Parasitology summary of lectures

Inhoudsopgave

,Introduction: Classification, life cycles &
definitions
Definitions

Parasites: Eukaryotic organisms that have to live in or on other organisms to complete their natural
life cycle.

Medical parasitology: Eukaryotic organisms that infect humans hosts.

Symbiosis

Symbiosis: Organisms living together  Close association between two organisms; Microorganisms
and host. Three forms of symbiosis:
 Commensalism: Benefit is uni-directional  Microorganism benefits, host is neither harmed
nor helped.
 Mutualism: Benefit is bi-directional  Microorganism and host benefit.
 Parasitism: Benefit is uni-directional  Microorganism benefits, host is harmed

Type of parasites

Endo- vs ectoparasites

 Endoparasites: Live IN the host  Leads to infection.
o Protozoa (unicellular):
 Size: 1-100 uM
 Intra- or extracellular
 Sexual or asexual reproduction in host
 Increase parasite load
 Classification based on movement organelles and type of reproduction
 Types:
 Rhizopods
 Cilliates
 Flagellates
 Sporozoans
o Metazoa (multicellular):
 Size: mm – meters
 Hermaphroditic or males and females
 Reproduction: Eggs or larvae
 Stable population
 Classification based on morphology, sex and digestive tract
 Types:
 Trematodes (flukes)
 Cestodes (tapeworms)  Flat worms and hermaphroditic (they can
still produce off-spring with only one egg)
 Nematodes (roundworms)

 Ectoparasites: Live ON the host  Leads to infestation.

, o Insecta (lice, fleas)
o Arachnida (mites, ticks)

Obligate vs facultative parasites

 Obligate: The parasite cannot survive without its host.
 Facultative: The parasite may exist in a free living state.

Type of hosts

 Definitive host: In which sexual reproduction takes place, that is, the fertilization process.
Two gametes form a zygote. This is preceded by sexual multiplication/gametogony
(formation of gametes). If in a host only sexual multiplication takes place (such as in malaria)
and not fertilization, it’s not a definitive host but an intermediate host.
Sexual multiplication and fertilization are two distinct processes in sexual reproduction.
 Intermediate host: In which asexual reproduction takes place.
 Accidental host: Parasite is seldom found in this host (host was infected by accident) 
These hosts are often a dead end and the lifecycle stops here. In these hosts, often disease is
caused which is neither beneficial for the hosts nor the parasite as disease can cause death.

Life cycles

 Direct: Parasite needs only one host to complete its life cycle.
o Enterobius vermicularis  Fecal-oral auto-infection.
 Indirect: Parasite needs two (or more) hosts to complete its life cycle (often an intermediate
and definitive host).
o T. saginata and solium  Humans are the definitive host. Fecal-oral auto-infection is
possible.

,Lecture 2: Diagnostic Challenges in
Parasitology
Diagnostic Research in Parasitology

 Intestinal protozoa
 Malaria
 Soil Transmitted Helminths
 Schistosomiasis

Characteristics of protozoa

 Single cell organism: 1-100 uM  You need a microscope to visualize them.
 Intra- and extracellular.
 Multiply within host

Lifecycle and reproduction

 Schizogony/sporogony: In malaria; or binary fission: In Entamoeba/Giardia (intestinal
protozoa). In Giardia:
o Trophozoites:
 Vegetative: motile, feeding
 Asexual reproduction.
 Causing the pathology.
 Disintegrate: it can’t survive outside
the host and therefore is difficult to
detect in feces.
o Cyst formation (= dormant stage):
 Decreased metabolism, rounding,
external cell wall
 Resistance to environment 
Transmission of infection (auto-
infection or to another person).
 Cysts differ from oocysts  Oocysts
are not dormant.

Characteristics of helminths

 Mm to meters.
 Multi-cellular adult stage.
 Multiple organs.

Lifecycle and reproduction

 Adult worms: Hermaphrodite or separate sex.
 Produce eggs or larvae.
 Eggs or larvae leave the host  To complete the lifecycle (sometimes elsewhere, such as in
an intermediate host).
 It’s a stable population, unless there’s repeated exposure. Quantification is possible.

,  Often cause chronic disease.
o Depending on the worm load.

Intestinal protozoa

 Diarrhea causing protozoa:
o Entamoeba histolytica
o Giardia lamblia
o Cryptosporidium parvum/C. hominis

Entamoeba histolytica Giardia lamblia Cryptosporidium parvum
 Travellers  Endemic in Netherlands  Endemic in Netherlands
 Relatives, family members (children) (children)
 Travellers  Water-related outbreaks
 Potentially invasive 
Bloody diarrhea (only E.  Gastro-intestinal complaints  Gastro-intestinal complaints
Clinical aspects
histolytica, E. dispar is not  Malabsorption  Sticking to  Chronic diarrhea
invasive) intestine wall  Self-limiting in healthy
 Abdominal pains  (Chronic) diarrhea individuals
 Liver abscess  Severe in non-immuno
competent




Trophozoites can be visible when
stool is very fresh or is fixed.
Then the trophozoites won’t
Microscopy
disintegrate.
diagnosis




Cysts of different species of
Entamoeba can’t be
differentiated under the
Microscopic microscope. This also goes for
problems the small trophozoites. Large
trophozoites can ingest red
blood cells. Only histolytica can
do this.
Diagnosis 1. Microscopy:
Advantages: Available, affordable, specificity (not for E. histolytica: false positive for one of the types).
Disadvantages: Sensitivity (false negatives; need at least 3 stool samples for confirmation), specificity for

, E. histolytica (no distinction possible between species), and staining procedures.

2. Serology (Antibody detection: ELISA)  A reaction of the host to the invaders and this takes some
time:
Advantages: Fast, sensitive
Disadvantages: Only for invasive species (such as E. hystolitica), miss non-targeted parasites (because it’s
very specific), however a-specific after treatment  Cross-reactivity with other antibodies and the
presence of antibodies for a long time (false positives), and not always sensitive.

3. Antigen detection (ELISA or Rapid Test)  Molecules excreted by the pathogen:
Advantages: Fast, sensitive (that is, more than microscopy; only 1 stool sample required), less training
required, reproducible.
Disadvantages: Sensitivity still not optimal (some infections are still missed; false negatives), miss non-
targeted parasites and costs.

4. Molecular diagnosis (q-PCR)  Pathogen DNA:
Advantages: Fast, sensitive & specific, multiplex setting, reproducible, (semi) quantitative, high
throughput, QC options.
Disadvantages: Molecular set-up of laboratory. This is not feasible in developing countries.


Microscopy can be an additional tool for travelers:




Malaria

 Plasmodium falciparum is the most dangerous species.
 Mainly in tropical regions.
 Children, pregnant women and travelers are at risk, because they are immunological naive.
Seroconversion (production of Abs) is too late (approx. 10 days) to fight the disease.

Microscopy

Detection of the parasite in a blood sample:
1. Thick blood smear: