External Parasites
INAG 120 – Equine Health
Management
November 21, 2011
Ectoparasites
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= parasites that attack skin and
body openings
Flies
Black Flies/Midges
Ticks
Mosquitoes
Lice
Mites
Mechanism of blood feeding
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Females: Blood = Protein
Males generally subsist on sugars
from nectar, etc.
EXCEPT: stable flies and horn
flies
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Both sexes feed on blood
Flies can detect and follow an “odor
plume” at great distances
Mechanism of blood feeding
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Most flies can detect Carbon Dioxide
Flies are also sensitive to heat and
moisture
Mouth-parts differ between species
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Blade- or sword-like with serrated edges
Once blood starts flowing, fly secretes
saliva that prevents coagulation
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Saliva is allergenic and causes swelling and
irritation
Life cycles
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Four major phases of life:
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Egg
Larva
Pupa
Adult
Lifecycles vary in timing and
duration depending on species
Disease transmission
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Insects that transmit diseases = vectors
Two types of transmission:
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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
Flies
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Horseflies
Deer flies
Stable flies
Horn flies
Face flies
Bot flies
Horseflies &
Deerflies
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Tabanid species
Breed in boggy areas
Active only during the
day in warm weather
Deerflies have
patterned wings and
are smaller
Horseflies have
transparent wings
Horseflies & Deerflies
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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!
Stable Flies and Horn Flies
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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
Stable Flies and Horn Flies
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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
Face Flies
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Non-biters
Closely resemble
house flies, larger
than horn flies
Feed on mucous
secretions around
eyes, nose, mouth
Lay eggs in fresh
manure
Can transmit eye
problems
Bot Flies
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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
Bots
Black Flies/Midges - Onchocerca
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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
Ticks
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Lyme disease
Rocky Mountain Spotted Fever
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Not generally a problem in horses
Lyme Disease
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Spirochetal (corkscrew-shaped)
bacteria – Borrelia burgdorferi
Transmitted through the bite of a
deer or black-legged tick
Endemic areas for Lyme disease:
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Northeast
Mid-Atlantic
Northern Midwest states
Northern California
Borrelia burgdorferi
Lyme Disease Transmission
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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:
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Larvae, nymph, adult
Must have a blood meal before they
can molt to next stage
Two-Year Life Cycle of Deer Tick
EGGS
MEAL 1
• Mouse
• Bird
LARVAE
NYMPHS
Eggs laid
adults die
Meal 3 (for
adults that
didn’t feed
in fall)
• Person
•Deer
•Horse
MEAL 2
Peak Feeding in
people, horses, mice
SPRING
WINTER
Nymphs molt
into adults
Larvae molt
into nymph
stage
SUMMER
FALL
ADULTS
Meal 3
• Person
•Deer
•Horse
Nymphs dormant
Lyme Disease Transmission
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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 = whitefooted mouse
Host of nymph ticks = humans,
rodents, dogs, cats, birds, etc.
Host of adult = deer plus others
Lyme Disease
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Multisystem disease!
Clinical Signs:
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Joints
Musculoskeletal system
Neurological system
Subclinical infection is common!
Development of clinical signs only
occurs in ~10% of infected animals
Lyme Disease and Horses
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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
Lyme Disease and Horses
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Diagnosis is difficult – VERY political!
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1.
2.
3.
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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
Lyme Disease Treatment
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Antibiotics –
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“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!
Lyme Disease Prevention
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No Vaccine licensed for horses
TICK CONTROL!!
Daily grooming and removal of ticks
Tick repellents applied to head, neck,
legs, belly and under tail
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Permethrin or DEET are particularly effective
Keep pastures mown
Remove brush, woodpiles, etc. to
decrease rodent nesting areas
Mosquitoes
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May be
encountered day
and night
Many different
species
Attracted to
incandescent light
but not to
fluorescent light!
Lice
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Most common of external parasites
Two varieties:
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Chewing/Biting – feed on skin cells
Sucking – feed on blood
Horse with lice:
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Heavy dandruff
Greasy skin
Bald spots
Lice
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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
Mites
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Microscopic!
Can cause mange
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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”
Mange…
 Psoroptic Mange
Chorioptic Mange 
Fly Control
More Fly Control
Premise Control
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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
Fly Parasites
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Gnat-sized parasitic wasp
Female wasp lays eggs in larvae of
stable/house flies
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Eggs hatch  feed on developing
larvae
Release more fly parasites every 34 weeks to keep up with flies
Effective, but depends on
neighbors!