English summary of the life cycles of parasites: includes classification, description of life cycle, localization, symptoms and diagnostic techniques.
Includes pictures of the life cycles and parasites.
Used for the course Parasitology (AM) for the Master Biomedical Sciences or the Master Heal...
1. What kind of parasite is this? Protozoa (rhizopod, ciliates, flagellates, sporozoan); helminth
(nematode, trematode, cestode); or arthropoda?
2. Describe the life cycle (with correct terminology)
3. What kind of life cycle? If >1 host: what is the definitive or intermediate host?
4. What are the symptoms of an infection for this parasite? Which stages are responsible for
pathogenesis, how long does it take to occur? Or is it non-pathogenic?
5. Where is this parasite endemic?
6. Are there any methods of lab diagnosis? What sample would you use?
,1. Plasmodium spp.
Four species cause disease in humans: P. falciparum, P. vivax, P. ovale and P. malariae. They use
humans almost exclusively as a natural intermediate host.
P. knowlesi is a zoonotic species, can be found in humans periodically.
Classification: Protozoa, sporozoan.
Type of life cycle:
Indirect. Human is the intermediate (only asexual replication) host and the mosquito is the
definitive host.
Target: liver and red blood cells in humans.
Life Cycle
1. During a blood meal, a malaria-infected female Anopheles mosquito injects sporozoites into
the human host.
2. Sporozoites infect liver cells.
3. Sporozoites in the liver cells mature into schizonts.
4. Liver cells with schizonts rupture and release merozoites.
5. These merozoites infect RBCs and become trophozoites (ring stage).
6. The ring stage trophozoites mature into schizonts, which rupture releasing merozoites that
can again infect RBCs.
7. Some trophozoites in RBCs differentiate into sexual erythrotic stages and now are
gametocytes.
- P. vivax, ovale and malariae have big round gametocytes, P. falciparum has bean-like.
, 8. The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested
by an Anopheles mosquito during a blood meal.
9. While in the mosquito’s stomach, the microgametes (become flagellated?) enter the
macrogametes (female) generating zygotes (sexual replication).
11. The zygotes in turn become motile and elongated and are now ookinetes.
12. Ookinetes invade the midgut wall of the mosquito where they develop into oocysts.
13. The oocysts grow, rupture and release sporozoites which make their way up to the
mosquito’s salivary glands. Injection of the sporozoites into a new human host can start the
life cycle again.
Within the life cycle of plasmodium spp. There are 3 separate cycles.
A. Exo-erythrocytic schizogony cycle. This is the initial replication cycle in the liver of the
human host. This takes in step 1-4 of the life cycle. (Outside of the RBCs).
B. Erythrocytic schizogony cycle. This is the asexual multiplication in the erythrocytes in the
human. These are the blood stages and take up step 5-7 of the life cycle.
C. Sporogonic cycle. The parasites’ multiplication in the mosquito or sexual replication. These
are steps 8-12 of the life cycle.
Symptoms of infection, incubation time and responsible stages
- All clinical manifestations are caused by the blood stage parasites (erythrocytic).
- Symptoms can be non-specific: fever and chills during rupture of red blood cells during the
erythrocytic stage accompanied by headache, myalgias, arthralgias, anemia, thrombocytopenia,
diarrhea, hypoglycemia, pulmonary or renal dysfunction and neurological changes.
- Clinical presentation can vary substantially depending on the infecting species, the level of
parasitemia, and the immune status of the patient.
- Plasmodium falciparum specifically causes the most fatal forms with nervous system
involvement (Cerebral malaria), acute renal failure, severe anemia, or acute respiratory distress
syndrome. This variant is therefore the most deadly.
- Complications of P. vivax malaria include splenomegaly (with, rarely, splenic rupture), and those
of P. malariae include nephrotic syndrome.
- Untreated malaria can progress to severe forms that may be rapidly (<24 hours) fatal.
- Of note, in P. vivax and P. ovale a dormant stage (step 3 in life cycle) (hypnozoites) can persist in
the liver and cause relapses by invading the bloodstream weeks, or even years later.
Endemic regions
Tropical and subtropical areas and altitudes below 1500m.
Plasmodium falciparum is the predominant species in the world.
Diagnostic method and sample
- The golden standard is microscopy. For this you use a thin or thick smear of blood. Use
Giemsa, Wright or Wright-Giemsa stains.
- difficult, time consuming and expensive.
- It’s also possible to diagnose through molecular diagnosis using species-specific PCR diagnosis.
Via the extraction of genomic DNA. This determines the different species of malaria.
- Antigen detection is used in the field through rapid diagnostic tests that detect enzymatic
activities associated with the parasite.
- Antibody detection is also possible using the indirect fluorescent antibody test (IFA).
- Making antibodies takes time
+ useful for screening blood donors or test treated patients
- not useful for testing a sick patient since they will die before antibodies have been made.
, 2. Leishmania spp.
Classification of the parasite: Intracellular, protozoa
(Category: Flagellates)
Type of life cycle
Complete, indirect life cycle. The human is the
intermediate host and so is the sandfly.
Obligate parasite and unable to complete the life cycle
without the host.
Reservoirs: Humans. Or small rodents, dogs, gerbils. Can
become chronic parasite that helps the parasite survive
within them.
Human infection is caused by about 21 of 30 species that infect mammals. These include the L.
donovani complex with 3 species (L. donovani, L. infantum, and L. chagasi); the L. mexicana complex
with 3 main species (L. mex-icana, L. amazonensis, and L. venezuelensis); L. tropica; L. major; L.
aethiopica; and the sub-genus Viannia with 4 main species (L. (V.) braziliensis, L. (V.) guyanensis, L.
(V.) panamensis, and L. (V.) peruviana). The different species are morphologically indistinguishable,
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