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Parasites & People - Host Parasite Relationship - Rumala
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Parasites & PeopleHost - Parasite Relationship
Department of ParasitologyFaculty of MedicinePeradeniya
Objectives:
• Define the following terms
Parasite
host,
endoparasite,
ectoparasite, pathogen, commensal, obligatory parasite, facultative parasite, definitive host, intermediate host, reservoir host
Describe the
growth, survival and spread of protozoan and helminth parasites
within and outside the human host
Symbiosis: "life together two organisms live in an association with one another.
RELATIONSHIP BETWEEN HOST AND THE PARASITE
MUTUALISM: Both members of the
association benefit.
Lactobacillin vagina
COMMENSALISM: parasite derives the benefit without causing any injury to the host
PARASITISM:
Parasites derives benefit from the host causing injury to the host
Parasite: An organism that lives in or on and takes its nourishment from another organism. A parasite cannot live independently.
RBC
Parasitized RBC
PARASITISM- association between parasite and host(MAN)
P
P
Ectoparasite (on the host- -skin,hair eg. arthropods)
Endoparasite(inside host-extra cellular/intra cellular)
MAN
ECTOPARASITE
A parasite which lives on the outer surface of the host. Eg : Lice
EndoparasitesParasites which lives within the host
exist in one of two forms:
• Extracellular (inhabiting spaces in the host’s body) or
• intracellular (inhabiting cells in the host’s body)
Intracellular parasite Extracellular parasite
Parasites can also be classified as
OBLIGATORY PARASITE : Parasitic throughout their life cycle. Cannot exist without a parasitic life and totally dependent on the host FACULTATIVE PARASITE: live a parasitic or free-living existence when the opportunity arises ACCIDENTAL PARASITE: A parasite which attaches to a unusual host
HOST:
Organism which harbours the parasite and
provides nourishment and shelter
The hosts are relatively larger in sizein comparison to their parasites
HOSTS
Intermediate host : Asexual reproduction/larvalstages
Definitive host: Sexual reproduction/adult stage
Reservoir host: Vertebrates which harbour the parasites in nature. Act as a sourceof infection.(not affected by parasite)
The host in which the parasite is not usually found
Accidental host:
Infection : The growth of a parasitic
organism within the body.
Pathogen : Parasite that causes
damage to the host
Virulence: Ability of an organism to
cause disease
Multiplication: Like prokaryotes, splits into 2 but more complex All genes copied, each daughter cell receivesa complete duplicate set - mitosisother structures eg. cytoplasm splits as well
into TWO - binary fissioneg. Intestinal protozoa (30 min)
or into several (hundreds) - multiple fissioneg. malaria, other tissue protozoa
ONCE THEY ENTER, WHAT HAPPENS NEXT?
Asexual reproductionmalaria parasite in liver cell
RBC
1 parasite x 60,000 in 5-6 days
No mixing/shuffling of genesBut allows rapid increase in numbers
Methods of multiplication - to allow shuffling of genes
1. Conjugation: cells in intimate contact exchangegenes and later divide
2. Sexual reproduction: cells with 2 sets of genes (Diploid)divide into female and male cells,each with only one set (Haploid) These unite to form the diploid cell = fertilizationResult - a combination from 2 parent cellsIncreased possibility of genetic variationand accelerated evolution
Human host is constantly in contact with microorganisms only few of them are able to establish in human tissues WHY?
RESISTANCE: Ability of host to prevent infection through Defense Mechanisms
Immune evasion - survival strategy of parasites
Successful parasites need to persist in host for transmission - with minimal damage to itself or the host….
These parasites devise strategies of evading the host immune response
HOW DO PARASITES OVERCOME HOST RESISTANCE
unlike prokaryotic microorganisms….
parasites are eukaryotes more complex morphology with multipleorganelles - wide array of Ag diversity
longer generation timemulticellular parasites eg. helminths
some helminths’ life span is 15-20 ynot much less than life span of host
Immune evasion strategies of parasites help in survival within the host
many are common with those adopted by prokaryotes such as bacteria, viruses
at all levels of the immune response both non-specific and specific
but some adapted and exploited successfully by parasites during evolution
Avoidance of recognition Immune modulation (change)
Key strategies
Successful parasites have ‘learnt’ to exploit the fundamental basis of immune response- ‘recognition as foreign’
Mechanisms of avoidance of recognition: antigenic variation
host mimicry
sequestration/intracellular retreat
classic mechanism by which parasites confound the immune systemto promote it’s own survival
Antigenic variation
shown by many blood parasites eg.Trypanosomes, Plasmodium
Switching surface antigensthat have induced protective immune response,to new antigens which are unaffected by that response.
some protozoa adapted to live intracellular inmacrophages
Molecular mimicry
Schistosomes live inside blood vessels for years unaffected by host defense
main strategy: expressing antigens similar/identical to host molecules
Malarial parasite
Invades erythrocytes RED BLOOD CELLS – no organelles,incapable of phagocytic activity,only a food store
Transmission of infection
Successful parasitism requires ability toinfect host and transmission to another host
Simple life cycles: no developmental stage during transmission, resistant stages - spores, cysts
Complex life cycles: some protozoa & all metazoasexual + asexual cycles often in different hosts
PORTALS OF ENTRY- of parasites
SKIN•direct contact•wound infection•injection by vector/human•active penetration by agent
Tissue communicating with outside eg. Resp Tract, GIT, GUT
Placenta - congenital transmission
Blood/ blood products/ tissue /organ transplant
HOW DO PARASITES ENTER INTO HOST?
VECTOR
invertebrate transmitting parasite from one host to another
Biological vector:
Mechanical vector:
development or/andmultiplication of parasite in vector
parasite does not undergo biological change in vector
HP vector
ZOONOSESZOONOSES
“diseases and infections that are naturally transmitted between vertebrate animalsand man”
dynamic From domestic or wild animals Importance depends on human contact
VERTEBRATE ANIMALMAN
Anthropozoonoses
Zooanthroponoses
Amphixenoses
Infections transmitted to man from lower vertebrates
Infections transmitted from man to animals
Infections maintained in both man and lower vertebrates, and may be naturally transmitted in either direction
LIFE CYCLES of parasites
A clear an accurate knowledge of LC’s essential for understanding -
pathogenesis, transmission, diagnosis, treatment, control & prevention of diseases caused by parasites
Protozoans & metazoans: Life cycles have several stages that vary in structure = morphology
Intestinal protozoans- Trophozoite- growing form
Cysts- resistant forms can survive in environment (stage of transmission)
Protozoan life cycles
Tissue protozoans-Trophozoites- various stages,
single/dividing nuclei
Gametocytes = gamete forming cells= sexual forms
Helminthes have complex life cycle
Reproductive stages egg/larva released develop into adults in another host - Intestinal nematodes
Egg/larva has to undergo further development in one or more intermediate hosts
- tape worms & flukes or in insect vector to become adults
- tissue nematodes
Tape worms (Cestodes): Adults in small intestine of vertebrateslive for several years (10-12 y)absorbs nutrients through their integument (wall)
Life cycleAdultIn Man/VertebrateDefinitive host
LarvaInvertebrate/Vertebrate/Man
Egg
Helminthes- multiplication factor
One infective stage one adult
Adult thousands of infective stages during life span
Eg. round worm - 250,000 eggs/day for 6 m
tape worm - 50,000 eggs/day for >10 y
Arthropods:A few have adapted to live as ectoparasites
eg. lice, mites
use humans as source of food - blood / tissue fluid
Many act as vectors of disease
Some are intermediate hosts to parasitesMy grateful thanks to Dr. Devika Iddawela for permission
to use some of her slides