Since we are currently doing a giveaway of an iRobot Roomba for clients who buy 12 months of Heartgard and 12 months of Nexgard, it seems like a great time to do a blog post on heartworm disease. Here at Dogwood Vet Clinic, we do our best to try and fully explain to you all aspects of your pet’s preventative care. But those of you who have already seen us know there is a lot to cover during an annual exam, especially if it’s your first visit to Dogwood Veterinary Clinic. There’s a chance you may not know much about heartworm disease. This blog post explains the high points of heartworm disease and why prevention is so important for your pet.

What is heartworm disease?

Heartworm disease is caused by Dirofilaria Immitis, a parasite that lives in the bloodstream. The disease is transmitted to dogs, cats, and ferrets through the bite of an infected mosquito, the intermediate host that is required for transmission. The disease is not contagious, patients only become affected if they are bitten by a mosquito containing the L3 infective larvae. Once the pet is bitten by the mosquito, the larvae continue to molt into adults within the host. Circulating worms become sexually mature 4 months after infection, and there are usually circulating microfilaria (offspring of the adult heartworms) by 6-9 months post infection.

How do heartworms affect your pet?

Let’s start by talking about dogs since dogs and cats are affected very differently. Immature adult heartworms find their way to the pulmonary arteries by day 120. At this time the worms measure 1 to 1.5 inches in length. When these young worms first enter the lungs the blood flow pushes the worms to the smaller pulmonary arteries. Even these early stages are causing damage to the lungs. The female worms can grow up to 10-12 inches in length. As the worms increase in size and number they can be found in the lobar arteries, main pulmonary artery, and right ventricle. Dogs with high worm burdens are more likely to be affected with caval syndrome, this is when worms are found in the right ventricle, right atrium, and vena cava. This is blocking blood from flowing adequately through the right side of the heart. This can lead to hemolysis (the lysis of red blood cells), organ dysfunction, and right sided heart failure. If the disease is caught early, sometimes the pet will not be showing any clinical signs. As the disease progresses, clinical signs may present as exercise intolerance, lethargy, coughing, respiratory difficulty, signs of heart failure, and radiographic changes. Caval syndrome is the most severe manifestation of heartworm disease. AIthough not all dogs affected with heartworm disease will get caval syndrome, if the disease is left untreated, it will eventually cause multiple organ dysfunction and death.

Cats are a completely different story when it comes to heartworm disease. Cats are susceptible to the disease, but adult worms are less likely to live in the cat. Although the worm burden is usually much less, it can still cause severe disease. The main reason heartworm disease is treated differently in the cat is there is no cure. Treatment to kill adult worms is not recommended due to the anaphylactic type reaction cats have when the adult worms die. In dogs we have a treatment, so we want to diagnose the disease as soon as possible in order to stop the progression of damage to the cardiopulmonary system. In cats, we have no way of treating the disease, making prevention even more important. When the heartworms first make their way to the pulmonary arteries, the tissue undergoes an inflammatory response known as HARD, heartworm associated respiratory disease. Cats can overcome this and have resolution of clinical signs, but when the adult worms die they are likely to cause another episode of pulmonary inflammation or thromboembolism. Thromboemobolism is usually fatal. Caval syndrome is much less likely in the cat with the low worm burdens. But even one worm can interfere with the function of the valves, resulting in a heart murmur. It is more difficult to get a reliable test result in cats than it is in dogs, and due to the difference in treatment options we only test when clinical signs are present. Clinical signs can present in a variety of ways in cats including vomiting, lethargy, inappetence, respiratory distress, heart murmur, and sometimes neurologic signs and even sudden death.

For our feline friends we encourage once monthly prevention, and if clinical signs are present, we will test for the disease and discuss management. With dogs we are much more diligent with our prevention, surveillance, and treatment of the disease  because we have a better treatment option than we do for cats. For both species, the most important thing to know about heartworm disease is that it is completely preventable! In dogs, we recommend monthly prevention and annual testing. Your pet is at risk even if one dose is slightly late or skipped, and if your dog is infected, we want to start treatment as soon as possible. It is not impossible for a pet to become infected while on preventative medication, although it is extremely rare. With the preventative product we carry, if we show record of sale of these preventatives after a negative heartworm test, the company will pay for your pet’s treatment should they become infected while using their product. This only applies if the preventative is purchased from a veterinary hospital. Heartworm disease is present in all 50 states, and although transmission does decrease in winter months, the risk of transmission still exists, especially in a place like Louisville,  Kentucky with such fluctuant temperatures in the wintertime.

While trying to keep this somewhat brief, I think we have covered why heartworm disease is so dangerous to your pet’s health and how we manage this disease and its prevention here at Dogwood Veterinary Clinic. When you are here for a wellness exam, we would be happy to answer any questions you may have about heartworm disease and prevention.