Heartworm Disease
The Good News, and the Bad News.
The days are getting longer, the air is getting warmer, and before we know it the mosquitoes will be out! That means it’s time to talk about heartworm disease. Should we start with the good news, or the bad news? Let’s get the bad news out of the way.
Heartworm disease is spread by mosquitoes. Mosquitoes consume the blood of an infected dog, pick up the immature heartworm (called microfilaria) and then carry it to a new host and infect the new host. The microfilaria mature into adult heartworms, which reside in the large vessels around the heart of the infected dog. Infected dogs do not show any signs until the worm burden is big enough to cause secondary complications, such as heart failure. Dogs can carry an infection without showing any signs for years before the heart and lungs stop being able to compensate, and the dog becomes sick. Signs of heart failure include coughing, shortness of breath, respiratory distress, exercise intolerance etc. Once enough damage to the heart and lungs is present, the damage is often irreversible. Clinically ill dogs that survive treatment can have a persistent cough or congestive heart failure for the rest of their lives. Those who have heart failure will have a shortened life expectancy.
Hang in there with me, we have to go through a few more items of bad news. Heartworm disease was once primarily found in the southeastern united states where the climate is warmer and never gets a really good freeze. However, heartworm has now been diagnosed in all 50 states. The endemic area keeps moving further north and west due to climate change and the transportation of dogs from southern states to other parts of the country for adoption.
Diagnosing heartworm disease is complicated. Since most dogs do not show clinical signs, we rely on screening tests to catch dogs who are infected. The test requires a small amount of blood. It takes approximately 6 months from initial infection for a heartworm test to be positive. This means that a dog is positive for 6 months before we have the ability to diagnose the disease! Using preventative in a positive dog does not treat the disease. In fact it can perpetuate resistance, and it can result in false negative heartworm tests. In a dog with a high enough microfilaria count, administering heart worm preventative can result in a reaction.
And finally, let’s touch on treatment. Treatment involves a 365 day treatment protocol. Day 0 starts a 30 day course of oral medication. Intramuscular injections are given on day 60, 90, and 91. Strict exercise restriction has to happen from day 60 until day 120, with gradual return to normal activity over the next 4 weeks. Day 365 the patient is tested for heartworm, and if positive the dog receives another intramuscular injection. Treatment is involved, and costly. Dogs that are not yet showing clinical or radiographic signs of heartworm infection generally handle treatment well. Dogs who are already showing complications of the disease have a much worse prognosis and a significantly longer recovery period.
BUT, we have saved the good news for last. Heartworm prevention is very successful, and heartworm prevention options have come so far! There are many excellent options that fit each patient’s specific needs. Heartworm preventatives are all only under the direction of a veterinarian because the disease is so difficult to diagnose and to treat. The preventatives themselves, when used according to direction, are some of the safest medications used in veterinary medicine. These preventatives work by killing the immature heartworms, the microfilaria, before it can mature. When given consistently and year round, these medications are very successful at preventing infection. Most drug companies offer a guarantee that will cover the cost of treatment in the event of a breakthrough infection as long as the preventative has been documented to have been purchased from a veterinarian, given consistently, with a documented yearly heartworm test.
You might be wondering where this information comes from! The American Heartworm Society routinely puts out guidelines. The most recent revision was done in 2024. Based on the recommendations from the American Heartworm Society, Highland Pet Hospital recommends a yearly heartworm test and year round prevention with an approved heartworm preventative. Many different preventative options exist, and you can talk with your veterinary care team about which option is best for your dog.
At first glance the bad news might seem to outweigh the good, but the good news is far more simple! Test your dog yearly and keep your dog on a routine heartworm preventative year round, and you are offering the best protection you can. Talk with your veterinary care team if you have questions, we are always happy to answer them!
