Parasites Flashcards
What is the definition of a parasite?
A parasite is an organism that lives in or on another organism (host) and derives its nutrients at the expense of this host.
How are parasites classified?
Life cycle and cellular structure.
Define a vector
An organism which acts as an intermediate host for a parasite.
Name the three broad groups of parasites
Helminths (worms)
Protozoa
Ectoparasites
What are Helminths and Protozoa?
Endoparasites, because the live within the host
Where are ectoparasites found?
On the outside of the body
What are the characteristics of Protozoa?
Microscopic, single celled organisms
Can live outside of a host
They multiply in humans
How can Protozoa be transmitted?
Faecal-oral route if they live in the human intestine
Or
via an arthropod vector (usually an insect) if the protozoa lives in blood or tissue.
Describe briefly the life cycle of a protozoa
Cysts enter the host, as cysts are tough, so can survive harsh environments (e.g. stomach acid)
The cysts contain sporozoites which are capable of infecting the host.
Sporozoites then mature in to merozoites (in the liver in Malaria).
Sexual form is then taken up by the vector or expelled from the body in the faeces/urine.
What are the three types of helminths?
Nematodes (round worms)
Cestodes (“tapeworms”)
Trematodes (flatworms/flukes)
What are the characteristics of Nematodes?
Transmitted via faecal-oral route
Ingested eggs hatch in the intestine and can be seen in faeces using microscopy.
Give an example of a nematode
Enterobiasis (pineworm)
Is very common in children in the UK, it causes anal itch, particularly at night.
What are the characteristics of Cestodes
Cysts are ingested and eggs hatch in the intestine.
Have at least one intermediary host, e.g. pigs, cattle or dogs/wolves/foxes.
What are the main differences between Nematodes and Cestodes?
Nematodes are round, while cestodes are generally ribbon like and segmented.
Cestodes have at least one intermediary host but nematodes generally do not.
Describe the outline of a Cestodes’ life cycle
Ova are excreted in human faeces and are either eaten or picked up by the intermediate host.
The cysts then develop and live in the intermediate/secondary host.
The human then ingests cysts, which pass through the digestive system to the intestine, where they hatch and develop in to adults.
The proglottids/segments (which contain eggs) then get excreted in faeces.
Give three examples of Cestodes and name their intermediate hosts.
Taenia saginata- beef intermediate host, cysts for in beef muscle and are eaten when raw or undercooked beef is consumed
Taenia solium- cysts form in pork muscle and are consumed when raw or undercooked pork is eaten
Echinsococcus- carried around by dogs/wolves/foxes
What are the characteristics of Trematodes?
They are flat/flattened oval shaped.
Have at least 2 hosts in their life cycle.
Describe the life cycle of Schistomaisis
Eggs hatch and release a miracidia in the water
Miracidia penetrate snail tissue and develop in to Sporocysts
Sporocysts develop in to cercariae which are release in to the water
Cercariae penetrate human skin.
During penetration of the skin, Cercariae lose their tails to become schistosomula
Schistosomula enter circulation and migrate to the portal blood in the liver and mature in to adults.
The adults the migrate to the mesenteric venules of the bowels/rectum
The adults shed eggs which circulate to the liver and are shed in stools.
What is a common example of a trematode?
Schistomaisis
What are characteristics of Ectoparasites?
Live outside the body/host
Are mostly insects or arachnids
Some are vectors as well as parasites themselves.
What are common example of ectoparasites?
Ticks, lice and mites
Where are enteric parasites found?
In the small intestine, so would use the faecal oral route of transmission.
Give examples of enteric parasites
Protozoa- Amoebic dysentery
Nematodes- Enterobiasis (pinworm) and Ascaris lumbricoides
Cestodes -Taenia saginata
Taenia solium
Echinococcus
Trematodes- SchistosomaisisGive examples of Blood-borne parasites
Protozoa- Malaria
Leishmanaisis
What are the indirect diagnostic methods?
Serology
Rapid diagnostic tests
Testing eosinophil levels
Testing IgE levels
What diagnostic tests can be used for Helminth infections in particular?
Testing eosinophil and IgE levels as a Helminth infection is accompanied by high levels of both of these.
What are the direct diagnostic methods?
Blood microscopy (for blood-borne disease) Stool microscopy (for enteric diseases)
Explain what happens in Amoebic dysentery
Entamoeba histolytica invade the gut wall.
Causes:
bloody diarrhoea
intestinal and extra-intestinal infections
(later in disease) liver abscesses which are filled with red-brown pus
Explain Enterobiasis
A threadlike nematode, which is very common in children in the UK.
It causes anal itch, particularly at night.
In the morning the ova can be found in the perianal region.
Describe Ascaris lumbricoides
A nematode that can grown to be between 15-35cm and is often asymptomatic.
A mass of worms can block the small intestine or common bile duct.
It can lead to developmental delay in children.
Describe Taenia saginata (beef) and Taenia solium (pork) infections
These are long segmented “ribbon” like worms, that have secondary hosts (intermediate hosts) of beef and pork respectively, the cysts live in the meat/muscle of these animals which is then eaten by humans.
The cysts develop in to adult worms in the small intestine.
Ova (eggs) are then excreted in human faeces for other pigs or cows to eat.
What happens is a human eats eggs rather than cysts?
Cysts will form outside of the gut such a the brain (neurocysticerosis), which can lead to epilepsy and is very dangerous.
can cause death
What can a Schistosomaisis infection cause?
“swimmers itch”- where the cercariae have burrowed in to the skin
Can cause death
Describe Leishmaniasis
Caused by sand-fly bites
Can lead to fever, weight loss and hepato-splenomegaly (enlargement of liver and spleen)
Can lead to death
Describe Malaria
Commonest cause of death by parasitic infection Symptoms are: Fever Chills Flu-like illness
