Review of Common Internal Parasites of Equines

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Review of Common Internal Parasites of Equines
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Routine fecal examination of horses will reveal
parasitism by four types of parasites.
Strongyloides westeri, Parascaris equorwn,
Strongyles (large and small, sp. are not
distinguishable by eggs), and Anoplocephala sp.
(eggs of the two important sp. in the U.S. are
indistinguishable). However, other sp. of
parasites occur and will be reviewed. If a more
thorough explanation of the Salient Points or
Features of the Life Cycle are necessary
please refer to your corresponding lecture notes
from the second year or an appropriate text.
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Strongyloides westeri eggs (40-52 u x 32-40
u) Thin walled, larvated egg, typical of
Strongyloides
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Strongyloides sp. Life Cycle Parthenogenic
parasitic females are found in the small
intestine of foals, animals become immune and the
adults are not generally found in the intestine
after six months of age, infection occurs, via
milk, skin penetration and ingestion, these
parasites have a short prepatent period (5-6
days) and may be found in very young horses.
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Strongyloides westeri Salient
Points Pathogenicity is questionableRx, TBZ, IVM,
other HZ, but not FBZ.
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Parascaris eguorum eggs (90-100u) thick-shelled
,globular shape, eggs are not larvated in fresh
feces, some eggs here contain the infective
larvae.
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Parascaris equorum emerging from a perforated
portion of the small intestine, this may occur in
heavy infections. Pathology due to migrating
ascarid larvae is considered to occur .
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Parascaris equorum Salient Points An infection
of foals, sucklings, weanlings and to a lesser
extent in yearlings and 2 year olds. Prepatent
period 10-13 weeks, Rx Piperazine, Organic
Phosphates, PRT ,HZ, IVM.
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Gastrophilus
Almost all horses are infected with bots
Gastrophilus intestinalis sometimes with G.
nasalis and rarely in the U.S., G.
hemorrhoidalis. Bots cannot be diagnosed by fecal
examination. Finding eggs on the horse's hair is
an indication of internal infection. Unless
treated most horses are infected.  
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Gastrophilus spp. Life Cycle Features The spp.
vary slightly, in host site, larvae hatch (4-14
days), go through early tissue migration and
attach to mucosa of G.I.tract, live there 8-10
months, pass out in feces, pupate, adults hatch
from pupae (5-6 weeks) and deposit eggs on horse
hair .
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Gastrophilus sp. in stomach Heavy infection, G.
intestina/s attach preferentially to the white
cardiac portion of the stomach and G. nasalis
attach preferentially to the pyloric region. G.
hemorrhoidalis attach in the rectum. Large
numbers have been reported to cause mechanical
blockage and/or rupture of the stomach, but this
is unlikely.
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Gingival erosions -due to early Gastrophilw
migrations, pharyngitis may also occur
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Gastrophilus spp. Salient Points Fly season April
to Dec. in Louisiana, peak season is in the fall
Rx organic phosphates ,all stages, IVM, all
stages.
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Three sp. of spirurid nematode occur as stomach
worms in horses (Habronema mwcae, H. microstoma
and Draschia megastoma). These are difficult to
diagnose as internal infections because eggs or
larvae are rarely found by standard fecal
flotation methods. One sp. of Trichostrongyle
stomach worms also occurs in horses (T. axei)
eggs are simiar to strongyles, but vary slightly
in size (79-92 u x 31-41 u) and shape (slightly
flattened and pointed at one end).
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Cutaneous habronemiasis Summer Sore on the
prepuce of horse, develop from larvae being
deposited or escaping from vector flies feeding
around a wound or abrasion. May also cause
pulmonary granulomas if larvae invade the lungs.
In the stomach Habronema sp. produce mucus
exudate, and D.megastoma produce tumor- like
lesions along the margoplecates
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Salient Points -Horse Stomach Worms Indirect
life cycle, Rx IVM. T. axei -BZ, IVM.
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Oxyuris equi egg(42-90 u) Eggs are elongated and
slightly flattened, with a operculum like plug at
one end, these are rarely seen in feces.
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Oxyuris equi adult females More commonly found
in foals, females rupture during their migrations
to the rectum and anus, eggs are released and
attach to walls, fixtures, etc. , development to
infective larvae within the eggs is fast, (3-5
days), prepatent period -5 months
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Loss of Hair Due to pruritus caused by migrating
female 0. equi, this irritation and resulting
hair loss is the principle problem associated
with the infection, and the common means of
diagnosis
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Oxyuris equi Salient Points Pinworm infections
are mainly but not only a problem in stabled
horses, 8 Rx most all drugs.
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Anoplocephala sp. Eggs -(50-60 u) Eggs possess
pyriform apparatus, spp. identification is not
possible on egg morphology .
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• magna in horse intestine
• Largest (up to 12 in) of two common spp. in the
  U.S. found in posterior portion of small
  intestines may produce catarrhal or hemorrhagic
  enteritis

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Anoplocephala perfoliata adults Smaller than A.
magna (1-2 in.) occur in larger numbers more ,
pathogenic of the two spp. , produces ulcers, and
may induce occlusion of ileo-cecal valve, or
intussusception in this area.
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Anoplocephala spp. Salient Points Indirect
life cycle .orbatid mites are intermediate
hosts Rx, Pyrantel is effective against
Anoplocephala spp.
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. Strongyle egg -70-85 u x 40-47 u) Spp. cannot
be identified by egg morphology , egg here can be
compared to Parascaris equorwn and Strongyloides
westeri.
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The large strongyles are pathogenic because of
their larval migrations, adults attach to the
mucosa of the large intestine and cecum and suck
blood. Some Triodontophorus spp. produce mucosal
ulcers
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Strongylus vulgaris caused aneurism in
mesenteric artery due to larval migrations
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Strongylus vulgaris related intestinal
infarction Due to emboli produced by S. vulgaris
larval migrations in mesenteric, arteries. Other
large strongyle larvae (S. edentatus S.
equinus) produce lesions during their migrations
in liver, diaphram and other viscera. .
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Comparison of large and small strongyles
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Large strongyles Salient Points Infections by
ingestion occur early but eggs are not produced
until 6 months (S. vulgaris), 9 months (S.
equinu) and 11 months (S. edentatus) prepatent
periods are shorter for small strongyles (6-12
weeks). Rx -BZ, Pyrantel, IVM (adults).
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Cutaneous Onchocerciasis Chronic dry dermatitis
due to microfilarial presence in skin, life cycle
is filarid type, vector Culicoides sp. adults
occur in ligamentum nuchae.
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Onchocerca cervicalis Salient Points Dx
microfilaria in skin snip IVM as a
microfilaricide
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Babesia cabaIli Large sp. has been in the past
described in southern U .S. (F1a.), Dennacentor
nitans vector in U.S.
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Babesia equi (Small spp.) Only a few cases
have been described in the U.S., more pathogenic
than Babesia caballi - Babesia spp. Remain a
threat from horses imported from South America,
Africa, and Asia .
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Trypanosomes -T. evansi causes surra, (Africa,
Asia, Central South america) Transmission is
mechanical by biting flies. T. equiperdwn causes
dourine, (Asia, Africa, Europe, Mexico) a
venereal disease.
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Many other sp. . of larval and adult helminth and
protozoan parasites occur less commonly in
horses. A complete list of these can be found on
page 796- 797 in the 6th edition of Soulsby's
Helrninths. ArthrQ12Qds and Protozoa of
Domesticated Animals (Monnig) or in Drudge
Lyons, Internal Parasites of Horses.
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