PARASITOLOGY Medical parasitology: “the study and medical implications of parasites that infect humans”
Parasitism - a way of life
Parasite and Parasitism are terms that define a way of life rather than a coherent and evolutionary related group of organisms
Symbiosis, Commensalism, Mutualism, Parsitism
PARASITE – A living organism dependent on another living organism for its survival in the form of nutrition as well as shelter. Parasites may be simple unicellular protozoa or complex multicellular metazoa
Parasitism - a way of life
Symbiosis : “Any two organisms living in close association, commonly one living in or on the the body of the other, are symbiotic, as contrasted with free living.” De Bary 1879
Commensalism : Sharing the table. One partner benefits but the other is not hurt.
Mutualism : Both partners benefit.
Parasitism: One partner (the parasite) harms or lives on the expense of the other (host).
Who is a parasite?
Parasites are usually much smaller than their hosts, they also do no kill before eating.
Parasites are found in all groups of organisms
Parasites can be
ECTOPARASITE: live on, but not in their hosts (they can nevertheless cause severe illness). Eg lice
ENDOPARASITE: live within the body and tissues of their hosts. Trypanosomes (which cause sleeping sickness) within the blood of an infected animal .
Infection & infestation
Infectious diseases are caused by transmittable parasitic agents including bacteria, viruses, fungi, protozoa and a variety of metazoans commonly referred to as helminths or worms
Infection usually implies replication of the agent resulting in a growing number of pathogens
Infestation are characterized by a constant number of pathogens. Severity of disease often depends on infection dose.
Hosts and life cycles Host: “the organism in, or on, which the parasite lives and causes harm
The definitive host is by definition the one in which the parasite reproduces sexually
intermediate hosts the organism in which the parasite lives during a period of its development only”
Host which actively transmit parasites to humans are often called vectors
In paratenic or transport hosts no parasite development occurs
Reservoir host are alternate animal host from which the parasite can be transmitted to humans (zoonosis) or domestic animals
Accidental host , not suitable for parasite development, but can cause disease
Prepatency : infected but parasite presence can not be detected yet
Patency : established infection, parasite stages can be detected (malaria parasites in blood smears, worm eggs in feces etc.)
Incubation period : time between infection and the development of symptoms
Acute disease can lead to crisis which can resolve in spontaneous healing, chronic infection or death
Convalescence: Period after healing, absence of infectious agents, no symptoms, in certain case immunity to reinfection
Although parasitic infections occur globally, the majority occur in tropical regions, where there is poverty, poor sanitation and personal hygiene
Often entire communities may be infected with multiple, different organisms which remain untreated because treatment is neither accessible nor affordable
Effective prevention and control requires "mass intervention strategies” and intense community education. Examples include:
General improved sanitation : pit latrines, fresh water wells, piped water
Vector control : insecticide impregnated bed nets, spraying of houses with residual insecticides, drainage, landfill
Mass screening and drug administration programmes which may need to be repeated at regular intervals
Parasites --- two main groups:
Protozoa are further divided into:
Metazoa (helminthes ) are further divided into:
Free-living or parasitic
Cyst form (infective for humans) &
Protozoa are grouped as :
Blood and tissue protozoa
Entamoeba histolytica (ameba)
Giardia lamblia (flagellate)
Cryptosporidium parvum (sporozoan)
2. Urogenital Protozoa:
Trichomonas vaginalis (flagellate)
Blood & Tissue Protozoa:
Leishmania species (flagellate)
Trypanosoma species (flagellate)
Toxoplasma gondii (sporozoa)
Plasmodium species (sporozoa)
endemic in tropics & subtropics
ROUTE OF TRANSMISSION:
– fecal-oral route
BY contaminated food & water
Sources of infection – carriers (asymptomatic & convalescent), houseflies, cockroaches
Risks – poverty, lack of hygiene, poor sanitation, mental retardation, male homosexuals
E. histolytica has 2 stages:
Found in intestinal & extra-intestinal lesions
Motile (amoeboid movement) & ingest RBCs.
Mature trophozoite has a single nucleus, even lining of peripheral chromatin & prominent nucleolus (karyosome)
Infective form for humans
Mature cyst has 4 nuclei (diagnostic feature)
Found in non-diarrheal stools
Not killed by chlorination
Readily killed by boiling or filtration of water
E. histolytica causes:
Asymptomatic infection (90%)
Acute intestinal amebiasis (Amebic Dysentery)
Amebic liver abscess
Infrequently abscess in brain, lungs & other organs
Cysts (infective stage) in contaminated food or water – excystation in intestinal tract – trophozoite (ileum) --- each divides to form 8 trophozoites – colonize cecum & colon – invasion of epithelium– till muscularis layer (flask shaped ulcers) – reaches sub mucosa- --- portal blood– liver abscess.
Some trophozoites in intestine encyst – passed in stools – contaminate food & water.