Heartworms: Dangerous Parasites
Animal lovers are special people. We have unique hearts comprised of something more than the traditional four chambers and anatomically correct assortment of blood vessels. We also have an intangible place in our hearts that serves no physical function but is reserved entirely for the pets we love. Our pets have special hearts, too. They love us in return in their own unique way.
But besides being the symbol of love, hearts are essential for life. So, for the love of our pets, let’s focus on the physical nature of their hearts and how we can keep these loving hearts (and the rest of their bodies) healthy. Let’s consider a specific health threat—heartworm disease.
THE HEARTWORM LIFE CYCLE
The heartworm is a filarid or “threadlike” worm that needs a carrier or “vector” to infect an animal. The heartworm’s carrier is the pesky mosquito. When a mosquito takes a blood meal from an infected dog, it can ingest tiny, immature forms of the heartworm called microfilariae. Microfilariae are the first stage of the heartworm life cycle. These tiny larvae enter the mosquito’s body and grow as they molt from one stage to the next. After a sufficient period of development, the immature heartworms are ready to move to a new residence. With its incubation role complete, the mosquito can carry the larvae to a new home—an unprotected dog or cat—and literally drop them off.
The mosquito deposits the larvae onto the surface of the pet’s skin. Some of the larvae die here, but many make their way into the animal’s body through the mosquito’s bite wound and begin a long journey toward their final destination near the heart. Migrating through the subcutaneous tissues of the pet, the larvae continue to mature. The larvae then penetrate a thin-walled blood vessel (a vein) that serves as an expressway toward the animal’s heart.
The heartworms eventually grow into mature adult heartworms. Contrary to their name, heartworms don’t actually reside in the heart, but rather set up residence in the pulmonary artery, a major blood vessel near the heart. Once in their new home, the heartworms begin to wreak havoc on their unsuspecting host—an unprotected dog or cat.
Dogs and cats react very differently to the presence of heartworms. Our easy-going, willing-to-do-anything-to-please dogs accept their heartworm guests, providing them with a comfortable place to live and raise a family. Male and female adult worms breed and have children (microfilariae) that venture away from their comfy lodging in the pulmonary artery, enter the general circulation, and hitch a ride with the next hungry mosquito, thereby completing their life cycle.
Cats, on the other hand, are not hospitable to heartworms. In fact, cats often eliminate their guests before they reach adulthood by activating their own unique immune systems. The feline immune response reduces the number of worms that reside in the pulmonary arteries. Even with fewer worms than their canine counterparts, however, cats with heartworm infection can become quite ill.
SIGNS OF INFECTION
How does a pet owner know when his dog or cat has heartworms? Does the pet look different? No, she often looks just fine. We can watch for physical signs, but the infection may be well under way before the dog or cat becomes ill. It is far better to have your veterinarian perform the appropriate tests to determine your pet’s heartworm status before the damage is done. If your pet is exhibiting any of the symptoms described below, take her to your veterinarian as soon as possible.
Heartworm-positive dogs may become lethargic or exercise intolerant or have difficulty breathing. Dogs can develop cardiac problems and retain fluid in their lungs and abdomen due to heart failure. Live adult worms usually hold on tightly to their home in the pulmonary artery, but when they die, the worms can be swept into the bloodstream and eventually lodge in tiny blood vessels in the lungs. These dead worms or worm fragments essentially behave like blood clots, called pulmonary emboli, and their presence in the blood vessels of the dog’s lungs can be fatal. Also, if there are many adult worms in the dog, they can band together and block even larger vessels, resulting in a condition called caval syndrome, which may require surgical removal of the worms.
Cats, unlike dogs, rarely have heart problems as a result of heartworms, but they can suffer severe, even deadly, lung problems. Infected cats may cough or have difficulty breathing. Cats may also be lethargic or, in rare instances, suffer neurologic problems. Other cats exhibit gastrointestinal signs such as vomiting. If your cat experiences vomiting unrelated to eating, heartworms may be the culprit.
Also unlike dogs, cats are severely affected not only by the presence of adult heartworms but by the presence of immature heartworms as well. Although cats eliminate many of the immature larvae before they reach adulthood, the mere presence of these larvae poses a significant health threat to cats. The respiratory syndrome associated with immature heartworms is called HARD, or heartworm-associated respiratory disease, and can be debilitating or fatal to your cat.
ROUTINE TESTING
Besides physical signs, how does a pet owner know if his pet has heartworms? Should we wait until she becomes ill? Absolutely not! Early diagnosis of heartworms means early intervention— and early intervention means our pets have a greater chance of surviving heartworm infection.
To prevent illnesses, we should have our dogs and cats examined on a regular basis. Dogs and cats should be tested for heartworms at least once a year. Because cats are more difficult to diagnose, they may additionally require radiographs or cardiac ultrasounds to determine their heartworm status.
HEARTWORM PREVENTION
The devastating effects of heartworm disease are something we pet lovers never want our dogs and cats to endure—and they don’t have to. Heartworm disease is preventable. There are a variety of oral and topical medications that interfere with the development of immature heartworm larvae into adult worms. The interference occurs early enough in the heartworm life cycle to prevent HARD in cats as well.
Many medications are available, so it should be easy for you to find the one that is best for both you and your pet. Oral and topical monthly medications work well. Discuss the available products with your veterinarian, and you’ll learn that most of them prevent more than heartworms. Many also prevent intestinal parasites, fleas, and even ear mites.
The prevention of intestinal parasites is important—not only for our pets, but for us, too. Some of these intestinal parasites are zoonotic, which means they can be transferred from animals to humans. So for the love of our pets and the love of our human family members, we should keep our dogs and cats on monthly heartworm medication.
The most important thing regarding heartworm prevention is not which preventive we choose, but how we administer it. Monthly heartworm medications need to be given monthly. That means no skipping or being late. If you have an unfailing memory, that’s great, but if you’re like the rest of us, you may need a little monthly prompting. Try putting a sticker on your calendar or circling the due date for the preventive. Or you could give the medication when you pay the rent or mortgage, or when you get your paycheck. If you live electronically, program the reminder into your device. Your veterinarian may even offer personal email reminders.
We live in a very busy world, but even so, there is no reason to miss something as important as heartworm prevention.
Yes, we are true pet lovers. And we want our pets around to love us in return for a very long time. So if your heart holds a special place for dogs and cats, remember to protect their hearts (and the rest of their bodies) from the devastating effects of heartworm disease.
Heartworm Disease Is Preventable
The bad news is that heartworm infection can be devastating to dogs and cats. The good news is that heartworm infection is preventable. Compared with the risk and cost associated with treating heartworm-infected dogs (and the fact that no approved treatment exists for cats), heartworm prevention is safe, easy, and inexpensive. By giving prescribed medications monthly, we can protect our pets so they do not have to suffer the consequences of heartworm infection. Ask your veterinarian which preventive is best for your pet.
Cats Need Protection, Too
Most people know that dogs are susceptible to heartworm infection, but they don’t realize that cats can become infected, too. In fact, feline heartworm infection can be debilitating or even fatal, and there is no approved treatment for adult heartworms in cats. For this reason, the American Heartworm Society and the Companion Animal Parasite Council advise pet owners to give their dogs and cats monthly heartworm medication year-round.