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.
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.
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
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.