Parasitology Flashcards
What is a parasite?
A parasite is an organism that lives on or in a host organism and gets
its food from or at the expense of its host.
What are the three main classes of parasites that can cause disease in humans:
There are three main classes of parasites that can cause disease in humans:
- Protozoa
- Helminths
- Ectoparasites.
What are protozoa?
• Protozoa are microscopic, single-celled organisms that can be freeliving or parasitic in nature.
Protozoa - ability in humans
They are able to multiply in humans allowing serious infections to
develop from a single organism.
Protozoa - describe transmission
Protozoa living in the human intestine can be transmitted by the fecal-oral
route
Protozoa living in blood or tissues are transmitted by an arthropod vector
Protozoa are classified by
Protozoa are classified by mode of movement
Protozoa - list types with examples
Amoeba, e.g. Entamoeba Flagellates, e.g. Giardia, Leishmania Ciliates e.g. Balantidium Sporozoa – organisms whose adult stage is not motile e.g. Plasmodium, Cryptosporidium
List 9 medically important protozoa infections
- Entamoeba histolytica
- Giardia lamblia
- Trichomonas vaginalis
- Malaria (Plasmodium spp.)
- Toxoplasma gondii
- Cryptosporidium
- Leishmania spp.
- Trypansoma cruzi
- Trypansoma brucei (gambiense/rhodesiense)
What are helminths?
Helminths are large, multicellular organisms (worms) generally visible
to the naked eye in their adult stages. In their adult form, helminths
cannot multiply in humans.
The three main groups of helminths that are human parasites
There are three main groups of helminths that are human parasites:
- Nematodes (roundworms)
- Trematodes (flukes)
- Cestodes (tapeworms)
List 4 soil-transmitted helminths and name what group of helminths they are in
Nematodes:
- Soil-transmitted helminths
- Ascaris lumbricoides
- Trichuris trichiura
- Hookworm spp.
- Enterobius vermicularis
List 4 filarial parasites and name what group of helminths they are in
Nematodes:
- Wuchereria bancrofti
- Loa loa
- Onchocerca volvulus
- Dracunculus medinensis
List 4 trematodes
Trematodes:
- Schistosoma mansoni/haematobium/japonicum
- Clonorchis sinensis
- Fasciola hepatica
- Paragonimus spp.
List 3 cestodes
Cestodes:
- Taenia saginata
- Taenia solium
- Echinococcus granulosus
What are ectoparasites
Blood-sucking arthropods such as ticks, fleas, lice, and mites that
attach or burrow into the skin and remain there for relatively long
periods of time (e.g., weeks to months).
Medically important ectoparasites
Mites:
• Scabies
• Trombiculid
Ticks:
• Hard
• Soft
Lice: • Pediculus humanus capitis • Pediculus humanus humanus • Pthirus pubis
Flies:
• Botflies
Parasitic infections - effect on disease in terms of climate + give example
• Parasitic infections cause a tremendous burden of disease in both the
tropics and subtropics as well as in more temperate climates. Malaria
kills ~660,000 people each year.
The Neglected Tropical Diseases (NTDs) include parasitic diseases
such as
The Neglected Tropical Diseases (NTDs) include parasitic diseases
such as lymphatic filariasis, onchocerciasis, and Guinea worm disease,
and affect >1 billion people, largely in rural areas of low-income
countries.
Parasites - Type of host
Type of host
• Intermediate – host in which larval or asexual stages develop
• Definitive – host in which adult or sexual stage occurs
Parasites - Type of vectors
Vectors
• Mechanical when no development of parasite in vector
• Biological when some stages of life cycle occur
Relative wealth is primary determinant of
Relative wealth is primary determinant of distributions
of parasite infections in humans
Determinants of parasite infections
- Faeco-oral
- Food (Animals, government controls)
- Complex life cycles (distributions of vectors and intermediate/definitive hosts)
- Resources
- Education
- Regional/national control programmes
- Treatments (efficacy/cost)
- Constr./building regulations
- Residence - urban/rural
- Environmental sanitation
Life cycle of Trypansoma cruzi
- Triatomine bug takes a blood meal = passes metacyclic trypomastigotes in feces - enter wound/mucosal membranes)
- Metacyclic trypomastigotes penetrate various cells at bite site - inside they transform into amastigotes
- Amastigotes multiply by binary fission in infected cells
- IntraC A → TM = burst out of cell into bloodstream
- Triatomine bug takes blood meal = TM ingested
- Epimastigotes in midgut
- Multiply in midgut
- Metacyclic TM in hind gut
Repeat
Chagas - describe acute phase
• Acute
– Incubation 1-2 wks after bite
– Up to months after transfusion
– Trypanosomes in blood
Chagas - describe ‘interdeterminate’ chronic phase
Chronic ‘indeterminate’ • Lifelong infection • Generally trypanosomes not detectable but often positive for parasite DNA • Seropositive • 60-70% • Normal ECG and X rays
Chagas - describe ‘determinate’ chronic phase
‘Determinate’ Chronic disease – Seropositive – 30-40% of infected 10-30 years after infection – 5-10% develop chronic Chagas immediately after acute disease
Chagas - describe acute symptoms
• Generally mild or asymptomatic • Local swelling (Romaña) • Nodule or chagoma • Fever • Anorexia • Lymphadenopathy
Chagas - describe effect on digestive system
Develops in 10-15% of patients with chronic
infections
• Esophagus, rectum, and sigmoid colon most
affected
Chagas - describe effect on colon
Presentation
– Constipation
• Complications – Faecaloma – Obstruction – Sigmoid volvulus – Ulceration – Perforation
Chagas pathogenesis - for acute
Acute
• Tissue damage caused by inflammatory response to parasite in nests of
amastigotes in cardiac, skeletal, and smooth muscle
• Parasite killing by antibodies, activated innate immune response and Th1 pro- inflammatory cytokines
Chagas pathogenesis - for interdeterminate
Indeterminate
• Regulatory immune response characterized by IL-10 and IL-17
Chagas pathogenesis - for chronic
Chronic
• Chronic inflammatory response to persistent parasites in muscle and nerve
cells
• Autoimmune mechanisms
• May vary by parasite strain and tissue tropism
• Predominance of Th1 cytokines and CD8+ T cells
Leishmaniasis: life cycle
Sandfly bites into mammalian skin
Promastigote engulfed by histiocyte
Replicate by binary fission
Amastigotes present in histiocyte also amastigotes released and taken up by fly
Pathogenesis of cutaneous leishmaniasis for acute lesions
Acute lesions
• Tissue damage caused by inflammatory response to presence of parasites in
macrophages
• Parasite killing by Th1 pro-inflammatory responses and macrophage killing
Pathogenesis of cutaneous leishmaniasis for latency
Latency
• Parasites remain present long-term. Regulatory immune response characterized by
balance of Th1 and anti-inflammatory responses
Pathogenesis of cutaneous leishmaniasis for relapse
Relapse (rare):
Alteration in immune response (i.e change in Th1 vs. immune regulation secondary
to HIV, malnutrtition) may trigger relapse:
• Mucocutaneous disease associated with strong but inadequate inflammatory response to
parasites that have metastasized to mucosa
• Diffuse cutaneous leishmaniasis associated with uncontrolled parasite replication.
• Recividans – recurrence of lesions at old ulcer site.
