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Class notes parasitology AM_470052 (AM_470052)
- Instelling
- Vrije Universiteit Amsterdam (VU)
Parasitology notes from biomedical master's infectious diseases specialization.
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ParasitologyLecture 1. Ectoparasites. Exploiting the behaviour of wild malaria
vectors
INDEX
1. Definitions .................................................................................................................................................................................................. 1
2. Classification of parasites ........................................................................................................................................................................... 1
3. Life cycles ................................................................................................................................................................................................... 2
3.1. Direct ........................................................................................................................................................ 2
3.2. Indirect ..................................................................................................................................................... 2
4. Life cycle we have to know ......................................................................................................................................................................... 2
4.1. Plasmodium spp. ....................................................................................................................................... 2
Classification .............................................................................................................................................................................. 3
Cycle .......................................................................................................................................................................................... 3
Transmission.............................................................................................................................................................................. 3
Diagnosis ................................................................................................................................................................................... 3
Pathology................................................................................................................................................................................... 4
4.2. Leishmania spp.......................................................................................................................................... 5
4.3. Cryptosporidium spp. ................................................................................................................................ 6
4.4. Schistosoma spp. ....................................................................................................................................... 8
4.5. Dracunculus medinensis ............................................................................................................................ 9
4.6. Trypanosoma brucei ................................................................................................................................ 12
4.7. Giardia intestinalis .................................................................................................................................. 13
4.8. Echinococcus spp. .................................................................................................................................... 14
4.9. Trichuris trichiura .................................................................................................................................... 15
4.10. Trypanosoma cruzi ................................................................................................................................ 16
4.11. Entamoeba histolytica ........................................................................................................................... 18
4.12. Taenia solium/saginata .......................................................................................................................... 19
4.13. Hookworm ............................................................................................................................................ 21
4.14. Toxoplasma gondii................................................................................................................................. 22
4.15. Dientameoeba fragilis ............................................................................................................................ 25
4.16. Ascaris lumbricoides .............................................................................................................................. 26
4.17. Strongyloides stercoralis ........................................................................................................................ 28
,A parasite is an eukaryotic organism that have to live in or on other organisms (hosts) to complete its natural life cycle. Medical
parasitology: one stage in a human host.
1. DEFINITIONS
Symbiosis: organisms living together. It’s a close association between two organisms: microorganisms and the host. We have
three types:
- Commensalism: ‘eating at the same table’, the benefits are uni-directional
- Mutualism: a form of commensalism, the benefits are bi-directional
- Parasitism: benefit is uni-directional
Endoparasite: they live inside the host → infection.
- Protozoa (unicellular)
- Metazoa (multicellular)
Ectoparasite: they live on the host → Infestation. For example,
- Arthropoda
o Insecta: lice and fleas
o Arachnida: mites and ticks
Facultative vs obligate
Most of them are obligative, the parasite cannot survive without its host. The facultative may exist in a free living state.
Different types of hosts
- Definitive host: in which sexual replication takes place
- Intermediate host: only asexual replication takes place
- Accidental host: parasite is seldom found in the host, infected by ‘accident’
In the case of plasmodium, the host is human and the mosquito is the definitive, where the sexual reproduction takes place.
2. CLASSIFICATION OF PARASITES
Protozoa: unicellular eukaryotes like Leishmania, plasmodium, toxoplasma and trypanosomes. They can be intracellular or
extracellular. They can be classified on the basis of how the move, the motion, and how they reproduce (binary fission or
schizogony and the organelles of locomotion are flagella, ciliate or rhizopods). This way, we can distinguish:
- Rhizopods - Flagellates
- Ciliates - Sporozoans
Metazoan: multicellular eukaryotes like Schistosoma, Taenia and Enterobius. They are divided in groups:
- Trematodes (fluke)
- Cestodes (tapeworm)
- Nematodes (roundworm)
Morphology, sex and the alimentary tract are different and the parasites can be classified with these characteristics. It’s
important to see if they are male or female, Taenia for example, is hermaphroditic and they have both reproductive organs.
Protozoa Metazoan
- Size: mm – meters
- Size 1-100 µm
- Unicellular - Multicellular
- Intra-/extracellular - Hermaphroditic or female/male worm
- Reproduction in host: asexual and/or sexual - Reproduction: eggs of larvae
1
, - Increase parasite load - Stable population
It’s important to remember the stable population in helminths, while in protozoa they increase the number of parasites in the
host.
3. LIFE CYCLES
3.1. DIRECT
The parasite needs one host to complete its life cycle. For example, Enterobius vermucularis is endemic in the Netherlands and
we are infected by the egg, the larvae Is found in the intestine and the eggs are laid in the perianal regions, which itches. If you
get infected with this, as it highly infective, the whole family probably will have it.
3.2. INDIRECT
Two or more hosts, they can have more complicated life cycles, but also
simple ones.
For example, Taenia solium has the intermediate host in pigs. The
person gets cysticercosis.
In the case of T.solium, if we get infected with the eggs we can get
cysticercosis. They can go to the brain and develop neurological
symptoms.
4. LIFE CYCLE WE HAVE TO KNOW
4.1. PLASMODIUM SPP.
2
, CLASSIFICATION
Protozoa, sporozoan intracellular and it’s only briefly living extracellularly, thus it is difficult to make a vaccine against the
parasite. There are different forms in the parasite, which makes them have diverse biochemical characteristics and it makes it
more difficult to have an effective vaccine.
Blood parasites of the genus Plasmodium. There are approximately 156 named species of Plasmodium which infect various
species of vertebrates. Four species are considered true parasites of humans, as they utilize humans almost exclusively as a
natural intermediate host: P. falciparum, P. vivax, P. ovale and P. malariae. They can infect several animals, there are 5 species
that infect humans: P. falciparum is the deathliest form of the malaria and when left untreated the person will die.
CYCLE
The malaria parasite life cycle involves two hosts (indirect, the human is intermedia because there is asexual reproduction, the
mosquito is the definitive host with the sexual reproduction). During a blood meal, a malaria-infected female Anopheles
mosquito inoculates sporozoites into the human host (1). Sporozoites infect liver cells within 60 minutes (2) and mature into
schizonts(3), which rupture and release merozoites after 6 days(4). (Of note, in P. vivax and P. ovale a dormant stage
[hypnozoites] can persist in the liver and cause relapses by invading the bloodstream and invade the red blood cells during
weeks, or even years later.) After this initial replication in the liver (exo-erythrocytic schizogony (A)), the parasites undergo
asexual multiplication in the erythrocytes (erythrocytic schizogony The letter B). Merozoites infect red blood cells (5) and
transforms into the ring stage, the trophozoites mature that replicate the DNA and form the daughter cells. This can take 48-72h
and the schizont will burst. The merozonts will be able o infect the red blood cells again. → The erytrocytic cycle will happen
over and over again, associated with pathology (anemia, chills, fever) (6).
- In the liver the undergo erythrocytic schizogony, which is different from binary fission as the parasite multiplies the
DNA a couple of times and more, 10,000 times and after this, the division takes place.
- Blood stage parasites are responsible for the clinical manifestations of the disease. In the liver, an immunotolerant
environment, there is not much pathology associated with the cycle.
Some parasites will differentiate into the sexual erythrocytic stages (gametocytes) by a process names gametocytogenesis (7).
The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested by an Anopheles mosquito during a
blood meal and they are the infectious stages for the mosquito (8). The parasites’ multiplication in the mosquito is known as the
sporogonic cycle (C). While in the mosquito’s stomach, the microgametes penetrate the macrogametes generating zygotes (9).
The zygotes in turn become motile and elongated (ookinetes) (10) which invade the midgut wall of the mosquito where they
develop into oocysts (11). The oocysts grow, rupture, and release sporozoites (12), which make their way to the mosquito’s
salivary glands. Inoculation of the sporozoites into a new human host perpetuates the malaria life cycle. The sporozoites are also
the transmission stage
The cycle lasts from 9-10 days inside the mosquito.
TRANSMISSION
➔ The parasite has two transmission stages: one sporozoite and the gametocyte, both intracellular stages of the parasite
in the red blood cell.
➔ The red blood cell does not have a nucleus, no MHC complex, so the parasite is very well hidden from the immune
system.
Inside the blood cell it transports a surface protein to the surface of the red blood cell. This gives the infected blood cell a sticky
phenotype. → Infected red blood cells stick to the capillaries. It does this to avoid splenic clearance that would clear the blood
cells. At the same time, they make themselves visible to the immune system. They can escape this immune system with
antigenic variation. Therefore, humans are not able to induce an effective immune response against the parasite. At the same
time, the sticky infected red blood cells can obstruct the blood flow and there can be an organ failure.
DIAGNOSIS
The red blood cells are used to perform diagnosis. If we want to see if someone is infected, we can take a drop of blood and
stain it, under the microscope it is visible.
3
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