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Preventing Heartworms in Dogs

Heartworm prevention is a year-round task. That's because mosquitoes exist just about everywhere, even in regions that have long, cold winters. While we may think of mosquitoes as a mere nuisance that occasionally interrupts our outdoor plans, they can mean a different thing entirely for your dog: the threat of heartworms. Protecting your dog from this potentially fatal parasite is an essential part of being a responsible pet parent, and your pet depends on you for this. So before heading out with your dog for a game of fetch in your potentially mosquito-laden backyard, take a few minutes to learn about heartworms and see why prevention—rather than treatment—is essential.

What are Heartworms?

Heartworms (Dirofilaria immitis) are actually a variety of roundworm. These worms mature into adults, mate and produce offspring—all while living inside your dog's heart, lungs and associated blood vessels. Adult heartworms, which look like cooked spaghetti, can live inside your dog for many years; females can reach up to 12 inches in length.

The Heartworm Life Cycle

It's important to understand the heartworm's life cycle to see why prevention is so important. Heartworms go through a long series of larval (immature) stages before becoming adults. The first larval stage exists in dogs that already have an infestation of adult heartworms. When an affected dog is bitten by a mosquito, it is possible for these first-stage larvae to migrate to the mosquito. Then they complete two more larval stages before migrating to the mouth of the mosquito, where they can be transferred via a mosquito bite to another dog. (Heartworm cannot be transmitted from dog to dog). Once there, the larvae can continue their progression to adulthood, where they infect areas in and around the dog's heart and lungs. Breeding-age adult heartworms then produce a new set of first-stage larvae and the cycle begins again. Adult heartworms in dogs can lead to a wide range of health problems, and if left untreated, will almost always result in death.

Clearly, the mosquito plays a key role here, because the heartworm's life cycle requires the insect, and it is used as a means of transporting the disease to dogs (as well as cats and other pets). In the United States, the warm southern regions have historically seen a larger occurrence of heartworms, but the disease has been reported in all 50 states.

Prevention is Key

It can be difficult to determine whether a dog has adult heartworms just by observation, although a persistent cough—particularly during exercise—is a classic sign, and a cough combined with lack of energy during exercise can potentially be indicative of a more severe case. Veterinary blood tests and other examinations are used to make a definite diagnosis.

It's possible to cure a dog of an infestation of adult heartworms, but the process is time consuming, expensive and can be very hard on your dog. Treatment typically requires hospitalization while injections of heartworm adulticides are administered, along with several weeks of limited exercise. And this doesn't even address the issue of previous damage to the dog's body prior to treatment. You'll need to work closely with your veterinarian to determine the best treatment options for your pet.

In all cases, heartworm prevention is key. Since it isn't practical (or possible) to protect your dog from ever getting a mosquito bite, the best solution is to prevent potential heartworm larvae from completing their life cycle in your pet's body. Happily, such prevention is simple and inexpensive. Monthly or semi-annual treatments of prescription oral pills (aimed at killing heartworms at the larval stages) are extremely successful in preventing heartworms from infecting your dog. Topical skin products are also available.

While some may assume that dogs don't need protection during the winter months (after all, if temperatures are low enough, there are no mosquitos, right?), it is often recommended that you continue heartworm prevention year-round to guard against an accidental lapse in protection (forgetting to administer a pill for one month, for example). Additionally, many heartworm prevention products also provide protection against other parasites, so year-round use is needed for this reason as well. Again, it's best to ask your veterinarian about the best preventative solution for your pet.