Human parasites, micro-organisms and zoonoses 2025-2026
HUMAN PARASITES, MICRO-ORGANISMS AND ZOONOSES
PART 1: DEFINITIONS & CLASSIFICATIONS
DEFINITIONS
Any organism that spends a portion or all of its life intimately associated with another living organism of a
di7erent species is known as a symbiont (or symbiote), and the relationship is designated as symbiosis. Thus
‘living together’ of ≧ 2 di7erent species.
Beneficial to… Bacteria in microbiome eat together
Symbiosis Organism 1 Organism 2 Dependence Damage with you. Can be considered as no
damage but can become problematic
Mutualism + + Partial No if they are able to invade, so thin line
Commensalism + / No (no) between commensal and
Parasitism + - Yes Yes pathobiont!
• 50% of all animal species are parasitic
• 100% of animals and plants become parasitized
• Several parasites per host species
• Di7erent hosts for one parasite species ⇨ can have benefits: water hosts or land hosts so if the water
dries it can still survive
Parasitology = study of parasitic symbiosis = protozoa, helminths, arthropods
PARASITISM
Kind of parasite Definition Example
Obligate Parasitic stage required within cycle Ticks are obligate blood-feeding.
Facultative Independent, non-parasitic cycle possible Mosquito is facultative blood-feeding because males
don’t feed on blood and females can survive with
sugar but then they cannot lay eggs.
Permanent Total cycle on the host Lice
Temporary Part of the cycle on the host Fleas: pupae can survive for a long time in the
environment
Incidental Relatively rare in this particular host Anisakis can be found in raw fish. Humans are
incidental routes because normally seals or whales
eat the fish instead of humans.
Erratic Abnormal location within the host Some helminths stay in the skin causing cutaneous
larva migrans or CLM because they can only go to the
blood in cats and dogs e.g.
Stenoxene High host-specificity Human head louse has claws and Black Africans have
another diameter of hair so the lice cannot claw on
their hair.
Euryxene Low host-specificity Ticks
Homoxene One host within the cycle (direct cycle)
Heteroxene More hosts within the cycle (indirect cycle)
HOST
Kind of host Parasite present as the…
Definitive Adult stage: sexual multiplication
Intermediate Larval stage (with development)
Paratenic Larval stage (without development)
Vector Active role in transmission (biologic, mechanic)
Malaria mosquito: biologic because the parasite undergoes a whole cycle in the mosquito
Stable flies, horse flies for sleeping sickness: Mechanical because they are just for transmission.
1
,Human parasites, micro-organisms and zoonoses 2025-2026
OTHER DEFINITIONS
Prepatent period Period between infection and becoming infectious (1) For lice: If you use insecticide
treatment, only the adult lice will die. In
Patent period Infectious period (production of eggs, larvae etc.) (2) order to kill the eggs, you need to treat
again during the prepatent phase
because otherwise you risk having new
eggs again.
Endo-parasite Lives within the host Toxoplasma
Ecto-parasite Lives on the host Lice, ticks, mites, mosquitos
Meso-parasite Lives in external cavities (mouth, ear, sinuses, Trichomonas vaginalis
genitals…)
Reservoir Normal biotope of the parasite Place where the parasite persists in the
host and from where the parasite can
be transmitted. Often in the patent
phase.
Zoonosis Infection that is naturally transmitted between vertebrate animals and humans
Anthroponosis Infection transmitted from human to human.
The One Health approach means looking at human, animal, and environmental health together. Many parasites
are zoonotic, meaning they can pass between animals and people (e.g., from cats or dogs if not dewormed). By
keeping both humans and animals healthy (like deworming cats and dogs), we reduce infection risks and create a
safer, more stable environment for all.
LIFE CYCLE
Some parasites have a complex life cycle:
• Parasites often need several hosts (1–4) to complete their development.
• These hosts can be both invertebrates (like insects) and vertebrates (like humans or animals).
• They may switch between land (terrestrial) and water (aquatic) environments.
• They can spread in di7erent ways:
o Vertical transmission: from mother to o7spring (e.g., Toxoplasma).
o Horizontal transmission: from one person/host to another (e.g., malaria via mosquitoes, or
through blood transfusion)
• This connects to the next steps in parasitology: studying epidemiology (spread), clinical disease +
diagnosis, and designing prevention and control strategies. By knowing the di7erent parameters of a
parasite’s lifecycle, we can predict its disease in a certain way and also predict its clinical features and
therefore form a diagnosis based on these features. This provides further design of preventive and
curative control.
o Passive diagnosis: when doctors only find the infection after symptoms appear.
o Active diagnosis: when health workers actively search for infections (important, because many
would be missed otherwise), like screening programs
ADAPTATIONS
With typical anthroponotic diseases, the parasites are adapted well to humans so little symptoms could occur.
This is needed because if the host dies, the parasites die as well. Zoonotic diseases are very pathogenic in
humans compared to anthroponotic diseases.
Morphological Size, form, locomotion, attachment (hooks, suckers, claws), sensory function, digestive
system
Biological Increase of reproductive potential: egg production, hermaphroditism, parthenogenesis (egg
does not need to be fertilized to generate o7spring), loss of seasonality, short pre-adult
phase (thus short pre-patent phase thus very fast infectious), survival, inclusion of asexual
multiplication, inclusion of secondary and tertiary hosts (to survive in di7erent
environments etc.)
Immunological Absorption of ‘host’ antigen, antigenic variation, immunologic ‘sanctuary’ sites, disruption
of host immunity, ‘masking’ surface antigens
Biochemical Energy metabolism (aerobe/anaerobe, micro-aerophilic), uptake of nutrients, increase of
transporter systems
2
,Human parasites, micro-organisms and zoonoses 2025-2026
PARASITE-HOST INTERACTIONS
Via food, drinking water
Passive
Via direct contact
Infection mechanisms Vectors (sucking, biting)
Active Specific adaptations of infective larvae
Vertical transmission (rarely)
Ectoparasites Limited interaction with the host
Intracellular = immune avoidance
Extracellular = immune evasion
Survival & immunity
Endoparasites - Antigenic variation
- Incorporation of host Ag
- Formation of protective wall (cysts)
PATHOGENESIS
⇨ how a parasite causes disease in a host (human or animal)
The infection depends on both parasite virulence and host resistance. These are the factors on both sides that
influence infection and disease outcome:
Parasite factors
These determine how likely and how severe infection will be:
• Species → Di7erent parasites have di7erent pathogenic potentials.
• Infection pressure → How often you are exposed (e.g., mosquito bites). More exposure = higher infection
chance.
• Localization → Where the parasite resides (e.g., intestine, blood, liver) a7ects the type of damage.
• Stage → Some life stages of a parasite are infectious, others are not. (e.g.: In malaria, only certain stages
in the mosquito or blood are infectious.)
Host factors
These determine the body’s ability to resist or tolerate infection:
• Age → Young or old people may be more susceptible.
• Nutritional status → Malnutrition weakens defense mechanisms.
• Immune status → HIV patients (low CD4 count) have poor immune defense.
• Physiological condition → Pregnancy, cancer, or other stressors alter immunity. (e.g. Pregnant women are
vulnerable to Toxoplasma gondii infection.)
Mechanisms or eSect of disease
Once infection occurs, parasites can cause harm through several mechanisms:
1. Wasting / spoliation (direct or indirect)
o Direct: The parasite itself consumes or removes host nutrients or tissues.
e.g. Hookworms suck blood from the intestinal wall = anemia
e.g. Trypanosomes and malaria parasites destroy blood cells = cell loss
o Indirect: The infection causes metabolic or immune disturbances that lead to wasting.
e.g. cancer patients may waste because immunity is suppressed.
e.g. immune activation burns calories and proteins, causing muscle wasting
2. Formation of toxic products
o Parasites release toxic substances; for instance, red blood cells burst in malaria, releasing toxins.
3. Immunosuppression
o Parasite or host responses suppress immunity (can make host vulnerable to other infections).
4. Allergy and hypersensitivity
o Some infections trigger allergic or anaphylactic reactions.
5. Mechanical damage (pressure, obstruction, migration)
o Parasites may block ducts or blood vessels, or damage tissues while migrating.
6. Irritation (skin, organs) and tissue damage
o Chronic irritation leads to inflammation or fibrosis.
3
, Human parasites, micro-organisms and zoonoses 2025-2026
CLINICAL SIGNS
Often, an infection is subclinical, meaning the host is infected but shows no obvious clinical signs.
A host may have clinical signs, depending on the parasite load, specific immunity and host factors. Based on the
location of the parasite, you can already anticipate di7erent pathologies:
GI anorexia, diarrhea, constipation, vomiting, anemia, e.a.
Respiratory anorexia, sneezing, coughing, dyspnea, tachypnea, e.a.
Reproduction sterility, abortion, congenital disorders, e.a.
CNS anorexia, convulsions, paralyses, e.a.
Vascular anorexia, edema, anemia, e.a.
Skin itch, erythema, crusts, alopecia, e.a.
Liver fever, anorexia, icterus, edema, e.a.
Organs Specific organ failure
PREVALENCE AND GLOBAL HEALTH SITUATION
The life expectancy is much lower in central to south Africa and a bit lower in Asia compared to Europe and North
America. Also, diarrheal diseases are much more prevalent in children under the age of 5 as leading infectious
killers. Same goes for malaria.
The prevalence of parasites is particularly high in tropics and subtropics and in developing countries. Because:
• Poor living conditions and hygiene
• Inadequate disease control and treatment
• Malnourishment
• Poor health education
• Regional or ethnic (dietary) habits
• Climate conditions
• Immunosuppression (HIV) and opportunistic infections
On the exam: How would you approach something on a community level? You always start at the education level!
At the end of the course, we will be able to propose community level strategies for prevention and disease control
for each disease. It is important to start with education and then diagnosis, treatment, recognition of symptoms.
GENERAL CLASSIFICATION
Kingdom Protista Animalia
Platyhelminthes Nematoda
Phylum Protozoa Acanthocephala Annelida Arthropoda
(flatworms) (roundworms)
Class Sarcomastigophora Trematoda Rhabditidae Hirudinea Insecta
(Subclass) Apicomplexa Monogenean Secernentea Arachnida
Myxozoa Digenea Enoplea Crustacea
Microspora Eucestoda Adenophorea
Ciliophora
Order (ida) All living organisms are divided into taxa. The taxonomic hierarchy is based on (morphological,
Superfamily genetic) similarities between members of the same group.
(oidea)
Family (idea) Criteria for classification include phylogenetic, biological, morphological and genetic characteristics.
Subfamily
(inae)
Genus Nomenclature: genus with capital, italic or underlined
Species Always write the full name the first time! Schistosoma mansoni vs S. mansoni
Apicomplexa: Malaria parasite ⇨ “apical” complex because under the microscope it looks like a pear with vesicles on the apical side
Ciliophora means carrying cilia
Cestode = tapeworms, typically segmented
Arachnida includes ticks and mites
Hirudinea will not be discussed
4
HUMAN PARASITES, MICRO-ORGANISMS AND ZOONOSES
PART 1: DEFINITIONS & CLASSIFICATIONS
DEFINITIONS
Any organism that spends a portion or all of its life intimately associated with another living organism of a
di7erent species is known as a symbiont (or symbiote), and the relationship is designated as symbiosis. Thus
‘living together’ of ≧ 2 di7erent species.
Beneficial to… Bacteria in microbiome eat together
Symbiosis Organism 1 Organism 2 Dependence Damage with you. Can be considered as no
damage but can become problematic
Mutualism + + Partial No if they are able to invade, so thin line
Commensalism + / No (no) between commensal and
Parasitism + - Yes Yes pathobiont!
• 50% of all animal species are parasitic
• 100% of animals and plants become parasitized
• Several parasites per host species
• Di7erent hosts for one parasite species ⇨ can have benefits: water hosts or land hosts so if the water
dries it can still survive
Parasitology = study of parasitic symbiosis = protozoa, helminths, arthropods
PARASITISM
Kind of parasite Definition Example
Obligate Parasitic stage required within cycle Ticks are obligate blood-feeding.
Facultative Independent, non-parasitic cycle possible Mosquito is facultative blood-feeding because males
don’t feed on blood and females can survive with
sugar but then they cannot lay eggs.
Permanent Total cycle on the host Lice
Temporary Part of the cycle on the host Fleas: pupae can survive for a long time in the
environment
Incidental Relatively rare in this particular host Anisakis can be found in raw fish. Humans are
incidental routes because normally seals or whales
eat the fish instead of humans.
Erratic Abnormal location within the host Some helminths stay in the skin causing cutaneous
larva migrans or CLM because they can only go to the
blood in cats and dogs e.g.
Stenoxene High host-specificity Human head louse has claws and Black Africans have
another diameter of hair so the lice cannot claw on
their hair.
Euryxene Low host-specificity Ticks
Homoxene One host within the cycle (direct cycle)
Heteroxene More hosts within the cycle (indirect cycle)
HOST
Kind of host Parasite present as the…
Definitive Adult stage: sexual multiplication
Intermediate Larval stage (with development)
Paratenic Larval stage (without development)
Vector Active role in transmission (biologic, mechanic)
Malaria mosquito: biologic because the parasite undergoes a whole cycle in the mosquito
Stable flies, horse flies for sleeping sickness: Mechanical because they are just for transmission.
1
,Human parasites, micro-organisms and zoonoses 2025-2026
OTHER DEFINITIONS
Prepatent period Period between infection and becoming infectious (1) For lice: If you use insecticide
treatment, only the adult lice will die. In
Patent period Infectious period (production of eggs, larvae etc.) (2) order to kill the eggs, you need to treat
again during the prepatent phase
because otherwise you risk having new
eggs again.
Endo-parasite Lives within the host Toxoplasma
Ecto-parasite Lives on the host Lice, ticks, mites, mosquitos
Meso-parasite Lives in external cavities (mouth, ear, sinuses, Trichomonas vaginalis
genitals…)
Reservoir Normal biotope of the parasite Place where the parasite persists in the
host and from where the parasite can
be transmitted. Often in the patent
phase.
Zoonosis Infection that is naturally transmitted between vertebrate animals and humans
Anthroponosis Infection transmitted from human to human.
The One Health approach means looking at human, animal, and environmental health together. Many parasites
are zoonotic, meaning they can pass between animals and people (e.g., from cats or dogs if not dewormed). By
keeping both humans and animals healthy (like deworming cats and dogs), we reduce infection risks and create a
safer, more stable environment for all.
LIFE CYCLE
Some parasites have a complex life cycle:
• Parasites often need several hosts (1–4) to complete their development.
• These hosts can be both invertebrates (like insects) and vertebrates (like humans or animals).
• They may switch between land (terrestrial) and water (aquatic) environments.
• They can spread in di7erent ways:
o Vertical transmission: from mother to o7spring (e.g., Toxoplasma).
o Horizontal transmission: from one person/host to another (e.g., malaria via mosquitoes, or
through blood transfusion)
• This connects to the next steps in parasitology: studying epidemiology (spread), clinical disease +
diagnosis, and designing prevention and control strategies. By knowing the di7erent parameters of a
parasite’s lifecycle, we can predict its disease in a certain way and also predict its clinical features and
therefore form a diagnosis based on these features. This provides further design of preventive and
curative control.
o Passive diagnosis: when doctors only find the infection after symptoms appear.
o Active diagnosis: when health workers actively search for infections (important, because many
would be missed otherwise), like screening programs
ADAPTATIONS
With typical anthroponotic diseases, the parasites are adapted well to humans so little symptoms could occur.
This is needed because if the host dies, the parasites die as well. Zoonotic diseases are very pathogenic in
humans compared to anthroponotic diseases.
Morphological Size, form, locomotion, attachment (hooks, suckers, claws), sensory function, digestive
system
Biological Increase of reproductive potential: egg production, hermaphroditism, parthenogenesis (egg
does not need to be fertilized to generate o7spring), loss of seasonality, short pre-adult
phase (thus short pre-patent phase thus very fast infectious), survival, inclusion of asexual
multiplication, inclusion of secondary and tertiary hosts (to survive in di7erent
environments etc.)
Immunological Absorption of ‘host’ antigen, antigenic variation, immunologic ‘sanctuary’ sites, disruption
of host immunity, ‘masking’ surface antigens
Biochemical Energy metabolism (aerobe/anaerobe, micro-aerophilic), uptake of nutrients, increase of
transporter systems
2
,Human parasites, micro-organisms and zoonoses 2025-2026
PARASITE-HOST INTERACTIONS
Via food, drinking water
Passive
Via direct contact
Infection mechanisms Vectors (sucking, biting)
Active Specific adaptations of infective larvae
Vertical transmission (rarely)
Ectoparasites Limited interaction with the host
Intracellular = immune avoidance
Extracellular = immune evasion
Survival & immunity
Endoparasites - Antigenic variation
- Incorporation of host Ag
- Formation of protective wall (cysts)
PATHOGENESIS
⇨ how a parasite causes disease in a host (human or animal)
The infection depends on both parasite virulence and host resistance. These are the factors on both sides that
influence infection and disease outcome:
Parasite factors
These determine how likely and how severe infection will be:
• Species → Di7erent parasites have di7erent pathogenic potentials.
• Infection pressure → How often you are exposed (e.g., mosquito bites). More exposure = higher infection
chance.
• Localization → Where the parasite resides (e.g., intestine, blood, liver) a7ects the type of damage.
• Stage → Some life stages of a parasite are infectious, others are not. (e.g.: In malaria, only certain stages
in the mosquito or blood are infectious.)
Host factors
These determine the body’s ability to resist or tolerate infection:
• Age → Young or old people may be more susceptible.
• Nutritional status → Malnutrition weakens defense mechanisms.
• Immune status → HIV patients (low CD4 count) have poor immune defense.
• Physiological condition → Pregnancy, cancer, or other stressors alter immunity. (e.g. Pregnant women are
vulnerable to Toxoplasma gondii infection.)
Mechanisms or eSect of disease
Once infection occurs, parasites can cause harm through several mechanisms:
1. Wasting / spoliation (direct or indirect)
o Direct: The parasite itself consumes or removes host nutrients or tissues.
e.g. Hookworms suck blood from the intestinal wall = anemia
e.g. Trypanosomes and malaria parasites destroy blood cells = cell loss
o Indirect: The infection causes metabolic or immune disturbances that lead to wasting.
e.g. cancer patients may waste because immunity is suppressed.
e.g. immune activation burns calories and proteins, causing muscle wasting
2. Formation of toxic products
o Parasites release toxic substances; for instance, red blood cells burst in malaria, releasing toxins.
3. Immunosuppression
o Parasite or host responses suppress immunity (can make host vulnerable to other infections).
4. Allergy and hypersensitivity
o Some infections trigger allergic or anaphylactic reactions.
5. Mechanical damage (pressure, obstruction, migration)
o Parasites may block ducts or blood vessels, or damage tissues while migrating.
6. Irritation (skin, organs) and tissue damage
o Chronic irritation leads to inflammation or fibrosis.
3
, Human parasites, micro-organisms and zoonoses 2025-2026
CLINICAL SIGNS
Often, an infection is subclinical, meaning the host is infected but shows no obvious clinical signs.
A host may have clinical signs, depending on the parasite load, specific immunity and host factors. Based on the
location of the parasite, you can already anticipate di7erent pathologies:
GI anorexia, diarrhea, constipation, vomiting, anemia, e.a.
Respiratory anorexia, sneezing, coughing, dyspnea, tachypnea, e.a.
Reproduction sterility, abortion, congenital disorders, e.a.
CNS anorexia, convulsions, paralyses, e.a.
Vascular anorexia, edema, anemia, e.a.
Skin itch, erythema, crusts, alopecia, e.a.
Liver fever, anorexia, icterus, edema, e.a.
Organs Specific organ failure
PREVALENCE AND GLOBAL HEALTH SITUATION
The life expectancy is much lower in central to south Africa and a bit lower in Asia compared to Europe and North
America. Also, diarrheal diseases are much more prevalent in children under the age of 5 as leading infectious
killers. Same goes for malaria.
The prevalence of parasites is particularly high in tropics and subtropics and in developing countries. Because:
• Poor living conditions and hygiene
• Inadequate disease control and treatment
• Malnourishment
• Poor health education
• Regional or ethnic (dietary) habits
• Climate conditions
• Immunosuppression (HIV) and opportunistic infections
On the exam: How would you approach something on a community level? You always start at the education level!
At the end of the course, we will be able to propose community level strategies for prevention and disease control
for each disease. It is important to start with education and then diagnosis, treatment, recognition of symptoms.
GENERAL CLASSIFICATION
Kingdom Protista Animalia
Platyhelminthes Nematoda
Phylum Protozoa Acanthocephala Annelida Arthropoda
(flatworms) (roundworms)
Class Sarcomastigophora Trematoda Rhabditidae Hirudinea Insecta
(Subclass) Apicomplexa Monogenean Secernentea Arachnida
Myxozoa Digenea Enoplea Crustacea
Microspora Eucestoda Adenophorea
Ciliophora
Order (ida) All living organisms are divided into taxa. The taxonomic hierarchy is based on (morphological,
Superfamily genetic) similarities between members of the same group.
(oidea)
Family (idea) Criteria for classification include phylogenetic, biological, morphological and genetic characteristics.
Subfamily
(inae)
Genus Nomenclature: genus with capital, italic or underlined
Species Always write the full name the first time! Schistosoma mansoni vs S. mansoni
Apicomplexa: Malaria parasite ⇨ “apical” complex because under the microscope it looks like a pear with vesicles on the apical side
Ciliophora means carrying cilia
Cestode = tapeworms, typically segmented
Arachnida includes ticks and mites
Hirudinea will not be discussed
4
