Introduction to Parasitology

Parasitology: Its science which is studied the parasitic phenomena. Dr. Amal Hassan Parasitology: Its science which is studied the parasitic phenomena.   Parasite: It’s an organism depend on other organism (different species) to get food and shelter and produce harmful effect . Parasitism: A relationship between different living organism , the first host and the second parasite

Host: an organism which harbors the parasite.  Classes of parasites Parasite are mainly classified into following two catagories; along with some other types of Parasites; 1- Ecto-parasite (ectozoa): lives outside on the surface of the body of the host.  

2-Endo-parasite (entozoa): lives inside the body of the host: in the blood, tissues, body cavities, digestive tract and other organs. Temporary parasite: visits its host for a short period.  Permanent parasite: leads a parasitic life throughout the whole period of its life. Facultative parasite: lives a parasitic life when opportunity arises.  

Obligatory parasite: cannot exist without the parasitic life.   Occasional or accidental parasite: attacks an unusual host. Wandering or aberrant parasite: happens to reach a place where it cannot live.

Classes of host: Introduction and classification also includes; Definitive host: either harbors the adult stage of the parasite or where the parasite utilizes the sexual method of reproduction. In the majority of human parasitic infections, man is the definitive host; in malaria and hydatid disease, however, man acts as the intermediate host.   Intermediate host: harbours the larval or asexual stages of the parasite. In some cases larval developments are completed in two different intermediate hosts which are been referred to as first and second intermediate hosts respectively. Some parasites require two intermediate hosts in which to complete their life cycle. Paratenic host (a carrier or transport host): a host where the parasite remains viable without further development.

Classification of parasites The parasites which medical men have to deal are divided into three main groups: Phylum Protozoa- single-celled organism, multiply in human host, Medical Protozoology Phylum Platyhelminthes and Phylum Nemathelminthes- multicellular worms, do not normally multiply in human host, medical Helminthology. Phylum Arthropoda – Medical Entomology

Classification according movement Ciliates – the only parasitic ciliate that causes disease in humans in Balantidium coli Flagellates – three of the most common and medically significant include Giardia lamblia, Trypanosome sp. and Trichomonas vaginalis Amoeba –include the pathogenic amoeba Entamoeba which cause dysentery in humans

Examples of important intestinal protozoa Transmitted by the faecal-oral route and cause diarrhoea Giardia lamblia: world-wide distribution, lives in the small intestine and results in malabsorption Entamoeba histolytica: may invade the colon and cause bloody diarrhoea – amoebic dysentery. Also causes amoebic liver abscess. Cryptosporidium parvum: more prevalent in the immunocompromised Balantidium coli: a large motile ciliated parasite that lives in the colon of pigs, humans and rodents and can lead to colonic ulceration

Examples of important systemic protozoa Detected in the blood Plasmodium: the cause of malaria. There are 4 species that infect man: P. falciparum, P. vivax, P. ovale and P. malariae Toxoplasma gondi: transmitted by the ingestion of oocysts from cat faeces. Infection can lead to ocular problems and is also a cause of neonatal toxoplasmosis Leishmania: transmitted by sand flies, can lead to visceral, cutaneous and mucocutaneous leishmaniasis Trypanosoma: haemoflagellates cause In Africa - sleeping sickness (transmitted by the Tsetse fly) In South America - Chagas disease . Typical lesion of cutaneous leishmaniasis

Examples of important intestinal nematodes Ascaris (roundworm) Found world-wide in conditions of poor hygiene, transmitted by the faecal- oral route Adult worms lives in the small intestine Causes eosinophilia Enterobius (pinworm or threadworm) prevalent in cold and temperate climates but rare in the tropics found mainly in children Trichuris (whipworm) A soil transmitted helminth prevalent in warm, humid conditions Can cause diarrhoea, rectal prolapse and anaemia in heavily-infected people Ancylostoma and Necator (hookworms) A major cause of anaemia in the tropics Strongyloides inhabits the small bowel infection more severe in immunospressed people (e.g. HIV/AIDS, malnutrition, intercurrent disease)

Examples of important flatworms : cestodes Systemic Echinococcus granulosus (dog tapeworm) and Echinicoccus multilocularis (rodent tapeworm) Hydatid disease occurs when the larval stages of these organisms are ingested The larvae may develop in the human host and cause space-occupying lesions in several organs, e.g. liver, brain Intestinal :(“tapeworms”) Taenia saginata worldwide acquired by ingestion of contaminated, uncooked beef a common infection but causes minimal symptoms Taenia solium acquired by ingestion of contaminated, uncooked pork that contains cystercerci Less common, but causes cystercicosis – a systemic disease where cysticerci encyst in muscles and in the brain – may lead to epilepsy

Examples of important trematodes (flukes) Fasciola hepatica (liver fluke)- a parasite of sheep, humans become infected when ingest metacercariae that have encysted on watercress. The adult trematode lives in the intra-hepatic bile ducts of the liver. “Fascioliasis” can lead to severe anaemia in humans Clonorchis sinensis (liver fluke): Widespread in China, Japan, Korea and Taiwan, this parasite is acquired by ingestion of infective metacercariae in raw or pickled fish Schistosoma haematobium, S. mansoni and S. japonicum Intestinal Fasciolopsis buski : A common parasite of humans and pigs in South- east Asia. This parasite is one of the largest trematodes to infect man (8cm in length) and lives in the upper intestine. Chronic infection leads to inflammation, ulceration and haemorrhage of the small intestine Paragonimus westermani ( lung fluke)- Widespread in the Far East and South east Asia, the parasite is acquired by ingestion of infective metacercariae in raw or pickled crustaceans