The list of threats to your pet—bacteria, viruses, parasites—and the deadly diseases they cause may seem endless, but they often can be prevented. Heartworm disease tops the list of dangers to your beloved companion, with more than 3,000 cases diagnosed per year in California pets. We diagnosed a heartworm-positive dog in Monrovia two months ago. The threat is real—it is here in our town. Fortunately, heartworm disease is preventable.

What is heartworm disease?

Heartworms, or Dirofilaria immitis, are parasitic worms that live in the heart and lungs of infected dogs. The worms enter a dog’s body as microscopic larvae, but mature and grow up to 12 inches long over the next 6 to 7 months. The adult worms reproduce, exponentially increase in number, and begin to cause health problems, including:

  • Obstruction of blood flow through the heart and lungs
  • Inflammation of the lungs and blood vessels
  • Fluid buildup in the lungs

If heartworm disease is not treated, the heart and lung damage will progress and eventually lead to death.

How can my pet get heartworm disease?

Heartworms are transmitted by mosquitoes that have ingested blood from an infected dog. The microscopic larval worms, called microfilariae, develop in the mosquito’s gastrointestinal tract. When the mosquito bites another animal, it transmits the larvae and the heartworm’s life cycle continues.

Which pets can develop heartworm disease?

Dogs are the preferred heartworm host, although all canine species can be affected, including wild canids, such as wolves, coyotes, and foxes, that serve as an infection reservoir for pets. Heartworm disease is most prevalent in hot, humid areas, but has been diagnosed in all 50 U.S. states.

Although they are not the desired host, cats can also be affected by heartworms. The few larval worms transmitted by a mosquito can mature to adults, but cannot reproduce, making the cat a dead-end host. Heartworm disease is still a deadly threat for cats, because a single worm can cause severe inflammation in the cat’s heart and lungs.

What are the signs of heartworm disease in dogs and cats?

The inflammation and blood-flow obstruction that the worms cause lead to the following clinical signs in infected dogs:

  • Lethargy
  • Decreased appetite
  • Weight loss
  • Decreased activity
  • Coughing
  • Vomiting
  • Distended abdomen
  • Difficulty breathing

Heartworm disease can cause similar clinical signs in cats, but, unfortunately, sudden death often occurs with no prior signs.

How is heartworm disease diagnosed in pets?

Heartworm disease is diagnosed with a simple blood test that can be performed in our clinic. Because the test detects adult female heartworms, the result will not be positive until the larvae mature to adulthood, approximately 6 months after transmission. Since cats are infected with only a few worms, which can all be male, diagnosis in cats is more difficult. Another test that detects antibodies produced in response to the worms’ presence can be used, but is less reliable.

The Companion Animal Parasite Council recommends that every dog and cat be tested for heartworm disease annually as part of their wellness visit.

How is heartworm disease treated in dogs and cats?

The treatment plan for heartworm disease, which is based on infection severity and the extent of heart and lung damage, may include:

  • Intramuscular injections of medication, given over several months, to kill adult worms
  • Medication to kill developing microfilariae
  • Anti-inflammatories to decrease inflammation and damage caused by living and dying worms
  • Antibiotics to kill bacteria carried by worms that contribute to inflammation

Treatments are often administered over several months, and can be costly and risky. As worms die, they can break apart, move through the bloodstream, and lodge in lung vessels, causing a life-threatening pulmonary embolism. During the treatment period, dogs must be confined to reduce their activity and allowed short leash walks for bathroom breaks only, to decrease the embolism risk. Dogs with severe heartworm disease are at greatest risk of pulmonary embolism and sudden death.

No approved treatment is available to kill adult heartworms in cats. If your cat is diagnosed with heartworm disease, medications can decrease inflammation severity and the clinical signs until the few heartworms die, which can take several years.

How can I protect my pet from heartworm disease?

Many heartworm-preventive products are available, and our veterinary staff can recommend one that best fits your pet’s needs. Most heartworm preventives are combined with products that also prevent roundworms, hookworms, whipworms, and other parasites. Prevention should begin at 2 to 3 months of age, given year-round, and continued throughout your pet’s life. If your pet is not currently taking a heartworm preventive, ask our veterinary team how to get started.

If you have questions about heartworm disease treatment or prevention, or your pet has signs consistent with heartworm disease, contact our office and schedule an appointment with Dr. Gueniat.