Parasitology Notes

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. Non-permissive 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.

1. 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:

• Apicomplexa

• Microspora

• Ascetospora

• Myxozoa

• Cliophora

• Sarcomastigophora (some parasitic)

2. 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.

Nematoda are the major parasitic class in the Nemathelminthes phylum. They have non-segmented cylindrical bodies covered by a tough layer of skin called a cuticle. The nematode lifecycle has three stages – egg, larval and adult. The larval stage can be subdivided into …

• L1 – first stage larva develops within the eggs and hatches out. It then sheds its external cuticle to grow to the next stage (ecdysis).

• L2 – second stage larva eventually moults into the next stage.

• L3 – third stage larva is the infective stage as it infects the definitive host.

• L4 – fourth stage larva occurs when L3 exsheaths within the definitive host.

Platyhelminthes have flattened bodies, which are either leaf-like (Trematoda class) or resemble ribbon segments (Cestoda class). Their life cycle also has three stages (egg, larva and adult).

Trematodes are hermaphroditic. Snails serve as their first intermediate host, and other aquatic animals and plants as second intermediate hosts. Trematodes are further divided into monogenetic (require only one host to finish their life cycle) and digenetic (require two hosts to complete their life cycle).

Cestodes are classified into two main orders – Pseudophyllidea (tapeworms of carnivores) and Cyclophyllidea (tapeworms of humans and animals). They have a segmented body composed of proglottids. Each proglottid is a complete functional unit. Their life cycles are complex and require one or more intermediate hosts. Adult tapeworms are endoparasites.

Acanthocephalans are distinguished by the presence of an anterior, protrusible proboscis covered in hooks. The definitive host is infected when it eats and intermediate host containing the parasite. The adult ancanthocephalans occur in the intestine of the definitive host, and the females produce eggs which are passed in the faeces then consumed by an intermediate host.

b) ii. Arthropods

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