Sep 05 2012

Nasty Little Worms and Your Best Friend’s Heart

I’m sure you hear about heartworm disease every year at your pet’s annual veterinary appointment; and hopefully you have your dog and cat on monthly heartworm prevention. But how much do you really know about heartworm disease? Do you know how an animal gets heartworm disease? Do you know the signs and symptoms of the disease? Do you know what to do if your animal has heartworm disease? Continue reading, and you will learn the answers…

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms), specifically Dirofilaria immitis. Dogs and cats of any age or breed are susceptible to infection [1].

The disease is spread by mosquitoes that become infected with microfilariae while taking a blood meal from an infected animal. When the infected mosquito bites another dog, cat, or susceptible animal, the larvae are deposited on the skin and actively migrate into the new host. For about 2 months the larvae migrate through the connective tissue, under the skin, and then pass into the animal’s blood stream. There, they are quickly transported to the arteries of the lungs. It takes a total of approximately six months for the infective larvae to mature into adult worms that begin producing offspring, microfilariae. Adult heartworms can live for five to seven years in the dog [2].

The onset and severity of disease in the dog is mainly a reflection of the number of adult heartworms present, the age of the infection and the level of activity of the dog. Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes. The heart may enlarge and become weakened due to an increased workload and congestive heart failure may occur. A very active dog (e.g., working dog) is more likely to develop severe disease with a relatively small number of heartworms than an inactive one (e.g., a lap dog or couch potato). Occasionally, a dog with a large number of heartworms may not only have worms in the heart, but also in the caudal vena cava (large primary vein of the lower body) between the liver and the heart. If the heartworms are not removed surgically, this syndrome causes sudden collapse and death within two to three days [3].

In the cat, the larvae molt as well, but fewer worms survive to adulthood. While dogs may suffer from severe heart and lung damage from heartworm infection, cats typically exhibit minimal changes in the heart. The cat’s primary response to the presence of heartworms occurs in the lungs. Cats typically have fewer and smaller worms than dogs and the life span of worms is shorter, approximately two to three years, compared to five to seven years in dogs. However, heartworms do not need to develop into adults to cause significant pulmonary damage in cats, and consequences can still be very serious when cats are infected by mosquitoes carrying heartworm larvae (acute pulmonary inflammation response and lung injury). This initial phase is often misdiagnosed as asthma or allergic bronchitis but in actuality is part of a syndrome now known as Heartworm Associated Respiratory Disease (HARD) [4].

Although at the Valley Cottage Animal Hospital we believe that prevention is key, many owners unknowingly adopt dogs that are already infected with the disease; so what happens then? The only treatment that is approved by the FDA is the use of melarsomine, commonly known as Immiticide. This requires 3 injections over a course of 30 days, followed by strict rest for a total of 60 days. This is proven to kill 98% of the worms. In the past, many have tried using what is called a “slow-kill” method, by using long-term heartworm preventative, or macrocyclic lactones. Slow-kill methods using continuous monthly administration of prophylactic doses of any macrocyclic lactone are NOT RECOMMENDED. While effective in reducing the life span of juvenile and adult heartworms, it appears that the older the worms are when first exposed to macrocyclic lactones, the longer it takes for them to die. It may take more than two years of continuous administration before adult heartworms are eliminated completely, and rigid exercise restriction would still be required for the entire treatment period! Throughout this period, the infection would persist and would continue to worsen. Another potential concern in using macrocyclic lactones long-term in heartworm positive dogs as stand-alone therapy is the potential for resistance in sub-populations of heartworms [5].

Canine heartworm infection is widely distributed throughout the United States. Heartworm infection has been found in dogs native to all 50 states. All dogs, regardless of their age, sex, or habitat, are susceptible to heartworm infection. At the Valley Cottage Animal Hospital we have seen an increasing rate of heart-worm positive dogs within the last year; in fact, we have treated 6 already just this year! This is why we cannot stress enough the importance of monthly heartworm preventatives and yearly checkups. Clinical signs that owners may notice include, but not limited to, cough, exercise intolerance, dyspnea (difficulty breathing), syncope (temporary loss of consciousness due to poor blood flow to the brain), and ascites (fluid accumulation in the abdominal cavity). If you note any of these signs, it is important to call you veterinarian.

If you have more questions about heartworm disease, please feel free to contact us at the Valley Cottage Animal Hospital anytime, or call your regular veterinarian. You can also check out the American Heartworm Society’s website at

iStock_000013832378XSmall, Dog Covered in Mosquitos
Can you see the mosquitoes crawling on this poor pup? 


Originally from Ohio, Dr. Ritchie moved to New York City in 2000. While completing her pre-veterinary requirements at Columbia University and Hunter College, she worked at multiple veterinary practices as a technician to hone her skills. She then moved onto Ross University School of Veterinary Medicine, completing her clinical training at Texas A&M College of Veterinary Medicine and Biomedical Sciences in August 2010. While still in veterinary school, she co-authored an article entitled “The Anatomy and Physiology of the Avian Endocrine System” for the publication Veterinary Clinics of North America: Exotic Animal Practice.
Dr. Ritchie has a strong interest in internal medicine, especially endocrinology, soft tissue surgery, emergency & critical care, and avian/exotic medicine. As a former education director at a museum she is also well-qualified for the client education that is so important to her since, in her opinion, it is the cornerstone of a trusting relationship between clients and their veterinarians.

By Midge L. Ritchie, DVM

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