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• Taxonomy scheme
• Nematodes
• Trematodes
• Cestodes
• Arthropods
• Protozoa
• Techniques
• Review Question

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The taxonomy scheme on the card above is the one we will use in this course as it is the one used in the text book. Note that it differs from the one given in the introductory lectures in that the protozoa are listed in the animal kingdom and not in the Protista. Taxonomists are not in agreement as to the number of kingdoms and how organisms are divided among them, as can be seen by a the third scheme shown below where the prokaryotes are divided into two kingdoms and the eukaryotes are lumped into one.

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Members of the Phylum NEMATHELMINTHES have a straight intestine, composed of only epithelial cells, with a posterior anus. The body cavity is a pseudocoel (i.e. lacking a mesodermal lining).

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The nematodes (phylum NEMATHELMINTHES, Class NEMATODA) are cylindrical in shape tapering at both ends and have a complete digestive system. The body is covered by an acellular cuticle. These helminths (worms) are commonly known as "roundworms" because of the shape of their cross-section

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Ascaris suum - Adult nematodes from the small intestine of a pig. This species is one of the larger nematodes.

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Ascaris suum - The diagnostic stage of this nematode is the egg which is passed out in the feces. The eggs will float in a saturated salt solution, therefore a standard fecal floatation can be used to diagnose an infection with this worm.

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Members of the Phylum PLATYHELMINTHES (the "flatworms") are usually flattened dorso-ventrally and are bilaterally symmetrical. They lack true body segmentation, a body cavity and an anus.

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The Trematodes (Phylum PLATYHELMINTHES, Class TREMATODA), as adults, are flattened dorso-ventrally, have a blind gut and no coelom. They also have suckers for attachment to the host. They are commonly referred to as "flukes".

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Dicrocoelium dendriticum - This small trematode is found in the bile ducts of sheep and cattle.

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Dicrocoelium dendriticum - The egg, which passes in the feces, is the diagnostic stage of this trematode. This egg, like many trematode ova, is operculate (has a cap on one end) and contains the next life-cycle stage, the miracidium.

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Fasciola hepatica - An adult specimen taken from the bile duct of an infected sheep. Note the size, distinct cone at the anterior end, "shoulders", and the "leaf-shaped" outline.

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Fasciola hepatica - The adults of this trematode are found in the bile ducts of the liver. This specimen is from a cow, but these flatworms may also infect sheep, man, and most other mammals. Note the thickening of the bile ducts caused by the parasite infection.

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Fasciola hepatica - The diagnostic stage of this trematode is the egg which is passed in the feces. Like many fluke eggs it has an operculum (cap) at one end and does not float in the standard saturated salt solution. You must use a sedimentation method to concentrate the feces in order to diagnose an infection with this parasite.

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The Cestodes (Phylum PLATYHELMINTHES, Class CESTODA), as adults, are ribbon shaped organisms whose "body" is made up of many repeating segments. They have no digestive tract or coelom and the adults are always found in some part of the host's digestive tract.

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Taenia saginata - The diagnostic stage for an infection caused by the adult tapeworm in humans is usually the gravid proglottid (an egg filled segment of the worm) which passes out with the feces. Identification of the type of tapeworm can be made by examining the morphology of the proglottid and the eggs it contains.  This slide shows an egg which was removed from a proglottid (although these eggs sometimes may be found in the feces by a standard fecal flotation).

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Taenia saginata - This gross specimen shows the larval stages of T. saginata in the muscle of a cow. The larva is a cysticercus. The cysticerci appear as white, fluid-filled bodies throughout the muscle tissue.

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Taenia saginata - The adult is found in the small intestine of humans and may be quite long.

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Phylum ARTHROPODA

Arthropods are bilaterally symmetrical animals with paired, multi-jointed appendages and an exoskeleton (outer covering) of secreted, non-cellular material (usually chitin or calcium carbonate). They have a dorsal heart and a ventral nerve cord. Their bodies are more or less segmented, and they are cold-blooded. The veterinary medical importance of these animals stems from their ability to act as pathogenic ecto- and endoparasites and from their ability to transmit other parasites or microbial pathogens.

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Class HEXAPODA (INSECTA)

Insects have all of the characteristics of the phylum Arthropoda. Specifically, their exoskeletons are composed of chitin and cross-linked proteins. The adult stages have three pairs of legs, and their bodies are divided into three segments: head, thorax, and abdomen. We will study three groups or orders of insects in this course. These orders are constituted by the true flies, the fleas, and the lice.

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Gasterophilus sp. These insects are the larval stage of a fly. The adult flies are free-living, and the larvae live attached to the wall of the horse's stomach. (This museum specimen shows the larvae attached to the mucosa of a horse's stomach.)

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Culex sp. This mosquito is an insect that may act as a vector and intermediate host for several different parasites. The larva, pupa, and male and female adults are seen here.

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Class ARACHNIDA

Arachnids have all the characteristics of the phylum Arthropoda. They share a chitinous exoskelton with the insects, but differ from those animals in that they have four pairs of legs in the adult stage. Also, their bodies exhibit fusion of two of the insect  segments, the head and thorax, into a single body region known as the cephalothorax. We will deal with only one Order in the class Arachnida, the Order ACARINA, which includes the ticks and mites.

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Notoedres cati This tiny skin burrowing mite was recovered in a skin scraping from a cat exhibiting symptoms of mange as illustrated in the museum specimen to the right.

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Notoedres sp. Note the damage that this skin burrowing mite (arachnid) has caused to this cat's head.

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Dermacentor variabilis ("American Dog Tick") These adult ticks (arachnids) were removed from the skin of a dog. Ticks not only suck blood from their hosts, they also are vectors for several parasitic diseases, such as Rocky Mountain Spotted Fever.

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The protozoa (Phylum PROTOZOA) are single cell animals which fall into several classes. In general the classes of protozoa are organized around structures used for locomotion (flagellum, pseudopodium, and cilia) or the lack of such structures. Most of the protozoa are microscopic.

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Eimeria stieda This protozoan infects the liver of rabbits. The white nodules you see in this rabbit's liver are part of the pathology caused by this parasite.

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Eimeria stieda The diagnostic stage of this coccidian is the oocyst which passes in the feces, it will float in a standard salt solution.

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Haemoproteus The stage of the protozoan seen in this red blood cell of a pigeon is one of the sexual forms of the parasite. This parasite is in the same group of protozoans as malaria.

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BAERMANN APPARATUS This device is used to recover nematode larvae from a fecal sample. The larvae under the microscope at the right were recovered using a Baermann apparatus. (right: modified Baermann apparatus, left: Baermann apparatus).

Review Question This parasite was recovered from the bile duct of a deer. Its eggs (the diagnostic stage) are found in the host's feces. A.) To what Phylum and Class does this parasite belong? B.) Which of the diagnostic methods learned in today's laboratory would be the best one to use to detect the eggs of this parasite in the deer's feces. Click here for the answer.

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