Introduction to Parasites Flashcards Preview

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Define parasites

A parasite is an organism that lives in another organism (host) and gets its food at the expense of this host, cannot live independently


Name the different groups of parasites

Protozoa e.g. malaria, leishmaniasis, amoebic dysentery
Helminths e.g. nematodes and trematodes
Arthropods e.g. lice, ticks, mites


What is a protozoa?

Protozoa are a diverse group of unicellular eukaryotic organisms. Historically, protozoa were defined as single-celled organisms with animal-like behaviors, such as motility and predation.


Name some examples of disease caused by protozoa

Malaria - plasmodium species
Amoebic dysentery - Entamoeba Hystolytica
Leishmaniasis - Leishmania species


Describe the life cycle of plasmodium species

• Plasmodium species, with mosquito vector.
• Sporozoites are injected under the skin the mosquito
• They travel through to blood to the liver where they mature
• They then re-enter circulation as merozoites
• These invade and destroy RBC’s using them to multiply
• Sexual forms are taken up by mosquito


Describe malaria

• Malaria is a mosquito-borne disease caused by a parasite.
• Patient often experience fever, chills, and flu-like illness
• Left untreated, they may develop severe complications and die


What tests are used to diagnose malaria?

• Giemsa stained blood film shows infected red cells (parasitaemia – in bloodstream)
• Thick and thin blood films
• Variety of rapid diagnostic tests e.g. antigen tests, PCR, urine and saliva tests


Why do we use both thin and thick blood films when diagnosing malaria?

o Thin films are similar to usual blood films and allow species identification, because the parasite's appearance is best preserved in this preparation.
o Thick films allow the microscopist to screen a larger volume of blood and are about eleven times more sensitive than the thin film, so picking up low levels of infection is easier on the thick film, but the appearance of the parasite is much more distorted and therefore distinguishing between the different species can be much more difficult.


Which species of plasmodium is the most deadly?

Plasmodium falciparum


What is the vector of plasmodium species?

Mosquito vector (Anopheles mosquito) - females


How can we prevent the contraction and spread of malaria?

• Insecticide treated mosquito nets (ITNs)
• Prophylaxis e.g. anti-malarial tablets
• Malaria Vaccine in research


What is amoebic dysentery?

Diarrhoea with blood/pus
Entamoeba histolytica
Intestinal and extra intestinal infections
Liver abscess in later disease


What causes amoebic dysentery?

Entamoeba histolytica (protozoa)


What causes malaria?

Plasmodium species (protozoa)


How do you diagnose amoebic dysentery?

Microscopy: Cysts in formed stool


How is amoebic dysentery transmitted?

Amoebic dysentery is transmitted through contaminated food and water. Amoebae spread by forming infective cysts which can be found in stools, and spread if whoever touches them does not sanitize their hands. There are also free amoebae, or trophozoites, that do not form cysts, however trophozoites do not survive long outside the human gastrointestinal tract and are a purely diagnostic observation.


Describe Leishmaniasis

• Cutaneous or mucocutaneous Leishmaniasis
o Skin or mucosal ulceration
• Visceral Leishmaniasis (kala-azar)
o Fever, weight loss and hepato-splenomegaly
• Infection by bite of sandflies


How do people contract leishmaniasis?

Infected by bite of sand fly infected by Leishamania species


How do you diagnose leishmaniasis?

Diagnosis by histology of biopsy material


What are helminths?

Helminths, also commonly known as parasitic worms, are large multicellular organisms, which when mature can generally be seen with the naked eye. They are often referred to as intestinal worms even though not all helminths reside in the intestines; for example schistosomes are not intestinal worms, but rather reside in blood vessels.


What are the different classes of helminth?

Nematodes - round worms
Cestodes - tapeworms
Trematodes - flat worms, flukes


What are nematodes?

Round worms e.g. Enterobius vermicularis


What is Enterobius vermicularis?

Round worm (nematode group of helminths)
Sometimes called pinworm, 1cm, threadlike
Often found in children
Common in areas of poor hygiene
Causes itching and poor sleep, often asymptomatic


How do you diagnose Enterobius vermicularis infection?

o Press adhesive sellotape against perianal region in the morning
o Ova seen on microscopy


Describe the life cycle of Enterobius vermicularis

The lifecycle begins with eggs being ingested.
The eggs hatch in the duodenum (i.e., first part of the small intestine).
The emerging pinworm larvae grow rapidly and migrate through the small intestine towards the colon.
During this migration they moult twice and become adults.
The male and female pinworms mate in the ileum
The gravid female pinworms settle in the ileum, caecum, appendix and ascending colon, where they attach themselves to the mucosa and ingest colonic contents and expel eggs.
Eggs are then passed out in faeces to infect new host


What is Ascaris lumbricoides?

Giant roundworm (nematode, helminth)
Often asymptomatic
Transient pulmonary symptoms (Loefller’s syndrome)
Mass of worms may obstruct small intestine or common bile duct


Describe the life cycle of Ascaris lumbricoides

• Ingested eggs enter circulation via the intestines
• Embryos hatch and pass through lungs
• Enter stomach and move to small intestine as adults to breed


How is Ascaris lumbricoides diagnosed?

• Ova seen in faeces by microscopy


What is Loefller’s syndrome?

Transient pulmonary symptoms, caused by Ascaris lumbicoides


What are cestodes?

Tapeworms of Helminth family


Give some examples of cestodes

Taenia species
Echinococcus species


Describe Taenia species infections

• Taenia saginata (beef) and Taenia solium (pork)
• Larval cysts ingested in meat (intermediate host, doesn’t reach adult form here)
• Adult tapeworm in human (definitive host)
• Tissue cysts in humans infected with T.solium - “cysticercosis” (ingestion of T.solium eggs)


Where do we ingest Taenia saginata from?



Where do we ingest Taenia sodium from?



How do we diagnose Taenia species infection?

Ova in stools on microscopy


Describe Echinococcus infections

• Echinococcus sp – carried by dogs, wolves, foxes
• Humans ingest eggs (dog faeces)
• Eggs hatch and enter circulation
• Hydatid cyst forms in liver
• Surgical resection must involve whole cyst


How do we become infected by Echinococcus species?

Ingest dog faeces


What are trematodes?

Flat worms/flukes
E.g. schistosomes species


What are schistosomes?

Flat worms/flukes (trematodes of helminth family)
Cause schistosomiasis (bilharzia)


What is bilharzia?

Schistosomiasis, infection of trematode schistosomes


What are the three major species of schistosomes and where are they found?

o S. haematobium (bladder)
o S. mansoni (intestinal)
o S japonicum (intestinal)


Where is S. haematobium usually found?



Where is S. mansoni usually found?



Where is S japonicum usually found?



Describe the life cycle of schistosomes

• Eggs (ova) excreted in urine or faeces
• Miracidia released in fresh water
• Penetrate body of snail (intermediate host)
• Cercaria emerge from snails after 4 -6 weeks
• Penetrate human skin (swimmer’s itch)
• Migrate through lungs to liver
• Mature in liver into worms (schistosomes) that migrate to mesenteric or bladder venules
• Lay eggs that cause inflammation in bladder or intestinal wall


What signs and symptoms does schistosomiasis cause?

Swimmers itch on initial infection
Inflammation in bladders or intestines
Acute febrile episode 4 - 8 weeks on (oviposition) “Katayama fever”


What is Katayama fever?

Acute febrile episode 4 - 8 weeks on (oviposition) from Schistosomiasis infection


Which schistosome species causes the most severe infection?

Schistosoma japonicum


What are some symptoms of Schistosoma haematobium infection?

o Haematuria
o Bladder cancer

(due to location in bladder)


What are some typical signs of helminth infection?

o Eosinophilia - increased eosinophils in blood
o Elevated IgE


How can you definitively diagnose parasitic infections?

Identification of parasites in host tissue or excreta


What can you identify using microscopy?

o Parasites, cysts and ova (P, C and O) in faeces


How can you diagnose parasitic infections?

Microscopy: direct identification of organism
Serology: Detection of antibodies
Blood films (malaria only)


When is serology useful in diagnosing parasites?

Detection of antibodies can be very useful when parasite is located in deep tissue sites