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External Parasites. INAG 120 – Equine Health Management November 26, 2008. Ectoparasites. = parasites that attack skin and body openings Flies Black Flies/Midges Ticks Mosquitoes Lice Mites. Mechanism of blood feeding. Females: Blood = Protein

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External Parasites

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External Parasites

INAG 120 – Equine Health Management

November 26, 2008

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  • = parasites that attack skin and body openings

  • Flies

  • Black Flies/Midges

  • Ticks

  • Mosquitoes

  • Lice

  • Mites

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Mechanism of blood feeding

  • Females: Blood = Protein

  • Males generally subsist on sugars from nectar, etc.

  • EXCEPT: stable flies and horn flies

    • Both sexes feed on blood

  • Flies can detect and follow an “odor plume” at great distances

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Mechanism of blood feeding

  • Most flies can detect Carbon Dioxide

  • Flies are also sensitive to heat and moisture

  • Mouth-parts differ between species

    • Blade- or sword-like with serrated edges

  • Once blood starts flowing, fly secretes saliva that prevents coagulation

    • Saliva is allergenic and causes swelling and irritation

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Life cycles

  • Four major phases of life:

    • Egg

    • Larva

    • Pupa

    • Adult

  • Lifecycles vary in timing and duration depending on species

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Disease transmission

  • Insects that transmit diseases = vectors

  • Two types of transmission:

    • Mechanical

    • Biological

  • Deerflies, horseflies, stable flies are thought to be able to transmit anthrax on their mouthparts

  • Mosquitoes and ticks serve as biological reservoirs for other diseases

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  • Horseflies

  • Deer flies

  • Stable flies

  • Horn flies

  • Face flies

  • Bot flies

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Horseflies & Deerflies

  • Breed in boggy areas

  • Active only during the day in warm weather

  • Deerflies have patterned wings and are smaller

  • Horseflies have transparent wings

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Horseflies & Deerflies

  • Larvae overwinter in the soil

  • Prefer wet mud near or under ponds, marshes, or streams

  • One cow can lose one quarter liter of blood per day in heavily infested areas!

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Stable Flies and Horn Flies

  • Introduced from Europe

  • Spend almost entire adult lives on their host (horses and cattle)

  • Stable flies look like house flies

  • Bite ankles of people, legs of horses

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Stable Flies and Horn Flies

  • Mouth parts are jabbed into skin like a needle

  • Curved spines at the tip move back and forth making hole deeper and wider

  • Larvae develop in manure and decaying vegetation

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Face Flies

  • Non-biters

  • Closely resemble house flies, largerthan horn flies

  • Feed on mucoussecretions aroundeyes, nose, mouth

  • Lay eggs in freshmanure

  • Can transmit eyeproblems

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Bot Flies

  • Lay their eggs on legs and chests of horses

  • Horses lick that area ingest eggs

  • Eggs hatch in intestines

  • Internal/External parasites!

  • Deworming program to control bots

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Black Flies/Midges - Onchocerca

  • Spread a parasitic roundworm, Onchocerca, which causes bumps to form in skin, can also be found in the eye!

Onchocerciasis in the eye of a horse. By permission from Knottenbelt DC, Pascoe RR, Diseases and Disorders of the Horse, Saunders, 2003

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  • Lyme disease

  • Rocky Mountain Spotted Fever

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Lyme Disease

  • Spirochetal (corkscrew-shaped) bacteria – Borrelia burgdorferi

  • Transmitted through the bite of a deer or black-legged tick

  • Endemic areas for Lyme disease:

    • Northeast

    • Mid-Atlantic

    • Northern Midwest states

    • Northern California

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Borrelia burgdorferi

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Lyme Disease Transmission

  • Larval deer ticks can become infected with bacteria if they take a blood meal from a rodent already infected

  • Transmit disease with subsequent blood meals

  • Ticks have 3 developmental stages:

    • Larvae, nymph, adult

    • Must have a blood meal before they can molt to next stage

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Two-Year Life Cycle of Deer Tick


  • MEAL 1

  • Mouse

  • Bird



Eggs laidadults die

MEAL 2Peak Feeding inpeople, horses, mice

Nymphs moltinto adults

  • Meal 3 (for adults thatdidn’t feedin fall)

  • Person

  • Deer

  • Horse

Larvae moltinto nymphstage






  • Meal 3

  • Person

  • Deer

  • Horse

Nymphs dormant

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Lyme Disease Transmission

  • Ticks live for 2 years

  • Must attach to animal host and feed for 12-24 hours before the bacteria can be transmitted to new host!

  • Natural host of larval ticks = white-footed mouse

  • Host of nymph ticks = humans, rodents, dogs, cats, birds, etc.

  • Host of adult = deer plus others

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Lyme Disease

  • Multisystem disease!

  • Clinical Signs:

    • Joints

    • Musculoskeletal system

    • Neurological system

    • Subclinical infection is common!

    • Development of clinical signs only occurs in 10% of infected animals!

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Lyme Disease and Horses

  • Spring and Fall  adult tick most active

  • Found commonly around head, throatlatch area, belly, under tail

  • Prompt removal of tick reduces risk of infection

  • Most common signs = behavioral changes and shifting lameness

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Lyme Disease and Horses

  • Diagnosis is difficult – VERY political!

    • Blood test detects antibodies/exposure to bacteria

    • History of tick exposure (or endemic area)

    • Veterinary clinical exam suggestive of Lyme disease

    • Elimination of other possible diagnoses (lameness exams, x-rays, blood work for other diseases, etc.)

    • Positive blood tests for Lyme Disease

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Lyme Disease Treatment

  • Antibiotics –

    • “Gold Standard” = IV Tetracycline (6.6 mg/kg) for 10 days followed by oral doxycycline for 30 days

    • Oral doxycycline alone more common (10 mg/kg 2x per day)

    • Several weeks – with response to therapy within 2-5 days

    • Monitor titers

  • Anti-inflammatories

  • Pro-biotics to replenish gut microbes killed by antibiotics

  • Side Effects!

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Lyme Disease Prevention

  • No Vaccine licensed for horses


  • Daily grooming and removal of ticks

  • Tick repellents applied to head, neck, legs, belly and under tail

    • Permethrin or DEET are particularly effective

  • Keep pastures mown

  • Remove brush, woodpiles, etc. to decrease rodent nesting areas

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  • May be encountered day and night

  • Many different species

  • Attracted to incandescent light but not to fluorescent light!

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  • Most common of external parasites

  • Two varieties:

    • Chewing/Biting – feed on skin cells

    • Sucking – feed on blood

  • Horse with lice:

    • Heavy dandruff

    • Greasy skin

    • Bald spots

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  • Can cause weight loss, general unthriftiness, anemia

  • Winterspring problem!

  • Lice are host-specific and spend their entire lives on the animal!

  • Transmitted by direct contact

  • Control with pesticide

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  • Microscopic!

  • Can cause mange

    • Sarcoptic mites (head neck, shoulders, flanks, abdomen)

    • Psoroptic/scab mites – skin surface  gooey scabs and crusts

    • Chorioptic mites  most common, seen on skin, cause scaling on legs “Clydesdale itch”

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Psoroptic Mange…

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Fly Control

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More Fly Control

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Premise Control

  • Control standing water

  • Compost manure far away from animals

  • Chain-drag fields and paddocks

  • Feed pelleted feed vs. sweet feed

  • Stall fans

  • Spray barn with Permectrin or Buzz Off

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