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Lab 3 Appendix: Ascarids, Oxyuris and Trichocephalids

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The ascarids are large parasitic nematodes that usually live in the small intestine of their host. All ascarids have 3 lips around the mouth opening and no buccal capsule. Oxyuris is a pinworm and the female worms, as the common name suggests, have a long narrow tail. The pinworms live in the large intestine of their host. The Trichocephalids (the name describes the worm's shape: most have an anterior end that is narrower than the posterior) are some of the most important nematode parasites. All members of this group contain a structure call the "stichosome" that is part of the esophagus. The stichosome is composed of a long slender tube surrounded by a row of large cells (stichocytes).

Objectives Checklist

Be able to identify the following:

9 The eggs of the following Ascarids: Toxocara canis, T. cati, Toxascaris leonina, Ascaris suum, Parascaris equorum. (Hint: knowing you have an ascarid egg and the host it came from, usually will identify the parasite, as most hosts only have one major ascarid parasite. An exception to this are dogs and cats, for which you will have to tell Toxocara eggs from those of Toxascaris.)

9 Adult ascarids to species (except those of dogs and cats).

9 The eggs of the following Trichocephalids: Trichuris vulpis, Trichuris spp. & Capillaria spp.

9 The eggs and adult female of the horse pinworm: Oxyuris equi.

9 Be able to identify the L1 of Trichinella spiralis in a "squash-prep" of muscle.


Lab Exercises


Ascarids are large nematodes that usually live the in small intestine. All ascarids have three lips around the mouth opening and have no buccal capsule. Eggs are thick-shlled and unsegmented when passed. They embryonate in feces or fecally contaminated soil. Infection is by ingestion of the embryonated egg, ingestion of larvae in a paratenic host, or by vertical transmission: in utero (especially in dogs) or transmammary (especially in cats).

Pigs: Ascaris suum

Ascaris suum

Anterior end view
Like all the ascarids, this adult worm has 3 lips. The dorsal lip (second lip) is out of focus in this picture.

Ascaris suum from pig intestine
Ascaris suum adults from the small intestine of a pig.  They can be up to 40 cm long.
Ascaris suum eggs in fecal float
Ascaris suum eggs in a fecal float. The eggs measure 60 x 45 µm.
Ascaris suum in lung
Ascaris suum larva in the lung of a guinea pig.

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Horses: Parascaris equorum

Parascaris equorum Parascaris equorum - largest nematode of the horse (up to 40 cm long); similar to A. suum in appearance.  This picture shows the lips around the mouth.
Parascaris equorum in fecal float Parascaris equorum egg from a fecal float.  It measures 90 to 100 µm in diameter.

Cats and Dogs: Toxocara canis, Toxascaris leonina, Toxocara cati

Toxocara canis Toxocara canis -
a large (up to 10 cm) ascarid of the small intestine of the dog.

Note the presence and shape of the cervical alae (clear cuticular flanges running along the anterior lateral margins of the worm).

Toxocara egg

Toxocara canis egg

  Note the rough surface, this egg measures 80 x 75 µm.

Toxascaris leonina
an ascarid found in
both dogs and cats.

Grossly, the adults are morphologically similar to those of Toxocara canis in the dog, but are easily distinguished from Toxocara cati of cats by the shape of the cervical alae. In T. leonina the alae terminate gradually, merging into the cuticle, giving the anterior end of the worm a "lance-like" appearance.

Toxascaris egg Toxascaris leonina eggs

Note the smooth surface and the empty space within the egg, these eggs measure 80 x 65 µm.

Toxocara cati

Toxocara cati
an ascarid of the cat.

The cervical alae of this worm differ from those of T. leonina, the other ascarid of cats. The alae are broad and end abruptly, giving the anterior end an "arrow-head" appearance.

Toxocara egg

Toxocara cati egg

Note the rough surface, this egg measures 75 X 65 µm.

Click here to link to the Toxocara canis life cycle.

Raccoon and Dog: Baylisascaris procyonis

Baylisacaris procyonis egg Baylisacaris procyonis egg (40X). Baylisacaris procyonis egg Pitted surface of Baylisacaris procyonis egg (40X).

Poultry: Heterakidae - Ascaridia galli and Heterakis gallinarum

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The pinworms are nematodes found in the large intestines of their hosts. The name "pinworm" comes from the long pointed tail of the female nematode of some, but not all species.

Horse: Oxyuris equi

Oxyuris equi Oxyuris equi  adults

Note the males are smaller than the females which have a long pointed tail.

Oxyuris equi eggs Oxyuris equi eggs.

Note the operculum (cap) at one end. Eggs may be found in the feces, but since the female worm normally deposits them on the skin of the perianal area, scrapings of this region are more likely to reveal the infection. They measure 90 x 42 µm.

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The common morphological feature of these worms is the presence of a "stichosome" which constitutes part of the esophagus. The stichosome is a structure constituted of a long slender tube surrounded by a row of large cells (stichocytes).

Mammals: Trichinella spiralis - the causal agent of trichinosis

Trichinella spiralis histology

left image:
First stage larvae (L1) within modified muscle cells ("Nurse cells").


right image:
Histological section showing only a little inflamation around the encysted L1.

Trichinella spiralis histology
Trichinella spiralis female Trichinella spiralis male Adult worms recovered from the small intestine. The female measures about 3 mm long while the male is only 1 mm long. The male has papillae on its tail (arrow), the female does not.


Dogs and Cats: Trichuris vulpis - canine whipworm (T. serrata - very rare feline whipworm)

whip worms whip worms in situ whip worms histology
Adult males and females
Adults in situ. The thick posterior end remains in the lumen of the large
intestine, while the thin anterior end is embedded into the tissue.
Histology of the worm in situ. Again note that the posterior is free in the intestinal lumen while the anterior is in the tissue.


Trichuris vulpis egg Trichuris vulpis
Bipolar plugged egg, with a smooth surface.
The egg measures 75 X 40 µm.
Trichuris vulpis float

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Foxes, Dogs and Cats: Eucoleus (Capillaria) aerophila - lung worm found embedded in the mucosa of the lungs.

Capillaria aerophila
Bipolar plugged with
a rough surface.
Each egg measures 70 X 35 µm.
Capillaria aerophila egg
Capillaria aerophila float

Foxes, Dogs and Cats: Pearsonema (Capillaria) plica - worm found in the urinary bladder, thus the eggs are found in the urine

Capillaria plica egg Capillaria plica egg
This egg is similar to the egg of E. (C.) aerophila, but is found in the urine.


Dogs and wild carnivores: Dioctophyma renale

kidney worm kidney worm in situ D. renale egg
Dioctophyma renale adults can measure up to 60 cm in length.
Adult worm in situ.  Note that the tissue of the kidney has been destroyed.
Egg found in the urine (compare to P. (C.) plica, which is also found in the urine).
This egg measures 65 X 42 µm.

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Copyright © 2008 - University of Pennsylvania School of Veterinary Medicine, All rights reserved.
Faculty: Dr. Thomas Nolan
Students: Takis Weekes V'11, Molly Church V'09, Diana Knight V'08, Douglas Gilson V'05, Chris Dykhouse V'04, Kimberly Mah V'00

Comments or Questions contact Dr. Tom Nolan at: