Parasitology - Urogenital (ie, more mammalian Coccidia)

Question 1

Why is Toxoplasma gondii, a cyst-forming protozoa, included in the the Urogenital section instead of Mammalian Coccidiosis in the Alimentary Parasites section?

Answer 1

Toxoplasma gondii** has life-cycle differences from **Eimeria, even though it’s also a protozoa that affects mammals .

Toxoplasma is:

Faculatively heteroxenous - it can use an intermediate host. Eimeria is homoxenous; it doesn’t use an intermediate host but has a direct life cycle.

Cyst-forming with bradyzoite & tachyzoite life stages - the intermediate-host immune system wall off the bradyzoite (slow-growing) stage by forming a CYST, which can persist in host tissue in dormant form for a very long time. Eimeria do not form cysts, but Neospora do form cysts that persist in intermediate-host tissue.

Cause of abortion in sheep - Intermediate hosts in rapid multiplication occurs to form tachyzoites, which burst parasitized cells and enter blood stream (acute phase), then form CYSTS with bradyzoites. In abortion PM, focal necrotic placentitis occurs - white lesions in cotyledons & foetal tissues.

Question 2

What is another cyst-forming coccidial protoza in the Urogenital section that causes abortion in intermediate hosts, aside from Toxoplasma gondii?

Answer 2

Neospora caninum

Question 3

How do Toxoplasma gondii & Neospora caninum cause abortion in their intermediate hosts?

Answer 3

**T. gondii & Neospora caninum **oocysts or tissue cysts are eaten by sheep or humans (T. gondii) or cattle (N. caninum), or it’s spread to these intermediate hosts transplacentally during ACUTE PHASE of infection. N. caninum can spread transplacentally (vertically) to more than one generation, while T. gondii is self-limiting to one generation.

The protozoa multiplies in acute phase in tissue cells, such as placenta, to produce tachyzoites, which cause cells to burst, releasing more tachyzoites. These enter bloodstream, invading and bursting more placenta & foetal cells. In chronic phase, cyst forms around intracellular bradyzoites, which are always infective.

T. gondii mostly asymptomatic in sheep but abortion if
non-immune ewe is infected during pregnancy, depending on stage of gestation:

1. Days 0-40 (first 1/3 of gestation) - leads to foetal death
2. Days 40-110 - foetal death, abortion or mummified foetus

3.Days 110-147 - stillborn or
weak lambs

N. caninum is most common form of abortion in dairy cow:

• congenitally infected calves might have encephalomyelitis & paresis
• If contracted <5 months into pregnancy, foetus is resorbed
• If contracted >5 month into pregnancy, it’s aborted
• very high prevalence in Europe (10-90% in herd)

Question 4

What are the differences between Toxoplasma gondii & Neospora caninum in terms of intermediate hosts, final hosts, site of cyst formation & morphology of cysts.

Answer 4

Intermediate hosts:

sheep, birds, rodents, pigs - T. gondii (birds & rodents are the hosts that affect the final hosts, cats)

cattle, herbivores - T. neospora (not really sure how cattle become infected by dogs unless they graze on grass contaminated by dogs?)

Final hosts:

Cats - T. gondii

Dogs - N. caninum

Sites of cyst formation:

Muscles & heart tissue, a little nerves - T. gondii

Nerves, less muscles & heart tissue - N. caninum

Cyst morphology:

N. caninum cyst has thicker wall than T. gondii cyst

Cysts are same size, 50-100 µm

Question 5

What are the clinical differences in the pathology caused by Toxoplasma gondii and Neospora caninum in the final hosts?

Answer 5

Cats, the final hosts of T. gondii, are typically asymptomatic. Only the tips of the villi (microvilli) are affected during the intestinal phase of T. gondii infection, so not much pathogenicity.

Dogs, the final hosts of N. caninum, experience ascending paralysis in hindlimbs, often with muscle wasting. Sudden collapse due to myocarditis possible.

Question 6

What are the differences in vertical transmission in the final host and the intermediate host of *T. gondii & N. caninum? *

Answer 6

• T. gondii* in intermediate host is self-limiting after one vertical transmission. Ie., Mother can pass it transplacentally to offspring, but then they die (abortion). This is in sheep & humans.
• N. caninum* can be passed in both IH & FH to successive generations. This is in both cows and dogs.

Question 7

Can you protect against Toxoplasmosis in sheep.

Answer 7

• *Toxovax vaccine** - live, avirulent strain of Toxoplasma, BUT:
• vaccine strain doesn’t form bradyzoites / tissue cysts, ∴ only tachyzoites, so all the attentuated organisms are eventually killed by the host immune response
• single does given by injection at least 6 wks pre-tupping
• protects for at least two years in absence of natural challenge
• immunity boosted by exposure to natural challenge

Medicated feed decoquinate:
- can be given daily to prevent infection during main risk period (ie., 14 wks before lambing)