Frequently Asked Questions About Heartworm Disease

What is heartworm disease?

Answer: Heartworm disease develops when a pet becomes infected with parasites called Dirofilaria immitis that are transmitted through the bite of an infected mosquito. Heartworms are “foot long” worms that live in the heart and major blood vessels of the lungs. The disease is spread by mosquitoes to dogs, as well as other animals, such as foxes, coyotes, and wolves, which act as reservoirs and help spread the disease. Heartworm infection often leads to severe lung disease and heart failure and can damage other organs in the body as well.

HWHow can I tell if my pet has heartworms?

Answer: Early on, many dogs show little or no symptoms. The longer the infection persists, the more likely symptoms will develop. Clinical signs depend on the number of these “foot-long” worms in the dog’s heart and major blood vessels, the dog’s lifestyle, and any other health issues the pet has. More active dogs, pets heavily infected with heartworms, or those with chronic disease often show prominent clinical signs.

Do you need a prescription for heartworm preventive medication? If so, why?

Answer: Yes, heartworm preventives must be purchased from your veterinarian. Health Canada labeling on heartworm preventives indicates that the medication is to be used by or on the order of a licensed veterinarian. This means a veterinarian must have a doctor-client-patient relationship in order to write a prescription to ensure safe use of this product.

Flea-Treatment-for-DogsHow do monthly heartworm preventives work?

Answer: Whether the prevention you choose is given o  rally or topically, all approved heartworm medications work by eliminating the immature (larval) stages of this parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the molted fourth larval stage which develops inside the animal. Unfortunately, within 2 months, immature heartworm larvae can molt into an adult (fifth) stage, which cannot be effectively eliminated by preventives. Because heartworms must be eliminated before they reach this adult stage, it is extremely important that heartworm preventives be administered strictly on schedule.

Why do dogs need to be blood tested before starting heartworm medication?

Answer: Preventive medications can be harmful or even fatal to a dog that has an adult heartworm infection, so it is very important that all dogs be tested before starting a preventive program. Adult heartworms reproduce by releasing millions of microscopic “babies” (called microfilaria) into a dog’s bloodstream. When we give a preventive medication, these microfilariae can suddenly die and a shock-type reaction can occur. This reaction can be triggered within a matter of minutes or hours and without prompt veterinary attention, death can occur. But even if an infected dog does not have an immediate “reaction” to preventive medication, the pet often remains infected for several years. As long as adult worms remain, they will continue to cause progressive disease of the heart, lungs, and pulmonary arteries which can lead to irreversible life-threatening problems.

This information has been obtained from the Heartworm Society.

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