Introduction to parasitology

Introduction
Parasitism - an association where one of two organisms (parasite) benefits, lives and gain shelter and nutrition at the expense of the other (the host). The host may suffer from wide range of functional and organic disturbances E.g. worms like Ascaris lumbricoides reside in the gastrointestinal tract of man, and feed on of intestinal food causing various illnesses.
Parasitology: is the science of parasitism and parasites, its study of host-parasite relationships.
Medical Parasitology: is the branch of medical sciences dealing with organisms
(parasites) which live temporarily or permanently, on or within the human body (host). Medical Parasitology consists of: medical protozoology, medical helminthology and medical arachnoentomology.
Parasite It is organisms lives upon or within another living organism (host), it obtains its nourishment and other needs from a host; the host supports the parasite
Types of Parasites
1-Location
- An ectoparasite (External parasite) a parasitic organism that lives on the outer surface of its host on skin or hair of host e.g. lice, ticks, mites etc.
- An endoparasite (Internal parasite) a parasitic organism that lives inside the body of the host may be just under the surface or deep in the body e.g. Entamoeba histolytica.
2– Required or not
- Obligatory parasite this parasite is completely dependent on the host during a segment or all of its life cycle i.e can’t live apart from its hoost e.g. Plasmodium spp. can not survive without a host i.e. completely adapted for parasitic existence.
- Facultative parasite is an organism that can exist in a free living state or as a parasite and hence does not absolutely depend on the parasitic way of life. Its can change its life style between free-living in the environment and parasitic according to the surrounding conditions. e.g. Strongyloides stercoralis.

3- Amount of time spent
- Temporary parasite lives free of its host during part of its life cycle, spends only a short time on a host
- Permanent parasite lives in its host from early life until maaturity or death.
- Accidental parasite – when a parasite attacks an unnatural host and survives.i.e Toxocara canis (a dog parasite) in man


- Host - “the organism in or on, which the parasite lives and causes harm”, the host providing food and/or protection. Some parasites require more than one host to complete their life cycle; or may not require a host during some stages.
Types of Hosts
- Definitive host: a host that harbours a parasite in the adult stage or where the parasite undergoes sexual method of reproduction. e.g. man is DH for Schistosoma haematobium, while female Anopheles mosquito is DH for Plasmodium species (malaria parasites).

- Intermediate host: The host in which the larval stage of parasite lives or the asexual
multiplication takes place. It’s required in the life cycle of parasite, the parasite lives during a period of its development only. In some cases, larval development is completed in two different intermediate hosts, referred to as first and second intermediate hosts.
- Reservoir host: the host harboring a parasite in nature, serving as a source of infection for other susceptible hosts. Reservoir hosts show no sign or symptom of disease. It maintains the life cycle of the parasite in nature such.g. sheep are RH for Fasciola hepatica.
- Vector: “carrier” of a parasite from one host to another. Often an insect. (e.g.an arthropod) that transports a pathogenic organism from an infected to a non-infected host”.
Modes of parasitic Infection
- Filth-borne or contaminative - where personal hygiene and community sanitation lacking. Infectious stages remain viable for long periods in contaminated soil.
- Soil or water-borne - water or dirt which can contain eggs, etc.; Larvae can penetrate skin of bare feet.
- Food-borne Ingestion of contaminated food and water or undercooked meat in which the infective stage has developed.
- Arthropod-borne through the bite or faeces of infected vector. It is the most difficult of all to control such as mosquitoes transmitting malaria, etc.
- Congenital from mother to foetus.
- Inhalation of dust carrying the infective stage of parasite.
Source of parasitic infection
1- Food -meat ( T.saginata, T. spiralis)
2- Vegetables(Ascaris, E.histolytica)
3- Water - (protozoal, cysts) (E.histolytica, Cryptosporidium)
- Cercaria (Schistosoms)
- Cyclops (D.medinensis)
4- Soil contaminated with faeces Ancylostoma ,Strongyloides .
5- Association with domestic animals
-Dog :(hydatid disease ,toxocariasis ,leishmania)
-cat : (Toxoplasma oocyst).
6- Arthropods :
- Blood sucking (malaria,trypanosome,leishmania).
- Mechanical transmission (ova, protozoal cysts)
- Myiasis (invasion of tissue by larvae of flies)
7- Blood transfusion : (erythrocytic stages of Plasmodium).
8-Congenital transplantation : (toxoplasma, Plasmodium).
9-Sexual intercourse : (Trichomonas vaginalis).
10-Inhalation of dust: (Enterobius ova).