Practical Veterinarian


Appearance and Morphology

• Variable length, 1 mm to several meters.

• Body covered with cuticle, may form specialized structures (e.g., alae).

• Usually sexually dimorphic with males smaller than females.


Nematodes are divided among two classes: the Secernentea and the Adenophorea. The Adenophorea contains the whipworms, capillarids, and Trichinella spiralis, while the Secernentea contains the remainder of the parasitic nematodes. Numerous advancements have been made in our understanding of the relationships of many of these parasites. As a consequence, some nematodes have been reassigned to different genera (Table Reclassification of Some Parasites Important to Veterinary Medicine). While some consider presenting current and future veterinarians with proper taxonomic nomenclature to be impractical, others consider not to do so to be a disservice to their continuing education and development. Therefore, those changes in species level nomenclature which have achieved general acceptance among the scientific community are presented herein. Tables Classification of Nematodes Encountered in Pigs, Classification of Nematodes Encountered in Horses, Classification of Nematodes Encountered in Ruminants, Classification of Nematodes Encountered in Dogs and Cats, and Classification of Nematodes Encountered in Poultry and Other Gallinaceous Birds present the orders, families, and species of the parasitic nematodes by host group.

Table Reclassification of Some Parasites Important to Veterinary Medicine

Old Name New Name Intermediate/ Paratenic Host Definitive Host Site of Infection
Capillaria aerophila Eucoleus aerophilus Earthworm Canids, felids, mustelids Lungs
Capillaria bohmi Eucoleus bohmi None known Fox, dog Nasal, paranasal sinuses
Capillaria plica Pearsonema plica Earthworm Canids, mustelids Urinary bladder
Capillaria feliscati Pearsonema feliscati Earthworm Felids Urinary bladder
Capillaria putorii Aonchotheca putorii Earthworm Mustelids, raccoon, pig, cat Stomach, small intestine
Capillaria bovis Aonchotheca bovis None known Cattle Small intestine
Capillaria annulatus Eucoleus annulatus Earthworm Chickens, wild game birds Esophagus, crop
Capillaria contortus Eucoleus contortus None or earthworm Wide variety of birds Esophagus, crop, mouth
Capillaria obsignata Baruscapillaria obsignata None Wide variety of birds Small intestine
Dipetalonema reconditum Acanthocheilonema reconditum Fleas, lice Canids Subcutaneous tissues
Toxocara cati Toxocara mystax Rodents Felids Small intestine
Ostertagia circumcincta Teladorsagia circumcincta None Ruminants, especially sheep Abomasum
Babesia equi Theileria equi Ixodid ticks Horses Red blood cells


Table Classification of Nematodes Encountered in Pigs

Class Order Family Species
Secernentea Rhabditida Strongyloididae Strongyloides ransomi
Strongylida Trichostrongylidae Hyostrongylus rubidus
Strongylidae Oesophagostomum dentatum; 0. brevkadum
Syngamidae Stephanurus dentatus
Metastrongylidae Metastrongylus apri; M. pudendodectus; M. salmi
Ascaridida Ascaridiidae Ascaris suum
Spirurida Spirocercidae Ascarops strongylina
Physocephalus sexalatus
Adenophorea Enoplida Trichuridae Trichuris suis
Trichinellidae Trichinella spiralis


Table Classification of Nematodes Encountered in Horses

Order Family Species
Rhabiditida Strongyloididae Strongyloides westeri
Halicephalobus (=Micronema) deletrix
Rhabditis (=Pelodera) strongyloides
Strongylida Dictyocaulidae Dictyocaulus arnfieldi
Trichostrongylidae Trichostrongylus axei
Strongylidae Strongylus vulgaris; S. edentatus; S. equinus
Cyathostomum spp. and other small strongyles
Ascaridida Ascaridiidae Parasea risequorum
Oxyurata Oxyuridae Oxyuris equi
Spirurida Habronematidae Draschia megastoma
Habronema muscae; H. microstoma
Thelaziidae Thelazia lacrymalis
Onchocercidae Onchocerca cervicalis


Table Classification of Nematodes Encountered in Ruminants

Class Order Family Species
Secernenta Rhabditida Strongyloididae Strongyloides papillosus
Strongylida Trichostrongylidae Ostertagia ostertagia;*
Teladorsagia circumcincta
Trichostrongylus axei; T. colubriformis
Haemonchus contortus; H. placet
Cooperia spp.
Molineidae Nematodirus spp.
Strongylidae Oesophagostomum radia-tum; 0. venulosum; 0. columbianum
Chabertia ovina
Ancylostomatidae Bunostomum phlebotomum
Dictyocaulidae Dictyocaulus filaria; D. viviparus
Protostrongylidae Muellerius capillaris
Protostrongylus spp.**
Spirurida Thelaziidae Thelazia skrjabini; T. gulosa
Gongylonematidae Gongylonema pulchrum; G. verrucosum**
Onchocercidae Setaria labiatopapillosa
Onchocerca gutterosa; 0. lienalis
Filariidae Stephanofilaria stilesi
Adenophorea Enoplida Trichuridae Trichuris bovis; T. ovis; T. skrjabini
Capillariidae Aonchotheca bovis

* Several other species of Ostertagia exist and may cause or contribute to disease.

** Not mentioned further in this book.

Table Classification of Nematodes Encountered in Dogs and Cats

Class Order Family Species
Secernentea Rhabditida Strongyloididae Strongyloides stercoralis
Strongylida Ancylostomatidae Ancylostoma caninum; A. tubaeforme; A. braziliense
Uncinaria stenocephala
Molineidae Ollulanus tricuspis
Angiostrongylidae Aleurostrongylus abstrusus
Angiostrongylus spp
Filaroididae Filaroides osleri; F. hirthi
Crenosomatidae Crenosoma vulpis
Ascaridida Toxocaridae Toxocara canis; T. mystax
Ascaridiidae Toxascaris leonina
Baylisascaris procyonis
Spirurida Spirocercidae Spirocerca lupi
Physalopteridae Physaloptera spp.
Thelaziidae Thelazia californiensis
Onchocercidae Dirofilaria immitis
Acanthocheilonema reconditium
Dracunculidae Dracunculus insignis
Adenophorea Dioctophymida Dioctophymatidae Dioctophyme renale
Enoplida Trichuridae Trichuris vulpis; T. campanula; T. serrata
Capillariidae Aonchotheca putorii
Eucoleus bohmi; Eucoleus aerophilus
Pearsonema plica; Pearsonema feliscati


Table Classification of Nematodes Encountered in Poultry and Other Gallinaceous Birds

Class Order Family Species
Secernentea Rhabditida Strongyloididae Strongyloides avium
Strongylida Syngamidae Syngamus trachea
Heterakidae Heterakis gallinarum
Ascaridiidae Ascaridia galli
Adenophorea Enoplida Capillariidae Eucoleus annulatus
Eucoleus contortus
Baruscapillaria obsignata


Life Cycle

• All nematodes, whether parasitic or free-living, have the same stages in the life cycle: egg, four larval stages (L1 molts to L2 molts to L3 molts to L4), immature adult (sometimes called L5), which matures into adult.

• Stages occurring in the external environment are subjected to stresses (temperature, desiccation, sunlight, etc.) that may kill or delay development (developmental times presented throughout the book are minimum times required under optimal conditions).

• For most parasitic nematodes, the third-stage larva is the infective stage (i.e., the stage that initiates infection in the definitive host); infective larvae cannot feed and are usually ensheathed — the third-stage larvae do not completely shed the cuticular sheath of the second-stage larvae until some point after being ingested; the sheath provides protection against adverse environmental conditions; after the infective L3 sheds the sheath of the L2, they are referred to as parasitic L3.

• Transmission may be direct (no intermediate host required) or indirect (intermediate host required or paratenic host involved).

• Development to adulthood may proceed normally or, under certain circumstances, larvae may arrest development and remain quiescient until reactivated at a later time.

• Adults infect a variety of organs; larval migration may or may not occur depending on species and site of infection.