Dogs get heartworm disease when adult heartworms in the heart mate, producing microfilaria. If a mosquito feeds on infected dogs, it will ingest the microfilaria. If the mosquito feeds on the dog again, it will transfer the molted microfilaria back to the dog and is now infectious. Heartworm preventatives kill the microfilaria after the mosquito transfers it back to the dog. If the preventive is not given to a dog with microfilaria at that stage, the dog will get heartworm disease.

On the Other: Heartworm Preventives Can be Fatal

If a dog that already has heartworm disease is given heartworm preventives, the dog might go into shock, and if not treated for this, it could die. Therefore, before starting a heartworm prevention program, you must have your dog checked for heartworm. It is wise to retest the dog every year or two in case you forgot to give the treatment one month.

Bottom Line

When a dog gets heartworm, it can be treated with varying results depending on the stage the disease is in, according to the Westwood Animal Hospital. There are four stages of heartworm, and if the dog is in stage four, there is not anything medically that can be done. Heartworm preventives will ensure your dog does not get heartworm disease.

Source:

Westwood Animal Hospital: Canine Heartworm FAQ