Samenvatting
Summary All lectures diagnosis and virulence sessions
- Vak
- Parasitology
- Instelling
- Vrije Universiteit Amsterdam (VU)
Summary of all the lectures, including the diagnosis and virulence sessions.
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Voorbeeld 3 van de 23 pagina's
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Voorbeeld van de inhoud
1. Introduction lectureParasites: eukaryote organisms that have to live in or on other organisms (hosts) to complete its
natural life cycle.
Medical parasitology: eukaryote organisms that have human hosts.
Three different types of symbiosis (association between species):
1. Parasitism: parasite benefits, host is harmed (benefit is uni-directional).
2. Mutualism: both host and parasite benefit (benefit is bi-directional).
3. Commensalism: parasite benefits, host is not harmed (benefit is uni-directional).
Facultative: the parasite may exist in a free living state.
Obligate: the parasite cannot survive without its host.
Hosts:
1. Definitive host: sexual replication takes place
2. Intermediate host: only asexual replication takes place (no sexual replication)
3. Accidental host: parasite is seldom found in this host (host was infected by ‘accident’)
Ectoparasites live on the host, this is called infestation.
Insecta: lice, fleas.
Arachnida: mites, ticks.
Endoparasites live in the host, and cause infections (e.g. malaria).
Protozoa (unicellular): are classified based on organelles of locomotion and reproduction
- Rhizopods
- Cilliates (have cilia)
- Flagellates (have flagella)
- Sporozoans
Metazoan (multicellular)
- Trematodes (fluke)
- Cestodes (tapeworm): hermaphrodites. Usually only one tapeworm is present in the
host, therefore it needs to fertilize itself to produce eggs.
- Nematodes (roundworm)
Protozoa Helminths
Size: 1-100 μm Size: mm-meters
Unicellular Multicellular (also multiple organs)
Intra-/extracellular hermaphroditic or ♀/♂ worm
reproduction in host: asexual (binary fission or reproduction: eggs of larvae
sporogony) and/or sexual
increase parasite load stable population (eggs usually leave host)
Life cycles:
Direct: parasite needs one host to complete its life cycle (e.g. Enterobius vermicularis).
Indirect: parasite needs two (or more) hosts to complete its life cycle (e.g. tapeworms)
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,2. Diagnostic challenges in parasitology
Toxoplasmosis is the most common parasite in the Netherlands. Also ectoparasites, such as lice, are
common in the Netherlands.
Intestinal protozoa, like Entamoeba and Giardia, have usually to different states:
- Trophozoites: active state, asexual reproduction, cause clinical symptoms, vegetative, cannot
survive outside the host for a long time, usually also not visible for diagnosis.
- Cyst formation: dormant state, decreased metabolism, resistance to environment,
responsible for transmission.
Diagnostic sampling for parasites: faeces, blood, urine, biopsy, CSF, saliva, sperm. Diagnosis of
infected individuals is important for diagnosis (clinic), research (academic), screening (public health),
and developing treatment (industry). Only diagnosis needs to be performed as soon as possible, for
other purposes samples can be frozen and collected over a longer period.
Characteristics of ideal diagnostic assay:
- Sensitive and specific
- Test results: reproducible, simple, uniform (quantitative?)
- Capable to process large sample numbers
- Fast
- Easy to perform (labour?)
- Field applicable
- Affordable (costs, labour)
- Available
Quantitativity is important for some parasite infections. For malaria the parasitic load within
individuals is important, while for helminths especially the quantitativity on population level is
important.
Diagnosis of parasitic infectious diseases:
1. Microscopy
2. Serology (antibody detection)
3. Antigen detection
4. Molecular diagnosis (q-PCR)
Microscopy is seen as the gold standard, also in developed countries such as the Netherlands.
Advantages:
- Fast
- easy to get samples
- broad (you can see all parasites, PCR is selective for one parasite)
Disadvantages:
- Difficult to distinguish between pathogenic and non-pathogenic species (Entamoeba
histolytica and E. dispar).
- Observer dependent
- Lacking sensitivity, therefore 3 samples are needed).
- Staining can increase sensitivity but increases labour. The background story of the patient
determines the staining that is used.
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, Background on diarrhoea causing protozoa:
- Entamoeba histolyticaI is not common in the Netherlands, and is only seen in travellers and
their relatives. It is potentially invasive, and can cause abdominal pains and liver abscess.
- Giardia lamblia is endemic in the Netherlands, especially in children, but is also seen in
travellers. Infection results in gastro-intestinal complaints, malabsorption, and (chronic)
diarrhea.
- Cryptosporidium parvum/C. hominis are endemic in the Netherlands, especially in children,
and is usually caused by water related outbreaks. Infection results in gastro-intestinal
complaints and chronic diarrhoea. In healthy individuals it is self-limiting, but in
immunocompromised patients it can result in severe symptoms.
Diagnosis of Entamoeba histolyticaI and Giardia lamblia is done by microscopy. Permanent staining of
fixed stool by patients themselves makes it possible to detect both trophozoites and cysts. To increase
the sensitivity, three samples need to be taken. There is one big problem with identification of
Entamoeba histolytica because it cannot be distinguished from E. dispar by microscopy.
Diagnosis of Cryptosporidium is also done by microscopy. However, acid-fast staining of stool is
required to detect oocytes in stool. This staining is not routinely performed in most laboratories,
therefore Cryptosporidium infections can be missed.
Diagnosis of diarrhoea causing protozoa by microscopy.
Advantages Disadvantages
Available Sensitivity
Affordable Specificity for E. histolytica
Specificity (not for E. histolytica) Staining procedures
Serology cannot be used to detect parasites, when they remain in the intestine, because no
antibodies will be formed against those parasites. It can be used for E. histolytica.
Diagnosis of diarrhoea causing protozoa by antigen detection.
Advantages Disadvantages
More sensitive than microscopy Sensitivity still not optimal
Less training required Costs
Reproducible Miss other parasites
Molecular diagnosis using q-PCR is thought to be a good sensitive and specific methods, which is
more and more affordable. However, PCR is still facing some problems:
- Clinical relevance: when only a few parasites are present, the PCR will be positive even when
it is not causing symptoms.
- Limited targets: for some rare parasites no targets are available, those cannot be identified
using PCR.
- Expensive compared to microscopy.
- Genetic variation might cause false-negative results.
In the Netherlands microscopy is often replaced by PCR. The patients comes with symptoms to the
physician, who combines this information with exposure risk. Then, a microbiologist will let a sample
be tested by routine diagnostic laboratory. If a sample is repeatedly negative, it is sent to a
parasitology reference centre.
Background on malaria: there are multiple malaria subspecies, but Plasmodium falciparum is the
most important (big killer). Mosquitos inject parasites, which will reproduce in the liver, after which
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