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 3 main classes of parasites that 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 free living or parasitic in nature
Describe the mechanism of protozoa infections
They are able to multiply in humans allowing serious infections to develop from a single organism
How does protozoa transmission occur?
- Protozoa living in blood / tissues are transmitted by an arthropod vector
- Protozoa living in human intestine can be transmitted via fecal-oral route
How are protozoa classified?
Protozoa are classified by mode of movement
Name different classifications of protozoa
- Amoeba: Entamoeba
- Flagellates: Giardia, Leishmania
- Ciliates: Balantidium
- Sporozoa: Plasmodium, Cryptosporidium
What are sporozoa?
Protozoa organisms whose adult stage is not motile
Name examples of medically important protozoa infections
Entamoeba histolytica Giardia lamblia Trichomonas vaginalis Malaria (Plasmodium spp.) Toxoplasma gondii Cryptosporidium Leishmania spp. Trypanosoma cruzi Trypansoma brucei (gambiense/rhodesiense)
What are helminths?
Helminths are large, multicellular organisms (worms) generally visible to naked eye in adult stages
In adult form, helminths cannot multiply in humans
What are the 3 groups of human parasitic helminths?
- Nematodes (roundworms)
- Trematodes (flukes)
- Cestodes (tapeworms)
Outline the clinically significant soil-transmitted helminths
- Ascaris lumbricoides
- Trichuris trichiura
- Hookworm spp.
- Enterobius vermicularis
What are the filarial helminth parasites?
- Wuchereria bancrofti
- Loa loa
- Onchocerca volvulus
- Dracunculus medinensis
Name other medically important helminth parasites
Toxocara canis/cati
Trichinella spiralis
Outline trematodes
- Schistosoma mansoni/haematobium/japonicum
- Clonorchis sinensis
- Fasciola hepatica
- Paragonimus spp.
Name some common 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).
What are the different types of mites?
- Scabies
* Trombiculid
Describe the different types of ticks
- Hard
* Soft
Name the varying types of lice
- Pediculus humanus capitis
- Pediculus humanus humanus
- Pthirus pubis
- Flies
- Botflies
What is the significance of parasitic infections?
Parasitic infections cause tremendous burden of disease in both tropics and subtropics as well as in more temperate climates
e.g. Malaria kills ~660,000 people each year
What are NTDs?
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.
What are the different types of parasitic hosts?
Intermediate – host in which larval or asexual stages develop
Definitive – host in which adult or sexual stage occurs
What are the 2 types of parasitic vectors?
Mechanical: no development of parasite in vector
Biological: some stages of life cycle occur
What is a determining factor of parasitic infection presence in humans?
Relative wealth is primary determinant of distributions of parasitic infections in humans
How does wealth effect distribution of parasitic infection?
Parasitic infections are more common in poorer countries
How does geographical location effect parasitic infection distribution?
There are fewer infections in the extreme areas (further North/South and sub-saharan Africa) as climate becomes inhabitable for both humans and parasites
What are the 6 most common NTDs?
- Guinea worm disease
- Lymphatic filariasis
- Onchocerciasis
- Schistosomiasis
- Soil-transmitted helminths
- Trachoma
How are parasitic infections transmitted from animals?
Many parasite infections are endemic in animal populations but there are limited opportunities for transmission to humans
How are parasite transmitted faeco-orally?
- Household sanitation
- Access to clean water
- Personal hygiene behaviours
How are parasitic infections transmitted from food?
- Animal husbandry
- Surveillance
- Regulations and government controls
What makes parasitic life cycles so complex?
Distributions of vectors and intermediate/definitive hosts
What other factors determine spread of parasitic infections?
- Government resources and level of human development/per capita income
- Education
- Country-level and regional control programmes
- Availability of cheap and efficacious treatments
- Construction and building regulations (eg Chagas)
- Urban vs. rural residence
- Environmental sanitation
Outline the life cycle of Trypanosoma cruzi (Chagas Disease)
Triatomine bug feeds on human at night and defecates - mode of transmission
Parasite enters and multiplies through nerve and muscle cells
How many cases of Chagas disease are there currently worldwide?
Currently 10m infected in endemic areas
- 325,000 cases in USA
- 100,000 cases in Europe
- 87% in Spain
Describe the acute phase of Chagas disease
Incubation 1-2 weeks after bite
Trypanosomes in blood
Describe the chronic ‘indeterminate’ phase of chagas disease?
- Lifelong infection
- Generally trypanosomes not detectable but often
positive for parasite DNA - Seropositive
- 60-70%
- Normal ECG and X rays
Explain what the chronic ‘determinate’ phase of Chagas is?
Seropositive
30-40% of infected, 10-30 yrs after infection
5-10% develop chronic
Chagas immediately after acute disease
When does acute chagas occur?
occurs within 3 weeks Symptoms last 8-10 weeks
Outline the signs and symptoms of acute chagas
Generally mild or asymptomatic Local swelling (Romaña) Nodule or chagoma Fever Anorexia Lymphadenopathy
What are the rarer symptoms of acute chagas?
Hepatosplenomegaly
Acute myocarditis
Meningoencephalitis
Fatality of <5% of symptomatic
After how long does chagas become chronic?
10-30 years can gradually develop chronic disease
What causes chronic chagas to occur?
Due to a change in immune response where parasite is recognised again to cause further inflammation
> reactivation of latent infection
What are the cardiac consequences of chronic chagas
Causes damage to conduction system of the heart leading to arrhythmias
Heart muscle damage (cardiomyopathy) can cause heart to become enlarged
Apical aneurysms are also common as well as strokes due to thrombus formation
What % of patients develop digestive problems in chronic chagas?
Develops in 10-15% of patients with chronic infections
What part of the GI tract is affected by chronic chagas?
Oesophagus, rectum and sigmoid colon most affected as peristalsis is impaired
Megacolon presentation: Constipation
What are the digestive complications of chronic chagas
- Fecaloma
- Obstruction
- Sigmoid volvulus
- Ulceration
- Perforation
What is acute chagas?
Acute illness; (a)symptomatic
Mild illness
Associated with parasite in blood
Summarise Indeterminate chagas
Immune response determines parasite no
Over couple of years;
- chronic chagas
- cardiac problems
What is the pathogenesis of acute chagas?
Tissue damage caused by inflammatory response to parasite in nests of amastigotes in cardiac, skeletal and smooth muscle
How does the immune system fight acute chagas?
Parasite killing by antibodies, activated innate immune response an dTh1 pro-inflammatory cytokines
What is the immune response to indeterminate chagas?
Regulatory immune response characterised by IL-10 and IL-17
What is the pathogenesis of chronic chagas?
Chronic inflammatory response to persistent parasites in muscles and nerve cells
How does the immune system respond to acute chagas?
Autoimmune mechanisms
May vary by parasite strain and tissue tropism
Predominance of Th1 cytokines and CD8+ T cells
Outline the strains of visceral leishmaniasis and where they’re found
Visceral leishmaniasis
- Asia: leishmania donovani
- Middle East / Africa / Asia: L. infantum variants
- Latin America: L. chagasi
Where in the world is cutaneous leishmaniasis commonly found?
Largely in Africa & Asia as well as southern Europe
Also present in southern and Latin America
Which strains of cutaneous leishmaniasis are found in the ‘old world’?
Old world: Mediteranean / Middle East:
- L. infantum
- L. major
- L. tropica
Name the strains of cutaneous leishmaniasis found in the ‘new world’
New world: Central & South America
- L. brazilliensis
- L. amazonensis
- L. mexicana
Describe the lifecycle of leishmaniasis?
Caused by sandfly bite transmitting promastigotes
Promastigotes infect macrophages to form nests of amastigotes which are then released
Released amastigotes go on to infect other cells
What is the leishmaniasis vector?
Vector: Lutzomyia / Phlebotomus
How does cutaneous leishmaniasis present in patients?
Sandfly bite develops into a papule which spreads to form a lesion which becomes necrotic to forms ulcers
Certain strain of leishmania infects along lymphatic vessels
Is immunity developed against leishmaniasis?
Children often infected and if they remain in that area they can develop immunity
New strains can reinfect old scars of leishmaniasis to reactivate infection
What is the effect of leishmaniasis reinfection?
More disseminated forms can occur causing more ulcers
What is diffuse cutaneous leishmaniasis?
Some individuals are unable to generate an adequate immune response ∴ get packed with parasites
What is mucocutaneous leishmaniasis?
leishmania developed as a child but later on get stuffy noses as lesions develop around mucosal area - septal destruction occurs
Describe the pathogenesis of acute lesions of cutaneous leishmaniasis
Tissue damage caused by inflammatory response to presence of parasites in macrophages
Parasite killing by Th1 proinflammatory responses and macrophage killing
How does latency develop in cutaneous leishmaniasis?
Parasites remain present long-term
Regulatory immune response characterised by balance of Th1 and anti-inflammatory responses
What causes a relapse of latent cutaneous leishmaniasis?
An alteration in immune response (ie. change in Th1 vs. immune regulation secondary to HIV, malnutrition)
Describe the relapse in mucocutaneous leishmaniasis
Mucocutaneous disease associated with strong but inadequate inflammatory response to parasites that have metastasized to mucosa
When does relapse occur in diffuse cutaneous leishmaniasis?
Diffuse cutaneous leishmaniasis associated with uncontrolled parasite replication
What is recividans?
The recurrence of lesions at old ulcer site
