Heartworm Disease in Cats
By Emily Hoppmann, DVM
Heartworm disease is most commonly thought to be a dog-only problem; however, cats are also at risk of contracting heartworm disease (even indoor cats). Heartworm disease is a serious and potentially fatal disease in pets. It is caused by foot-long worms (heartworms) that live in the heart, lungs, and blood vessels. Heartworms are known causes of severe lung disease, heart failure, upper respiratory infection, difficulty breathing, damage to other organs in the body and even death. With cats, heartworms affect the body differently than in dogs and sudden death can be the only clinical sign that you see so it is even more important to have them on year-round protection. All of these issues that heartworms can cause are extremely scary, but the good news is that heartworm disease is 100% preventable if we keep pets on year-round, monthly heartworm prevention!
You may wonder how exactly heartworms can infect animals and why we have so many pets with heartworms in South Carolina – the answer lies in the fact that heartworms are transmitted via mosquitoes and there is no shortage of mosquitoes around here! The inside of a mosquito provides a place for the heartworm to go through different stages of development while it catches a ride from an already infected host to a new host. This is why one pet cannot directly infect another pet with heartworms through blood or saliva; the heartworm needs to go through the stages within a mosquito, but having pets around that have heartworms makes it more likely that another pet will be bit by an infected mosquito and infected indirectly with heartworms. Adult heartworms living in an infected animal produce microscopic baby worms called microfilaria that circulate throughout the blood. When a mosquito bites and takes a blood meal from an infected animal, it picks up the microfilaria. The microfilaria develop and mature into larvae while inside of the mosquito over a period of 10 to 14 days. In this life-stage, they are considered to be infectious from the mosquito bite to an animal. When an infected mosquito bites another dog, cat, or any other susceptible host (including humans, though this is rare and the heartworms do not tend to survive in humans), the infective larvae enter the new host through the mosquito’s bite wound. Once inside a new host, it can take approximately 6 months for the larvae to mature into adult heartworms. The most common heartworm screening test that veterinarians do for dogs can only detect female, pregnant, adult heartworms so it is possible to have a false negative if your pet has been infected within the past 6 months and the heartworms are not old enough to be detected by the test. This is also why the test cannot tell us how bad the infection by the heartworms is (how many worms the pet has) since it does not even take into account the male heartworms or the female heartworms that are not pregnant. The most common heartworm screening test that veterinarians do for cats actually looks for the cat’s response to the heartworms since cats tend to have less heartworms than dogs so the test that are used for dogs are not as accurate in cats.
Unfortunately, for us and our pets, living in the South means we have an optimal climate for pesky mosquitoes all year long and have a very high population of dogs and cats that are not on heartworm protection and are infected, and thus as a result, an extremely high incidence of heartworm disease here. Because infected mosquitoes can come inside, both outdoor and indoor pets are at risk—and therefore the need for even indoor-only cats or dogs to be on monthly heartworm prevention. In fact, they have found that indoor-only cats have almost as high of a chance of having heartworms as outside cats or outside/inside cats.
Heartworm disease in the cat presents differently compared to in dogs. Cats are not typical hosts for heartworms, but that doesn’t mean they aren’t as susceptible to heartworm infection or severe disease due to heartworms. Often times, heartworms in cats do not survive to the adult stage, but if a cat does end up with adult heartworms, they typically have only a few (1-6) worms – this does not mean that having heartworms in cats is any safer though. Medications used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the getting heartworms and if they are infected there is no cure, only medical management. Also, since many cats that are affected by heartworms have so few detectable adult worms testing is more difficult and this can result in heartworm disease often going under-diagnosed in cats. This gives people the misconception that heartworm disease is not present in cats, but that is far from true. It just takes different testing and careful attention to clinical signs at home and during physical examinations to detect heartworm disease in cats. It is important to know that even though heartworms are less likely to be high in number or even reach adulthood, immature worms circulating in the bloodstream can still cause real damage to your cat’s health and can even be fatal.
Signs of heartworm disease in cats range from subtle signs (such as sneezing) to extremely dramatic, life-threatening signs (such as sudden death). This is why it can be difficult without further diagnostic testing to know whether a cat has heartworm disease just based on what you are seeing at home. Symptoms can include vague signs such as coughing, sneezing, asthma-like attacks, difficulty breathing, periodic vomiting, loss of appetite, or weight loss. Occasionally a cat suffering from heartworm disease might have difficulty walking, will experience fainting or seizure spells, or even accumulate fluid in their abdominal cavity. Heartworms in cats also have the ability to travel to other areas of the body, such as the brain, eye, and spinal cord and cause disease in these areas (not just the heart and lungs). Severe complications such as blood clots in the lungs and inflammation can result when adult worms die in the cat’s body—and unfortunately, the first sign in some cases is collapse, or sudden death due to the cat’s body having an almost anaphylaxis shock-type response.
The preferred diagnostics for screening cats for heartworm disease includes the use of both an antigen and an antibody test (the antibody test is the test discussed earlier that most veterinarians run in-house that detects the body’s response to exposure to heartworm larvae). I also always recommend x-rays or an ultrasound to aid in detecting heartworm disease or to see the extent of damage once a cat tests positive for the disease. Cats, even those on monthly prevention, should be tested yearly to make sure that they do not have heartworms since early medical management of the infection is the best option to prevent long-term damage and fatal consequences. Early and continuous medical management is key to keeping our heartworm positive kitties as happy and healthy as possible since there is no approved drug therapy for elimination of heartworm infection in cats (unlike in dogs, where there is a very effective treatment to eliminate heartworms once they are detected). Since the drugs that are used to treat heartworm disease in dogs are fatal if administered to a cat, monthly year-round prevention is critical. Nevertheless, cats that test positive with heartworm disease can still live a good quality of life with the aid of routine veterinary check-ups, close monitoring at home, and medical management to help reduce the body’s inflammatory response to the invasion by the heartworms in all of its life-cycle stages.