Are rat droppings bad for dogs?

Leptospirosis can be transmitted from rats to dogs through contact with infected urine, which your dog might have encountered while sniffing around in their solid waste. Lepto is potentially deadly and transmissible to humans, though it is not prevalent in all areas, so this risk varies with your location. This risk would also depend on whether or not your dog has received the lepto vaccine, which is usually recommended for active dogs.

From PetMD:

The Leptospira spirochete infection mainly occurs in subtropical, tropical, and wet environments. Leptospira spirochetes are more prevalent in marshy/muddy areas which have stagnant surface water and are frequented by wildlife. Heavily irrigated pastures are also common sources of infection. The infection rate for domestic pets has been increasing in the U.S. And Canada, with infections occurring most commonly in the fall season. Dogs will typically come into contact with the leptospira bacteria in infected water, soil, or mud, while swimming, passing through, or drinking contaminated water, or from coming into contact with urine from an infected animal. This last method of contact might take place in the wild. Hunting and sporting dogs, dogs that live near wooded areas, and dogs that live on or near farms are at an increased risk of acuiring this bacteria. Also at increased risk are dogs that have spent time in a kennel.

Feces from poisoned rats could also be a concern, as traces of the poison could be present. If you don't have reason to believe the rats hanging out in your garage have been poisoned, I wouldn't be too concerned.

I would just keep an eye on your dog and get rid of the rat droppings as you find them. Wild rats can pass disease and traces of poison through their fecal matter. But if you're not sure if your dog got to it or not, I'd just keep a close eye on him/her. If your dog begins to show signs of not being well, take him to the vet. Otherwise, don't worry to much and try to prevent your dog from having access to rat droppings in the future.

I would not encourage them to eat it, but if they do happen to get ahold of it, don't freak out. Depending on the rats diet and what its eaten (hopefully nothing poisonous)

I wouldn't run to the vet if your dog seems healthy and is on parasite control, but you should be aware that there are many parasites and bacteria families that are shed in rat feces. Keep an eye out for any signs of distress, sickness, inappetence, and lethargy and consider bringing it up at your next vet wellness visit, just to be careful.