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Forty-one-year-old Adam said that the dog's colour had also changed, and he was in serious trouble. That's why, instead of taking any risks, he called up the vet to get them to have a look.
That decision may just have saved Dozer's life.
Adam explained: "We are very careful about walking him and we always take two or three bottles of water with us.
"It was probably even a shorter walk than normal and it was quite a mild day, certainly no heatwave, and this was out of the blue and totally unexpected.
"Even though we were in the shade a lot, he was breathing very heavily and kept having to sit down.
"At first it seemed just like a normal panting by a dog but then I could see he was increasingly struggling to breathe. I thought I could see his colour changing and it was really worrying - I didn't want to take any chances."
So, he called Vets Now and rushed him off too the clinic.
At the clinic, veterinarian Susana Jauregui said: "Dozer was very unstable when he was brought in and his blood pressure was dropping alarmingly,
"We were giving him rapid intravenous fluids to try and help but it wasn't looking good.
"When we tried to get him back to the kennel he went downhill really fast and we had to carry him back to the treatment room and get him back on oxygen.
"It was overnight and obviously we didn't sleep as we were waiting for word. When they told me he had pulled through and had made a good recovery it was such a relief.
"We just couldn't understand how this had happened."
So, what caused all of this panic? Well, it is believed that Dozer suffered from Brachycephalic Obstructive Airway Syndrome (BOAS), which is a particular problem for dogs with flat faces such as pugs and bulldogs.
Susana continued: "After four hours of one-to-one monitoring we removed the tube and connected a nasal catheter to the oxygen, and he started to become progressively more stable.
"He fought all night long to survive and was remarkably well the next morning.
"It was such a relief for everyone involved and we're so pleased to hear he's better now."
Now Adam is asking other pet owners - particularly those with animals like Dozer - to watch out for the signs, and react quickly if they are spotted.
He added: "If you see that your pet is starting to breathe differently or erratically, or their colour starts to change, then you've got to get them to the vet very quickly,"
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