Intro to Parasitology Flashcards
Define a parasite.
An organism living in/on another organism, obtaining all or part of its organic nutrients and energy from it and resulting in some cost (usually a negative effect) to its host.
What are the two main groups of parasites?
- Unicellular – microparasites.
a. Protozoa - leishmania, plasmodium, trypanosome cruzi, trypanosome brucei and giardia. - Multicellular – macroparasites.
a. Plathyhelminthes
- Trematodes – fasciola, clonorchis, schistosoma
- Cestodes – taenia, echinococcus
b. Nematodes –ascaris, hookworms, filarial worms.
what are the adaptations for successful parasitism?
Synchronisation with the host – physiological synchronisation. e.g. parasites transmitted by night mosquitoes will accumulate in blood at night to ensure successful transmission.
Recognition of habitat – they need to find the host, if they go into wrong environment there’s risk of being eliminated.
Maintenance of position – some places are better to stay than others, such as the gut which lacks good immune system.
Nutritional adaptations.
Counter the immune system.
What are the apicomplexa?
- 30,000 species of protozoa which are all parasites – they are an extremely large and diverse group.
- Several species infect humans and others are important pathogens of livestock.
- Alternative name is sporozoa due to the spores they produce.
- Include causative agent for malaria – plasmodium.
- Locomotion mechanism - there is actin-myosin action involved.
o Sporozoan parasites ‘glide’ over the host cell surface by a type of mechanochemical force transduction where movement is generated by dynamic interactions between charged sites on substrate and receptor on cell surface of the parasite.
o Cell motility involves complex of adhesive proteins that are moved along the cell by an actin myosin contractile system anchored to the inner membrane of the parasite. - Have an inner membrane complex associated with microtubules from the polar ring.
- They are distinguished by their anterior apical complex which functions in attachment and penetration of the host cell. It can’t be seen under light microscope.
In this structure there are rhoptries (secretory organelles) which contain enzymes that will erode the extracellular material and thus allow parasite to enter the cell.
What are three genetic elements present in the apicomplexa? Talk about plastids.
the nucleus, mitochondria and in plastids.
The plastid is surrounded by four membranes and are thought to be derived from choloroplasts – they choloroplasts that were taken into the apicomplexa and evolved by endosymbiosis.
Not surprisingly apicomplexa share a lot of features, specifically their metabolic pathway, with plants.
The plastid has allowed specific targets for chemotherapy – such as the 6th enzyme in the shikimate pathway, but we have to take in account the therapeutic index.
Describe the general apicomplexa life cycle
- Sporogony – occurs immediately after sexual phase and consists of an asexual phase that results in production of sporozoites.
a. Sporozoites are the invasive form that will invade cells. - Merogony – sporozoites will undergo another asexual replication and form merozoites.
a. There are often multiple rounds of merogony (which is production of more merozoites) – the merozoites, which are invasive, can re-invade cells and initiate another round. - Gametogony – the merozoites can develop into gametes as an alternative to asexual reproduction.
- The gametes will then fuse to form a zygote that will undergo sporogony.
List the three apicomplexa that cause disease
- Eimeria species -causes coccidosis.
- Cryptosporidium parvum - causes GI disease/diarrhoea
- Toxoplasma gondii
Describe Eimeria species of apicomplexa
About 300 species are known – five of which are highly pathogenic.
A major problem in poultry – causes the disease coccidosis. Poultry are of major value.
Involves cycles of asexual reproduction (merogony) in the host’s intestinal cells. Cysts in faeces from infected individuals will be ingested.
They invade the epithelial intestinal cells via apical complex digestion and then undergo asexual reproduction in those cells. They will then rupture out of the cells to go onto re-invade more intestinal cells. This leads to cycles of asexual reproduction which is why they are very pathogenic.
A direct life cycle – that means there is only one host.
What are the symptoms of coccidosis?
o Clinical signs of coccidiosis are due to destruction of the intestinal epithelium and, frequently, the underlying connective tissue of the mucosa.
o This may be accompanied by hemorrhage into the lumen of the intestine, catarrhal inflammation, and diarrhea.
o Signs may include discharge of blood or tissue, loss of appetite, emaciation and dehydration.
Describe Cryptosporidium parvum and disease it causes.
Human parasite that is important cause of diarrhea, especially in HIV+.
Leading cause of gastrointestinal infection in the UK, it’s also common in young cattle.
Transmission is usually via contaminated food and water or person-person contact. (fecal-oral type of transmission).
Self-limiting parasite, usually last 2 weeks then you’ll self-cure – the immune system will limit rounds of merogony. They can lead to rapid epidemics.
However in immune-compromised patients it can turn very serious.
No safe or effective treatment.
Life cycle involves sexual cycle followed by asexual cycle. When swallowed, the cyst of the parasite is weakened by the stomach’s acid to release the 4 sporozites which will then go onto invading the intestinal lining.
Oocysts are very common! There is on average 69 in one gram of cow manure.
Describe Toxoplasma gondii and disease it causes
- Coccidian parasite that affects humans.
- Generally asymptomatic but in 10-20% of people there are symptoms of enlarged lymph nodes and flu-like symptoms.
- Problem in infected pregnant women, where it can lead to congenital toxoplasmosis and cause still births or defects in born babies.
Life cycle -
- Sexual cycle takes place exclusively in intestines of cat and rodent family, who then defecate. Humans are infected from oocysts/cysts in uncooked meat. Because cats eat rodents we can say there is tropic link in transmission.
- Sporozoites are resistant to environmental damage and can persist for years in a moist environment.
- Toxoplasma was shown to induce risky behavior in humans - more traffic accidents observed in those infected.
How does the parasite ensure its own transmission?
Rats have innate defensive reaction to predator odours (i.e. cats)
However infected rats would seek out cats
Describe congenital toxoplasmosis
o Fetal toxoplasmosis is acquired if woman contracts disease for first time during pregnancy. Most severe in first trimester.
o In adults, infection will stimulate the proliferation of cytotoxic T cells which will help macrophages to kill parasite.
o If the parasite is in a cyst however, it will survive – once every often it will break out of the cyst and this maintains the T cell levels to keep the parasites at bay.
o In fetus however only maternal antibodies can protect against the parasite which will cross over the placenta. T cells however cannot pass into fetus. Lack of immune response will lead to dangerous symptoms - the proliferation of the parasite will be left unchecked.
o The same problem is seen in HIV infected and immunocompromised
