Canine Heartworm Prevention
Canine heartworm prevention is important and includes the use of preventive heartworm medicine for dogs such as Interceptor. This page looks at some common preventive heartworm medicine for dogs.

Heartworms in dogs can cause serious health damage, such as congestive heart failure and liver problems to heavily infested dogs. So, it is important that a heartworm prevention program is in place especially for dogs living in areas where mosquitoes are aplenty.

Successful treating heartworms in dogs depends on many factors. For example, the number of heartworms, their location, the age and the condition of the dog, and whether there are complications such as congestive heart failure or liver disease.

The objective of treating heartworms in dogs is to kill all the adult heartworms, as well as the larvae (microfilaria), and then to initiate preventive measures.

However, killing adult heartworms involves the use of a very strong heartworm medicine (Immiticide®) that contains arsenic. The medicine can cause (though not frequently) such toxic reactions in dogs as appetite loss, vomiting, diarrhea, jaundice, kidney failure, and sometimes even death.

In addition, heartworm treatment can cause thromboembolism in dogs, a complication caused by the death of adult worms. The dead worms are carried by the bloodstream into the pulmonary circulation, where they obstruct the blood flow and cause the vessels to clot.

In view of the potential danger (and the costs involved) in treating heartworms in dogs, it is much better and safer to prevent infestations in the first place.

Canine Heartworm Prevention – Preventive Medicines

Heartworm preventive measures should be taken for the whole life of a dog. But before starting on a canine heartworm prevention program, be sure to have an antigen test done on your dog if he is 7 months or older.

This test should also be repeated every year even if your dog is on a heartworm prevention program.

The reason for the test is that, many heartworm preventive medicines can cause rather serious illness if given to a dog with circulating heartworm larvae (microfilaria).

There are several types of heartworm medicine for dogs. Some heartworm medications are administered monthly for 7 to 8 months of a year.

Many veterinarians as well as the Amercian Heartworm Society is now recommending year-round prevention, even in seasonal areas. Discuss with your vet the most suitable preventive program for your dog.

Below is a list of the most common medicines for canine heartworm prevention:


This heartworm medicine contains ivermectin which acts on the L4 heartworm larvae. Even dogs already infected with heartworms for as long as two months before treatment will be blocked from developing heartworm disease. Heartgard® is to be given orally once a month.

Heartgard® Plus

This chewable tablet contains ivermectin and pyrantel pamoate, and can prevent not only heartworms but can also control roundworms and hookworms.


This heartworm medicine for dogs contains milbemycin oxime which also acts on the L4 heartworm larvae. It also controls hookworms, roundworms, and whipworms. It is to be given orally once a month.


This heartworm medicine contains selamectin and is a liquid heartworm preventive applied once a month to the skin of the dog’s neck (between the shoulder blades). Besides preventing heartworms, it also controls fleas, prevents flea eggs from hatching for a month, treats ear mites and sarcoptic mites.

ProHeart 6®

This is an injectable form of heartworm medicine for dogs which contains moxidectin. One injection is said to provide protection against heartworm for 6 months.

However, some veterinarians are concerned about the potential side effects that can be caused by ProHeart 6. Some potential side effects include facial swelling, diarrhea and vomiting, fever, rash, weakness and lethargy.

In more severe cases, seizures and even death may occur. The rate of these side effects seems rather high compared to other heartworm preventives.

Ivermectin and Certain Dog Breeds

Because of a defect of a gene (gene MDR-1), some dog breeds are susceptible to the potential toxicity of the drug ivermectin (Heartgard® and Heartgard® Plus).

These dog breeds include most herding breeds such as the Collies, Border Collies, Shetland Sheepdogs, Australian Shepherds, Old English Sheepdogs, and longhaired Whippets.

Although the recommended dose of ivermectin for heartworm prevention is generally safe to use even on these breeds, for peace of mind it is advisable to use other alternatives that are available for canine heartworm prevention among these dog breeds.

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