Is it unusual for chihuahuas to get cavities?

If its baby teeth they will fall out.
Most dogs who eat table food will have faster rotten teeth than the ones who eat dry.

Brush the dogs teeth and your next visit to the vet tell the vet to look into it, but if you arent due for a while, why dont you just pop into the vets and just ask, they wont charge you to ask.

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Dog's teeth are just like ours…if not taken care of they will rot.

I just paid 564.87 to get my chihuahua's teeth cleaned! The vet told me that if i started brushing his teeth when I first got him. His back molars were abscessed, and I'm guessing was in serious pain.
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If you do not brush your dog's teeth daily and keep up with its dental hygiene, then no, it isn't unusual. The treatment is generally to remove the infected tooth in a dog. Rates vary by vet and whether it is done in conjunction with another procedure. Talk to your vet for an accurate quote.
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Feed you dog mores bones of all types like chicken necks and beef briskit as well as something like a beef chuck bone as they have wierd angles and will help to clean the teeth.

Check the spot after a few bones, if it is a cavity you should see that it is indented through the enamel.. You can get doggy dental work done but it has to be done under anesthetic - so not something to rush into, if it doesn't look serious just get it checked out by the vet at your next visit.

You could also brush your dogs teeth with a dog flavoured toothpaste (don't use human stuff) and special dog toothbrush - one that slips over your finger like a finger puppet… That might help keep the teeth healthy.
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Chihuahuas are a "dry mouth" breed, without much saliva, so their teeth are very prone to building up placque, or getting cavities, or bad teeth for other reasons, like crowding or retained baby teeth.
One of my vets, has a Chihuahua who is a tiny one, about 2 1/2 lbs, and about 6 years old, and my other vet has had to pull just about everyone of her teeth, despite daily brushing, and twice a year dental cleanings.
My chihuahuas, have good teeth, with just occassional cleaning, and dry super super premium foods. This is because I add a product to their drinking water to fight placque. I'm not allowed to advertise my websites or products I use here.

Dental cleanings, with maybe an extraction or two usually run from
$ 80. to $ 130 depending on your vet, and how much needs to be done, because dogs need to be sedated for the proceedure.

I'm the founder of SugarNSpice Chihuahuas, Champions and GrandChampions produced, puppies shipped to 9 countries. I've owned as many as 75 at one time. I manage a 150 dog pet resort.

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35 years experience breeding, training , boarding, showing, rescuing dogs.