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Let’s dig into heartworms!

Posted 10.21.15 by Emily Lindsey

What are heartworms?

Simply put, heartworms are THE ENEMY! Heartworms are nematode worms that live in an animal’s circulatory system and can cause serious illness and even death. This disease is primarily a canine issue, but cats can also be affected. House pets are not the only carriers, either. Wolves, coyotes, and foxes can also carry this disease and serve as a reservoir for infection.

How Do Dogs Get Heartworms?

Short answer: MOSQUITOS TRANSMIT HEARTWORMS. A dog or cat must be bitten by an infected mosquito to contract heartworms. One animal cannot directly give it to another. If your dog lives in a kennel or neighborhood with an infected dog nearby, it is at an increased risk for heartworms because mosquitoes, like hipsters, like to “eat local”. There are many stages of heartworms. I like to use a human analogy to explain:

Adult heartworms come in male and female versions. They live in the large blood vessel that leads from the heart to the lungs (the pulmonary artery). Both sexes must be present in the same animal to make baby heartworms called microfilaria, which circulate in the blood.  A mosquito comes along and takes a delicious microfilaria-filled blood meal. This mosquito then becomes a heartworm nanny, fostering the little baby parasites until they are toddlers. The warmer it is, the faster those rugrats –er, rugworms? – grow! That’s why summer is peak transmission season. Then, a couple of weeks later, the mosquito bites a new dog and infects it with these toddlers. Over the course of several months, the toddlers grow up and migrate through the skin and tissues, into the circulation, and finally, to the large vessel leaving the heart. Most heartworm tests detect only these ADULT WORMS, so if your dog was bitten by an infected mosquito yesterday, it won’t TEST POSITIVE FOR 6-7 months! That is why YEARLY HEARTWOMR TESTING IS REQUIRED! We want to pick up new infections as soon as possible because of:

dogs with heartworms

What happens to an animal with heartworms?

Those worms in the pulmonary artery (as it exits the heart) release some nasty stuff that flows to the lungs and causes a lot of inflammation. Despite the name, heartworms really irritate the lungs and their blood vessels first and foremost. Over time, this lung damage causes abnormal blood flow, and can ultimately lead to heart failure, but this takes a long time to develop. Dogs can develop coughing, difficulty breathing, weight loss. Our goal is to find the worms before they can even cause these signs, which is why we require heartworm testing EVERY YEAR. This testing is also required in order to be covered by heartworm prevention warranties. (More on that later). Cats seem to be even more sensitive than dogs to the lung inflammation caused by heartworms. They can develop an asthma-like disease that lasts for years. Some cats show no signs but then die suddenly, and the only abnormality on post-mortem might be a single heartworm. Bottom line: heartworms are bad to get. So:

How do we prevent heartworms?

There are several products on the market to prevent canine and feline heartworm infection. Some are monthly pills or topicals, and one (Proheart6) is an every 6 month injection for dogs only. ALL OF THESE PRODUCTS ARE BY VETERINARY PRESCRIPTION ONLY! This is important since some over the counter flea and tick preventatives have similar sounding names and packaging to the heartworm preventatives. I’ve had clients mistakenly think they were buying heartworm prevention off the shelf at a big box store (and saving a few bucks!) only to discover that their pet has not been protected from heartworms at all!

There are online pharmacies that sell heartworm preventatives at a slight discount, but UNLESS YOU BUY DIRECTLY FROM A VETERINARIAN, YOU ARE NOT COVERED BY THE HEARTWORM PREVENTION COMPANY’S WARRANTY! This warranty is important, because if you test annually and purchased 12 doses per 12 months, the company will help pay for heartworm treatment, which can cost over a thousand dollars. Though prevention failure is rare, some resistant heartworm strains are developing, especially against some of the older products on the market. Many heartworm preventatives also have GI dewormer and/ or flea prevention built in. Talk to your veterinarian about which product might be best for your pet.

How do we get rid of heartworms?

The recommended treatment for adult heartworms in dogs involves giving injections of an arsenic-based compound that will SLOWLY kill the adult worms exiting the heart. Where do these worms go? They break off and go to the lungs, where they cause MORE inflammation. This is why the process must be slow and controlled, and dogs must be strictly rested for the entire 2+ month treatment window. If large amounts of dead worms hit the lungs all at once, dogs can have serious side effects to treatment. Dogs can rarely have severe reactions to the drug itself, but more frequently they are very painful at the injection site. Needless to say, this is not the most pleasant thing to put your dog through, so please remember that PREVENTION IS KEY! For cats, there is no current treatment for getting rid of adult worms in the heart, so for cats, PREVENTION IS ALSO KEY!

For more information on heartworm disease in pets, including some terrifying pictures and incidence maps, please go to:

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