Heartworms are parasites that dogs, and even cats, can catch from being bitten by a carrier mosquito. The worm is injected into the pet by the mosquito, and it matures into an adult worm living inside the chambers of the heart. After the worm matures in the heart and reproduces, the immature worms are picked up by another mosquito. The cycle is continued in the next animal the mosquito bites. Over time these worms can cause severe damage to the heart which can lead to serious disease and even death. Cat's don't seem to be as susceptible to infection. Infection in cats also tends to be less severe than that in dogs. For this reason the degree of disease these worms cause in cats tends to be lower as well.

In some parts of the country the risk of catching heartworms is large. In Idaho the risk of catching the parasite is relatively small. I personally have only seen a few cases in Boise over the last 11 years. These were all in dogs that had moved from other parts of the country, where exposure to the worms is more likely, bringing the infection with them. Last Spring there was an article in the Statesman about heartworms which told of cases discovered in Eastern Idaho. These dogs apparently had never been out of Idaho and therefore caught the parasite in Idaho. The risk in Idaho is still low, but as the number of animals moving into the area which might be already infected grows, so does the risk to our pets.

When a dog has heartworms they can be treated, and the treatment is best done early in the infection. Treatment for cats with heartworms is too risky because of the toxicity of the treatment medication. The treatment for dogs is involved, and not without complications. The ideal course of management for heartworms is to prevent infection, both in cats and dogs. Prevention is simple and involves giving a medicated treat once a month during the warmer months of the year when a mosquito bite is possible. The medication is safe and effective as long as there are no heartworms present before preventative medication is given. If heartworms are present the medication might kill the worms in the heart too rapidly and lead to serious complications. For this reason each animal starting on prevention should be given a blood test to ensure they don't already have an infection. One other benefit of heartworm prevention is that the treat also contains additional worm medication to control roundworm and hookworm infections which are more prevalent in Idaho.

The position I have always taken on heartworms has been to blood test and give the prevention medication to pets that are traveling outside the area. I have also tried to test all pets I see moving into the area to make sure they haven't brought the parasite with them. I have not, however, actively encouraged clients with pets that don't leave Boise to use preventative medication. I still think that the risk of catching the parasite in Boise is very small, but as more and more infected dogs move into the area the risk is going to go up.

The question we each have to ask is should we wait to start a prevention program until the risk goes up or should we be proactive and start one now? The problem with waiting is that there will be a definite lag of months to years between the time the increased risk is here and when it is identified through testing. This lag period will be greater if no prevention program is in place because relatively few dogs will be tested, and in that time some pets will have already contracted the parasite needlessly. The problem with starting a preventative program now is the risk might be low enough at this time that it isn't necessary quite yet.

My purpose in writing this letter is to inform you of all the facts so you can make the best decision for you and your pet. If you have any further questions I would be happy to talk with you and try to answer them. If you wish to have a heartworm test and preventative program started we will be happy to start one for you at any time.

Thank you,

Walter Wethered, DVM