Signs of heart worm for dogs

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They can vary, depending on the severity of the infection.
Heartworms are spread when a mosquito bites an infected dog and picks up some of the microscopic larvae called microfilariae. In the mosquito, the microfilariae go through a couple of developmental stages until they reach the infective stage. When the mosquito bites a new dog, some of the infective larvae enter that dog's bloodstream. From there they migrate into the right side of the heart where they take up residence and start to grow. At maturity they can reach a foot long. The signs the dog shows will depend mostly on how many adult worms are present, and exactly where they are located. If some of the worms make their way backward they can congregate in the vena cava where there is less room, and where they can interfere with the movement of one of the heart valves. This can cause something called "caval syndrome" which can lead to sudden death.
More commonly, the worms just build up in the heart, doing their thing, taking up space and causing havoc with blood flow.
The resulting signs are those of right-sided heart failure: cough, shortness of breath, decreased energy, and sometimes a distended belly. If this goes on long enough, it will eventually start to mess up the function of the left side of the heart as well--more severe cough, blue tint to the mucous membranes and reluctance to move. Blood clots are also possible. The liver and kidneys can become damaged too.
An important thing to know is that many heartworm positive dogs do not show any signs at all.
Your vet will be able to determine if your dog has heartworm with a quick blood test. He will probably be able to give you the results within 15 minutes.
Hopefully he will be negative, and you will be able to place your dog on a monthly preventative medication.
If the test is positive, your vet will discuss treatment options with you.
As usual, an ounce of prevention is worth a pound of cure!