(O) 29: Parasitology I Flashcards
What is a parasite?
any organism that takes METABOLIC ADVANTAGE of another organism
- not necessarily harmful
- include animals, plants, fungi, bacteria and viruses (host-dependent guests)
- parasitism is one of most successful ways of life
Forms of parasitism
- facultative parasites
- obligate parasites
- endoparasites
- ectoparasites
Facultative parasites
can live as parasites but are NOT dependent on parasitic mode of life
does not need to exploit host
Obligate parasites
organisms that cannot complete their life-cycle without exploiting a suitable host
Endoparasites
live WITHIN a suitable host
Ectoparasites
live on the SURFACE of a suitable host
ex. ticks, fleas
What domain is mostly studied in parasitology?
Eukaryota
- single-celled (protozoa) + multi-celled
Classification of eukaryotic parasites
Protozoa - MICROscopic, single-celled
Metazoa - MACROscopic, multi-cellular (helminths - worms, arthropods - insects and arachnids)
Parasitic Protozoa + Helminths
Eukaryotic parasites are different from viruses/bacteria by their higher complexity = slower reproduction + less genetic flexibility
Protozoa: single-celled eukaryotes (either free-living or parasitic) that feed on organic matter like organic tissues and debris
Helminths are worms (either free-living or parasitic)
Why are there so many parasitic diseases?
we come into contact w/ them
- we drink to survive
- we eat to survive
- we mist interact w/ our environment to survive
Parasitic protozoa
all protozoans are EUKARYOTIC SINGLE-CELLED organisms
Classified based on mechanism of motility:
- flagellates (use flagella)
- amebae (use plasma memb + projections - pseudopodia)
- sporozoa or apicomplexa (gliding)
- ciliates (move cilia - hair-like projections)
Mechanisms of parasite entry
- oral (Giardia lamblia)
- sexual (trichomonads vaginalis)
- inhalation (toxoplasma gondii) - widespread from cat litter
- direct contact (trypanosoma cruzi)
- arthropod vectors (plasmodium falciparum - causes MALARIA)
Division + Reproduction of parasites
Asexual, sexual and both
Life cycles
- monoxenous
- diheteroxenous
- triheteroxenous
Monoxenous
Life cycle
- infects only one host
- does whole life cycle in one host
Diheteroxenous
- needs two hosts
- asexual reprod. in intermediate host
- sexual reprod. in final host
Triheteroxenous
has a first and second intermediate then final host
increased rate of survival
Survival of parasites
Immune evasion
- antigenic variation (immune system doesn’t recognize it)
- niche selection
- inhibition of host protective immune responses
- entering long-lasting dormant stage
Non-immune evasion
- enter environmentally resistant stage
Pathogenesis of parasites
causes
- cell death
- virulence factors (ex. enzymes)
- biproducts of parasite metabolism (ex. hemazoin)
Malaria
“bad air” b/c ppl thought it was air-borne
- passed by bite of infected mosquito
- spread by Plasmodium (single-celled parasite - sporozoan)
Plasmodium
- a sporozoan (single-celled parasite)
- parasite that causes MALARIA
4 species of Plasmodium infect humans
- P. FALCIPARUM (most fatal, in Africa)
- P. vivax (Asia + South America)
- P. ovale (Africa)
- P. malariae (causes CHRONIC infection)
P. knowlesi infects macaques which is a cause of zoonotic malaria in humans (progresses quickly(
Anopheles mosquitoes
- Human malaria is transmitted only be FEMALES of genus Anopheles
- female mosquitoes TAKE BLOOD MEALS FOR EGG PRODUCTION + these blood meals link humans and mosquitoes
Global distribution of malaria
- presence of malaria depends mainly on climatic factors like temp, humidity and rainfall
- transmitted in tropical/subtropicla areas
Malaria parasites can complete their growth cycle IN mosquitoes (extrinsic incubation period) - we are intermediate host
Mosquitoes reproduce in standing after
- humans facilitate reproduction w/ blood
Malaria Life Cycle - Mosquito (Sporogony)
Definitive host
- Gametocytes in infected human blood
- Female mosquitoes ingest gametocytes ingested w/ bloodmeal from infected human. Gamete form sin the stomach
- Sexual Reproduction – male gametocytes undergo EXFLAGELLATION and fertilize female gametocytes creating diploid cells in the stomach of the mosquito called OOKINETES
- Ookinetes migrate between stomach lining and undergo asexual reproduction (sporogony) to
form an OOCYST containing sporozoites (haploid). - Sporozoites form and release occurs in the stomach
- oocysts grow, rupture, and release sporozoites into the mosquito’s hemolymph. - Within the hemolymph, sporozoites migrate to the salivary glands and into the salivary duct
- Sporozoites in the mosquitoes saliva is injected into another human when
the infected mosquito takes a second blood meal
Microgametocyte vs Macrogametocyte
Microgametocyte: MALE gametocyte
Macrogametocyte: FEMALE gametocyte
P. Falciparum infection - Human
- Sporozoites are injected into human through the saliva of an Anopholes mosquito during a second blood meal.
- Sporozoites enter the bloodstream and invade liver cells where they reproduce asexually (schizogony) to form CRYPTOZOITES (sporozoites in the liver - can’t infect liver)
- The schizont in the infected liver cell grows and ruptures, releasing MEROZOITES (cryptozoites in blood) into circulation.
- Merozoites infect RBCs and become ring stage “signet” TROPHOZOITES.
- trophozoites divide asexually (schizogony) in RBC to form a schizont.
- some of the cells will differentiate into male (micro) or female (macro) gametocytes
- When RBC ruptures, these cells are released into circulation—many infect other red blood cells in circulation - Mosquitos who bite an infected human will ingest the gametocytes during the blood meal.
Hemozoin
stacking of heme groups
- makes humans sick + releases free heme into the body
biocrystal made by Plasmodium in humans
merozoites (sporozoites in the blood) use RBCs as a source of nutrition
- heme is released to kill parasite but P. falciparum stacks heme = bursting of RBCs = spread of merozoites
Other modes of malaria transmission
- vertical transmission from mother to her fetus before or during delivery
- blood transfusion
- organ transplant
- shared use of needles/syringes
Symptoms of malaria
ANEMIA + JAUNDICE (caused by loss of RBCs by bursting + liver being affected)
- also flu-like symptoms, nausea, vomiting, diarrhea
Diagnosis of malaria
- microscopic exam of thick and thin blood smears
- PCR is more sensitive and specific than microscopy but results take longer
Treatment of malaria
should be guided by these 4 factors:
- infecting Plasmodium species
- clinical status of patient
- expected drug susceptibility of infecting parasite based on geographical area
- previous use of antimalarials
Incidence of malaria
dependent on biological and human factors:
- density of suitable habitats for mosquito vector breeding
- prevalence of infected humans that mosquitoes can feed on
- susceptibility of humans bitten by an infected mosquito
Malaria antagonistic interactions
Sickle cell anemia (Hb mutation = malformed RBC) - clogs blood vessels
If one is heterozygous for sickle cell (no symptoms), they are at an advantage if infected w/ malaria
- not enough good RBCs for malaria to work well
- lower parasitic load b/c of lower number of healthy cells
- can still be infected but LESS symptoms
Malaria Prevention
Vector control + personal protection
- case management
- insecticide-treated nets (ppl cannot touch net b/c mosquitoes can bite btwn)
- indoor residual spraying
- prophylaxis treatment (pregnant women + infants)
