If there is a possibility that your dog ate a mouse that ingested D-con, you need to get your pup to an emergency clinic as soon as possible.
Just like in mice, the rodenticide is an anticoagulant that 'messes' with the clotting factors of your dog's blood system. This can cause your dog to literally bleed to death.
There is treatment, but it needs to happen soon after ingestion, so please, find an emergency clinic in your area and get your pup there.
With all respect to the previous expert's answer, I have some additional light to shed on the topic. The active ingredient in D-con, and other rodenticides is a chemical (brodifacoum or other similar chemicals) that interferes with blood clotting. However, the way that it interferes is by blocking the liver's ability to make a specific chemical that aids the blood in clotting. We are getting small injuries to our blood vessels all the time, but our body makes clots that stop the bleeding almost immediately. The way the body does this is first by using a cell called a platelet to essentially plug up the hole in the vessel, and then uses proteins called clotting factors to help seal it in. These are the chemicals that rat poison prevents the body from making. When these chemicals are not present, abnormal bleeding develops. This can show up as bruising under the skin, bleeding into the chest or abdomen, or any other number of presentations.
The good news is in the form of 2 bits of information:
First, the toxic effect of rat poison is dose-related, and the amount that it takes to kill a mouse is significantly less than what it takes to affect a dog. In most cases, the amount of rat poison ingested by a dog that eats a poisoned mouse is small enough that it will not cause any symptoms.
Second, even if a pet ingests rat poison directly, there are enough clotting factors in the bloodstream to last most dogs 3-5 days before they start showing any symptoms.
Now, if your pet were to directly ingest a large or unknown amount of rat poison directly, there is benefit from getting to a veterinarian if it is within an hour or two of the ingestion, so that they can induce vomiting and try to get some of the poisonout before it is absorbed by the system.
The good news is that the odds of your pet having ingested enough rat poison to cause serious problems by eating a mouse are extremely low.
As a precaution, I would still follow up with your regular veterinarian. If you want to be extremely cautious and proactive, your veterinarian will prescribe Vitamin K in an oral form, which is the antidote for rat poisoning. This is a prescription medication, so it would have to be prescribed by your vet.
You can also contact the ASPCA Animal Poison Hotline if there is ever a question about a toxic exposure. They have information about toxic doses of different chemicals and products. There is a fee for the service, and you must have a credit card when you call, but the peace of mind is well worth it.
You can get more information at: www.aspca.org