Introduction to Parasitology

The Spectrum of Parasitism

Parasites are an extremely varied group. They range from flies, such as the blood-sucking mosquitoes, nematodes such as the heartworm of dogs, liver flukes of cattle and sheep, fleas commonly found on dogs and cats, lice and ticks found on almost all domestic animals and protozoa such as Giardia which are found in most domestic animals but are of particular significance in cattle and dogs. The table below illustrates some of these parasites.

A female mosquito blood feeding. Mosquitoes serve as intermediate hosts of other parasites such as Dirofilaria immitis the dog and cat heartworm and  Plasmodium species causing malaria in humans and birds. They are also vectors of  viruses   causing yellow fever and  encephalitis. A cluster of nematodes, the  roundworm of dogs, Toxocara canis. This parasite is  common in puppies and may be transmitted transplacentally as well as  to nursing pups in their mother's milk. This parasite has public health importance as a cause of visceral larva migrans in man.

Fleas are common parasites of dogs and cats. They bite their hosts and feed on blood. Fleas are intermediate hosts of the tapeworm Dipylidium caninum and the filarid nematode  Dipetalonema reconditum. The cat flea is  a vector of feline parvovirus. Fasciola hepatica, the liver fluke of ruminants. The parasite has a complex life cycle involving snail intermediate hosts. Migration of developing flukes in the host liver provokes an intense inflammatory reaction  with severe liver damage.

 Hematopinus suis, the blood-sucking louse of swine is common in pigs raised indoors with transmission readily occurring from pig to pig. Infested pigs are restless and rub their skin frequently to relieve the itching. Anoplocephala perfoliata,  a tapeworm of horses, is often found in clusters at the ileo-cecal junction. It is widespread in distribution and usually benign.  However, cecal abscesses, and intussusceptions have been reported.

The nematode Dirofilaria immitis, the heartworm  has a complex life cycle involving mosquitoes as intermediate hosts. These worms are found in the cardio-pulmonary circulation and may cause severe heart disease in dogs and cats. The protozoan Giardia is  important as a cause of diarrhea in dogs and cattle but is also found in other domestic animals as well as man. The trophozoite stage, shown here, attaches to the mucous epithelium cells of the small intestine.

An important feature to note about parasites is that they are not equally parasitic. Parasitism is seen as a spectrum. It includes organisms at one of the spectrum that spend most or all of their lives as independent free-living creatures, seeking a host only to feed.  The other end of the spectrum includes parasites that spend their entire lives in or on a host and cannot survive at any stage of their life cycles without a host. Between these two extremes we see a whole host of parasitic configurations with differing degrees of host dependency.

At one end of the spectrum, we see flies such as mosquitoes which are the least parasitic and visit their hosts only to feed. Fleas are slightly more parasitic in that they feed and often lay eggs on the host. Coccidial protozoans are even more parasitic with all the life cycle stages except two occurring inside a host. Ascaris suum, a nematode of pigs has two well defined phases to its life cycle - a parasitic phase in the swine host and a preparasitic phase consisting of three stages found free-living in the external environment. At the other

 end of the spectrum, we find the lice which spend their entire lives on the hair or feathers of their hosts. Off the host they will survive for only a short period of time, at most one or two days. Dirofilaria immitis, the dog heartworm is  entirely parasitic with all stages of the life cycle occurring either in the definitive host (dogs)  or in the mosquito intermediate host. 




Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000