If your dog isn't already on heartworm preventative all year round (and they should be), make sure you get them on preventative NOW!
Heartworm is transmitted by mosquito bites. The mosquito bites a dog infected with heartworm, then bites your dog and transmits it to them. If you give your dog an effective heartworm preventative within a month of the bite, the heartworm will die.
If the dog is not taking an effective preventative (or if you forget to give it), the microscopically small heartworm larvae transmitted by the mosquito will grow up to a worm large enough to easily be seen by the naked eye which lives in the pulmonary artery coming off the heart. If untreated, these worms can cause heart failure and death.
If you discover that your dog is positive for heartworm, the treatment is a series of painful injections in the back muscles with a drug containing arsenic. What this does is kill the adult worms, and what you are left with is dead worms in the pulmonary artery. These can also cause death by pulmonary embolism (blood clots to the lungs).
It is much better and easier to prevent heartworms than it is to treat them after they have developed to the adult stage.
Heartworm is common in this area. The local resevoir is the dogs that have the adult form of the disease and the coyote population. Mosquitoes bite these animals then bite your dog, transmitting hearworm larvae.
Come in now, get your dog tested for heartworm and pick up some heartworm preventative.