Heartworms are transmitted to the dog through mosquito bites so if your dog has been around mosquito's and have been bitten by a mosquito then chances are your dog will likely get heartworm from the mosquito.
Symptoms of heartworm disease in dogs are.
A persistent cough. Your dog may develop a cough that is mild but persistent, your dog may resist exercise and also have a loss of appetite, the dog may be fatigued as well and lose weight.
If the heartworm gets severe enough it can be fatal and your dog will have a swollen belly due to excess fluid built up in the abdomen.
You should always treat your dog for heartworms and do your best to prevent the heartworm disease before it can happen to your dog.