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The Yucky Truth about Canine Heartworm Disease

Well, it is Spring Break this week, and spring has decided to go into hiding. She will soon be peeking her head around the corner again. When she comes, we will get warmer weather, rain (we hope), humidity, and unfortunately mosquitos.

Now, I know you may wonder why I am talking about heartworm disease this week since this isn’t a disease that normally gets referred to an internist. So, let me explain. Typically, one of the questions that I routinely ask as I take a history is whether or not a dog is on heartworm preventive. It never ceases to amaze me how many people are willing to come see a specialist and diagnose and treat their dog for an uncommon disease, but they aren’t protecting their dogs from common diseases with heartworm preventive, flea/tick preventive, and vaccines.

Heartworm disease is a serious and potentially fatal disease that is caused by the heartworm, Dirofilaria immitis. The dog and other wild canines are considered the natural host for this parasite, which means the worms can live, mature, and reproduce in the dog. Heartworms can also infect other species, including cats and humans. Heartworm disease affects blood vessels, lungs, and in cases of severe infection, the heart.

To understand how to prevent and diagnose heartworm disease, you need to understand the life cycle of the worm. Mosquitos play a huge role in heartworm infection because they ingest microfilariae (baby worms) from the blood of infected dogs. The microfilariae then molt into an infective larva (L3) inside the mosquito. When the mosquito bites a susceptible dog, the L3 larva is deposited on the skin, and it migrates through the puncture wound created by the mosquito into the dog. Once the larvae are inside the dog, they molt and mature into adult heartworms. The worms can mature and produce microfilariae as early as 6 months although most need 7 – 9 months. The adult worms can then live in the dog for 5 – 7 years, and they will continue to produce more microfilaria, increasing the risk of other dogs to be exposed to heartworms. Adult heartworms can grow to nearly a foot! I love trivial facts, and here is one for you: trapped mosquitos in heartworm endemic areas have infection rates of 2 – 19%. In a kennel structure with a known heartworm positive dog, mosquito infection rates increase to 30 – 74%. This is a huge increase in risk to healthy dogs when they live is close proximity to a heartworm positive dog.

Dogs with heartworms may have few to no signs in the early stages of disease. As the disease progresses, they can develop a cough, exercise intolerance, or heart failure with a distended abdomen. Dogs that are extremely active or have heavy worm burdens are the most likely ones to show clinical signs.

Although the dog is the natural host for heartworms, heartworm infection is preventable. Puppies should be started on preventive by 8 weeks of age. As long as they are started on preventive before 7 months of age, they do not need a heartworm test before initiating therapy. If they are 7 months or older, they should be tested before starting medication, and they should be retested 6 months later (to insure that they didn’t have a developing infection when started). Then, they should be tested yearly. Yearly tests are recommended in all dogs to make sure that they remain disease free. Even when preventive is given year-round, heartworm preventive is not 100%, and infection can occur if a dose is late, a pill is spit out, or a topical product is rubbed off.

Heartworm preventives can be given in 3 forms – oral, topical, or injectable. The oral drugs are ivermectin (Heartgard®) and milbemycin (Sentinel® and Trifexis®). Both of these preventives kill hookworms and roundworms, and milbemycin kills whipworms as well. Moxidectin (Advantage-Multi®) and selamectin (Revolution®) are available as topical products, and moxidectin is available in a injectable sustained-release product (Pro-Heart® 6) that is given every 6 months. As many of you are aware, several dog breeds, including collies, are susceptible to side effects from certain drugs, and heartworm preventives are on that list. However, all of the above preventives have been found safe in these breeds in standard doses.

Heartworm disease has been diagnosed in all 50 states. If you look at the incidence map below, you can see that we are living in a heartworm endemic state in Oklahoma. Because mosquitos can be seen even in the winter in our temperate climate, year-round heartworm preventive is recommended.

Obviously, there are multiple drug options for heartworm prevention, and you should talk to your primary care veterinarian to see what they recommend. For those of you who have a hard time remembering to give medications every 30 days, the 6-month injection may be a good option. Many veterinarians now use an online store, and you can buy your heartworm dose once a month instead of buying a 6- or 12-month supply all at one time The online store will bill and ship you at 30 day intervals to help you remember to give your medication on time. I think this is a phenomenal option! In addition, there truly is an app for that, and multiple drug companies (Merck and Virbac) have reminder apps for your smart phone. There are also apps from human pharmacies with drug administration reminders that can be adapted for heartworm preventives.

Back to heartworm testing, there are two types of tests that are used. One is a test that looks for protein secreted by an adult female heartworm. This is called an antigen test, and this is the most sensitive way to test for heartworm disease. Another test looks for circulating microfilariae in the blood. This test is called a Knott’s test. Most practices routinely perform the antigen test, but the American Heartworm Society actually recommends that both tests be performed annually. If you miss a dose of preventive you should talk to your veterinarian, re-start monthly preventive, and test for heartworm infection 6 months later, since heartworms take 6 – 9 months to mature, so that the infection can be diagnosed.

Heaven-forbid, but if your dog tests positive for heartworm disease, the first thing to be done is to repeat the test for confirmation. This will usually be done with a different test, and it is often sent to a reference laboratory. The second most important thing is to restrict your dog’s activity to minimize the damage from the heartworms. The third step is treatment. Your veterinarian will follow the guidelines of the American Heartworm Society, and these include the administration of monthly heartworm preventive, doxycycline (an antibiotic that helps manage heartworm disease in infected dogs and decreases the risk of infection to other dogs), anti-inflammatory drugs, and an adulticide to kill the heartworms. Ideally, the adulticide will be given in a 3-injection series to improve heartworm kill to 98%. Slow-kill methods of killing heartworms with continuous administration of a monthly preventive are not recommended by the American Heartworm Society. Lastly, a heartworm test will be performed 6 months after treatment to make sure all adults have been killed. Preventive will be given year-round, and yearly tests will be performed to be certain that your dog has not contracted heartworm infection again.

Because many heartworm positive dogs are coming out of shelter/rescue situations, you and your veterinarian may be faced with a decision to proceed with an elective procedure (spay, neuter, or dental). Dogs with no to mild clinical signs had no increase in complications in a study performed in 2014 by Peterson et al. Dogs with more advanced disease should avoid procedures for 6 months after adulticide therapy.

So, to wrap this up, please test your dogs yearly for heartworm disease, and please put them on a year-round preventive regime. This infection is easy to prevent, challenging to diagnose, and dangerous to treat.

For more information, you can visit the American Heartworm Society website, ask your regular veterinarian, or send a message to our website.

Photo Credits:


Life Cycle of Heartworm:


Heartworm Incidence 2013

When To Do A Heartworm Test:

Puppy Heart:

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