Hello XXXXXXXXXX,
This is Dr. Christian. Is your dog neutered? Is there any blood in the urine? Is he trying to go numerous times without producing much urine? Is he on any medications?
Usually when a dog strains to urinate there are 2 main reasons: pain or blockage. Pain can be due to a urinary or prostate infection. Basically the urinary tract is painful enough that even a small amount of urine causes the dog to feel it has to urinate, hence it tries and isn't very productive. The second reason is a blockage. A blockage in male dogs is much more common than in females. The main cause of blockage is a urinary stone that lodges itself in the urethra on the way from the bladder to exiting the body. Urinary stones can be present for very long periods of time without symptoms. I once had a sheltie with what could be described as a handful of gravel in her bladder and had never shown symptoms until I saw her. The problem with a blockage is that it is an emergency. The longer a dog is blocked the longer the urine builds in the bladder and then backs up in to the kidneys and then in to the bloodstream. These dogs can be very toxic, vomit and become lethargic. The bladder can even rupture if left too long. It may be that with the earlier incident he managed to clear the blockage out on his own. I wouldn't count on that happening this time. While this may only be painful urinations, I would recommend you get him to a veterinarian as soon as you possibly can. Don't wait. If he is blocked and not treated in a timely manner the complications could be deadly.
Dog Veterinarian
12 years of veterinary experience in canine surgery, medicine and behavior