Parisitology 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.
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.
They are able to multiply in humans allowing serious infections to develop from a single organism.
Transmission:
- Protozoa living in the human intestine can be transmitted by the faecal-oral route
- Protozoa living in blood or tissues are transmitted by an arthropod vector
What are the types of protozoa?
Amoeba, e.g. Entamoeba- move by pushing out a pseudopodia
Flagellates, e.g. Giardia, Leishmania- use flagella
Ciliates e.g. Balantidium- use cilia
Sporozoa – organisms whose adult stage is not motile
- e.g. Plasmodium, Cryptosporidium
What are some medically important protozoa infections?
Entamoeba histolytica- invades intestine and forms ulcers
Giardia lamblia
Trichomonas vaginalis- sexually transmitted
Malaria (Plasmodium spp.)
Toxoplasma gondii- spread through cat feaces causes cysts in internal tissues
Cryptosporidium- causes diarrhoea
Leishmania spp.- transmitted by sand flies
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.
There are three main groups of helminths that are human parasites:
1. Nematodes (roundworms)
2. Trematodes (flukes)
3. Cestodes (tapeworms)
What are some medically important nematodes?
Soil-transmitted helminths - Ascaris lumbricoides - Trichuris trichiura - Hookworm spp. - Enterobius vermicularis Filarial parasites - Wuchereria bancrofti - Loa loa - Onchocerca volvulus - Dracunculus medinensis Others - Toxocara canis/cati - Trichinella spiralis
What are some medically important trematodes and cestodes?
Trematodes
- Schistosoma mansoni/haematobium/japonicum
- Clonorchis sinensis- food borne from freshwater fish
- Fasciola hepatica- from eating contaminated grass
- Paragonimus spp.- lungworm transmitted through poorly cooked crustacean
Cestodes
- Taenia saginata- cow worm
- Taenia solium- pig worm
- Echinococcus granulosus- causes large lesions in the viscera filled with cyts
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 some medically important ectoparasites?
Mites - Scabies - Trombiculid Ticks - Hard - Soft Lice - Pediculus humanus capitis - Pediculus humanus humanus - Pthirus pubis Flies - Botflies
What are the different types of hosts and vectors that parasites deal with?
Type of host
- Intermediate – host in which larval or asexual stages develop
- Definitive – host in which adult or sexual stage occurs
Vectors
- Mechanical when no development of parasite in vector
- Biological when some stages of life cycle occur
What are some determinants of parasite infections?
Depends on mode of transmission and opportunities for transmission
Faeco-oral
- Household sanitation
- Access to clean water
- Personal hygiene behaviours
Food
- Animal husbandry
- Surveillance
- Regulations and government controls
Complex life cycles
- Distributions of vectors and intermediate/definitive hosts
Others
- 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
What is the life cycle of Trypansoma cruzi like?
Not transmitted through bite, transmitted through its faeces when you scratch your skin and you get a break in skin
Or if it bites close to mouth or eye it can be directly transmitted through its faeces
You get infection with metacyclic trypomastigotes which infect many types of mammalian cells but predominantly muscle and nerve cells
Inside the cells they become amastigotes which multiply by binary fission, transform into trypomastigotes and then burst out of the cell releasing them into the blood where they can infect more cells
What are the phases of Chagas disease?
Acute
- Incubation 1-2 weeks after bite
- Up to months after transfusion
- Trypanosomes in blood
Chronic ‘indeterminate’
- Lifelong infection
- Generally trypanosomes not detectable but often positive for parasite DNA
- Seropositive
- 60-70% develop indeterminate chagas
- Normal ECG and X rays
‘Determinate’ Chronic disease
- Seropositive but remain negative in blood
- 30-40% of infected 10-30 years after infection
- 5-10% develop immediate chronic Chagas immediately after acute disease which can result in severe or end organ damage
What are the signs and symptoms of acute Chagas?
Occurs within 3 weeks Generally mild or asymptomatic - Local swelling (Romaña) - Nodule or chagoma - Fever - Anorexia - Lymphadenopathy 1-2% diagnosed Symptoms last 8-10 wks Rarely (young and IS) - Hepatopsplenomegaly - Acute myocarditis - Meningoencephalitis Fatality <5% of symptomatic
What are the cardiac effects of chronic Chagas?
Trypomastigotes tend to target the autonomic nervous system
In the heart the protozoa infect the Purkinje fibres, causing damage to the conduction system through inflammation and fibrosis which results in a wide number of arrythmias
Once the cardiac muscle cells are infected it causes the muscle to become thinner and thinner which causes the heart to become distended, so you get cardio myopathy
You can get an aneurysm at the apex of the heart and insufficiency of the heart valves
