Some dogs don't show symptoms but are able to infect other dogs for weeks. (mylifewithdogspdx.com)
Unfortunately, an infected dog is very likely to infect other dogs in the period before he shows any symptoms. (happytailsspa-blog.com)
The bacteria that cause the illness are carried primarily by rats and other rodents, but dogs that are infected with the disease can infect other dogs as well. (dogbreedz.com)