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Flashcards in Parasitology - Other protozoa Deck (18)
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Histomonas meleagridis


are four other protozoa that cause disease in animals (and humans in the cases of cryptosporidium & giardia).

What is the host range of Cryptospordium? Why is it important to study for both veterinarians and medical doctors?


Cryptosporidium has a wide host range that causes diarrhoea in young animals.

It is a common cause of calf scours. Adults might asymptomatic carriers.

It also causes diarrhoea in humans and is common infection in AIDS. 


What is special about the Cryptosporidium oocyst?

It's already infected/sportulated when it is passed in faeces.

They're also very small - only 4-5 µm, compared with coccidia oocyts that range from 15-50 µm.


Cryptosporidium is different from coccidia in terms of where it "lives". Coccidia (Eimeria & Isospora) tend to attach the epithelial cells of the small intestine and the caecum. Where do Cryptosporidium trophozoites, merozoites and gametocytes lives?

Microvillous brush border of the alimentary AND the respiratory tracts - ie. they DON'T penetrate host epithelial cells like coccidia.


What is the main Cryptosporidium species that infects domestic animals and humans?

Cryptosporidium parvum


How is Cryptosporidium diagnosed in the live animal?

Faecal smear. Remember the oocysts are microscopic, smaller than RBCs, so you need to stain them. 

Like Mycobacterium tuberculosis, which are also acid-fast, they only absorb Zhiehl-Neelsen stain (red against blue-green background - mycolic acid in membrane).


How would you prevent cryptosporidium infection in calves? 

1. Isolate/quarantine brought-in; treat signs of diarrhoea

2. Good hygiene, adequate bedding

3. Treat with haloguginone (Halocur, Intervet), oral dose


What is special about the morphology of the protozoan Giardia

It's a flagellate protozoan (as a trophozoite) that's pear-shaped with eight flagella. See photo. It is also a large adhesive disc.


Giardia is the commonest cause of protozoal diarrhoea in man in the UK & is very common in domestic animals, affecting 15% of British puppies.  

What are the routes of transmission?

1. Oral-faecal

2. Water-borne (eg. contaminated streams & water supplies)


What are Giardia oocysts called? What is special about their excretion and what implications does this have on diagnosis of Giardia by oocyst?

They're known as cysts. They're immediate infective when they're passed in faeces. 

They're passed only INTERMITTENTLY, so faeces must be collected on at least three consecutive days.

Cysts are also HEAVY, so higher-density fluids must be used to float them (ie., don't use NaCl solution)

Can also use commercial immunoassays



What does Histomonas meleagridis cause? What is the pathogenesis of the disease?

Blackhead in turkeys aka histomomosis

Head and wattle of turkey become cyanotic in proportion of cases. 



Histomonas meleagridis is a flagellate protozoan that uses the caecal nematode, Heterakis gallinarum as its vector. 

What is the life cycle? How many vectors does it have?

It has two vectors.

The protozoa M. meleagridis, is carried by the larval stage inside the egg of Heterakis gallinarum into the intestine of an avian host, such as the turkey. 

When the H. gallinarum egg hatches in the caecum, the larvae release the histomonas, which penetrate the caecal wall, where they travel to the liver via the hepatic portal vein. 

Some of the histomonas incorporate into the Heterakis larvae's ovaries & end up in the next generation of Heterakis eggs.

These Heterakis eggs with Histomonas inside are eaten by earthworms, which carry the larvae inside, so now Histomonas can survive in the environment, inside the earthworm.

The turkey then eats the earthworm and the cycle starts again.


What kills turkeys affected by Histomonas meleagridis?

Liver damage. Caecal also becomes necrotic along with liver.


Aside from blue wattles, what other symptoms can indicate Histomonas infection in turkeys?

Yellow droppings


What is the mortality rate for young turkeys infected by Histomonas meleagridis protozoan?



Is there any treatment against Histomonas meleagridis? Preventative measures?

No chemical control. 

Don't rear turkeys on ground used by chickens, which act as symptomless carriers.


How does the morphology of the protozoan Balantidium compared with Histomonas and Giardia?

Balantidium is ciliated while Histomonas and Giardia are flagellated (Histomonas has one flagellum and Giardia has eight flagella).


Balantidium protozoan is usually a commensal on what species of domestic animal?

Pigs - in their large intestines


What problems can Balantidium cause in pigs and humans? 

Ulceration & dysentery if large intestine mucosa is damaged.

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