Parasitology

define the term parasite

two -species association in which one species (the parasite) lives on or in the second (the host) for a significant period of its life and obtains nourishment from it

always involves two species and the parasite is always the beneficiary

microparasites

can replicate in the host so infection levels can rise rapidly after a single infection - malaria

macroparasites

generally cannot replicate in the host so the levels of infection arendetermined by the number of infection events and number of infective stages

parasite lifecycle that never leaves the host

parasite is never exposed to the external world and completes its development and reproduction in a single host - transmission is by predation /scavenging - rare

eg thricinella spiralis - the spiral threadworm infects host when infected meat is eaten

parasite never exposed but development takes place in two or more host species

species in which larval, juvenile or non sexual stages develop is the intermediate host

where one transmits the parasite directly to another ins known as a vector

plasmodium (malaria) , wuchereria (elenphantitis)

the parasite is exposed to the external world for parts of its lifecycle but not free living

use cycsts or egg shells to protect parasite when it is outside of the host

direct life cycle

direct life cycles when only one host is involved - e.g ascaris

indirect lifecycle

involving intermediate and definitive hosts or different species - eg tapeworms

the parasite is exposed to the external world as a free living organism between hosts

reentry to the host can be passive (eating) or by penetration this includes hookworms and schistosomes

role of evolution

evolution acts on the host parasite and on the host parasite relationship

arms race

a series of escalating mutual counter adaptations by 2 lineages to exploit or inhibit exploitation

symbiosis (living together)

a relationship between two dissimilar species living together

• commensalism = no metabolic dependence (clown fish and anenomea)

• mutualism = obligitory relationship - both benifit

• parasitism = one partner benifits a, metabolic dependence may cause harm to the host

if the intimate relationship is onesided then this is parasitic

remarkably common most species have parasites - stralings 126

humans 402

nematodes

range from parasites of plants and animals to free living nematodes

evidence of parasitism from free living and parasitic nematodes

comparing DNA sequences

usually nematodes are only parasitic part of life have some outside life cycle

parasitism evolved at least 7 times in this group

principles of the evolution of parasitism

loss of otherwise essential genes through mutualistic relationship with bacterial endosymbiont

horizontal gene transfer

parasitic feature evolving under free living conditions

Brugia malayi

A parasitic nematode responsible for lymphatic filariasis in humans, transmitted by mosquito bites.

example of a loss of essential genes

20% of gene predictions are B. malayi specific

indicate a huge pool of genes involved in nematode defence

genome shows loss of genes in the nematode encoding for enzymes required for biosynthesis or purine, heme and reboflavin

these vital components are likely sourced from bacteria - wolbachia pr host

pristionchus pacificus

A nematode species known for its role in ecological studies and as a model organism in evolutionary biology, exhibiting both free-living and parasitic lifestyles.

plant parasite. Eggs in root system of host. J2 develops in egg. J2 penetrates plant root; migrates between cells; induces multinuclear cells. Moults x3 to adult.

plant nemotode parasites

of agricultural importance

very different life style to animal parasites

all have a stylet to puncture the plant cell wall to provide muctinucleated plant cells which worms feed on

include root knot nematodes - Meloidogyne incognita  and M. hapla

horizontal gene transfer in Meloidogyne incognita  and M. hapla

genomes of the two species rich in cell wall degrading carbohydrate- active enzymes (CAZymes (eg cellulases, xylanases)

no counterpart in most ofther animals and is absent from free living nematodes so likely acquired horizontally from plants

parasites and bacteria

for plant nematodes rhizobial bacteria may have been a source of horizontal gene tranfer that enabled the evolution of plant parasitism in nematodes

pre-adaptations

what initiates the evolution from free living life cycles to a complex parasitic one with host switching

are there trends towards parasites and intermediate stages

has parasitism arisin through a series of adatpive steps each one conferring a fitness advantage

the thought is there is some sort of requirment for independent physiological and morphological alterations that enable parasitism

pre adaptations are adaptations to current environments that in the futrue might be coopted to a new function helping the organism in a new niche

ie - an adaptation that conferes fitness in one environment but can be exploited for survival in another

pre-adaptation example

parasite of the large intestine

swallowed during grazing on grass near dung

little diferent between dung ans colon contents

pre-adaptation - cuticle, tolerates toxic enzymes and anaerobic conditions

would allow for the evolution of parasitism

but pre-adaptation remains hypothetical

nematode associations with insects

may represent pre adaptations

• tolerance to toxic enzymes

• adapt to low oxygen conditions

• form dauer larvae

• phoretic associations

• necromenic association

not yet parasitic but could develop metabolic dependence on its host

dauer larvae stage

these are specialised, arrested non feeding stage - specialised for suvival and dispersal

phorecy- the use of insects for transportation (not species specific)

necromy - dauer associates with insects then waits for its death to feed on the microbes that grow on its carcass

is the dauer stage preadaptation?

both the dauer stage and parasitic larvae have speciealised cutical

both cannot feed and resume development in a later life cycle

molecular similarities - a conserved endocrine singalling mechanism involving dafachronic acid controls formation of dauer and infective larvae

caenorhabtitis elagans

L2 haas to sense the environment to decide wether to grow or arrest

dauer has to sense environment (eg temp) to decide when to resume developments

• C. elegans – development of dauer is default developmental behaviour.

• Specified when DAF-12 is not occupied by ligand

• Under inducing conditions- generate DA -binds to DAF-12 to specify non dauer larvae

strongyloides papillosus

has adult parasites and free living parasitic lifestyle

halfway house between free living and parasite

it has a free living developmental stage then a choice between a infective larvae and a non infective larvae

• Add DA to S. papillosus larvae passed from the host as well as the progeny of the free-living adults both resulted in the development of non-infective larvae suggesting that DA–DAF-12 signalling acts at the two places indicated.

anclyostoma (hookworm)

when the development of the parasitic stage is constitiutive not faculative can DA still turn it off

• The role (if any) of DA in the development of life cycles such as these, remains to be investigated.

• can DA be used to control nematode transmission?

 

pre-adaptation example

evidence from pristionchus pasificus

has a necromenic association with scarab beetles is this a intermediate between free living nematodes and parasites

it has a increase in detoxifying enzymes seen in lots of parasites

but it also has glycosyl hydrolase encoding genes in its genome - not found in any other non-parasitic nematode

malaria

found in areas above 15 degrees as the mosquito vectors require this higher temp

plasmodium

species resonsible for causing malaria

>100 species

4 are transmissible to humans

P. falciparum - most wide spread

P. vivax - worldwide - tropics

P. ovale - west africa

P. malariae - worlldwide but patchy

needs temps of over 15 degrees and cant survive above 3000 m

life cycle - simple

4 phases - 1 sexual and 3 asecual phases with multiplication

• injected into humans by a feeding female mosquito

• sporozoite is injected into humans and it turns into merozoites as it moves into the liver

• leaves the liver and moves into red blood cells so it can be taken up by the mosquito

• another replication stage and each merozoite turns into 30-40 more

• some turn into the sexual form and turn into gametocytes

• burst out of red blood cells and invade others in a synchronous event

• this mass bursting of red blood cells and release of waste is what produces the symptoms

• when fed on by another mosquito they are sucked up with the red blood cell

• digestion of the red blood cells in the mid gut of the fly triggeres the gametocytes to turn into zygotes and become sporozoites again

phases of the life cycle

1 - sexual phase occurs only in the mosquitoes

the second asexual phase is in the liver

third asexual pahas in blood repeated many times

every phase begins with feeding and growth

every phase ends when new invasive parasites appear

in the the third phase some parasites become sex cells the gametocytes

start a new cycle if taken up by anopheles mosquitos

test for infection

take a pinprick of the subject, stain it and look under the microscope

shizogony

a shizont = mass of parasite in red blood cells

shizogony = bursting out of the red cell

red blood phase starts to deform and look diferent

the sexual phase

the mosquito is the definitive host

• ingested gametophytes swell

• discharge osmophillic bodies in RBC

• disrupts RBC membrane and releases gametophytes

• male gametophyte DNA is replicated 3 times resulting in 8 sets of DNA

• 8 kinetosomes formed in microtubular organising

• kinetosome is the base for flagellum and 8 flagella are formed

• gamete explodes releasing flagellum

• called exflagellation = release of spermatozoa

• these then actively swim to female gametocytes and fertilize them

• forms a zygote that in the next 5-10 hours develops into an ookinete

• major changes occur and forms apical complexes

• ookinent penatrates the mosquito midgut wall, transforms into an oocyst. this is the first asexual phase

first asexual phase

• oocysts growsto 80 um in diameter

• oocyst prjects into haemocoel of insect and feeds on teh heamoglobin of the blood meal

• DNA replication then occurs

• each oocyst contains at least 1000 sporozoites

• burst out of sporocyst and migrate to mosguitos salivary gland

apical complexes

plasmodium is a single celled organism but has a front and back with a apical complex to invade cells

ookinete uses apical complex to digest epithilium of gut wall

second asexual phase

within 30-40 minutes after a mosquito bite they end up in your liver cells and go into macrophyte like cells called kupffer cells

then leave kupffer cells go into hepatocytes then go through 2-3 more kupffere cells before they stop

hepatocytes are nutrient rich

rapid growth = loss in sporozoite morphology rounds up and becomes a trophozoite

divides into 10-30,000 merozoites

at this point infected person shows no symptoms

i P. viavax there is a latent stage called hypnozoites that trophozoites can become and cause relapses years later

the parasite in cells

parasite moves into parasitophorus vacuole - is not free in the cytoplasm

the third asexual phase

in red blood cells

merozoites burst out of heptacytes and invade RBCs

once inside they ingest haemoglobin

2-3 days to grow and syncronously burst out in schizogony

after several blood cycle some become gametophytes and remain dormant in RBC until bitten by mosquito

how does the parasite know what cells to invade

• receptor recogition and binding

• erthrocyte deformation

red blood cell invasion

RBC is normally very rigid - not phagocytic

this is due to its cytoskeleton network

highly ordered and hard to disrupt

• parasites attach to red blood cell and roll around on the RBC surface for a while

• entry following the alignment of apical end

• AMA-1 and RON-2 secreted by the parasite - secretes its own receptor which binds to RBC

• in P falciparum PfRh5 from the parasite binds to basigin (blood group molecule) - essential for RBC invasion

• attachment of fibrils to parasite surface coat

• tight junction is formed subsequent invagination then the surface coat of the parasite is sloughed of and not engulfed in the parasitic vacuole

• invasion

• requires dramatic re-organisation of the cytoskeleton of both parasite and host cell

• to invade it needs only form

• rhopteries to form tight junctions and penetrate the RBC membrane.

• micronemes and secretory organelles that play a crucial role in parasite entry.

post entry of malaria

the development of feeding stages

trophozoites ptoduce enzymes to degrade haemoglobin

novel transporter system - maurer’s clefts

then you result in deformed shape of the RBC

RBC becomes deformed with lumps on its surface (knobs)

importance of knobs

Knobs are protrusions on the surface of infected red blood cells that facilitate adherence to blood vessel walls, enhancing parasite retention in the circulation and contributing to severe disease symptoms in malaria.

this keeps the parasite away from the spleen which controls malarial infections

can cause damage to brain and kidney esp P flaciparum

fatal consequences of cerebral malaria

toxoplasma gondii

single celled protozoa

known as the king of parasites as it can infect almost any warm blooded animal (birds and mammals)

the apicomplexan family

of parasites, including Plasmodium and Toxoplasma, characterized by an apical complex used for host cell invasion.

obligate intracellular protozoan parasites

affects 1/3 of humans

infects almost anything warm blooded

three strains of toxoplasma

members of the cat family are the only definitve host

history

• 1908 discovered in the gundi (rodent from middle east)

• 1939 - congenital infection in a child

• 1960 carnivorism (lab hypothesis)

• 1965 (carnivorism human)

• 1970 - found in cat faeces

• 1970 - life cycle found out by bill hutchison

Demonstrated that toxoplasma was a parasite of cats that shed oocystes in the faeces

lifecycle

• felines ingest tissue infected with bradyzoite cystes (or oocystes from another feline with the sporozoites)

• the cysts burst and bradyzoites invade the intestinal epithelium (as do sporozoites)

• both forms can differentiate into male and female gametes and after fertilisation become oocysts

• these oocytes contain sporozoites and are shed from the feline - several million a day for up to 3 weeks

if any warm blooded animal ingests thes the sporozoites will be released into the intestine where they invade it and diferentiate into tachyzoites

disseminate arount the body and become bradyzoite cysts again

if a cat eats these cysts the cyle begins

problems of toxoplasma

can form brachyzoites in the brain or in the fetus of pregnant women

can cause abourtion or defects

if you are immunosupressed the toxoplamosis can reactivate and migrate to the brain

stages of infection - tachyzoites

• intermediate hosts

• rapid intracellular growth and accumulation

• targets almost any nucleated cell

• secreted into the blood stream

• results in acut disease (parasitemia)

• limited by immune response and transfomation into cyst-forming bradyzoites

is probably that the host trying to control the tachyzoites results in transformation into bradyzoite cysts - particularly TH1 response

immune response keeps infection in check and dormant

toxo secretes molecules inhancing th1 response - cyclophillin 18

life cycle - bradyzoites

• intermediate hosts

• slow multiplication rate

• forms cysts in neural and muscular tissue

• persist and cause chronic disease

• if immunocompromised they can cause acute encephalitis

life cycle - sexual stage

• in definitive host (feline)

• in intestine oocysts form in the endotheluim

• unsporulated oocysts shed in faeces - 3-18d)

• sporulate in 3 week period

• contaminate water, soil, fruits and vegetables

• very stable in warm and humid environments

why is it only feline hosts

linolec acid is required for sexual reproduction of toxoplasma

felines are the only mammals that lack delta-6-desturase in their small intestine - enzyme that breaks down linoleic acid

thus cats have an excess of linoelic acid in their intestine allowing toxoplama to develop sexually

thus experimentally you can increase the levels of linoleic acid in other species and they become the definitie host

allows for easier study with mice as cats are not great in the lab

parasite movement

it glides

use genomic info to study hteir biology

a lot harder to do this in malaria parasites

movement relies on an intricate linear motor sytem which is sandwiched between the parasites plasma membrane and pair of membranes known as the inner membrane complex

apical complec - rhotperies and microneme deliver molecules to initiate cell invasion

actin and myosin together with special gliding associated proteins are involved

effects of toxoplasma

women are seropositive for toxomplasma have more sons than uninfected women

mice infected produce more males in litters

maybe this is as males are known to roam more than females = more likely to be eaten

treatment

many many people have this

treated with

• sulfonamides and pyrimethanime

• spiramycin to reduce congenital transmissions

• live vaccine of cystless strain available for sheep

• no human vaccine

• no drugs target encysted bradycytes

leishmania

incidence of 400,000 a year and a total 12 cases in the world

20,000 death/year in india

many diferent species

transmitted by sandfly vectors - major groups

• phlebotomus

• lutzomyia

obligate intracellular parsites

live in macrophytes

the forms infecting macrophages are very similar reult in diverese clinical manifestations

found as far north as the mediteranean basin

lifecycle

live in sandfly which are much smaller than mosquitors so the females saw into the skin to eat and deliver anticoagulant and parasite with it

initially the parasite has a glagellum (promastagotes) then lose it when they enter macrophages (amastigotes)

grow by binary fission in the macrophage then burst out

taken up by sand flies when feeding and become metacyclic form (ready to infect humans again