Practical Veterinarian


Endoparasites are those parasites that live within the body of the host. There are four major groups of endoparasites — nematodes, acanthocephalans, platyhelminths (trematodes and cestodes), and protozoans. Table Characteristics of the Major Helminth Groups provides distinguishing characteristics of the helminthic parasites.

Table Characteristics of the Major Helminth Groups

Characteristic Phylum
Nemathelminthes Acanthocephala Platyhelminthes
Cestodes Trematodes
Body shape Round, elongate, generally tapering at both ends, not segmented Round, elongate, anterior with spiny proboscis, appears to be segmented Dorso-ventrally flattened, elongate, segmented Dorso-ventrally flattened, leaf-shaped, not segmented
Coelem Pseudocoel Pseudocoel Acoelomate Acoelomate
Digestive tract Complete (mouth, esophagus, intestine, anus) Absent Absent Incomplete (mouth, esophagus, intestine)
Sexes Dioecious (separate sexes) Dioecious Monoecious (hermaphroditic) Monoecious (exceptions)


Acanthocephalans (Thorny-Headed Worms)

Appearance and Morphology

• Variable length, 6 mm to 30+ cm.

• Proboscis is retractable into body; body covered with tegument; absorbs nutrients.

• Usually sexually dimorphic with males smaller than females.


Higher-level classification of the acanthocephalans is not presented here because only one species, Macracanthorhynchus himdinaceus (swine) has been generally encountered in veterinary medicine. Recognition of the parasite in the host and understanding the characteristics of the phylum are sufficient for most practicing veterinarians.

Life Cycle

• All acanthocephalans have indirect life cycles; stages include egg, acanthor, acanthella, cysticanth, adult.

• Essential first intermediate host is some type of arthropod; simplest life cycle requires only one intermediate host, while more complicated cycles involve the addition of a second or even third intermediate host.

• Adults live in the intestinal tract only; extra-intestinal migration does not occur.




Protozoans are single-celled organisms, varying greatly in size and shape. Of all the protozoans in the world, only a small number are parasitic. Of these, many are not harmful to the host, although those that are pathogenic can cause severe and devastating disease. The pathogenic parasitic protozoans encountered in veterinary medicine are spread among three major taxonomic groups — Sarcomastigophora (flagellates and amoebae), Ciliophora (ciliates), and Apicomplexa (apicomplexans which include the coccidia). Table Parasitic Protozoans Encountered in Veterinary Medicine lists the protozoans that may be encountered in veterinary medicine.

Table Parasitic Protozoans Encountered in Veterinary Medicine

Protozoan Croup Species Host*
Flagellates Giardia duodenalis Mammals including humans
Histomonas meleagridis Gallinaceous birds
Trichomonas gallinae Pigeons, chickens, raptors
Tritrichomonas foetus Cattle
Ciliates Balantidium coli Pigs, rats, dogs, primates
Apicomplexans Eimeria spp. Ruminants, horses, pigs, poultry
Isospora spp. Dogs, pigs, cats
Toxoplasma gondii Cats, sheep, goats, pigs, humans
Sarcocystis neurona Horses, oppossums
Neospora caninum Dogs, cattle
Cryptosporidium parvum Mammals including humans
Cryptosporidium andersoni Cattle
Cytauxzoon felis Bobcat, cats
Babesia bigemina Cattle
Babesia caballi Horses
Theileria equi Horses
Babesia canis Dogs
Hepatozoon americanum Dogs

*Host in which the parasite is most often encountered.


• Possess at least one flagellum for locomotion in the trophozoite stage.

• Reproduce by binary fission.

• Some species form cysts that are resistant to external environmental conditions.


• Possess cilia for locomotion in the trophozoite stage.

• Possess two types of nuclei — macronucleus and micronucleus.

• Form cysts that are resistant to external environmental conditions.


• Obligatory intracellular parasites.

• Multiply through strict sequence of asexual and sexual reproduction.

• May have direct or indirect life cycles.