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
|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|
|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.
• 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
|Flagellates||Giardia duodenalis||Mammals including humans|
|Histomonas meleagridis||Gallinaceous birds|
|Trichomonas gallinae||Pigeons, chickens, raptors|
|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|
|Cytauxzoon felis||Bobcat, cats|
*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.