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Heartworm Preventatives: Do Dogs Need Them?


Every pet parent knows there are a lot of choices you have to make for your pet’s health. What kind of dog food should I buy? Should I have a fenced yard? Am I giving him too many treats?

With prime mosquito season on the way, the big questions on many dog owners’ minds are about heartworm prevention.

The majority of vets recommend putting your pet on a heartworm prevention medication all year long, and I’m sure one image of worms spilling out of a heart like spaghetti noodles was enough to convince you to take your vet’s advice.

But with the cost of such medications, having to remember to give it to your pet each month and fears of possible side effects, more and more people are beginning to question the need for heartworm pills.

To help you make an informed decision for your dog’s health, let’s take a closer look at heartworm and how it works.

What is Heartworm?

3543639799_133094920c_zPut simply, heartworm is a parasite that dogs (and other mammals such as cats, ferrets and some wild animals) can contract from infected mosquitoes. Given the right conditions, the worms can grow and mature within the host animal and travel through veins toward the lungs and heart, causing potentially fatal complications.

How Dangerous is Heartworm?

Left untreated, heartworm can be fatal. As worms multiply and grow, they can block blood vessels, causing reduced blood and oxygen levels, and even blood clots. It’s definitely a scary thought – especially when you consider that females can grow to over 10 inches long.

However, there is some good news!

  • There’s only one way your dog can become infected: Being bitten by a mosquito carrying heartworm larva. Your dog cannot catch heartworm from another infected animal.
  • If your dog is infected, it can’t spread the parasite to you.
  • The heartworm life cycle is long and complex. It takes about 90 to 100 days for them to reach the stage at which they can enter the circulatory system, so there’s plenty of time to catch and treat an infection.

How Common is Heartworm?

Heartworm frequency may not be as high as you think, though it depends on several factors.

Although the incidence of reporting the disease isn’t perfect, data collected by the American Heartworm Society show that many veterinary clinics throughout the U.S. had fewer than 50 cases in 2013. Much of the nation saw fewer than five cases per clinic. (Click here to see the map.)

And according to Banfield Pet Hospital’s State of Pet Health 2014 Report for over 850 clinics and about 2.3 million dogs nationwide, heartworm was not among the most common diagnoses. Poor dental health, being overweight, and age-related conditions were the biggest concerns. However, it should be noted that it wasn’t indicated how many of these dogs were on a heartworm preventative.

Since heartworm is spread by mosquitoes, the frequency will depend on the climate. If you live in an area where most of the year has hot, humid weather that mosquitoes (and the parasites they carry) love, the risk increases. Some areas with a higher rate of heartworm infection include coastal regions of Texas and Louisiana (along the Gulf of Mexico), areas in the South along the Mississippi River and the southern Atlantic coast.

The parasites require a host in which to mature and reproduce. The young released by females can only develop into their first larval stage if ingested by a mosquito feeding on an infected animal. So if there’s an abundance of reservoir species (like dogs, coyotes, foxes, etc) in your area, there’s more opportunity for the parasite to populate and be spread between animals.

Are There Risks to Using Heartworm Preventatives?

Some pet owners are hesitant about these medications, due to possible health risks posed by the drugs they contain.

8703778170_ebb3435a0c_zThe active ingredient in some of the most popular medications is ivermectin, which can be toxic to some dogs. (Click here for a list of the active ingredient of different medications.) However, the intolerance seems to be genetic and more common among breeds like Collies, Australian Shepherds, Shelties, Border Collies, Old English Sheepdogs, and English Shepherds. If you’re concerned about your dog’s reaction to any drug, consult your veterinarian.

And since heartworm medications don’t exactly prevent infection, but rather kill any worms in your dog before they can fully mature, some people have reservations about giving their pet something designed to kill.

Are There Alternatives to Heartworm Medication?

Currently the only proven way to prevent a serious infection – besides making sure your dog is never ever bitten by a mosquito – is through preventative medication. However, some pet owners (and some vets) feel comfortable forgoing the medication for some – or even the entirety – of the year.

Using Temperature as a Guide: According to an article originally published in Whole Dog Journal, you can determine how many months of the year you need to administer the medication based on temperature: “Heartworm larvae cannot develop to the stage needed to infect dogs until temperatures have been over 57 degrees Fahrenheit, day and night, for at least one to two weeks.”

The logic behind this method is that if it’s too cold for the heartworm larvae to mature and harm your pet, why do you need to give the medication?

It’s important to note, however, that heartworms can survive for some time inside their host, so you shouldn’t stop dosing as soon as the weather drops below 57 degrees. Read more about this method under the “Temperature and timing” section.

Using a Reduced-Frequency Approach: Heartworm medications are typically given every month, but many were designed to offer protection beyond 30 days in case a dose was missed or spit out or vomited up by a dog. According to the same article referenced in the above method, you may be able to give the medication every 45 days. Read more under “Frequency of Preventatives.”

Using the Testing Approach: Before giving a dog heartworm medication, they must be tested for heartworms. Although the medication is designed to kill heartworms when they’re young, if a dog has a more advanced case of infection, administering the pills could pose risks. A rapid die-off of larvae from adults in the animal’s system could cause an anaphylactic reaction. And of course, the dog will need a more advanced form of treatment.

Click here to see all or our products from Wondercide.

Though typically not recommended, it is possible to use heartworm testing as a form of prevention. Treatment is very successful and fairly simple if an infection is caught early. Therefore, some pet owners have their pet regularly tested for heartworm and treated if the test is positive, rather than giving them a medication every month they may not need.

Using Mosquito Repellents: Since no one likes being bit by a mosquito regardless of parasites they might be carrying, a good option is to use mosquito repellents – whether your dog is on a preventative medication or not. By limiting your dog’s exposure to mosquitoes, you can limit their exposure to heartworm. And don’t worry about dangers from Deet. There are now great, natural options available that are safe for you and your pet, including Wondercide’s Biter Fighter Deet Free Insect Repellent and their Repel Natural Bar Soap with Citronella and Geranium.

When making choices regarding heartworm protection, consider all the factors to make an informed decision for your pet. And be sure to choose a knowledgeable veterinarian you feel comfortable working with.

For more tips on keeping your pet healthy naturally, check out some of our other pet posts:








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Mosquito Feeding – Steve Begin

Collies at Play – Doug Brown