INTRODUCTION TO PARASITOLOGY

Symbiosis

Essence of living together or the association of two living organisms, each of a different species.

Commensalism

Association of two different species of organisms that is beneficial to one, neutral to the other.

Mutualism

Association of two different species of organisms that is beneficial to both.

Parasitism

Association of two different species of organisms that is beneficial to one at the other’s expense.

Parasitology

Area of Biology studying the phenomenon of dependence of one living organism on another.

• Endoparasite

• Ectoparasite

Type of Parasite - According to Habitat

Endoparasite

Parasite living inside the body of a host. It manifests as infection.

Ectoparasite

Parasite living outside of the body of a host. It manifests as infestation.

Pedinculus Rumanos Capitis (Head lice)

Example of Ectoparasite

• Obligate

• Facultative

• Accidental

• Spurious

Type of Parasites - According to Mode of Living

Obligate Parasites

It requires host at a certain stage of their cycle to complete their development. They are unable to live outside the host.

Facultative Parasites

It is a parasite that is capable of living independently of a host/ May exist in a free-living state or parasitic when the need arises.

• Naegleria fowleri

• Strongyloides stercoralis

Example of Facultative Parasites

Accidental Parasites

Parasite that is not common to a certain host.

Ancylostoma caninum - Dogs

Example of Accidental Parasite in Humans

Spurious Parasites

Free living organisms that passes through the digestive tract without infecting humans.

Monoxenous - one host only (ex. E. histolytica)

Heteroxenous - two or more host (ex. Trematodes)

Differentiate Monoxenous from Heteroxenous Parasites.

Definitive Host

The host in which the sexual maturity or adult of a parasite development occurs.

Intermediate Host

Host in which the asexual maturity or larval stage of parasite development occurs. Common in helminths.

Accidental or incidental host

Host other than the normal one that is harboring a parasite.

Reservoir Host

Host harboring parasites that are parasitic for humans and from which humans may become infected. It allows the life cycle of the parasite to continue, becoming an additional source of infection.

Paratenic Host

A host that harbors the parasite but it does not allow the life cycle to continue. The host only serves as the transport medium.

Diphyllobotrium latum pleocercoid larvae in Carnivorous fish

Example of a parasite that is seen in Paratenic Host

Transport Host

Host responsible for transferring parasites from one location to another.

Carrier Host

Parasite-harboring host that is asymptomatic but can infect others.

• Biological

• Mechanical

• Vehicle

Enumerate the types of vectors

Biological vector

The essential part of the parasite’s life cycle. A type of vector that harbors the pathogen inside their body.

Mechanical Vector

Only transports the parasite. A type of vector that harbors the pathogen on their body surface.

Vehicle

A nonliving entity that is contaminated with etiologic agent and can be a source of infection.

Female Anopheles mosquito

The biological vector of Plasmodium

Soil

Most common source of infection.

• Hookworms

• Ascaris lumbricoides

• Trichuris trichiura

• Strongyloides stercoralis

What are the parasites that can be seen in soil?

• Amoeba

• Naegleria fowleri

• Shistosoma cercariae

What are the parasites that can be seen in water?

Food

May contain the infective stage of the parasite, as exemplified by a number of trematode and cestode infections.

• Taenia solium - pork

• Taenia saginata - beef

• Paragonimus westermani - seafood

Parasites seen in food

• Cats - toxoplasmosis

• Rats - Hymenolepis

Animals that can harbor parasites.

Autoinfection

The infected person himself is the source of infection.

• Capillaria philippinensis

• Hymenolepis spp.

• Enterobius vermicularis

• Strongyloides stercoralis

Context: they lay embryonated eggs.

Parasites that can cause autoinfection.

• Hookworm

• Strongyloides

Parasites that penetrate from soil to skin.

Schistosoma

Parasites that penetrate from water to skin.

Toxoplasmosis

A parasite that crosses the placental barrier during pregnancy.

• Ancylostoma

• Strongyloides

Parasites that can cause congenital transmission through transmammary glands (mother’s milk)

Trichomonas vaginalis

Parasites that can be transmitted through sexual intercourse.

Protozoans

Single celled parasites

Metazoans

Multicellular worms