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Heartworm Prevention for Dogs: Which Medicine is Best?

What are Heartworms?

Heartworms are parasites that are injected through a mosquito bite into a healthy pet’s bloodstream. The parasites live in the heart or lungs of the diseased pet. Mosquitoes acts as a parasite-transmitting agent between the affected dog and a healthy dog.

Dogs are more prone to developing heartworm infection as compared to ferrets, cats etc. The disease acts like cancer that spreads throughout the body. Heartworms harm the vital organs of a dog and may lead to a dog’s demise.

Dog Heatworm Prevention Image

Mosquitoes suck up the blood of a diseased animal that carries heartworm larvae (microfilaria). Inside the mosquito, the microfiliaria passes through several stages of development before infecting a healthy dog. For completion of the life cycle of a larva, certain conditions must be fulfilled including 57 degree Fahrenheit temperature, through nights and days, for as much as 8 days. The larva stops its development at lower temperatures but restarts the growth as soon as the required temperature is attained. A larva turns completely infectious in about 8-30 days.

Once a larva is transmitted into the blood of a healthy dog, it begins to reproduce till the blood is completely filled with microfilaria to be passed on to another dog and to damage his heart and lungs. This maturity phase takes about six to seven months.

Prevention from Development of Heartworm Disease

Pharmaceutical companies manufacture preventives for heartworm disease that have been approved by FDA. Medicinal preventives have proved to be effective in the prevention of heartworms. The medicines kill heartworm larvae in the blood and interrupt their reproduction cycle.

Some of the preventive measures taken against heartworm disease include:

a. Mosquito-repellents: They are used to prevent mosquito bites (the transmitting agent of microfilaria). Mosquito-repellent produces such odors around the skin of dog that mosquitoes are repelled by the strong odors.

b. Routine Check-ups: The sooner the disease is diagnosed, the better. The larva takes months to mature so there’s still chance for the survival of your dog after getting infected.

Have your dog get a routine check-up every six months, including regular microfiliarial concentration tests or antigen tests. These tests diagnose the presence of immature or mature larva in the blood of your dog. Radiograph (X-ray) will further guide you and the doctor about the degree of infestation of the organs of dog.

Heartworm infestation can be easily treated at earlier stages of the disease and your pup can become healthy and happy again.

3. Seasonal or Monthly Medicinal Preventatives

dog sitting next to owner using laptop

Heartworm medicines are available in a huge range and to select the best preventive medicine, ask your vet to prescribe one. A single dose of the chewable medicine needs to be given after a month.

If you are not living in a highly-endemic area, you can enjoy the health of your pup more fully and for longer without a worry. In winters, you don’t have to worry about heartworms and may stop giving your dog heartworm-prevention drugs.

How are Heartworms Treated?

1. Drug Injections

Heartworms in dogs are commonly treated by injecting a series of drugs into the bloodstream of the dog to kill the adult worms. Regular treatments are done to monitor the intensity of infection or harm caused to the dog’s body.

After initial injections, the recovery period is approximately 4 to 6 weeks or longer. During the recovery phase, regular preventives are given to the affected dog to stop the reproduction of microfiliaria and formation of more baby worms. Few dogs also need to take additional antibiotics or other medicines to fight the heartworms.

After 6 months of the first heartworm treatment, the vet runs all the diagnostic tests again to know if the dog is safely out of the danger zone and that all the heartworms are dead by now. If heartworms are still diagnosed, the treatment procedure begins all over again.

2. Surgical treatment

In extreme cases of heartworm disease spreading in dogs, surgery is done to extract all the worms from a dog’s body.

3. Arsenic-based Medicines

The infected dogs are commonly given an arsenic-based drug. But, the side-effects are inflammation due to excessive killing of heartworms and sometimes respiratory failure. Survival during such treatments is difficult for dogs. That’s why it’s advised to prevent the disease rather than to treat it later on. After all, prevention is better than cure!

The Best Preventive Medicines for Heartworm In Dogs

vet dog

The news that your dog is suffering from heartworms and is at a critical stage of the disease can be devastating to hear for any dog-owner. I understand the feeling! As much as we care for our children, to provide them with the best possible thing in the world, we care for our pets equally. So, it’s imperative that we prevent it instead of treating it when it’s too late.

There are homeopathic or natural treatments for heartworm and for its prevention but there’s no 100% guarantee of being cured. The medicinal treatment has proved its worth and effectiveness during past years.

You may be curious about the active ingredients in the medicine and about the type of medicine that you may use to prevent the disease. There are three types of medicines: chewing pills that are to be given on monthly or seasonal basis, topical medicines that are applied to the skin for prevention if your dog hates pills and injections that are administered in severe conditions or when the two other options have been ruled out by your vet.

Above-mentioned queries have been answered via the list below. So, the top-rated, most-trusted and highly-effective preventive medicines for heartworms are:


Iverhart Plus (ivermectin/pyrantel) has been rated high for the effective protection it provides to dogs against heartworms, roundworms, hookworms and various other parasites. Its flavor is that of a pork’s liver. Dogs love to chew on it due to the juicy taste of pork’s liver. They are safe for use and can be given to your dog once every month without troubling your vet for dosage inquiry.


Heartgard Plus is another popular heartworm preventive. The oral medicine is for dogs and cats. Its individual flavor is that of beef. The chewable tablet is loved by a large population of dogs due to its flavor.

Heartgard Plus is a substitute drug for Iverhart Plus as they both contain similar major ingredients: Pyrantel pamoate and Ivermectin. Additionally, HeartGard Plus can treat roundworms, hookworms and a few other parasites.


Tri-heart is another top of the list medicine commonly prescribed by vets for prevention against heartworms. Tri-heart is more of a generic substitute for HeartGard Plus.

If you cannot afford to pay for the expensive HeartGard Plus the whole year round, you may use Tri-Heart that provides an equal amount of protection against heartworms.

As found in HeartGard and Iverhart, the two major ingredients of the chewable pill are: Pyrantel pamoate and Ivermectin. Additionally, Tri-heart protects your dog against hookworms, whipworms, and roundworms.

The effectiveness of these three drugs hasn’t shown a significant difference. The surety of protection against heartworms is 100% as they are highly reviewed preventive drugs with positive responses. If you want to buy expensive, you may go for it.

The best part of Tri-Heart is that it’s safe and effective for all dog breeds. It is devoid of ivermectin which has harmful side-effects in some breeds-in case of high dosage. ​It leads to maximal levels of dog health.


Advantage Multi is a topical medicine that is also used for heartworm treatment commonly. The dogs who do not agree to take the chewable pills are protected against heartworms using this medicine. There is difficulty in applying but for safety reasons, it’s something necessary.


ProHeart is a popular injectable that is used by those who cannot give their dog a monthly treatment against heartworms. The major ingredient of ProHeart is Moxidectin, which treats heartworms as well as hookworms. Injectables are never the best choice because of the possibility of side-effects like allergic reactions.

In old dogs, injectables like ProHeart are a complete NO option. In that case, ask for the advice of your veterinary for the treatment of heartworms.

Schedules are Important: Whether it’s a monthly dosage or regular visits to the vet for treatment, you should follow the schedules strictly to avoid more problems for your pet and for yourself.

Are There Any Risks of Medicinal Prevention of Heartworms?

Drugs for pets never come without side-effects or some risks, specifically, in terms of the dosage and regularity of medicines. The milder side-effects that have been identified in dogs include loss of appetite, nausea, and vomiting. However, some adverse reactions have also been administered in dogs who were taking heartworm medicines. They include tremors, seizures, shocks, lung failure, and coma. Severe conditions may cause death. But, such reactions are mostly associated with over-dose of heartworm medicines.

Despite the list of top-rated and highly effective drugs list for heartworm, I would suggest taking a prescription from your doctor. And I am sure that you will end up getting one of the above-mentioned medicines. Regularity in dosage, usage of one kind of drug and of an appropriate dose of medicine are some precautions that should be considered.

About the Author:

James Shore is a part-time dog-trainer and dog behavior consultant. He is a professional freelancer with years of experience in dog training. He is interested in finding out fun ways to handle dog behaviors, specifically, Labradors to help dog-owners enjoy their companions at all times. His pet-passion led him to develop https://www.labradortraininghq.com/ to help people.

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