• A dog infected with heartworms can look fine until the advanced phases of the condition.dog on boat image by JulianMay.co.uk from Fotolia.com
  
  Heartworms are parasites that are transmitted from a mosquito to a dog through a bite. These worms live primarily in the right ventricle of a dog's heart and can cause heart failure if left untreated. There are adulticide medications that can be administered to a dog to kill the adult worms and begin the recovery process. These medications are not without side effects.
  
  

Pulmonary Thromboembolism

• Pulmonary thromboembolism occurs when the parasitic worms begin to block the major arteries of the heart as they die. This condition can lead to rapid heart failure. According to the University of Michigan State, only severe cases of the condition are extremely dangerous, but there will be some sign of it with every effective heartworm treatment.
  
  

Allergies

• Some dogs are sensitive or allergic to the adulticide used to kill adult heartworms. These dogs may suffer rashes, irritation or even anabolic shock after receiving the medication. An alternative medication is necessary to stop the heartworm infestation if this is the case.
  
  

Inflammation

• While the worms in the dog's heart die, they can cause inflammation in the heart. A heart that previously had been stressed by worms may find the inflammation too much to handle. Without anti-inflammation medication, the dog could perish during heartworm treatment.