Avoiding Canine Heart Worm

Q: My 3-year-old Lab-border collie mix has been on Heartgard for 2 1/2 years. The manufacturer states that when administered in large doses, Ivermectin can cause adverse reactions, particularly in the collie breed.

To what kind of doses are they referring? Is the recommended dose over a long period of time harmful? If so, is the chance of contracting heart worm in Southern California substantial enough to risk any associated side effects? Does the Heartgard have to be administered continuously or is it possible to only administer it for part of the year during the mosquito “season” or just in such situations as vacations in susceptible areas?

Thank you for your help.

N.J. Neboc, Tustin

A: Heartgard is a canine heart worm preventive medication produced by the Merck Co. using the drug Ivermectin. It is a very effective drug and has the great advantage of once-a-month dosing, which is very convenient for pet owners.

There are three tablets, each of increasing strength, which are given to a dog within a particular weight range. The dosage is six micrograms per kilogram and this is used as the upper dosage of each of the three weight ranges listed by the manufacturer. Trials by the manufacturer indicate that toxic reactions occur when the dosage equals or exceeds 200 micrograms per kilogram.

The Merck Co. recommends giving Heartgard within one month of the beginning of the local mosquito season and continuing for one month afterwards. In most areas of the United States, this would be about six to nine months. Fortunately, in Southern California, our more arid climate and limited rainfall reduce the mosquito populations and tend to greatly reduce the chances for heart worms. Most recent studies or surveys indicate that heart worms are currently not a problem here although the potential for the future is always there.

The manufacturer’s literature does caution the use of Ivermectin in certain dogs, particularly the collie breed, because the drug’s concentration in central nervous tissue is apparently higher. These reactions have been reported at doses many times greater than the recommended low doses.

They advise close observation of collies for at least eight hours after giving the medication, watching for signs such as weakness, depression, ataxia, excessive salivation, paresis or coma. If such signs start to appear, they recommend immediate treatment by your veterinarian.

Since your dog has been on the medication for over two years with no adverse effects, you should have no need to worry. You should discuss this drug with your veterinarian who may suggest reducing the number of months that your pet is to take the medication or may even recommend stopping the treatment while your pet is living in the local area. If you do travel to heart worm areas, you will want to start medication about one month before and one month after your trip. Your veterinarian may be able to supply you with a package insert that explains the dosages, the precautions and any recommendations.