Modern Medicine, Old-Fashioned Care

Apr 1, 2013 | General Health

Why Prevention of Heartworm Disease is the Best Treatment

Heartworm-Cycle-2-290x274April showers bring May flowers, but couple that with a rather wet winter, and you can expect the mosquitos and ticks to show up too. These blood-sucking parasites love your pet as much as you do. If your dog or cat is left unprotected, these tiny thieves will steal your pet’s health.

This month is all about parasites, such as heartworms and ticks, and illnesses they can transfer to unprotected pets. As a pet owner, chances are you’ve heard about heartworm disease, but what do you really know about the disease and why prevention is so important? Let’s look at how this parasite affects dogs and cats, the symptoms and treatment, and how to prevent the disease.

Heartworm disease

Heartworm disease is caused by a parasite which is transmitted to dogs and cats by mosquitos. Dirofilaria immitis is a roundworm parasite that lives in the blood vessels and the heart. Mature heartworms release larvae, known as microfilaria, into the bloodstream. The microfilaria migrates through the lungs and settles into the heart during the adult stage. When a mosquito feeds on the blood of an infected animal, it can easily transfer the larvae to other dogs and cats.

The larvae need warmer temperatures to complete their development, and mosquitos are more prevalent in warm and moist conditions such as those found in the spring through the fall. No region is immune to the problem—heartworm disease has been diagnosed in all 50 states.

Even dogs and cats that spend the majority of time inside are at risk since mosquitos have a way of sneaking in open doors and windows.

Heartworm disease in dogs

Mature heartworms can be as long as 12 inches in dogs and live in the blood vessels in the lungs and in the heart. Large numbers of live or dead heartworms in the bloodstream can result in blockages in the blood vessels causing inflammation and serious damage. A blood test can confirm heartworm disease but early infection may yield a false negative result so repeating diagnostic testing months later is sometimes recommended. X-rays and ultrasounds may show changes in the heart and lungs caused by heartworm disease.

Symptoms include:

  • Coughing, rarely bringing up blood
  • Labored or rapid breathing
  • Weight loss
  • Lethargy
  • Nose bleeds and fainting
  • Abdominal fluid retention

Heartworm disease in cats

Heartworm disease is much different in cats than it is in dogs. As cats are not the natural host for heartworms, they are more resistant to infection but when they are infected, it’s very serious. The number of worms found in a cat is generally lower than in a dog, but only a few worms can cause severe disease.

Diagnosis is more difficult to confirm in cats because the blood test used to detect heartworm infection for dogs is unreliable for cats. A blood test looks for a protein (antigen) produced by the parasite. A low number of worms or female-only infections may not produce sufficient antigen to be accurately detected. For cats, testing includes a combination of blood testing, x-rays and an ultrasound for a positive diagnosis.
Some cats may not show signs of infection or symptoms could be mild or sporadic which can result in misdiagnosis. Initial symptoms can disappear and return with more severity. Dead immature worms cause inflammation in the lungs which leads to Heartworm Associated Respiratory Disease (HARD), but can be mistaken for asthma or allergic bronchitis.

Symptoms in cats include:

  • Coughing or labored breathing
  • Vomiting
  • Lethargy
  • Weight loss
  • Sudden breathing problems, collapse and rarely death


Treatment options for dogs are guided by several factors, including:

  • Number of heartworms present
  • Location of the heartworms
  • Age and health of the dog
  • Presence of microfilariae

Getting rid of the parasite includes elimination of the adult worms and killing the immature microfilariae. Depending on the severity of the disease, these treatments could happen concurrently. However, complications can occur with any treatment. Potential complications can include:

  • Drug toxicity – Two commonly used medications use are FDA-approved but they do contain arsenic and can lead to several reactions ranging from vomiting to death.
  • Passage of dead worms into the lungs causing respiratory failure. Elderly dogs and high-risk dogs such as those with severe infestation, congestive heart failure or kidney disease may not be good candidates. Surgical treatment is only for chronic cases, especially where drug therapy is not an option, and would still require drug treatment after the dog’s condition improves.

Heartworm disease in cats is very serious, often leading to death. Treatment is challenging because there are no products approved to treat cats and the dead worms resulting from drug treatment are highly toxic to cats. Supportive therapy of prednisone, antibiotics, heart and lung medications or intravenous fluids is usually recommended as the disease progresses.


The treatment of heartworm disease in dogs and cats is challenging and unpleasant with no guarantees for complete recovery. The best treatment option is prevention! Avoiding this deadly parasite is better for your pet’s overall wellness—and preventive treatment is easy for you. Test your pet annually or as recommended by your veterinarian. Ask which medication is the best and most convenient option for preventing heartworm disease in your dog or cat. The cost of prevention is low compared to the cost of treatment!