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What are some important protozoan parasites?

Phyla; protista:
- Trichomonas vaginalis (urogenital)
- Giardia lamblia (intestinal)
- Entamoeba histolytica (intestinal)


What is the name of the infection that the protozoan parasite Trichomonas vaginalis causes?
(How prevalent is it? Who it found in? Transmission?)

• Trichomoniasis
- Most common treatable STI with worldwide prevalence > 170 million/year
- Also infects males (despite Trichomonas vaginalis name); residing in the urethra and prostate
- Transmitted by unprotected sexual intercourse


What is the lifecycle for the protozoan parasite, Trichomonas vaginalis?

1) Trophozoite resides in female lower genital tract (vaginal secretions), male urethra and prostate
2) Replicates via binary fission (no sexual reproduction; no eggs, cysts etc.) - does not survive in external environment as a result of lack of cysts
3) Trophozoite then transmitted via sexual intercourse (humans only host)
>>> Trophozoites is infective AND diagnostic stage (1, 3)


What is unique about the structure of Trichomonas vaginallis?

- 4 free flagella (with one attached to cell membrane)
- Rigid axostyle other end; for attachment to epithelium


What are the symptoms of Trichomoniasis infection? Who presents?

- Women mostly symptomatic (or become so within 6 months)
- Men mostly asymptomatic (but still can transmit parasite)

- Irritation e.g. 'Strawberry cervix' - characteristic
- Malodorous vaginal discharge
- Itching
- pH increases to 5 or higher (normal pH < 4.5)

>>> Increases risk of HIV and HSV2 (Herpes-Simplex 2) acquisition and transmission


What may rise in pH of the vagina in Trichomoniasis lead to?

- Secondary infection w/yeast or bacteria
>>> Loss of protective acidic pH (normally < 4.5)


What are the reproductive outcomes of Trichomoniasis infection for men and women respectively?

- Pregnant women w/ trichomoniasis can deliver/LBW babies

- Trichomonas vaginalis can decreases sperm counts and motility
- Prostatitis
- Urethritis


How is diagnosis made for Trichomoniasis infection?

Criteria for diagnosis different in each gender:
- Wet mount (swab + microscopy) in both M & F; BUT not very sensitive
- Culture; more sensitive (GOLD STANDARD), but more expensive and time-consuming (2-7 days growth)
- Polymerase chain reaction (PCR); expensive
- Point-of-care tests (RDTs - dipsticks); results in less than 1 hour


How is Trichomoniasis infection treated?

• PO Metronidazole (nitroimidazole drug)
- Single 2000mg dose or 400mg BD for 7 days
- If metronidazole ineffective; tinidazole
- Metronidazole also used to treat H. pylori
- Sexual contacts should be treated simultaneously; stop fueling cycle of resistance


What is the advice of Metronidazole treatment for Trichomoniasis in pregnant women?

- Can be taken
- BUT, not recommended as unclear from studies whether teratogenic


How can Trichomoniasis infection be prevented?

- Treat sexual partners; partner-referral scheme (PR) or patient-delivered partner treatment (PDPT)
- Nonoxynol-9 (spermicide in condoms) has anti-Trichomonal activity


Why is repeat infection of Trichomonas vaginalis common?

- Reinfection from untreated partner
- Resistance to drug (e.g. Metronidazole; try Tinidazole)


What is it about Trichomonas' physiology that allows activity against it from Metronidazole, but doesn't result in toxicity in humans?

- Trichomonads possess organelle called hydrogenosome; not possessed in humans
- Used to metabolise pyruvate to gain energy, releasing hydrogen (hence the name)
- Metronidazole is activated in the hydrogenosome, leading to nitroso (NO) free radical


What are some key facts re. the intestinal protozoa, Giardia lamblia?
(Resides? Infective stage? Symptoms?)

- AKA G. duodenalis, or G. intestinalis
- Resides in small intestine; most common protozoan isolated from faeces
- Infection from cysts
- Does not penetrate epithelium; often asymptomatic (local infection, does not disseminate)
- Not specific to humans; can infect other animals


What are the symptoms of Giardia lamblia infection (when symptomatic)?

- Chronic or acute diarrhoea
- Flatulence
- Bloating
- Abdominal cramps


What is the Giardia lamblia life cycle, and how does this relate to its infection of humans?

- Cysts are resistant forms of the Giardia; responsible for transmission of giardiasis.

1) Cysts are 'hardy'; can survive several months in cold water. Infection occurs via ingestion of cysts in contaminated food, water, or by faecal-oral (hands or fomites)
2) In the small intestine, excystation release trophozoites (each cyst producing 2)
3) Trophozoites multiply by binary fission, remaining in lumen of small intestine by ventral sucking disk to mucosa
4) Encystation occurs as parasites transit towards colon
5) Cysts mainly passed in non-diarrhoeal faeces. Trophozoites also passed; but they do not survive the outside environment


What is unique about Giardia's structure that prevents its loss in diarrhoea?

- Ventral sucking disk/adhesive disk
- Allows trophozoite to remain attached to the intestinal brush border of the small intestine
- Strong attachment 'like glue'


How is giardiasis infection diagnosed? Why is one not always reliable/requires repeating?

- Copromicroscopical examination; detection of cysts in stool by microscopy (wet mounts or stains)
> Not always reliable as intensity of cyst output can vary greatly
> THUS, 3 stools need to be taken with intervals of at least 2 days

- Enterotest; gelatin capsule containing nylon string taped to patient face and swallowed.
> Retrieved after 4 hours - overnight
> Distal (end) of string scraped and used for wet mount microscopy and permanent slide staining
> Gelatin dissolves in stomach, nylon string unwinds
>>> More reliable than boggo stool examination


Why is giardiasis underdiagnosed in the UK?

- Myth of it being a disease that is only contracted abroad
- ONLY 3000-4000 reported cases annually to PHE
- Incidence likely to be much higher; but usually not looked for if patient does not have recent travel history


How is giardiasis infection treated?

- Metronidazole given over multiple days = 85% cure rates (same drug as used for Trichomonas vaginalis

Also effective:
• Tinidazole single dose
• Quinacrine (antimalarial, also antiprotozoal)
• Paromomycin (broad spectrum aminoglycoside antibiotic; not absorbed, remains in the gut [where parasite is]; also can be used during pregnancy)


What are the key facts WRT Entamoeba histolytica? 'tissue destruction'
(Hosts? Transmission? Resides? Symptoms?)

- Causes Amebiasis (amebic dysentery; massive diarrhoea)
- Humans are ONLY hosts
- Transmission; faecal-oral
- Parasites resides in colon (unlike Giardia lamblia; small intestine)
- Generally asymptomatic/mild symptoms, but CAN BE FATAL if spread to other organs


What is the life cycle like for Entamoeba histolytica?

1) Cysts and trophozoites passed in faeces; cysts in formed stool, trophozoites in diarrhoea.
2) Infection occurs by ingestion of mature CYSTS; faecally contaminated food, water, hands
3) Excystation in small intestine; trophozoites released, multiply by binary fission etc.
4) Produce cysts, migrated to large intestine; where both stages and then passed in faeces (depending on stool type)

>>> Most cases, trophozoites remain in small intestine lumen (3); but E. histolytica trophozoites can invade the intestinal mucosa too, or reach extraintestinal sites in the bloodstream e.g. liver, brain, lungs = pathogenic manifestations.


What is the infective stage for Entamoeba histolytica? What about the other stage?

- Cysts; can survive for days-weeks in external environment, due to their protective walls
- Trophozoites passed in stool are rapidly destroyed once outside the body, and do not survive gastric passage (mostly confined to intestinal lumen)


What is the difference between Entamoeba histolytica and Entamoeba dispar?

E. histolytica:
- Invasive form

E. dispar:
- Noninvasive form


How is E. histolytica/dispar infection diagnosed? How to distinguish?

- Copro-microscopical detection (faeces)
- E. histolytica has amorphous shape
- E. histolytica and E. dispar morphologically indistinguishable unless E. histolytica is observed w/ingested RBCs (erythrophagocytosis; can have like 6 RBCs etc.)


What complications may occur w/E. histolytica infection (amoebiasis)?

- ACute amoebic colitis (colon inflammation)
- Fulminant colitis w/perforation; lethal; gut bacteria spreads into body
- Pericardial amoebiasis; entamoeba spread to heart (high mortality)
- Peritoneal amoebiasis
- Cerebral amoebiasis (brain)
- Cutaneous amoebiasis (skin; not as serious)


What is the treatment for amoebiasis?

Two types of treament:
- Luminal amoebicides;
• For infections confined to the intestinal lumen (E. dispar, some E. histolytica)
E.g. Diloxanide furoate, Paromomycin (suitable for Giardiasis, as well we preggerz), Diiodohydroxyquinoline (active against cyst + trophozoites in gut lumen)
• Tetracycline also used

- Tissue amoebiasis (extraintestinal);
• Metronidazole (or tinidazole, related agents)
• Followed by luminal agent to eliminate infection; metronidazole is less effective against parasite in the gut lumen and not active in cysts (just mitigates mortality risk first and foremost)


Should tetracyclines be used in pregnancy to treat luminal amoebiasis?

- Tetracycline binds to growing bone; stains foetal teeth brown-green (but just milk teeth)
- Can be used to label bone growth
- Fine in adult teeth as not growing


What can Trichomonas vaginalis, Giardia lamblia and Entamoeba histolytica all be treated by?

- Metronidazole
>>> But should be followed by luminal amoebicides in amoebiasis
>>> All three can be asymptomatic; but also cause mild-severe symptoms, or death.


What other protozoan parasites are of note, that induce a greater pathology?

- African Trypanosomiasis (sleeping sickness);
- American Trypanosomiasis (Chagas disease);
- Leishmaniasis (visceral, cutaneous, mucocutaneous);