Parasitology Exam 2

Direct Life cycles

• Requires only a single host

• Can reinfect the same host at different stages

  • ex: adult ascaris may be residing in a person’s intestines, and that same person may then ingest some eggs from contaminated water

Indirect life cycles

• life cycle that requires two or more hosts

Definitive host

Parasite reaches sexual maturity and sexual reproduction may take place

intermediate host 

Parasite larvae undergo required developmental steps and may be accompanied by asexual reproduction (but no sexual reproduction)

Paratenic host

nonessential hosts that increase the likelihood of transmission to the definitive host (no reproduction or development)

To do list

• achieve transmission

• enter its host

• migrate to appropriate tissues or site of host’s body

• maintain position within the host

• find a mate

• reproduce and release progeny from the host

• Undergo successful developmental changes within the host

• Cope with varying physiological conditions within the host

• Evade destruction by the host immune system

Transmission

• The passage of parasite to a host

• most fundamental part of being a parasite

• almost all organisms must disperse from site of origin to successfully grow and reproduce

• parasites have wide variety of strategies to achieve transmission

Mode of transmission

• the way a parasite moves from host to host 

• potentially different modes at different life cycle stages

Fecal-Oral transmission

• Most intestinal protozoa and helminths utilize this type of transmission

• propagules (the stage responsible for achieving transmission) are released in the host’s feces

  • food and water can be contaminated: consumption by host 

  • parasite usually in a metabolically inactive stage (egg or cyst)

    • these are hardy and can withstand harsh environmental conditions

Propagules

what is the name of the stage responsible for achieving transmission in fecal-oral transmission

Giardia

• Example of fecal-oral transmission

• Live in hosts small intestine

• Parasite stages

  • trophozoite - feeding stage in intestines that replicate by binary fission

  • cyst: dormant form of parasite released in feces

    • physiological conditions cause some trophozoites to become cysts

• Cyst formation for transmission:

  • not fully understood 

  • two potential mechanisms

    • decreased lipid concentrations and increased pH in the host’s intestine

    • Density of trophozoites in host: quorum sensing- a mechanism of regulating gene expression that depends on the density of cells in each area

• During cyst formation 

  • flagella lost

  • Cyst wall proteins are excreted via exocytosis

  • Once in the environment, it can survive for months until a host consumes it 

Quorum Sensing

A mechanism of regulating gene expression that depends on the density of cells in each area

Trophic transmission

• Parasite takes advantage of an established predator-prey relationship 

  • definitive host consumes intermediate host 

• This can be couples with other mode of transission 

• ex: the rat tapeworm 

  • rat defecates and eggs are in feces

  • insect intermediate host ingests eggs in feces

  • larvae hatch in insect

  • back to definitive host when rats eats insect

Manipulation of a host

• phenomenon to increase trophic transmission

• Example: Pomphorhynchus laevis (a thorny-headed worm) uses the aquatic amphipod Gammarus pulex as its intermediate host and a freshwater fish (like perch) as its definitive host 

• Uninfected vs infected amphipods:

  • uninfected- stay in darker, rocky water and are hidden from fish

  • infected- swim in open, light water and are easy to prey for fish

Direct penetration

• Once the parasite finds their host, they bore their way in 

• sarcoptes scabi (itch mite)

• considered an ectoparasite even though the adult female burrows into skin to lay eggs

Sexual transmission

• Some eukaryotic parasites can be transmitted by sexual contact 

• ex: Trichomonas vaginalis 

• protozoan that is sexually transmitted in humans

• public lice 

Vertical transmission

• The transmission of parasites from mother to offspring across the placenta, through breast milk (mammals), or via infected gametes

• ex: alaria americana

  • trematodes larvae can be transmitted via breast milk

  • recall life cycles has canid as a definitive host, sail as first intermediate host, and tadpole as second intermediate host

• toxoplasma gondii

  • from cat feces 

• Rhipcephalus 

  • one host tick: remains on the same host for the larval, nymph, and adult stages, only leaving the host prior to laying eggs

  • Limits their ability to transmsit disease, except in the case of transovarian transmission

  • larvae already are infected when they hatch!

Vector competence

• The ability of a particular vector to acquire, maintain, and transmit a specific pathogen

• Not all blood-feeding arthropods can serve as vectors. The parasite must…

  • survive the vector’s digestive tract

  • penetrate the gut wall

  • reproduce and/or develop within the vecotor

  • migrate to the appropriate location for transmission (like salivary glands)

  • achieve transmission when the vector next feeds

Vector capacity

• A measure of transmission potential of a parasite from a vector population to a host population

  • high ___ = a good vector for the parasite

    • primary vs secondary vectors

• Facts that contribute to a high ___

  • regular feeding on the host in question

  • feeding for an extended period and taking a relatively large blood meal

  • abundance

  • good dispersal ability

  • a life span long enough to allow the parasite to reach its infective stage

Mosquitoes

• Only females blood feed and transmit disease

• over 3700 types (live over most of the world)

• considered deadliest animals

• primary vector for many parasitic diseases

Mosquito life cycle

• eggs → larvae → pupae → adults

• where eggs are laid and what they look like depend on the species 

Tsetse fly

• both males and females blood feed

• transmits trypansosoma brucei (african sleeping sickness)

Sand fly

• only females blood feed

• transmits leishmania (leishmaniasis)

Black fly

• only females blood feed

• transmits onchocerca (nematode causing river blindness) and leucocytozoon (malaria - like parasite of birds)

Kissing bugs

• these bugs are widespread in south and central america and a fiar amount of noth america

• about 155 species (most of which transmit trypanosoma cruzi causing chagas disease)

• both male and female ___ take blood meals

  • they are called this name because they like to food at night on soft tissues (around lips and eyes)

fleas

• males and females take blood meals 

• ectoparasite but can also transmit some pathogens

  • transmits causative agent of plague, murine typhus, and some tapeworms of veterinary importance

• some parasites using the ___ as a vector can block its glands and induces starvation

  • once glands unblocked, starving fleas bite repeatedly, increasing transmission

Ticks

• These are arachnids and both males and females blood feed

• burrow into skin and mouthparts, may go unnoticed for days but they feed continually

• 2 main families

  • Hard __: presence of scutum and visible caputulum

  • soft ___: no scutum and capitulum on underside of body

• Saliva of some ___ can produce paralysis especially in children; recovery follows removal of __

• some people also develop and allergy to red meat after being bitten (alpha-gal syndrome)

• makes for great vectors for disease

  • lyme disease, RMSP, relapsing fever, tularemia, ehrlichiosis, red water fever

fecal oral, sexual, vertical, direct penetration

List 4 general modes of transmission parasites utilize to infect their hosts

site specificity

• The requirement of many parasites to arrive at a specific anatomical location on or within a particular host to grow or reproduce 

  • ex: Plasmodium restricted to hepatocytes or erythrocytes depending on developmental stage in the mammal

  • ex: the flagellated protozoan, Histomonas meleagridis, which infects different types of poultry, prefers the cecum

• Each host offers several different habitat options

  • Each of these potential habitats differs in pH, nutrient availability, oxygen concentration, and other factors

• Parasites are competing against the host’s immune system

  • Immune responses vary from location to location

• Potential competition (especially with high infection levels) with other parasites 

• Not all parasites are site-specific

  • ex: cyst stages of some tapeworms can be found in various tissues and organs

Feather lice

• Very site specific

• some __ attach to feathers with barbs of a certain diameter, matching the width of their appendages

affinity between surface molecules of host tissues and parasites

• high importance for protozoan parasites that are not highly mobile

• ex: toxoplasma gondii

  • parasite initially binds intestinal epithelium of intermediate host 

  • toxoplasma microneme protein (mic2) bind to host protein intracellular adhesion molecule 1 (ICAM-1)

  • leads to diapedesis (squeezing between epithelial cells) allowing the parasite to infect macrophages and dendritic cells

  • invasion with apical complex (actin/myosin motors)

  • phagocytic cells further disseminate parasite throughout host 

habitat options, battling immune system, potential competition

2 reasons why parasites have evolved to have site-specific adaptations

hold position

• digestive tract- compete with peristalsis

• circulatory system - rushing blood

• ectoparasites- environmental factors including wind, rain, and host grooming

Intestinal parasites

• invasion of epithelial linnings

• adhere with adhesive disks or hook on with proboscis or scolex

trematodes

• aka flukes

• use of strong, ventral suckers

• sexual reproducers- males and females form pairs, and male’s power musculature (couples with suckers)

plasmodium falciparum adherence

• inside eythrocytes, the parasite produces the protein ___  erythrocyte membrane protein 1 (pfEMP1)

• along with other parasite and host proteins, causes formation of “knobs” that bind to capillary epithelial cells

• __ avoids transport to the spleen, where parasite infected cells might be detected and destroyed

• parasite infected erythrocytes may accumulate in the capillaries of the central nervous or renal system, resulting in much of the severe pathology associated with __

parasitic copepod

• external segmentation lost

• locomotory appendages largesly vestigial

  • females have developed anterior anchors

  • each anchor bifurcates, allowing her to embed tightly into blubber

Reproductive rates

• high investment in ___ common when chances of progeny survival are low

• high egg-laying capacity

  • some nematodes (roundworms) and cestodes (tapeworms) can shed between 200,000 - 700,000 eggs per day

• Cestodes and some trematodes (flukes) are monoecious

  • proglottids of tapeworms equipped with both testes and ovaries

  • Don’t have to reuly on both sexes being present

asexual reproduction

• substantial ___ can occur in many species of parasites

• this can take place in the definitive and/or intermediate host

  • including biological vectors!

Apicomplexans

• __ have complex life cycles that alternate between sexual reproduction and asexual reproduction

• specifics vary between species, but general features remain the same:

  • sexual reproduction

    • gamete formation

  • asexual reproduction

    • sporogony

    • merogony

chemoattractants

• chemical compounds can originate from the host and/or the parasite itself

• not all factors are known or well understood

• disrupting this signaling could help decrease infection rates

Complications

• co-infection with other parasites

  • ex: mice co-infected with plasmodium and trematodes tend ot produce more plasmodium gametocytes → higher infectious load to mosquito → pass on more parasites to next mouse

• multiple modes of transmission

  • mode of transmission can change based on environmental factors

• abiotic and biotic factors impact transmission of parasites released by their hosts

  • abiotic- temperature, pH, salinity

  • biotic- likelihood of hyperparasitism, predation on eggs/larvae, or encountering/entering inappropriate host 

portal of exit

• the anatomical structure through which propagules move

  • for GI parasites, usually the anus

  • for vector-borne parasites, usually the vector for both ingress and egress

  • sexually transmitted parasites, ingress and egress via genital contact

• portal of exit not always same structure/tissue as where parasite enters

  • hookworm and schistosome larcae can burrow throughouh skin → intestine → eggs released in feces and/or urine

transcriptomics

• indentification of environmental signals that parasites use

• how these signals are detected and impact gene expression

Trypanosoma brucei

• __ changes between slender (left image) and stumpy forms

  • stumpy form infectious to tsetse fly vecotr

• regulated by parasites themselves by monitoring their density in human blood

• like quorum sensing, slender forms release stumpy induction factor (SIF) 

  • threshold level of SIF → slender to stumpy form

  • tsetse fly infection → human immune system clears most parasites → parasite surface protein change to increase parasite numbers again

hypobiosis

• the cessation of development, during which the organism remains able to reinitiate development in response to specific environmental stimuli 

• stimuli are factors such as temperature, soil, moisture, changing photoperiod, other abiotic factors

  • hookworms and strongyloid are good examples that can undergo hypobiosis

change in environments

• plasmodium falciparum human to anopheles mosquito

  • pre made mRNA molecules held quiescent in gametocytes in P granules

  • Environmental changes between host induce translation for needed proteins for new developmental stages

    • drop in temperature

    • rise in pH

    • presence of xanthurenic acid

mechanisms responsible for differential gene expression

• many plants release a hormone through their roots called strigolacone, which attracts fungi that establish a mutualistic relationship

• striga seeds have around adozen figgerent strigolacone receptors

• when the receptors bind the released hormone, it triggers germination of the parasitic flowering plant