by Christopher Bern, DVM
Here’s a rather scary set of data to ponder. Heartworm infection has been diagnosed in all 50 states and is considered at least regionally endemic in every U.S. state and protectorate except Alaska. Yet only about half of U.S. dogs are regularly on heartworm prevention and this number has been declining over the last several years. We don’t have good information on the number of cats on prevention, but it’s certainly much lower. These numbers must change!
Why do we veterinarians talk so much about this? And why does it worry us to see so few pets on prevention? It’s because of how extremely serious heartworm disease can be and how incredibly preventable it is.
Heartworms are nasty little creatures. These parasites are carried by mosquitos, meaning that your pet doesn’t have to have any contact at all with other dogs and cats in order to become infected. Think your indoor cat or small dog isn’t at risk? Think again. Most dogs go outside at least a little bit and this puts them at risk. Even if a pet is 100% indoors and never goes outside (well, they go to the vet, don’t they? and isn’t that outside?), we have all seen mosquitos in our homes. In a study several years ago, 60% of cats who were diagnosed with heartworm disease were considered “indoor only” by their owners. All it takes is one mosquito with heartworm larva to bite your pet, and you suddenly have a serious situation on your hands.
These worms are almost a foot long and about the size of a piece of spaghetti. They live in the right ventricle of the heart and the vessels leading to the lungs. In the early stages of infection you won’t see any symptoms at all! Signs of more advanced heartworm disease normally start with coughing and exercise intolerance (getting tired after activity more quickly than previous times) and if untreated will progress to fluid accumulation in the lungs and abdomen. Infected dogs will die slowly of congestive heart failure and treatment is very risky in the later stages of disease. In early stages treatment is possible in dogs, but still carries some significant risks and can be extremely expensive.
Heartworm infection in cats is possible though less likely than in dogs. Unfortunately, the disease is harder to detect in cats, so routine blood testing may not always catch the disease early enough. One of the more common signs of heartworms in cats is sudden death – obviously something we want to avoid. There is no approved treatment in cats, so if a cat does develop infection it is much harder to deal with. Basically, cats are more resistant to heartworms than dogs, but if they do get it the situation is normally far worse.
Thankfully, prevention is readily available, easy to administer and inexpensive. Options for prevention include oral and topical medications, and in dogs an injection called ProHeart 6, which lasts for six months. The American Heartworm Society and most veterinarians recommend year-round prevention because of often unpredictable weather patterns and the serious nature of the disease, regardless of where you live. Most pets can be fully protected for around $100 or less per year. This works out to less than $10 per month! The majority of people I know spend more than this on lattes and fast food. Still think that’s too expensive? For the cost of treating heartworm disease one time you could have purchased enough prevention for around 10 years of your pet’s life.
Another current recommendation of the American Heartworm Society is annual testing for heartworm disease. There are two main reasons for this line of thinking. First, as I mentioned earlier, there are no symptoms in the early stages of disease. Most of the pets I have personally diagnosed have been in this category and we detected the infection on a routine test. The earlier we can detect disease, the better chance we have of treating it. The second reason is that heartworm positive dogs can have a reaction to certain preventives. If we don’t know their infection status, we may recommend a preventive that can actually be harmful.
“But my pet is always on heartworm prevention! Why do I still need to have her tested every year?” Several studies have shown that no matter how good someone thinks they are at giving prevention, around 80% of owners accidentally miss at least one dose throughout the year. There have also been some recent concerns about regional resistance of heartworms to common preventives, though one in-depth study showed that even in these cases there was considerable inconsistency in giving prevention, and the majority of positive dogs were off prevention during a higher risk period.
In today’s world there is no reason why pets should not be consistently on year-round heartworm prevention. This is one of the most important preventive measures you as a pet owner can take.