What could be the cause of my dog's death?

[vlvt2]

I would like to have an idea of why my dog died.  The first day I noticed symptoms, he had two different instances of diarrhea overnight (with blood in the stool).  He didn't want his food the following morning.  He ate a little in the evening and drank water, but he vomited once (it was clear vomit).  I could also hear his breathing.  It sounded raspy, and he was breathing faster than normal.  The second day, he took a turn for the worse.  His breathing was the same, and he would not eat even his favorite food.  By that afternoon, he acted like he could not get comfortable.  He would get up and lay back down.  At this point I was extremely worried.  I called the vet and they said it was common for dogs to have the doggy flu and to make sure he got plenty of water.  I set an appointment for the next day and tried to give him water.  He would not drink the water (or even open his mouth) so I laid him in his bed and hoped he would start feeling better.  Within an hour of laying him down, he died.  About 15 minutes before he died, he expelled a lot of red blood from his anal area and was now limp.  I wrapped him up and tried to take him to the emergency clinic, but I didn't make it in time.  He convulsed twice and sounded like he gasped for air, then he passed away.  (And he lost more blood during this time.)

Throughout his life, my dog had always had a bigger belly than I thought he should, but I had given him heartworm tablets (and he had visits to the vet).  During his later years, when I tried to play with him, he would always have a brief coughing fit (with hacking and sometimes spitting a small amount of spit, but the spit wasn't very often).  I just can't believe he died so fast, and what could be the cause of the blood?    
    

[Jim Humphries, B.S., D.V.M.]

I am very sorry this happened.  It is so hard to lose a beloved family friend.  As you probably know, any guess on our part would be a simple guess / speculation.  No professional wants to engage in speculation.  

Passing blood like that may have been an indicator of some serious intestinal problem.  Infections, virus, cancer, toxicity and the list goes on.  A cough can also indicate a primary lung or respiratory problem, or it can be secondary to heart failure.  If the dog had been receiving regular check ups the heart problem would likely be discovered.

At the first signs of blood in the stool I would like to have seen the dog get a veterinary exam and treatment.  that is a very serious symptom and should not be ignored. And canine flu does not cause blood in the stool.  

If you are very curious as to cause of death, a veterinarian can perform an autopsy and perhaps give you several likely senarios.  

We are sorry you suffered this loss, and also sorry we cannot speculate as to the cause.    

[vlvt2]

Thank you for replying so quickly and with such good information.  I am kicking myself for not taking him to the vet sooner.