If You See a Blue Line Tattooed on a Dog, This Is What It Means

Updated: Apr. 03, 2023

That little blue tattoo conveys a lot of information about your dog’s health

Dogs come in all shapes and sizes, from adorable small dog breeds to big, lovable large dog breeds. And they each have different natural markings, from brindle or fluffy coats to two different-colored eyes or black spots on their tongues. As pet owners, we also have ways of “marking” our dogs, whether it’s with a red dog collar, a winter coat or a dog spay tattoo.

A tattoo? Yes, you read that right! A dog spay tattoo on your pet’s belly is a common part of a medical procedure. But do all dogs get them, and are they necessary? Find out what that little blue line means and how it can save you and your pet from costly, painful interventions in the future.

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Why do some dogs have a line tattooed on them?

A blue line tattooed on a dog’s belly is the operating veterinarian’s way of signaling that the dog has been spayed or neutered. The tattoo is done when the animal undergoes its sterilization surgery, usually with a tattoo gun owned by the veterinary hospital, explains Cherice Roth, DVM, chief veterinary officer at Fuzzy, an online pet-health company. “Alternatively,” she says, “we may also just add sterilized ink or dye directly into the incision. The tattoo is placed after the surgery is complete but before the pet has woken up from anesthesia.” Because the dog will be on pain medication during and after the surgery, it won’t feel any discomfort as a result of the tattoo.

Wondering if your dog has this mark? If you have a rescue dog, there’s a good chance the answer is yes. Nearly all animals adopted from shelters undergo mandatory sterilization, and the spay tattoo practice is much more common among rescue dogs and those sterilized at free or low-cost spay/neuter clinics. According to a 2022 report published in the Veterinary Journal, at least 80% of shelters tattoo the animals they sterilize, while only a small percentage of private veterinary practices do this type of tattooing.

What does a dog spay tattoo look like?

“Most veterinarians are not known for their artistic flair,” jokes Dr. Roth, who explains that a dog spay tattoo is typically just a simple line near the incision. It may appear on the incision itself, be a single line on one side or appear as a line on both sides of the incision—that just boils down to the veterinarian’s preference. And while spay tattoos are usually blue, they may occasionally be done with green or black ink. While the ink may get lighter over time, it will remain as a visual indicator of the dog’s sterilization status.

Why do vets tattoo dogs when spaying them?

As a pet owner, you might wonder why your pet, or any other, needs a dog spay tattoo. After all, can’t you just tell the vet about your pet’s spay/neuter status if there’s an issue? Well, it isn’t always that simple, and there are several good reasons this is a smart move. Most involve some “what if” scenarios, which, while sad, need to be considered:

  • Your pet has an emergency that you’re not there for, and you aren’t able to talk to the vet.
  • Your dog becomes permanently lost or separated from you and winds up in a shelter waiting for adoption.
  • Life events necessitate that you surrender your pet to a shelter, where your pet’s medical records may or may not follow.
  • If you die and there’s no one to care for your pet, it may end up in a shelter.

Here are some other reasons, related and unrelated to the ones above, that spay/neuter tattoos are a good idea for pets.

Scars may fade

When a dog is sterilized, a scar will be left on its belly. But scars may fade, especially if the dog is very young when the surgery is performed, explains Dr. Roth. A tattoo at the incision site lasts forever. Plus, because tattoos are only applied here to indicate that sterilization has occurred, there’s no doubt about their meaning.

For male dogs, it may seem like there’s an obvious way to tell if they’re neutered—whether or not they have testicles. But even this isn’t a sure thing. Some dogs suffer from retained testicles that don’t “drop” into the scrotum and therefore are never visible on the outside of their bodies.

Scars don’t tell the whole story

A scar on a dog’s belly probably means it’s been sterilized, but it could also mean that it has had another type of surgery. So just the scar alone doesn’t confirm spay or neuter status, but a dog spay tattoo does.

Dog spay tattoos save money and painful intervention

Here’s where the dog spay tattoo really makes a difference. “We tattoo so that it’s clear the pet has been spayed or neutered without having to open the pet surgically,” says Dr. Roth. “There are other methods to check for spay/neuter status, such as hormone blood tests or an abdominal ultrasound, but they are cost prohibitive for most families.” An ultrasound can cost between $300 and $500 with a private vet, and they, along with blood tests, may not be conclusive. Dr. Roth explains that in many cases, it may be more cost-effective to pay for the surgical procedure, during which a vet discovers whether the dog is sterilized only after it goes under the knife.

“The rationale behind the tattoo,” Dr. Roth explains, “is that we want to prevent the pet from being put under anesthesia and having their body opened needlessly.” This is especially relevant for shelter dogs, who often come in as adults or strays and without any medical history.

Pet spay tattoos also help veterinarians rule out certain diseases and conditions related to the reproductive organs, says Dr. Roth, including pregnancy, a uterine infection or testicular cancer. And if your pet comes in with some urgent distress, a quick check will at least let the vet know whether the problem might or might not be related to this. “Knowing the spay/neuter status of a pet helps us to better help the pet,” Dr. Roth adds.

Do vets always tattoo spayed dogs?

No, but blue neuter/spay tattoos are becoming more popular. “Nearly all shelters tattoo sterilized pets,” says Dr. Roth, adding that they also usually require that animals be neutered before they can be adopted. In private settings, though, the procedure is still relatively uncommon—perhaps because both pet owners and private-practice vets are less likely to consider scenarios where a beloved pet becomes homeless. Even if that seems like an impossible scenario, ask your vet if tattooing is an option. “I wish that every veterinary practice did it so that we could help to prevent needless procedures,” says Dr. Roth.

Next, find out the answers to other canine mysteries, including why dogs get the zoomies, eat grass and engage in other weird dog behavior.


  • Cherice Roth, DVM, chief veterinary officer at Fuzzy
  • Veterinary Journal: “Identification of spayed and neutered cats and dogs: Veterinary training and compliance with practice guidelines”