Internal Parasites in Horses
Created by: Tracey Hoffman
Topic #3080
Parasite
• Organism that lives off another organism at
the expense of the host
Major Equine Parasites
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Strongyles
Roundworms
Bots
Pinworms
Strongyles
• Includes both large and small strongyles
• Most common
• Commonly referred to as bloodworms
– Because of color after it ingests blood from
host animal
Life Cycle
• Larvae develop to infective stage within one
week
• Larvae crawl up blades of grass and are
ingested by the horse
• Once ingested, larvae travel through the
digestive system to the large intestine
– S. vulgaris migrate to the anterior mesenteric
artery
Life Cycle
– S. endentatus to the liver and flank area
– S. equinus migrate to the liver and pancreas
• Adult large strongyles live in the cecum and
large intestine
• Eggs are laid in the cecum and large
intestine and passed through the feces
• Superior mesenteric artery
Adult Strongyle Damage
• These worms are aggressive feeders
• After feeding, move to a fresh site leaving a
bleeding crater
• Craters ulcerate and heals, leaving scar
Large Strongyles in Mesenteric
Artery
• Large strongyles can live in mesenteric
artery, which supplies blood to the intestine
• Inflame artery wall
• Create blood clots that interfere with
oxygen and nutrients to/from the intestine
• Leads to severe and often fatal colic
Small Strongyles
• Also called cyathostomes
• Cause under-performance
– Loss of condition
– Feed inefficiency
– Predisposition to secondary diseases
• Are extremely prolific
• Emerge from lining of intestines and leave
an ulcerated gut wall
Roundworm Life Cycle
• Host foal ingests infected eggs from contaminated
environment and/or feed. Eggs pass through oral
cavity to the stomach and into the small intestine.
(Solid Arrows)
• Eggs hatch in intestine and migrate through the
liver to the lungs via the circulatory system.
Larvae then travel to the pharynx and are
swallowed (tracheal migration shown by the
interrupted arrows). Larvae mature to adult state
in the small intestine
Roundworm Life Cycle
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Adult roundworms in small intestine
Eggs are laid in the small intestine
Eggs are excreted in feces
Larvae develop to infective 2nd state eggs
within two weeks
• Emaciated infected foal
• Adult roundworms clustered in ruptured
small intestine
Roundworms
• Adult roundworms are visible to the eye
• Roundworm larvae migration through the
lungs and liver is most dangerous
• Often causes foal pneumonia – potentially
fatal
Large Roundworms
• Most common in young horses
• Intestinal infections may show no clinical
signs
• Can decrease gut motility
• Intestinal impactions from clustered
roundworms may cause rupture to the
intestine and produce peritonitis
Bot life Cycle
• Pupae form in loose soil
• Adult bot fly emerges from pupa in 3-10 weeks.
Female posterior is long and pointed to deposit
eggs.
• Common bot fly (G. intestinalis) lays eggs on leg
hairs. Eggs must be licked to hatch larvae. After
ingestion, larvae tunnel in epithelium of tongue and
form pockets between upper molars. One month
later, larvae migrate to stomach or duodenum
Bot life Cycle
• Throat bot fly (G. nasalis) lays eggs on
hairs around side and back of mouth. Eggs
are ingested with food and water.
• Nose bot fly (G. hemorroidalis) lays eggs
around nose and mouth. Eggs ingested with
food and water.
• Larvae of bot fly cluster on linings of
stomach and proximal duodenum
Bot life Cycle
• Larvae passed in feces
• Common bot fly larva
• Throat bot fly larva
Bots
• Eggs are yellow in color
• Eggs are difficult to remove
• Each female bot fly may lay as many as 1,000
eggs
• Removal of eggs on hair coat when present is
essential
• Use lvermectin or Mexidectin 1 month after first
frost and in spring to eliminat larvae in stomach
Pinworm Life Cycle
• Sticky eggs will adhere to walls, fences,
bedding, and will be found in water or feed
• Larvae develop into infective stage within
the eggs
• Infective larvae are ingested by the host.
Larvae travel through digestive system to
the cecum and large intestine where they
mature in four or five months.
Pinworm Life Cycle
• Adult pinworms reside in the lumen of the
cecum and large intestine
• Adult female crawls to the skin around the
anal area to deposit eggs in clusters
Pinworms
• Major sign is horse rubbing his tail or rear
quarters on any available surface
• Caused by females depositing eggs
• Eggs are pale yellow to cream colored
• Contain fluid
• Need a microscope or hand lens to view
them
• Cleanliness is essential prevention