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Important Diseases

  • Tetanus

  • Rabies

  • Influenza

  • Eastern and Western Encephalitis (EEE, WEE)

  • West Nile Virus

  • Rhinopneumonitis (Equine Herpes Virus 1)

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  • Caused by Clostridium tetani

  • Abundant in environment, manure

  • Often fatal

  • Infection through wounds

  • Yearly booster

    • New study suggests that yearly booster is essential for protection in horses

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  • Caused by a virus

  • Always fatal in infected horses

  • Humans can get disease from horses

  • Infection from bite of infected animal

    • Skunk, raccoon, bats, etc.

  • Yearly booster-Pastured horses??

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  • Caused by a virus

  • Endemic in the US, stabled horses?

  • New horses brought onto premises

  • Respiratory disease, 3 day incubation

  • Booster every 6 to 12 months

    • Depending on exposure

    • Killed vs. Modified live vaccine

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Eastern and Western Equine Encephalitis

  • Caused by a virus-”sleeping sickness”

  • Spread via mosquitoes

  • Regional areas

    • WEE-West of Mississippi

    • EEE-East of Mississippi (2003 outbreak in SE US)

  • Fatality- EEE=75-100%; WEE=25%-50%

  • Boosters every 4-6 months during mosquito season in endemic areas

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Eastern Equine Encephalitis

  • Over 200 cases reported this year in ten US States

    • Florida, Alabama, Georgia, Tennessee, North Carolina, South Carolina, Mississippi, Virginia, Indiana, and Wisconsin

    • Florida outbreak = 66% fatalities

    • All have residual neurologic deficits

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West Nile Virus

  • Caused by a virus

  • Spread via mosquitoes

  • Throughout the US

    • 2003 – over 4,000 cases in horses

    • 2002 – over 14,000 cases in horses

  • Fatality-30%, 40% may have lasting deficits

  • **Horses over 18 years had a 2.8X greater likelihood of dying or being euthanatized

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West Nile Virus Transmission Cycle

Mosquito vector

Incidental infections

West Nile


Incidental infections


reservoir hosts

Incubation period -- 3 to 15 days

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  • Horses cannot spread West Nile Virus to other horses, people, or pets

  • Dead-end Host

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Prevention of West Nile Virus

  • Client Education a must

    • Not just a source of vaccine and meds

  • No stagnant water

    • 4 days required for mosquito breeding

    • Old tires--and other Redneck landscaping materials

    • Gambusia fish eat larvae in tanks and ornamental pools

  • Topical spraying on horse—permethrins

  • Mosquito Magnet (

  • Stalls at night

    • fans in stalls

Mosquito Magnet

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West Nile Virus Vaccines (USDA Approved)

New Vaccine

  • “Innovator vaccine is 92% effective”

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  • Caused by a Equine Herpes Virus 1

  • Respiratory disease is most common

  • Neurologic, abortion, and weak foal forms

  • Latent infections in horses

    • May be reactivated by vaccination in older horses

    • May precipitate neurologic disease

  • Not recommended in older horses because most horses have been exposed to virus

  • **Broodmares should be vaccinated during pregnancy to prevent abortion

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Vaccine Protocol: Older Horse

Spring: (March 15-31)

  • 4 – Way Vaccine

    • Tetanus

    • Eastern Equine Encephalitis

    • Western Equine Encephalitis

    • Influenza

  • Rabies

  • West Nile Virus

    Summer: (July 31-August 15)

  • Eastern Equine Encephalitis

  • West Nile Virus (most cases in Sept. and Oct.)

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Vaccine Protocol: Optional for Older Horse

Spring: (March 15-31)

  • Potomac Horse Fever

    • Pastured horses on endemic farms

    • Questionable effectiveness

      Summer: (July 31-August 15)

  • Influenza

    • Stabled horses with new arrivals

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  • Resistance-Many of today’s parasites are resistant to some dewormers

  • Adaptation: faster reproduction, environmental changes, to dewormers

  • Parasites must pass into environment to complete life cycle

  • Infection in related to quantity of parasites in environment that horse is exposed to!!

  • Older horses are more susceptible to infections

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Parasites-What problems do they cause??

  • Colic

  • Diarrhea

  • Poor hair coat

  • Weight loss

  • Suboptimal performance

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  • Minimize environmental contamination of the parasite offspring

  • Treatment vs. Prevention

  • Preventing infection of larval stages is key

  • Killing adult parasites important because it prevents them from laying eggs

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Specific Parasites

  • Large Strongyles

  • Small Strongyles

  • Bots

  • Tapeworms

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Large Strongyles (blood worms)

  • Strongylus vulgaris

    • Most common large strongyle

    • Larva stages lodge in main blood vessel that supplies the small intestines

  • Strongylus equinus

  • Triodontophorus spp.

  • Discovery of avermectins (Zimecterin®, Equvalan®, Quest®) have pushed to near extinction

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  • Gastrophilus intestinalis

  • Gastrophilus nasalis

    • Larva stages live in stomach and small intestine

    • Larva released in manure in spring and become flies

    • Flies lay nits (eggs) on front legs of horses

    • Not a big problem in horses

    • Associated with ulcers?

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Small Strongyles (most important)

  • Cyathostomum spp.

    • Affect all grazing horses

    • Infections are acquired from pasture only not stalls or dry lots

    • Eggs hatch at temperatures 45 F to 85 F

    • Temperatures > 85 F larva die

    • Horses are safe from infection during summer in Southeastern United States

    • Eggs resistant to freezing

    • Encyst in large intestine

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Small Strongyles

Normal Large Intestine wall

Encysted Stage

Inflamed Large Intestine Wall

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  • Anoplocephala perfoliata

    • Most common tapeworm

    • Can reach up to 3” in length

    • Ileocecal junction

  • Anoplocephala magna

  • Paranoplocepha mamillan

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  • Life Cycle:

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  • Clinical signs, What to look for in your horse?

    • Colic

      • Spasmodic – 22% associated with tapeworms

      • Ileal impaction – 80% associated with tapeworms

      • Intussusception – blockage associated with telescoping intestines

    • Weight loss

    • Abnormal hair coat

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Tapeworm Treatments

All are FDA approved

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Specific Deworming Agents

  • Benzimadazoles

    • Fendbendazole (Panacur®) - Resistance

  • Pyrantel pamoate (Strongid®) - Resistance

  • Avermectins

    • Ivermectin (Zymecterin®), moxidectin (Quest®)

  • Cestode dewormers

    • Praziquantel (Contained in Zimectrin Gold®, Quest Plus®, and Equimax®)

  • Daily dewormers

    • Pyrantel tartarate (Strongid-C®)

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Don’t be fooled – Read the label!!

These all contain Ivermectin and Praziquantel

Get all parasites including Tapeworms

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Suppressive Deworming

  • Deworming should be based on:

  • Egg Reappearance Period (ERP)

    • Length of time in which the eggs appear in the manure after deworming

      • Panacur and Strongid = 4 weeks

      • Ivermectins = 6 to 8 weeks

      • Moxidectin = 8 to 10 weeks

    • Scheduling deworming based on ERP

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Protocol for Strategic Deworming

  • September 1st

    • Ivermectin + Praziquantel (Zimecterin Gold)

  • October 24th

    • Moxidectin (Quest)

  • December 17th

    • Ivermectin (Zimecterin)

  • March 1st

    • Ivermectin + Praziquantel (Zimecterin Gold)

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Protocol for Strategic Deworming (cont)

  • May 1st through August 31st

    • Limited exposure to develop immunity

    • Parasites cannot live on pastures in summer

  • Fecal egg counts can be done to determine resistance of parasites

    • Egg count before treatment

    • Egg count 7-10 days after treatment

    • Deworming can be adjusted based on egg counts

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Strategic Deworming-Alternative 2

  • September 1st

    • Moxidectin + Praziquantel (Quest Plus)

  • November 1st

    • Ivermectin

  • December 25th

    • Strongid Paste

  • January 20th

    • Panacur

  • February 15th

    • Ivermectin + Praziquantel (Zymectrin Gold)

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Strategic Deworming-Alternative 2 (cont)

  • March 1st through August 31st

    • Limited exposure to develop immunity

    • Take advantage of summer heat

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Strategic Deworming-Alternative 2

  • September 1st through August 31st

    • Strongid C daily dewormer

    • Colic protection policy by Pfizer


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When horses dream??

I am vaccinated and I have no parasites