Protecting Your Pet from Heartworm Disease

By Joelle Abrams, DVM

1. What is Heartworm disease? And how can my pet get it?
   Heartworm disease is caused by infection with the spaghetti-like worm Dirofilaria immitis. The adult worms reside in the pulmonary artery and right ventricle (heart) of pets. When a female and male heartworms are present in the same pet, they can produce microfiliariae (tiny little baby worms) that are discharged into the bloodstream. Those microfiliariae are then ingested by mosquitoes when feeding on a pet. The microfilariae undergo some development within the mosquito (become infective L3 larvae) and get deposited on the skin of your dog or cat when a mosquito feeds again. A single mosquito can transmit 10-12 larvae! It takes several months for those larvae to become adult worms. About 6 months after infection with the L3 larvae if both male and female heartworms develop, the cycle repeats itself and microfilariae are produced. Adult heartworms can live 3-5years while microfilariae live about 1-3 years.
   
   
2. Heartworm disease is for dogs only, right?
   WRONG. Both cats and dogs are at risk of heartworm disease. The disease is a bit different between the two species but is still a problem in cats.
   In cats, the worms are usually smaller and there might only be 3 or 4 worms in the heart which is why the heartworm test in cats is not as sensitive as for dogs. Also for some reasons, in cats, it’s not unusual to have only males (or only females) heartworms which means no microfilariae will be produced.
   In dogs, the disease is directly related to the number of worms (i.e. the more worms the more sick a dog will be) and how long your dog has been infected.
   
   
3. What are signs of heartworm disease?
   Dogs can initially show no signs at all to mild coughing to more severe exercise intolerance, and heart failure. Cats are a bit trickier in the sense that their signs can be more vague: coughing, vomiting, slight to more severe difficulty breathing...
   When the adult heartworms start dying, this is when signs of heartworm disease get worse.
   
   
4. How do I know if my pet already has heartworm disease?
   A simple blood test at your veterinarian should help diagnose heartworm disease. In some cases, a test is not sufficient and your veterinarian might recommend chest radiographs, ultrasound or even other diagnostics tests.
   
   
5. What can I do if my pet is diagnosed with heartworm disease?
   Untreated heartworm disease can be fatal. Treatment for the disease is expensive (several thousands of dollars) and can lead to lots of complications including death, which is why prevention is easier. There is no immunity to heartworm disease. So even if your pet recovered from the disease, he will still need to be on heartworm prevention. If your pet is diagnosed with heartworm disease, your veterinarian will most likely hospitalize your pet and treat with an adulticide (to kill the adult worms). If your pet is very sick, he might need to be referred to a specialty or a 24 hour-care facility.
6. How can I prevent my pet from getting heartworm disease? Heartworm disease used to be confined to the states bordering the Mississippi river but in the past few years, things have changed drastically and heartworm disease is everywhere as mosquitoes have traveled considerably. The easiest and most effective way to protect your pet is use a monthly heartworm preventive (contact your veterinarian for a list of approved prescription products for dogs and cats). Trying to do mosquito control whenever possible can also help: mosquitoes breed in stagnant water so make sure to avoid those areas.

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