Feline Heartworm Disease: Separating Fact from Fiction
The Heartworm Hotline column is presented in partnership between Today’s Veterinary Practice and the American Heartworm Society (heartwormsociety.org). The goal of the column is to communicate practical and timely information on prevention, diagnosis, and treatment of heartworm disease, as well as highlight current topics related to heartworm research and findings in veterinary medicine.
Routine wellness visits present the ideal time to discuss the importance of feline heartworm prevention with clients. Although most veterinarians recognize that feline heartworm disease can be serious and even life-threatening, far too few clients invest in heartworm prevention for their cats.
Veterinarians can grow weary of what seems like an uphill battle to convince clients of their cats’ need for heartworm prevention. If you’ve heard one of the following statements from a colleague or client—or even had the same thoughts yourself—chances are you are not alone. Separating the facts about feline heartworm disease from misperceptions is key to educating clients about this often-overlooked disease.
“Cats rarely/never get heartworm disease.”
FACT: Although it is true that the prevalence of disease from adult heartworms is less common in cats than in dogs, it may be more accurate to characterize feline heartworm disease as different from canine heartworm disease, rather than rare.
Unlike dogs, which can harbor large numbers of adult worms, cats are susceptible but imperfect hosts for Dirofilaria immitis. Most heartworms in cats die 3 to 4 months after infection, when they are still immature adults—long before infection can produce a positive result on an antigen test.
However, before they die, these immature worms cause disease and damage—a condition known as heartworm-associated respiratory disease, which is characterized by coughing, wheezing, nausea, vomiting, and inappetence. The arrival of these immature worms into the small arteries of the lungs triggers a severe inflammatory response that damages the arteries, bronchioles, and alveoli. Cats are most likely to demonstrate clinical signs of heartworm infection at 2 times: 1) when the immature adult worms reach the pulmonary vasculature and subsequently die and 2) when the mature adult worms die, usually 2 to 4 years after infection.
“I hardly ever see a heartworm-positive cat.”
FACT: The diagnosis of feline heartworm disease is complicated because no single test can consistently detect the various stages of heartworm disease in cats. Often, the combination of serology and imaging results, along with clinical signs, is needed to confirm the diagnosis (TABLE 1).
Antigen tests most accurately detect mature female heartworms when they are producing microfilariae. However, not only do cats have more male-only and immature heartworms—neither of which are detectable with antigen tests—their burden of adult heartworms is smaller than that of dogs (FIGURE 1). These factors make the detection of heartworms in cats challenging. Even when cats are infected with adult heartworms, these infections are commonly missed by antigen screening.
Antibody tests can detect the presence of infections, both past and present, with male or female worms, as early as 2 months after infection, making them a helpful tool for assessing exposure risk. However, the limitation of antibody testing is that it cannot determine if the cat is currently infected. Because an estimated 10% to 20% of antibody-positive cats are infected with adult heartworms, you should perform antigen testing for these cats at 1- to 2-month intervals. If you suspect adult heartworm infection but the antigen test results are repeatedly “negative,” you may be able to increase test sensitivity by heat pretreatment of the serum sample. In cases where soluble antigen is bound to endogenous antibodies, forming an insoluble unit in the bloodstream known as an “immune complex,” the antigen is essentially blocked from detection. Heat pretreatment is a process that denatures the proteins within the circulating immune complex so that antibodies precipitate and the antigen is freed, making it available for detection.
Because of the challenges in diagnosing feline heartworm disease—and the potential for cats to experience disease and damage from even immature worms—the possibility of heartworm infection in a coughing, asthmatic cat with “no antigen detected” should not be dismissed.
Radiography and Echocardiography
These 2 modalities can confirm heartworm diagnosis and/or assess the likelihood of infection. Imaging can be helpful when the index of suspicion in a cat is elevated on the basis of clinical signs but serology results are inconclusive (FIGURE 2).
“The risk for heartworms in cats is minimal because most cats live indoors.”
FACT: Outdoor cats are obviously at greater risk of being bitten by infected mosquitoes than are their indoor counterparts, but an indoor lifestyle is no guarantee of safety.
According to a 2014 Vetstreet survey (vetstreet.com/our-pet-experts/do-you-let-your-cat-roam-outside-we-polled-veterinary-professionals-and-readers), roughly three quarters of cat owners house their cats indoors, where they might assume that the risk for heartworm transmission is minimal. However, the Mosquito Control Association (mosquito.org/page/faq) notes that mosquitoes are adept at coming indoors through screens, open doors, attic soffits, and bathroom exhaust vents and often congregate in garages and near doors, seeking indoor entry. Meanwhile, many “indoor” cats may be allowed outdoors on a protected patio or may occasionally slip outside despite their owner’s intentions.
An indoor life can help protect cats from many threats, but it is no panacea when it comes to heartworm prevention. The only way to protect cats from this serious threat is year-round administration of heartworm preventives.
“I can’t justify the cost of feline heartworm prevention.”
FACT: The adage “an ounce of prevention is worth a pound of cure” applies here.
When clients understand that heartworm infection in cats can only be prevented, not cured, and that just 1 adult heartworm can cause their cat to die suddenly, they can better justify the price of heartworm prevention. Because heartworm preventives for cats cover multiple parasites—some internal only, others internal and external—it makes sense to stress the added value of broad-spectrum protection. Again, indoor cats are not exempt from risk; parasite eggs can be tracked into the house on the soles of shoes or even carried indoors on the coat of a dog that has rolled in infested grass. Pointing out that people can contract intestinal parasites from pets is a compelling message, especially for clients with young children.
Because cats are susceptible but imperfect hosts for heartworm disease, their worm burdens are much smaller than those of dogs. However, because of cats’ relatively small body size, infections with just 1 or 2 worms can be life-threatening.
Determining the exact prevalence of feline heartworm in your practice area can be difficult, mainly because of testing limitations. A simple approach can be to base your recommendations for heartworm prevention in cats on the comparative risk for heartworm infection in dogs in your area. If canine heartworm disease is endemic to your area, it can be assumed that heartworm prevention for both species is warranted.