Heartworm Disease: Heartworm Preventatives & Treatments

Mosquitoes and target practice. Us versus them. We swat them, slap them, spray them, and curse their very existence—and yet they continue to vex us! Sadly, this is not just a burden us humans to endure; it’s also true for our animal friends as well. Furthermore, mosquitoes pose more than a mere nuisance; they’re also a potential source of serious health risks. Mosquitoes serve as the gateway—the incubator, the means of delivery—for a list of nasty infections and diseases. And for dog owners, one of the most dreaded conditions they can spawn is heart disease in dogs.

What is heartworm disease?

The heartworm disease cycle involves a number of players and events, but those pesky, blood-sucking mosquitoes are the common denominator. The disease begins with previously infected animals (usually domestic dogs, foxes, coyotes, and wolves), whose blood—again, when infected—contains microscopic baby heartworms called microfilaria. When a mosquito bites an infected animal, it sucks up these microfilaria and carries them inside its body. There, the baby worms develop, over a course of 10-30 days, into “infective stage” larvae. And when the mosquito bites its next victim, these larvae are transferred into the host’s bloodstream.

After about six months, the larvae will mature into adult heartworms. (Females are about 6-14 inches long; males about half this size.) Once inside a living host, these parasites have the capacity to live 5-7 years, and will start reproducing, Meaning an infected dog’s body could harbor several thousand worms, setting the stage for their disease.

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Why is heartworm disease in dogs dangerous?

When these parasites multiply and grow inside your dog’s body, they infest and clog up the heart and major blood vessels. This blockage reduces blood supply to other major organs (especially the lungs, liver, and kidneys). As they obstruct arteries, the heart has to exert itself more to pump blood through the lungs to receive oxygen—eventually enlarging the heart and leading to organ failure.

Most diagnoses of heartworm disease in dogs occur when the patient is between two and eight years of age, because of the time involved in producing adult heartworms. (It is rare in puppies as the microfilariae have generally not had time to mature in a host that young.) Because of this, the disease may be well advanced by the time it is found.

The importance of testing for heartworm disease.

The American Heartworm Society (AHS) strongly recommends annual testing as part of a complete heartworm prevention program. According to their research, only half of the dogs in areas prone to the disease receive proper preventative medicine, and only about 75% of canines receive their required monthly doses.

The AHS recommends the following guidelines for proper heartworm disease prevention:

• ALL dogs should be TESTED ANNUALLY for heartworm infection. This will allow a veterinarian to catch heartworms earlier for effective management.
• Dogs should be tested for heartworm if an owner is going to switch from one preventive to another. If a pet owner selects another heartworm preventive option, there are specific time periods in which the pet should be retested in order to ensure the dog is protected. It will be necessary to test more often.
• Puppies can begin taking heartworm preventives without testing first. This can begin under the age of seven months since it takes about six-and-one-half months after being bitten by an infected mosquito before the dog will test positive. Testing should occur four to seven months after starting the heartworm medication to determine the presence of a heartworm infection.

As always, consult with your veterinarian about testing schedules and options.

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Testing Options

A blood test can help detect/confirm heart disease.

Antigen tests look for specific antigens from adult female heartworms and have proven very successful in confirming heartworm infections in dogs. These tests are performed on-site by a local veterinarian or at many veterinarian reference laboratories. These tests are efficient at detecting female heartworms at least seven or eight months old, but less likely to find worms younger than five months old.

Developed in the late 1970s and early 1980s, antibody tests have largely been replaced by the more effective antigen test. However, the antibody test is more commonly used to find heartworms in cats.

What should I do if my dog tests positive for heartworms?? 

If a dog is diagnosed with heartworm disease, consult your veterinarian and follow their instructions to the letter. The vet will need to assess what stage the infection is in and look for underlying conditions which may hinder the dog’s recovery (such as heart and lung damage as well as liver and kidney function). These problems will have to be dealt with first before drug treatment can begin.

A continuing shortage of a drug called melarsomine hydrochloride (marketed as Immiticide®) has in some cases forced veterinarians to look for other primary solutions in treating heartworms in dogs. Two of these are ivermectin and Noromectin. A vet may prescribe one of these and give it in a series of deep muscle injections in the dog’s back over a 24-hour period (or two or more shots given a month apart). Someone will need to watch for side effects, and a dog will be required to maintain strict bed rest or short-term hospitalization to watch for signs of shock or other adverse results. (The dog may also be sore and stiff at the injection site, so a prescription for pain medication may also be required.)

During the four weeks it takes for the worms to die, the dog must be kept from running and playing to avoid dead/dying worms from migrating to the heart or lungs and causing another life-threatening blockage. Monthly heartworm medicine will also be administered during this time to catch other worms in development. Abnormal symptoms during the treatment—like diarrhea, coughing, vomiting, and depression—should be reported to a vet.

Generally, the success rate for dogs treated for heartworms is about 98% and most will not require additional intervention. A few may require additional treatment if heartworm tests still indicate a presence six months after the initial injections. It can take months before a dog will have a negative heartworm antigen test.

Due to Immiticide’s shortage, some vets have also used a “slow method” of treatment using Heartgard and doxycycline (prescription required) with positive results. Always consult with a veterinarian about the best treatment route for your dog.

What are the symptoms of heartworm disease?

Initial symptoms of an infected dog may be mild and subtle and are often chocked up to aging. Most dogs do not show any early signs of the disease, which means by the time it is caught it is usually in an advanced state—making it riskier and more difficult to cure. Heartworm disease is divided into four stages.

Stage One: Most dogs will appear healthy and happy, and blood tests may even be negative at this phase. A slight cough may be noticeable. However, many pet owners and vets will not realize there might be indicators of more serious issues. One veterinarian did mention seeing signs of premature aging in dogs with gray around the muzzle and forelegs as a possible sign, but this has not been clinically supported.

Stage Two: Symptoms may have progressed enough to be detected during testing at this point. Other indicators may be fatigue after exercise or a lingering dry cough. This cough results from three things: bronchitis that develops as pieces of dying worms become trapped in the lungs; fluid accumulation in the lungs as the heart fails; and the enlarged, damaged heart pressing on the dog’s esophagus. Signs of heart disease will appear on an x-ray. Lab tests may show mild anemia, and urine analysis may indicate some minor protein loss.

Stage Three: At this point, the disease will show up well on x-rays. Dogs will continue to cough and exhibit tiredness from exercise or avoid exercise completely. Dogs may also experience trouble breathing. Weight loss and severe vascular damage on an x-ray will appear. Lab work may show acute anemia and marked urinary protein loss.

Stage Four: Untreated, this stage of heartworm disease is fatal. The impact of the disease will be very noticeable with breathing issues, coughing, and marked exhaustion or hesitancy to exercise. Dogs can appear lethargic for extended periods of time as well. At this point, heartworm disease will have made an impact on other organs such as the lungs, kidney, and liver. A dog’s stomach may look pear-shaped or pot-bellied as his liver enlarges and fluid accumulates in the abdomen.

If a dog experiences the Caval Syndrome, it will collapse in shock, and urine will turn dark brown. Heartworms will be visible by ultrasound and blood work will be abnormal. This dog is dying and the surgical removal of adult heartworms via an incision through the jugular vein is the only way to save it. If a dog survives this medical emergency, any further heartworm infection treatment must wait until the dog is stable enough to return to an earlier disease stage.

Every stage of treatment for heartworm disease carries some risks. The longer heartworm disease goes undetected, the more difficult it is to cure. The best defense, of course, is a good offensive attack through a regularly applied regimen of heartworm preventive medicine.

How can I prevent heartworm disease in my dog?

While manufacturers and ingredients will vary from brand to brand, pet owners must take into consideration what works best for his or her canine companion. Make sure the product label specifically states it is chemically engineered to kill parasitic worms like heartworms, roundworms, and hookworms. It is always good to consult with a veterinarian about the best plan of action, but take into account if a spot-on treatment might be best if your dog is not a fan of chewable tablets. Both spot-on and tablet choice provide protection against heartworm disease. All of these chemical components work with varying capacities to prevent heartworm formation.

Four primary chemical-based treatment options exist for heart disease protection:

• Ivermectin-The primary ingredient in Heartgard® Plus and several other brand names
• Selamection-The principal ingredient in Revolution®
• Moxidectin-The primary ingredient in Advantange Multi®
• Milbeymycin Oxime-This combination is used by Sentinel®, Sentinel® Spectrum and Trifexis®

Not sure what product would work best for you? Consult with your vet to get the best possible product for your canine.

Take Preventive Action

Most vets agree on three things about preventive action, follow the product’s directions and administer it monthly on a regular basis AND maintain heartworm medication year-round. Cases of heartworm have appeared in all 50 states so even dogs that live in the northern United States are still at risk. Heartworm treatments are usually priced in the $30 to $40 range for a 6 to 12 month supply. Most veterinarians will begin heartworm preventive treatment as young as six or seven months in puppies to ensure life-long control, but always work with your vet to find the best course of action for a pet.

While people can take direct action against mosquito attack, pets must depend on humans for assistance when it comes to heartworm. Keep that bull’s eye off your dog when a mosquito is looking for a target and help him live a longer, happier life in return.