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Parasitology Notes

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Parasitology Introduction

1. Parasitism A parasite is an organism that is metabolically and physiologically dependent on another organism. All parasites are eukaryotic, but may be unicellular or multicellular. In some cases, two or more parasites may occur in the same organism and this is known as polyparasitism.

2. Types of parasites Parasites can be divided into ecto- and endoparasites. Ectoparasites (external) feed or live on the body surface of the host. Most ectoparasites are arthropods. Based on their life cycle, parasites can be divided into facultative or obligatory parasites. Facultative parasites can live freely and complete their life cycle without a host under certain conditions. They enter the body of the host and cause infection. Obligatory parasites must enter a host to complete their lifecycle. Obligatory parasites can be further divided into monoxenus or heteroxenous according to the number of hosts needed to complete their life cycles. Monoxenous parasites need only one host to complete their life cycle, whereas heteroxenous parasites need two or more.

3. Parasite life cycles Many parasites are polymorphic - have different body forms during life cycle. Parasites may exist in the following forms ...


Permanent parasites - spend most of their life cycle with their host.


Temporary parasites - visit their hosts occasionally at intermittent times to take their meal.


Periodic or seasonal parasites - found on their host at certain time/times of year.


Incidental parasites - found in hosts other than their normal host.


Erratic parasites - found in their normal host but in unusual organs or tissues which they are not adapted to live in.


Specific parasites - adapted to live in a specific host in a certain part of the body.

Parasites with a direct life cycle have parasitic stages that occur in or on only 1 host. Parasites with an indirect life cycle require an intermediate host for the development of some stages.

4. Hosts A host is a living organism that affords subsistence or lodgement to the parasite under natural conditions, but at the same time suffers harm or injury caused by that parasite.

A definitive host is the host in which parasitic sexual reproduction occurs. An intermediate host is one in which parasite asexual reproduction occurs. Permissive hosts are not utilised by the parasite but still favour life cycle completion. Nonpermissive hosts are ones in which the parasite cannot complete their life cycle. A reservoir host is a temporary host used in the absence of the natural host. This is often applied to zoonotic infections. A paratenic host is one used for transport. Paratenesis is the passage of an infective stage larva by a paratenic host to the definitive host.

5. Classification of parasites Generally, parasites are classified as protozoa or metazoa. a) Protozoa Protozoa are unicellular eukaryotic organisms. Their protoplasm is enclosed by a cell membrane and contain numerous organelles. Their morphology is varied, as is their metabolism and physiology. They are usually aerobic and contain mitochondria. They can reproduce asexually by binary fission, multiple fission, budding or spore formation, or sexually by conjugation. Some protozoa produce gametes which fuse to produce a diploid zygote. Pathogenic protozoa may damage the host by direct damage to tissue or through systemic inflammatory response by haematogenous dissemination. Parasitic phyla from the Protista kingdom include:












Sarcomastigophora (some parasitic)

b) Metazoa Metazoa are classified within the kingdom animalia. They are multicellular, and have tissue and organ systems. They are classified into two groups - helminths and arthropods. b) i. Helminths Helminths are complex, multicellular organisms. The outer surface may be covered with a protective cuticle, which is acellular and may be smooth or contain ridges, spines, scales or tubercles. They often possess attachment organs, usually located anteriorly. The helminths can be separated into three phyla: Nemathelminthes, Platyhelminthes and Acanthocephla.

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