Parasitology Flashcards
What is a parasite?
An organism which lives briefly/permanently, on or with the host.
Parasite classification
Ecto – live on the outer surface of host
Endo – Reside inside body
Obligate – totally dependent on their host during a certain stage
Zoonotic – Primarily infects animals and also is transmissible to humans
Facultative – display both parasitic and non-parasitic modes of existence
Accidental – attacks an unnatural host and survives
Erratic – wanders into an organ in which it is not usually found
Classification by parasite role in lifecycle
Definitive host - habours sexually mature stages of the parasite
Intermediate - harbours larval and sexually immature stages of the parasite
Reservoir host - carries same species and same stages of parasite as human, maintains the life cycle of parasite in nature and is a reservoir source of infection for man
Paratenic/transport - does not undergo development but remains alive and infective to another host
Vector - arthropod that transmits parasites from one host to another
Host-Microbe relationship
When two organisms interact together - known as symbiosis - 3types
Mutualism - generally obligatory relationship, both partners benefit.
Commensalism - one partner benfits from the association but host is neither helped nor harmed.
Parasitism - one organisms either harms or lives at expense of host
Modse of transmission
Fecal-oral transmission - egg/cysts contaminate food and consumed by host
Trophic - Parasite makes use of established predator-prey relationship that is present between definitive and intermediate host
Vertical - Transmission from mother to offspring, either via placenta or breast milk
Direct penetration
Vector transmission -
Parasites move between vector and host as vector takes blood meal
Vector competence and capacity
Not all blood feeding arthropods can act as vectors: Parasite must be able to penetrate vector gut wall, reproduce/develop/migrate to salivary glands/resist selective pressure of immune system and achieve transmission
Vector Competence - ability of particular type to become infected with a specific pathogen
Capacity linked to extrinsic factors - population size, feeding freq and time, life span, dispersal ability
Site specificity
Originates from fact that different parasites prefer to survive in different environmental conditions
Parasitics strategies to maintain position on/within host
Endo-parasites need to resist intestinal peristalsis – must cope with blood surge
Ecto-parasites – wind, rain and grooming to contend with
Methods to maintain their position –
invasion of intestinal tissue by Toxoplasma gondii
Adhesive disks – Giardia lamblia
Scolex – tapeworms
P. Faliciparum – erythrocyte membrane protein 1 – protrudes from membrane of infected cells and forms nobs
Parasite environmental response
Constant surveillance of environmental signals by parasites
Change in transcritptome and proteome – genes differentially expressed in a stage of specific manner are required for nutrient acquisition, calc signalling, lipid metab, and host immune system escape
Plasmoidum species alter production of transmissible gameteocytes in response to host stress hormones.
Trypanosoma brucei – changes morphology from long slender form to shorter form by detecting its density in the blood
Prevention Methods
General improved sanitation
Vector control
Mass screening and drug admin programmes
Food Safety
Many parasites consumed from raw/undercooked meat - beef Tapeworm, lung fluke from crabs
Food safety practices - toxoplasma gondii reduced due to increased consumption of frozen meat
Prevention through modern farming practices
Proper sanitation
Hygenic disposal of human/animal waste, provision of uncontaminated food and water cuts transmission cycle of parasites relying on fecal-oral transmission
Chemical prevention
DDT - opens sodium ion channels in neurones lading to unregulated neuron firing, spasms and death
DDT resistance - upregulation of P450 genes results in enzymatic oxidiation of certain molecules including DDT in certain insects
Organophosphates - malathion - inhibits acetylcholinesterase
Carbamate - bendiocarb - toxic to mammals
Pyrethoids - deltametrin - disrupts sodium ion channel proteins on neurons - toxic to aquatic life
Biological Control
Biological control – purposeful introduction of a biological agents to control or eradicate a pest
Preys upon/competes against pest
Cheap, environ friendly + resis development is less likely
Transgenic vectors
Transgenic vectors – reduced vector capacity
Reduction strategy – relies on vector introduction with lower fitness – mating does not lead to progeny
Replacement – Introduction of a vector resistance to disease agent – replaces wild type.
Sterile-male technique
Limitations - transgenic vectors less competitive/fit than their wild type counterparts
Helminths
Multicellular, billateraly symmetrical parasites
Cestode - tape worms
Trematode -flukes
Nematode - round worms
Trematodes (Flukes)
Vetabrate parasites – require one or more intermediate hosts
Eggs pass through environment and larva hatch and swim away to infect intermediate host (a species of mollusk)
Larva known as cercariae are produced
Infiltrate via skin and develop into adults which enter an intermediate host and wait to be ingested – metacercariae
Attach to vegetation, secrete a resistant cyst wall and wait to be eaten
Shistosamiasis - Bilharzia
Human infection takes place by direct penetration of cercariae
Exposure to freshwater contaminated with eggs
Symptoms include rash/itchy skin, fever, chills, cough, muscle eggs
Symptoms cause by bodies reaction to eggs produced by worms not the worms themselves
Definitive diagnosis by identifying the schistosome
Praziquantel used for therapy
Cestodes
Cestodes – adult worms found in small intestine
Larval tapeworms – extraintestinal tissues and produce systemic infections with clinical effects related to the size, number and location of cysts
Eggs hatch in gut releasing onchosphere that migrates through gut wall and blood vessels to encysts in muscle – forms cysticerci (scolex containing cysts)
Cestode examples
Intestinal - Tapeworms
Taenia saginata - worldwide, uncooked contaminated beef, common infection causing minimal symptoms
Taenia solium - worldwide, uncooked pork containing cysticerci, less common causes cysticercosis
Systemic - Echniococcus granulosus (dog tapeworm) and Echinicoccus multicocularis (rodent tapeworm) -
hydatid disease occurs when larval stages of these organisms are ingested
Larvae develop in human host and cause space-occupying lesions in several organs
Taenia solium/saginata (tapeworm) lifecycle
Eggs passes in faeces (in solium ingestion or release of eggs in upper intestines can cause cysticercosis)
Cattle (saginata) and pigs (solium) infected from ingesting eggs/proglottids
Eggs hatch releasing onchosphere which migrates through gut wall and blood vessels to encyst in striated muscle, forms cysticerci (cysts containing scolex).
Raw/undercooked meat ingested
Ascaris lumbricoides - life cycle
Adult worms in small intestine lay large numbers of eggs
Eggs passed with faeces and contaminate veg
Larvae develop inside eggs, swallowed in undercooked food
Eggs enter small intestine where larvae hatch
Larvae penertrate gut mucosa – carried via hepatic circulation to heart and lungs
Develop and escape into alveoli
Re-enter stomach via trachea/oesophagus – pass into small intestine and grow to adulthood
Toxocara canis/cati
Eggs from dogs/cats passed in faeces and mature in soil
Eggs ingested and hatch in small intestine
Larvae penertrate intestinal mucosa, enter bloodstream and carried to tissues
In humans – larvae become arrested in tissues – gives visceral larva migrans/ocular larva migrans
