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Feline heartworm disease: an emerging problem
An ounce of prevention is worth a pound of cure for your kitty
I thought it would never happen but the weather is warming into spring. That means my cats will be itching to go outside. Previously I had authored an article on canine heartworm disease.
More recently, there has been more of a focus on feline heartworm disease in veterinary medicine. Dirofilaria immitis, or heartworm, is a blood parasite transmitted by mosquitoes. This parasite lives in the heart and vessels and cannot be found on a fecal analysis. A blood test is required for diagnosis. In dogs and cats the parasite is injected into the capillaries of the skin when the mosquito feeds. However, the maturation of the parasite is very different in each of these species.
Heartworm always reaches maturity in dogs once it migrates to the heart. If undiagnosed and untreated the worms proliferate in the right ventricle (the lower chamber of the heart), pulmonary artery and the lung tissue itself. Not only does this pathology lead to heart failure, emboli and ultimately death, it also forces the infected dog to become a breeding reservoir to infect other dogs. Fortunately, heartworm testing has been considered part of the annual exam for over a decade and safe, effective heartworm preventatives are easy to obtain and relatively inexpensive.
The parasite takes a much different progression in cats. In fact, the parasite rarely reaches maturity. After infection the larval stage, or microfilariae, usually only progress to young adulthood before the cat's immune system kills it. "Great," you say, "my cat will kill it off before adulthood therefore eliminating the need for testing, treatment or preventative."
Unfortunately, this is not the case. Once the young adult dies (usually in the smaller arteries leading to the lungs) it triggers an inflammatory response that permanently changes the vessel and surrounding lung tissue. This leads to a debilitating asthma type syndrome and, in some cases, sudden death. The lung's main job is to exchange gases — to take in oxygen and release carbon dioxide. In order to do that the small arteries and veins must run right next to the alveoli (the portion of the lung where actual gas exchange takes place). After the worm dies it leads to what is called an infiltrate of white blood cells into the lining of the blood vessels. This infiltrate not only damages the blood vessel, it also leads to inflammation of the airway constricting it and leading to secretions. Many veterinary pathologists feel that feline heartworm is not a heartworm at all, but rather a lungworm.
So how do we treat this disease? The saying, "an ounce of prevention is worth a pound of cure" plays a crucial role here. Once the worm has died naturally and the inflammatory response has started, the best we veterinarians can do is manage the situation. Anti inflammatories in the cortisone family and bronchodilators (medications to relax and dilate the airways are used to alleviate the cat's symptoms but a true cure is not possible. Similar to a person who has smoked for years and quits later in life, the damage is already done. Instead, there is a push to place cats on heartworm preventative (especially those cats that go outside).
Luckily, the same heartworm preventatives that are used in dogs on a monthly basis can be used in cats. These medications are just as safe and effective and not any more costly. I have four cats. Three of them would not notice if I placed a small pill (especially beef flavored) in their food once monthly. However, the fourth would be an exercise in frustration. Luckily, there are topical medications that prevent heartworm disease in cats. These once monthly topical medications not only prevent heartworm disease, but also prevent intestinal worms, ear mites, fleas and ticks and other parasites.
If your cat does go outside make sure to not only speak to your veterinarian about appropriate vaccinations, but also appropriate heartworm preventative.
Dr. Kearns is a veterinarian with a special interest in emergency and critical care. He has been in practice for eight years. Dr. Kearns is pictured here with his son Matthew and his cat, The One Eyed Guy.