Heartworm Prevention and Medicine for Dogs

Treating a heartworm infestation is difficult and dangerous. It is far easier and more effective to prevent the problem in the first place. In theory, the best way to prevent heartworms is to keep your dog from being bitten by a mosquito. Unfortunately, preventing mosquito bites can never be 100 percent effective. Dogs can be reasonably protected if they remain indoors in the late afternoon and evening, when mosquitoes are feeding.

Areas of most frequent heartworm infestation are along coastal regions, where swamps or other brackish water provide ideal conditions for mosquitoes to breed. Since mosquitoes have a flight range of one quarter mile, spraying around the yard and kennel and removing standing water can be partially effective, but they will never eliminate the threat.

If you live or travel with your dog in an area where heartworm is endemic, your dog should be on a heartworm prevention program. Ask your veterinarian about local prevalence and follow their recommendations for prevention. Most dogs should be on a heartworm preventive program.

A prevention program should be started at 6 to 8 weeks of age in endemic areas, or as soon thereafter as climate conditions dictate. In the Deep South, where mosquitoes are a year-round problem, dogs should be kept on preventive drugs all year long. In areas where it is not necessary to administer the drug year round, start one month before the mosquito season and continue one month beyond the first frost (generally from May or June to November or December). Heartworm prevention is important for the dog's whole life. Some owners may elect to give heartworm preventives year round for zoonotic parasite protection and to reduce the risk of breakthrough heartworm disease in case they miss a monthly dose. All dogs 7 months and older should have an antigen test for heartworms before starting a prevention program. If the test is positive, a microfilaria concentration test should be performed. The antigen test should be repeated annually or as frequently as your veterinarian recommends-even if the dog is on a heartworm prevention program. Many heartworm preventives can cause illness if given to a dog with circulating microfilaria.

There are a number of drugs currently in use as heartworm preventives. They include ivermectin (Ivomec, Heartgard), milbemycin oxime (Interceptor), and selamectin (Revolution).

Heartgard is an effective preventive that is given once a month. This drug acts on the L4 larvae. It has the advantage that dogs do not have to be heartworm-free to initiate therapy; dogs infected for as long as two months before treatment will not develop heartworms. If a monthly dose is missed, restart the drug and obtain a heartworm antigen test seven months later. Heartgard is marketed in chewable tablets of different sizes, depending on the weight of the dog. The recommended dose is generally considered to be safe to use on Collies and other herding breeds. However, with safer alternatives available, most owners avoid this for the breeds with the genetic defect that causes sensitivity to ivermectin.

Heartgard Plus is a popular chewable tablet that combines ivermectin with pyrantel pamoate. This combination prevents heartworms and also controls roundworms and hookworms.

Interceptor (milbemycin oxime) is another orally administered once-a-month heartworm preventive that also acts on the L4 larvae. Like Heartgard, this drug is marketed in different dosages based on the weight of the dog. Interceptor also controls hookworms, roundworms, and whipworms. This drug is safer to use on Collies and Collie crosses.

Selamectin (Revolution) isa once-a-month liquid heartworm preventive applied to the skin of the dog's neck between the shoulder blades. It is available from your veterinarian in premeasured doses based on the dog's size and age. A principal advantage of selamectin is that it also controls adult fleas and prevents flea eggs from hatching for one month. In addition, it treats ear mites and the mites that cause scabies.

Ivermectin Sensitivity

Some breeds show an increased susceptibility to the potential toxicity of ivermectin and similar drugs. In these dogs, there is a defect of the gene MDR-1. This is a multi-drug transporter gene that influences the movement of drugs across the blood brain barrier. Dogs who are homozygous for this autosomal recessive trait will have severe, potentially fatal reactions to some drugs, including ivermectin. Dogs who are heterozygous may be able to safely take the medications, but may pass on the defective trait to their offspring.

A genetic test for this defect is available through Washington State University's Veterinary Laboratory of Clinical Pharmacology. A cheek swab can be sent to determine if your dog has the gene. This testing is recommended for Collies, Border Collies, Shetland Sheepdogs, Australian Shepherds, Old English Sheepdogs, and longhaired Whippets. In breeds such as the Collie, more than 70 percent of the dogs tested are either homozygous for this defect or carry it.

This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.

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