Parasites of Veterinary Interest, Parasitic Diseases

Parasite is an organism that is metabolically and physiologically dependent on another organism. Parasite exploits the host for development and survival during one or more stages of its life cycle. Some parasites are unicellular, others are multicellular. They range in size from tiny protozoa as small as the size of many bacteria to tapeworms that can measure several meters in length. In some cases, two or more parasites can occur in the same host and this phenomenon is called polyparasitism.

Based on the site of infection, parasites can be divided into ectoparasites (external), such as lice, fleas, ticks, bugs, flies and mosquitoes) and endoparasites (internal). Endoparasites are further divided into enteric parasites that occupy the digestive tract; hemoparasites that are found in blood and blood forming organs and venereal parasites that infect reproductive organs.1 It is important to understand the different classifications of parasites because they all have different life cycles and respond to different therapeutics.2


Protozoa are unicellular organisms. They include many groups with important parasites of animals and humans.


Helminths are a diverse group of parasitic worms, encompassing nematodes (round worms), cestodes (multisegmented worms) and trematodes (flukes), and constitute a major health problem for humans and animals in many parts of the world. Although their diseases impact could be reduced dramatically by improved sanitation for humans and pasture control in domestic animals, such methods are not sufficient to eradicate these parasites. In the absence of vaccines, control of these parasites is reliant on chemotherapy to ease symptoms and reduce transmission. The intensive use of drugs in the live-stock industry has led to widespread resistance to all current anthelmintics. With few new drugs or vaccines, the fight against parasites could become a losing battle. Helminthic diseases are treated with a variety of drugs including macrocyclic lactones, benzimidazoles, imidazothiazoles and praziquantel. In animals, resistance to anthelmintics occurred rapidly after their introduction. In small ruminants, anthelmintic-resistant nematodes are already a serious problem. In Australia, for example, the prevalence and severity of resistance threatens the profitability of the entire sheep industry. Resistance has arisen to all of the major families of broad spectrum anthelmintics. Of greater concern is the spread of resistance to triclabendazole, the main drug used to treat fluke infections because of its high activity against the migrating immature stages.(3

  • Acanthocheilonema (Dipetalonema) reconditum - tropical filarial worms parasitic in humans and monkeys.
  • Ancylostoma duodenale - common hookworm in humans. A. braziliense, A. ceylonicum, and A. caninum occur primarily in cats and dogs, but all have been known to occur in humans.
  • Ancylostoma caninum
  • Ancylostoma tubaeforme - a hookworm that infects cats
  • Angiostrongylus cantonensis - rat lungworm, causes eosinophilic meningitis; transmission vectors: slugs.
  • Ascaris suum
  • Baylisascaris procyonis
  • Capillaria
  • Cercopithifilaria
  • Coenurus cerebralis (the larval stage of the tapeworm Taenia multiceps)
  • Dicrocoelium dendriticum - the lanceolate fluke; ruminants are the usual definitive hosts for lanceolate fluke, although other herbivorous animals, carnivores, and humans can serve as definitive hosts; in heavy infections, symptoms may include inflammation of gallbladder, liver abscesses and abdominal pain.
  • Dictyocaulus bovis
  • Dipylidium caninum
  • Dirofilaria immitis - long, thin parasitic roundworms that infect a variety of mammals. Infection is transmitted by mosquito bites; in dogs causes "heartworm disease."
  • Dirofilaria repens - the leading cause of human dirofilariasis in Europe; not found in the United States.
  • Echinococcus granulosus
  • Fasciola hepatica
  • Fascioloides magna
  • Gnathostoma - parasitic roundworms; mammals and man become infected by eating raw or undercooked freshwater fish, eels, frogs, birds, and reptiles.
  • Habronema muscae - internal stomach parasite that is most commonly found in horses.
  • Habronema majus- parasite in the stomach of horses.
  • Haemonchus contortus - barber's pole worm; very common parasite and one of the most pathogenic nematodes of ruminants.
  • Metastrongylus
  • Muellerius capillaris - hair lungworm, parasite of small ruminants.
  • Onchocerca (skin worm)
  • Oslerus osleri
  • Ostertagia ostertagi -brown stomach worms
  • Paragonimus
  • Schistosoma bovis - liver fluke infecting cattle
  • Strongyloides canis
  • Strongylus vulgaris - (blood worms, palisade worms, sclerostomes, or red worms); roundworms that infect large intestine of horses.
  • Syngamus trachea - gapeworm of poultry; found in the trachea of chickens, turkeys, guinea fowl and many species of wild birds.
  • Taenia pisiformis
  • Taenia saginata - tapeworm that infects cattle; it is acquired by humans through the ingestion of raw or insufficiently cooked beef.
  • Taenia solium - tapeworm that infects swine; i is acquired by humans through the ingestion of cured or undercooked pork.
  • Thelazia callipaeda - a parasitic roundworm and the most common cause of eyeworm infestation in humans, dogs and cats.
  • Toxocara canis
  • Trichinella spiralis
  • Trichobilharzia regenti - a nasal parasite of birds that causes dermatitis in humans.
  • Trichostrongylus axei - small stomach worm (hairworm) of horses.
  • Trichuris


Arthropoda of veterinary importance include many insect species and Acari (ticks, fleas, and mites)

  • Dermanyssus Gallinae Bird Mite - ectoparasite of poultry, pigeons, sparrows, and other birds. Spreads animal and human diseases.
  • Sarcoptes equi
  • Pneumonyssoides caninum
  • Psoroptes ovis - a mite that causes a contagious, itchy disease in sheep called "sheep scab."
  • Ixodes ricinus
  • Dermacentor marginatus (Ornate sheep tick) - found in Morocco, Spain, Italy, South France, Switzerland, West Germany and Poland; transmits several diseases: canine babesiosis, Q fever, tularemia, Boutonneuse fever, Russian spring-summer encephalitis virus and Siberian typhus.
  • Caligus clemensi (sea louse) - a parasite of marine fish; can cause deadly infestations of both farm-grown and wild salmon.
  • Cimex colombarius (pigeon bug) - parasite of poultry and pigeon nests; may transfer to humans after their usual hosts leave their nests.
  • Cimex lectularius - common bed bug
  • Culex pipiens - common house mosquito; vector of some diseases, such as Japanese encephalitis, meningitis, and urticaria.
  • Culicoides imicola
  • Demodex spp.
  • Gasterophilus intestinalis
  • Haematobia irritans - horn fly, the European genus of bloodsucking flies.
  • Hypoderma bovis - warble fly, a parasite on cattle and deer.
  • Knemidocoptes mutans
  • Lepeophtheirus salmonis (sea louse) - a parasite living on salmon.
  • Lucilia sericata - common green bottle fly; common sheep parasite (the fly will lay its eggs in sheep wool)
  • Musca domestica
  • Nosema spp.
  • Notoedres cati
  • Oestrus ovis
  • Otodectes cynotis
  • Ornithonyssus sylviarum mite
  • Phlebotomus - sand flies that transmit leishmaniasis.
  • Pulex irritans
  • Rhipicephalus sanguineus
  • Sarcophaga Flesh Flies
  • Tabanus atratus - black horse fly that attacks cattle and other livestock; transmits bacterial, viral, and other diseases such as surra and anthrax, to both humans and other animals through its bite.
  • Triatoma - kissing bugs that transmit Chagas disease
  • Ctenocephalides canis (dog flea)
  • Ctenocephalides felis (cat flea)


  1. Essentials of Veterinary Parasitology. Hany Elsheikha, Naveed Ahmed Khan
  2. Clinical Pathology and Laboratory Techniques for Veterinary Technicians. Anne M. Barger, Amy L. MacNeill (editors)
  3. Anthelmintics Resistance; How to Overcome it?

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