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.
There are three main classes of parasites that can cause disease in humans:
- Protozoa – unicellular organisms
- Helminths
- Ectoparasites – fleas, scabies, etc.
Describe protozoa.
Protozoa are microscopic, single-celled organisms that can be free-living (like amoeba) 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
List the different types of protozoa, and give an example of each.
Protozoa are classified by their mode of movement.
We have:
- Amoeba, e.g. Entamoeba (moves around via pseudopodia)
- Flagellates, e.g. Giardia, Leishmania (propelled by flagella)
- Ciliates e.g. Balantidium (use the cilia around them)
- Sporozoa e.g. Plasmodium, Cryptosporidium (organisms whose adult stage is not motile)
List some medically important protozoa infections.
- Entamoeba histolytica – causes amoebic dysentery (infection of the gut)
- Giardia lamblia – common cause of diarrhoea
- Trichomonas vaginalis – causes vaginal discharge
- Malaria (Plasmodium spp.)
- Toxoplasma gondii – infection from cat faeces, tend to infect children
- Cryptosporidium – cause of epidemic diarrhoea world-wide, and a cause of growth-stunting/delay
- Leishmania spp. – causes severe systemic illness, also causes cutaneous ulcers
- Trypansoma cruzi (Chagas disease)
- Trypansoma brucei (gambiense/rhodesiense) – cause an acute inflammatory illness
Describe helminths and the main groups.
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:
- Nematodes (roundworms)
- Trematodes (flukes)
- Cestodes (tapeworms)
List some medically important helminths (of the nematodes).
SOIL-TRANSMITTED HELMINTHS:
- Ascaris lumbricoides - the most common helminth parasite (can go up to 20-30 cm)
- Trichuris trichiura – 3-4 cm, lives inside the large intestinal mucosa
- Hookworm spp. – causes anaemia, they feed off of the blood supply of the small intestinal mucosa
- Enterobius vermicularis – ‘itchy bum worm’, common
FILARIAL PARASITES:
- Wuchereria bancrofti – lives in the blood, enters lymphatics and causes inflammation, causing elephantitis
- Loa loa – eye worm
- Onchocerca volvulus - can cause blindness
- Dracunculus medinensis – guinea worm infection, lives in superficial tissues, female causes itching which causes a blister; victim goes to water to relive itching, which is when female releases her larvae
OTHERS:
- Toxocara canis/cati - get from cats/ dogs
- Trichinella spiralis – comes through contaminated meat, causing trichonosis
List some medically important helminths (of the trematodes and cestodes).
TREMATODES:
- Schistosoma mansoni/haematobium/japonicum – live in the mesenteric vessels
- Clonorchis sinensis – (fluke) lives in the bile duct, can cause cholangiocarcinoma, common in parts of asia, comes from eating poorly cooked fish
- Fasciola hepatica – (liver fluke) used to be common
- Paragonimus spp. – lung worm infection, can cause a TB-like illness
CESTODES:
- Taenia saginata – beef tapeworm, causes a chronic infection
- Taenia solium – pig tapeworm infection, if get infected at a certain lifecycle stage, can cause cycsticercosis, it has a world-wide distribution and is associated with epilepsy
- Echinococcus granulosus – causes Hytadid disease which causes very large cysts in the organs such as liver, infection of dogs
Describe ectoparasites.
They are 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).
List some medically important ectoparasites.
MITES:
- Scabies – causes an itchy rash between the fingers, wrists and flexors; the scratching of the skin can cause secondary infections
- Trombiculid – can get when you walk around in tropics in freshly grown grass, often causes cellulitis due to amount of itching
TICKS:
- Hard
- Soft
LICE:
- Pediculus humanus capitis
- Pediculus humanus humanus
- Pthirus pubis
FLIES:
- Botflies – cause myaisis
List some general comments about parasites.
- 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 lymphatic filariasis, onchocerciasis, and Guinea worm disease, and affect >1 billion people, largely in rural areas of low-income countries.
- Parasitic infections also affect persons living in developed countries.
Comment on the lifestyles of parasites.
Parasites often have complex life cycles.
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
Describe the life cycle of schistosomiasis.
Some water is contaminated with human faeces and the eggs of the schistosomiasis are in the water.
It develops into an intermediate stage called the miracidium, which infects the snail (amplifier).
They asexually reproduce in the snail, and release thousands of cercarie and this infects humans when they walk in contaminated water.
They develop into adults in the human (mesenteric system), undergo sexual reproduction and produce many eggs, which are then released into the environment again, continuing the life cycle.
What is the geographical distribution of parasitic infections?
Essentially, the poorer the country, the more parasites will be present.
You most likely won’t get NTDs (neglected tropical diseases) in the wealthier and colder parts of the world.
List some determinants of parasite infections.
Parasitic infections depend 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 as it lives in poorly constructed households)
- urban vs. rural residence
- environmental sanitation
Describe the life cycle of Trypanosoma cruzi (Chagas disease).
Chagas disease is caused by a parasite called Trypansoma cruzi.
The reduviid bug feeds on the human at night. It bites you, and defecates nearby (the parasite lives in the faeces). When you scratch the bite, you can scratch the faeces into your skin, or rub it onto your eyes in the mucosa.
The parasite enters and multiplies in the nerve and muscle cells. These cells rupture and release thousands of trypomastigotes.
