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INTRODUCTION TO PARASITOLOGY: MEDICAL PARASITOLOGY INTRODUCTION Medical parasitology deals with the parasites, which cause human infections and the diseases they produce. It is broadly divided into 2 parts : Protozoology Helminthology. PARASITES Parasites are living organisms, which de...

INTRODUCTION TO PARASITOLOGY: MEDICAL PARASITOLOGY INTRODUCTION Medical parasitology deals with the parasites, which cause human infections and the diseases they produce. It is broadly divided into 2 parts : Protozoology Helminthology. PARASITES Parasites are living organisms, which depend on a living host for their nourishment and survival. They multiply or undergo development in the host. The term ‘parasite' is usually applied to: Protozoa (unicellular organisms) Helminths (multicellular organisms) TYPES OF PARASITES Ectoparasite: inhabit only the body surface of the host without penetrating the tissue, e.g. lice, ticks, and mites Endoparasite: A parasite, which lives within the body of the host and is said to cause an infection is called an endoparasite. Free-living parasite: It refers to non parasitic stages of active existence, which live independent of the host  Endoparasites can be classified as: Obligate parasite: The parasite, which cannot exist without a host, e.g. Toxoplasma gondii & Plasmodium. Facultative parasite: Organism which may either live as parasitic form or as free living form. Accidental parasites: Parasites, which infect an unusual host are known as accidental parasites. Echinococcus granulosus infects man accidentally, giving rise to hydatid cysts. HOST Host is defined as an organism, which harbors the parasite and provides nourishment and shelter to latter and is relatively larger than the parasite. Definitive host: The host, in which the adult parasite lives and undergoes sexual reproduction is called the definitive host, e.g. mosquito acts as definitive host in malaria. Intermediate host: The host, in which the larval stage of the parasite lives or asexual multiplication takes place is called the intermediate host. In some parasites, 2 different intermediate hosts may be required to complete different larval stages. These are known as first and second intermediate hosts, respectively.  Paratenic host: A host, in which larval stage of the parasite remains viable without further development is referred as a paratenic host. Such host transmits the infection to another host. Reservoir host: In an endemic area, a parasitic infection is continuously kept up by the presence of a host, which harbors the parasite and acts as an important source of infection to other susceptible hosts, e.g. dog is the reservoir host of hydatid disease. Accidental host: The host, in which the parasite is not usually found, e.g. man is an accidental host for cystic echinococcosis. ZOONOSIS Protozoal zoonoses, e.g. toxoplasmosis, leishmaniasis, balantidiasis, and cryptosporodiasis Helminthic zoonoses, e.g. hydatid disease, taeniasis Anthropozoonoses: Infections transmitted to man from lower vertebrate animals, e.g. cystic echinococcosis Zooanthroponoses: Infections transmitted from man to lower vertebrate animals, e.g. human tuberculosis to cattle. HOST-PARASITE RELATIONSHIPS Host-parasite relationships are of following types: Symbiosis Commensalism Parasitism LIFE CYCLE OF PARASITES Direct life cycle: When a parasite requires only single host to complete its development, e.g. Entamoeba histolytica requires only a human host to complete its life cycle. Indirect life cycle: When a parasite requires 2 or more species of host to complete its development, e.g. malarial parasite requires both human host and mosquito to complete its life cycle. SOURCES OF INFECTION Contaminated soil and water Food Insect vectors Animals Other persons Self (autoinfection) MODES OF INFECTION Oral transmission Skin transmission Vector transmission Direct transmission Vertical transmission Iatrogenic transmission PATHOGENESIS Clinical infection produced by parasite may take many forms: acute, subacute, chronic, latent, or recurrent. Pathogenic mechanisms, in parasitic infections are: Lytic necrosis Trauma Allergic manifestations Physical obstruction Inflammatory reaction Neoplasia IMMUNITY Like other infectious agents, parasites also elicit immune responses in the host, both humoral as well as cellular. But immunological protection against parasitic infections is much less efficient, than it is against bacterial or viral infections. Several factors may contribute to this ??? (Homework) LABORATORY DIAGNOSIS Microscopy Stool Blood Urine Sputum Cerebrospinal fluid (CSF) Tissue and aspirates Genital specimens. LABORATORY DIAGNOSIS Culture: Some parasites like Leishmania, Entamoeba, and Trypanosoma can be cultured in the laboratory in various axenic and polyxenic media. Serological test: These tests are basically of 2 types: Antigen Detection: (ELISA) Antibody Detection: (CFT), (IHA), (IFA), and (ELISA) test. Skin test: Skin tests are performed by injecting parasitic antigen intradermally and observing the reaction. LABORATORY DIAGNOSIS Molecular method: polymerase chain reaction (PCR) Animal inoculation: It is useful for the detection of Toxoplasma, Trypanosoma, and Babesia from the blood and other specimens. Xenodiagnosis: Some parasitic infection like Chagas’ disease caused by T. cruzi can be diagnosed by feeding the larvae of reduviid bugs with patients blood and then detection of amastigotes of T. cruzi in their feces. LABORATORY DIAGNOSIS Imaging: Imaging procedures like Xray, ultrasonography (USG) computed tomography (CT) scan and magnetic resonance Imaging (MRI) are now being extensively used for diagnosing various parasitic infection like neurocysticercosis and hydatid cyst disease. Hematology: Anemia is frequently seen in hookworm infection and malaria. Eosinophilia is frequently present in helminthic infections. Hypergammaglobulinemia occurs in visceral leishmaniasis. Leukocytosis is seen in amoebic liver abscess.