No Siamese aer absolutely not 'dirty by nature' , where an earth would you get that idea.
The fact remains that a kitten or cat that cannot control faeces that are literally streaming away from it is not naughty, it has a health problem and needs proper treatment.
TF will not usually be picked up in ordinary samples sent away to the lab it needs a specific test, or a swab placed immediately under a microscope by the vet as soon as it's taken from the cat.
Giardia, I think can be detected in a lab sample, and you will only know if it is viral if it is actually tested, you can't know by just looking at the cat.

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