Vet’s Corner will feature articles on pet health, safety and well-being from contributing vets, animal practitioners and other contributing authors.

What is Heartworm?

Heartworm is a very serious disease that can kill animals. It is caused by worms that live in the heart, lungs and associated blood vessels of an infected animal and can cause severe lung disease, heart failure and organ damage. Heartworm disease affects both dogs and cats. Dogs provide an ideal ‘host’ for heartworms meaning the worms can easily and rapidly reproduce. Heartworms can cause long term damage to the dog’s heart, lungs and arteries even after the heartworm infection has been cleared. Early prevention is important to avoid any infection that could affect the overall quality of your dog’s life. A heartworm infection in cats can easily go undetected as cats do not provide such an ideal ’breeding site’ for the worms. However, a cat infected with heartworm can develop Heartworm Associated Respiratory Disease (HARD) which can cause long term damage for the cat’s health. Prevention is vital in protecting a cat from heartworm as medications used to treat heartworm infections in dogs cannot be used in cats.


How is Heartworm transmitted to my dog or cat?

Heartworm is transmitted by mosquito bites. Adult female heartworms living in an infected dog produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes blood from an infected animal, it picks up these baby worms, which develop and mature into infective stage larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog or cat, the infective larvae are deposited onto the surface of the animal’s skin and enter the body through the mosquito’s bite wound. Once inside the body, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.


What are the signs of heartworm disease?

In dogs, it is often not until the infection has progressed into a more widespread infection that signs will appear. Dogs that are very active dogs, have existing health problems or are heavily infected with heartworms will usually exhibit more noticeable signs. Signs to look out for include a persistent cough, loss of appetite, lethargy, weight loss and abnormal fatigue after moderate activity. As the infection develops, the dog may develop a swollen belly due to the accumulation of fluid in the abdomen. As the number of heartworms increase, so does the risk of the dog developing heart failure as blood flow to the heart is blocked. The dog will struggle to breathe, have pale gums and pass dark and/or bloody urine.
In cats, signs of a heartworm infection can appear as coughing, loss of appetite, vomiting, weight loss and episodes of asthma-like attacks. In severe cases, the cat may have difficulty walking, faint or have seizures or develop a swollen belly from fluid accumulation in the abdomen. Unfortunately, the first sign in some cases is sudden collapse of the cat, or sudden death.


How can I prevent my pet from being infected with heartworm?

If your pet is over 6 months of age and you have not already started them on heartworm preventative medications, it is recommended to have them tested to check for possible infection. Testing is done by taking a small blood sample which is checked under a microscope for the presence of worms. Although there is no vaccine available for the prevention of heartworm disease in dogs or cats, there are various preventative medications available which your vet can recommend to you. These include:

  • ProHeart— an injectable medication (for dogs only)
  • Heartgard Plus—a chewable medication (for dogs only). Heartgard Plus also offers protection against roundworms and hookworms
  • Advocate—a ‘once a month’ topical ‘spot on’ application available for cats and dogs. Advocate also offers protection against fleas, lice, gastrointestinal worms, ear mites, lungworms (in dogs), sarcoptic mange (in dogs) and Demodex mites (in dogs)
  • Revolution—a topical treatment available for cats and dogs. Revolution for dogs also offers protection against fleas, ticks, ear mites and sarcoptic mites. Revolution for cats also offers protection against fleas, ear mites, roundworms and hookworms.

Puppies and kittens can contract heartworm disease just as easily as adult cats and dogs so early preventative treatment is advised. Your vet will advise the most suitable medication to use. Annual testing for heartworm is advised even if your pet is on regular heartworm preventative medication.