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External Parasites

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External Parasites in Horses

External parasites do not usually cause the damage to horses that
internal parasites do. However, in some cases an infestation of a
particular external parasite may cause irritation, a dull hair
coat, anemia, weight loss and a general unthrifty condition.

Types of external parasites that may be found on horses include:
flies and gnats,
ticks, and

Many kinds of flies are found around or on horses. Flies that
suck blood are horse flies, deer flies, black flies, biting
midges, mosquitoes, horn flies and stable flies. There are four
stages in a fly's life cycle: egg, larva, pupa and adult. Growth
occurs during the larval stage. Transformation into the adult
stage occurs during the pupal stage.

Horse fly eggs are deposited in a variety of aquatic or
semiaquatic areas including water, water's edge, flooded forest
floor, salt marsh, plant debris and, although rare, pastureland.
Generation time varies from one year to two or three years.

Deer flies normally deposit eggs on vegetation around the water's
edge or in salt marshes. Horse and deer flies are painful biters,
causing extreme annoyance and blood loss (up to 0.5 cc per fly)
with more blood loss following the bite due to oozing. EIA
transmission by these flies is also a concern. Control is
difficult because of the larval habitat. These flies do not
normally enter structures. Provide pastured horses space and
structures for sanctuary during heavy attacks to help alleviate
the control problem.

Mosquitoes and black flies develop in water. Black flies require
moving water. Areas that can breed black flies are streams and
pond overflows. Black flies are seen biting horses on belly
lines, between the back legs and in the ears.

Mosquitoes breed in standing water, tree holes, flood zones, old
tires, cans and barrels. Control mosquitoes by eliminating their
breeding sites. Mosquitoes are vectors of equine
encephalomyelitis as discussed in the disease section.

The horn fly resembles a small stable fly and is present in large
numbers on horses maintained with or near cattle. Horn flies
deposit their eggs on fresh cow manure. Hundreds of flies may be
found feeding upon a horse's shoulders, neck, withers and
abdomen, taking 20 to 30 blood meals per day. These flies often
congregate around hair loss areas and cause severe dermatitis
identified by ulcers and crust.

The stable fly resembles the house fly but can be recognized by
its prominent bloodsucking mouth-part. The optimal environment
for stable fly larval development is hay contaminated with urine,
water and manure. Considerable blood loss and annoyance can be
produced by the stable fly. These flies may contribute to the
transmission of swamp fever and are vectors of "summer sore" as
discussed in the Internal Parasites section under stomach worms.
Stable flies feed on the legs and abdomens of horses.

Other biting flies such as biting midges can become quite
numerous on horses, especially in swamp and marsh areas where
these insects breed. These flies congregate mostly on the
underside of horses, where they cause extreme irritation and
blood loss. To achieve control, pasture horses away from wooded
areas during periods of heavy attacks and treat horses with
insecticides every day or two.

The house fly is a nonbloodsucking parasite. House fly larvae
develop best in manure but can develop in a wide variety of
organic debris. These flies transmit stomach worms and have been
incriminated in the transmission of more than 60 vertebrate
pathogens by body part contamination, regurgitation and
defecation. The house fly causes considerable irritation to the
horse, particularly when it feeds on lacrimal secretions around
the horse's eyes.

Stable fly and house fly control starts with good stable and
pasture management. Eliminate breeding sites of these flies when
possible. Remove breeding materials at least every seven days
because these manure breeding flies have life cycles of seven to
21 days. Inhibit fly development by keeping manure and other fly
breeding materials dry. Design paddock areas to promote adequate
drainage and eliminate wet areas where fly breeding is likely to
occur. Good drainage away from manure piles promotes drying and
helps reduce fly breeding. Properly maintain automatic waterers
to prevent leakage. If very carefully managed and made an
integral part of an overall Insect Pest Management Program,
parasites and predators of house flies can be used to aid in a
house and stable fly control program.

Face flies are non biting flies that breed in fresh cattle
droppings. They congregate around the horse's head, where they
feed on liquid secretions around the eyes and mouth. They are
associated with numerous eye problems, and large numbers result
in annoyance. Control face flies by treating a horse's face on a
daily basis with wipe-ons and sprays or exclude them with face

Lice can be found on all parts of the horse, but are usually
first noticed on the head, neck, mane and tail. There are two
types of horse lice. One is the horse-biting louse (Bovicola
equi). These lice are chestnut brown except for the abdomen,
which is yellow with dark crossbands. They are very flat with a
broad, rounded head and slender legs. The biting louse has
chewing mouthparts and feeds on dry skin scales and hair. Lice
spend their entire life cycle on the horse. Biting lice lay their
eggs around the angle of the jaw and on the flanks. Eggs hatch in
five to 10 days, and nymphs begin feeding immediately, maturing
in three to four weeks.

The second type of horse louse is the horse-sucking louse
(Haematopinus asini). This louse is about 1/8-inch long and is a
dirty gray color. It has a very broad abdomen that contrasts with
its long narrow head. Eggs will hatch in 11 to 20 days. The newly
hatched nymphs suck blood immediately and complete their
development to maturity in two to four weeks. Sucking lice are
more common than biting lice. Sucking lice are very irritating
and cause the horse to rub excessively, often rubbing off patches
of hair. When large numbers of lice are present the horse may
become anemic. Louse infestations are more commonly seen on
horses fed inadequate diets. They cause weight loss and stunted
growth. Lice are not considered important in the transmission of
equine pathogenic agents.

Mange or itch mites can infest horses and heavy infestations
result in severe dermatitis because these mites tunnel in the
upper skin layers and suck blood and cause secondary infections.
Careful treatment is necessary for control. The infested area and
sur-rounding areas on the horse must be thoroughly treated
initially and again seven to 10 days later.

Ticks, unlike lice, are not species specific, so the same types
of ticks that attack other animals will also attack horses. Ticks
cause irritation, restlessness and spread of horse diseases.
Ticks can be vectors of sleeping sickness, Lyme disease,
piroplasmosis and EIA. Heavy tick infestations may cause anemia.
One species of tick is a frequent invader of the ears. Because of
the irritation, the horse may have droopy ears and become head
shy. Adult female ticks lay eggs on the ground and then die. The
eggs develop into larvae (seed ticks) that climb up on grass or
shrubs where they latch on to a passing host. As each develops
into a nymph, it sucks blood. Adults may mate on or off the host
and ticks can survive for long periods off the host. Remove
single ticks by swabbing them with cotton soaked in alcohol or
chloroform. Because ticks breathe through spiracles (holes in
their abdomens), they are suffocated by the alcohol or
chloroform. Insecticides can also be used to treat ticks
directly. Ticks should not be grasped with an unprotected hand;
instead, use tweezers to remove ticks.

Insecticides - Wettable powder formulations (WP) are preferred
over emulsifiable concentrate solutions (EC) because certain
solvents in EC solutions, even upon dilution with water, may
cause damage and skin irritation to some horses. Exercise caution
when purchasing or using any insecticide on horses. Make sure the
insecticide is labeled for use on horses and follow label
directions closely. Many horse owners complain there are no
effective insecticides, particularly fly sprays, for horses. This
is probably the result of a resistant fly population. To help
slow down the development of resistance, do not treat with two
classes of insecticides simultaneously, because flies may develop
resistance to both classes at the same time. Instead, treat with
a specific class of insecticide for several months to a year.
Then switch to another class of insecticide before resistance is
developed by the flies. As pointed out in the discussion of
stable flies and house files, control should involve more than
the use of insecticides.

Properly remove and dispose of fly breeding materials at least
every three to seven days.

Design stables, paddocks and pastures to allow easy and thorough
waste removal and proper drainage.

Correct improper drainage and structural problems that allow
waste accumulation in existing operations.
Remove manure and other fly breeding materials within three days
after animals are removed from barns, run-in sheds or dry-lots.
Ensure thorough cleanup of all potential fly breeding areas.
Use insecticidal control in conjunction with good sanitation,
moisture control and mechanical control. Insecticidal control
should never be the sole means of control.
Residual insecticides, space sprays, fogs, mists, fly baits and
fly traps aid in the reduction of adult flies. Begin their use
early in the spring when fly populations are small.

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