Introduction to parasitology

Parasites are organisms that live on other living organisms (hosts), such as animals and humans. There are many kinds of parasites that affect various animals and systems within the animals. Often parasites are specific to one kind of animal or a restricted group of animals, but some have a wide range of hosts. This article concentrates on common parasites in pets.

Worms (Helmints)


Cestodes are tape worms. They usually reside in the intestines of the definitive host (final host) and form flat chains of segments. The segments often resemble rice grains  that move upon elimination in the feces. The feces of affected animals contain microscopic eggs (invisible) within and outside of the segments. The eggs develop larval forms in phases involving intermediate hosts, which later are ingested by the definitive host. Ingestion of feces containing eggs does not transmit worms between pets. One of the most common tape worms is Dipylidium, which is transmitted to dogs and cat via ingestion of flea larvae. 


Nematodes are elongate worms. The include roundworms (Ascaris, Toxocara), Hookworms (Ancylostoma, Uncinaria), Whipworms (Trichuris), heartworms (Dirofilaria) and more. They are generally transmitted between the definitive hosts via ingestion of eggs or larvae from contaminated feces, environment or body secretions. The most common nematodes are roundworms which pass from mothers to offspring and in feces of affected pets. Heartworms have intermediate host and are transmitted by mosquito bites. They invade the heart and the vascular system.


Trematodes are flukes. They have a complex life cycle and are transmitted via ingestion of intermediate hosts. They are not as common in pets.

Protozoan parasites

Protozoan parasites are monocellular (one cell) parasites. They are common in pets.


These parasites are characterized by flagellae- hair like prolongations used for propagation. A common flagellate is Giardia, which reside in the intestines and can cause severe watery diarrhea. Young animals are more sensitive. Transmission is via feces and contaminated environment.


These parasites invade the intestinal cells and some are very common in pets (mostly Isospora). Young animals may develop severs bloody diarrhea. Transmission can be direct (ingestion of feces or contaminated environment) or via intermediate hosts.  Cats can carry Toxoplasma and shed cysts in the feces.



These parasites are mostly external (ectoparasites) and include mites and ticks. They are usually transmitted directly between animals.

Mange mites

Mange mites affect the skin and can cause serious disease. Most mites are species or tissue specific, but there are exceptions. Common mange mites are Sarcoptes, which causes scabies in people and Demodex, which is occasionally life threatening in pets.

Other mites

Ear mites cause intense ear inflammation in pets. They are common in dogs, cats, rabbits and guinea pigs.

Skin mites are common in many pets including reptiles. They can cause skin irritation in people. Face and leg mites are a potential serious problem in pet birds.


Ticks are very common in most pets. They are blood sucking parasites and sometimes cause serious conditions. They are important carriers of some serious diseases.


Lice tend to be more species specific and usually do not cause serious problems. 


Fleas are responsible for the most common serious parasitic problem in pets. They are blood sucking parasites and often cause massive infestation. They spend their entire life cycle on the pet. Flea bite allergies and complicated skin infections are common.


Many species of flies are parasitic and feed on animal material. Fly bite dermatitis and fly larva infestation are fairly common in warm conditions and can be a serious problem.

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