Summer is just about here, and so are the mosquitoes. Here’s why—and how—to talk to your clients about heartworm disease prevention in cats.
Spring and summer usher in a host of undesirable visitors that pester pets and people alike. As veterinarians, we cannot help but associate warm weather and mosquitoes with heartworm disease (HWD). For dogs, the evolution of macrocyclic lactone preventives has made available new products, such as ProHeart 12 (moxidectin) and Simparica Trio (sarolaner + moxidectin/pyrantel),1,2 both from Zoetis, to prevent canine HWD, but where does that leave cats?
Despite the availability of monthly heartworm preventives for cats, feline HWD remains a less common conversation in the exam room. With the high success of feline heartworm prevention in both experimental and field trials,3,4 veterinary teams should be more excited to share with clients our trust in these products to prevent the nightmare of managing a heartworm-positive cat.
Suspicion of feline HWD should be based on a combination of history, environment, clinical examination and diagnostic findings. Clinical findings may include acute or chronic respiratory distress (hemoptysis, dyspnea, orthopnea, cough); sudden death may also occur.5
Orthogonal thoracic radiographs may reveal dilated peripheral pulmonary arteries, a bronchointerstitial lung pattern and/or an enlarged cardiac silhouette.5 Less common radiographic findings reported by the American Heartworm Society include chylothorax, pneumothorax and ascites.5
The most trust in diagnosing feline HWD should be placed in serology: heartworm antigen and antibody testing. We must recall our epidemiology roots established in veterinary school to explain the limitations, and often frustrations, of these diagnostics to our clients. Specifically, a false-negative heartworm antigen test can occur with a same-sex male infection, a low worm burden, an early infection or operator error.5 However, an antigen-positive test is considered 100% specific in confirming HWD.5 Alternatively, a positive antibody test could suggest current infection or previous exposure to D. immitis larvae, but overall this is less sensitive than antigen testing.5 Experts recommend that we explain to clients that paired serology—both antigen and antibody testing—is better than a single test when screening for feline HWD.5
Our options for managing feline HWD are limited. The use of corticosteroids, surgical extraction of adult worms and benign neglect all offer much lower than the 99% success rate in eliminating adult heartworms with the melarsomine three-injection protocol used in dogs.6 (The melarsomine protocol is not an option for cats due to its reported toxicity in the species.6)
There has been some success with retrieving adult worms via cardiac catheterization; however, these were considered salvage surgeries.7 Both clients and veterinarians should understand and agree that prevention is a much better option than treatment when it comes to HWD in cats.
HWD can be scary for both clients and veterinarians. It is important to emphasize our trust in monthly chemoprophylaxis, given its great success in preventing HWD and its high margin of safety. Selamectin + sarolaner (Revolution Plus—Zoetis) and imidacloprid + moxidectin (Advantage Multi—Bayer) are the powerhouse macrocyclic lactones for preventing feline HWD. Both products offer 100% prevention of developing adult heartworms following exposure to D. immitus.3,4
In one experimental study,3 cats were injected with L3 larvae and treated with monthly Revolution Plus for one to three consecutive months. Four to six months following treatment, 100% of the cats had no adult worms on necropsy.
A 2015 study4 investigated the opposite pattern: application of Advantage Multi prior to inoculation with L3 larvae. All cats received four consecutive monthly applications of Advantage Multi, after which they were exposed to heartworm larvae every seven days for about one month. At the end of seven months, all cats were reportedly antigen- and antibody-negative.
Your clients don’t need to understand the nuances of these studies, but they should understand three simple words conveyed by your veterinary team: “These products work.” If the curious client would like to be pointed in the direction of these studies and product labels, we can confidently provide them with these resources.
One way we can connect to our clients is by being transparent about the unpredictability of feline HWD during annual wellness visits. For example, “Mrs. Dyer, unlike Chloe’s chronic kidney disease, which is manageable, treating heartworm disease in cats can be difficult. Let’s discuss ways to keep Chloe protected with a topical monthly preventive to avoid this.” This statement explains the difficulty in treating heartworm disease, provides a personal parallel to the patient and provides the client with a solution.
We can also rely on the feline HWD guidelines provided by the American Heartworm Society when talking with clients. In one small study on heartworm disease in the U.S., for example, a reported 25% of strictly “indoor cats” tested positive for HWD.8 This is a great opportunity to discuss why all domestic cats—even those that do not go outside—should be on year-round monthly parasite prevention. If the owner is unwilling to commit to this, share the AHS recommendation that heartworm prevention can be started one month before the onset of warm weather and continued three months after the warm season ends to accommodate the molting of larvae to adults.5 This conversation can be initiated by the nursing staff collecting information on indoor/outdoor status that the veterinarian can reference later during the examination.
At the end of day, we want to provide our clients with power. There are many ways to celebrate their decision to apply monthly feline HWD prevention. This includes educating them about rebate programs available through Zoetis for clients purchasing six to 12 months of Revolution Plus, hanging a wall display of the names of feline patients that are on monthly prevention or encouraging clients to post photos of “protected cats” on the practice’s social media platform, with a prize offered to one of those clients selected randomly. Whatever celebration your practice creates to motivate clients to purchase feline heartworm prevention, have comfort that there will be one fewer case of feline HWD to worry about!
Dr. Santos practices at Brick Town veterinary Hospital in Brick Township, New Jersey. She is a 2019 graduate of Purdue University College of Veterinary Medicine. Her interests include physical rehabilitation/pain management, endocrinology and client communication.
1. McTier TL, Kryda K, Wachowski M, et al. ProHeart® 12, a moxidectin extended-release injectable formulation for prevention of heartworm (Dirofilaria immitis) disease in dogs in the USA for 12 months. Parasit Vectors 2019; 12:369.
2. FDA approves new combination parasite preventive. dvm360.com website: dvm360.com/view/fda-approves-new-combination-parasite-preventive. Published February 28, 2020. Accessed May 10, 2020.
3. McTier TL, Pullins A, Chaplin S, et al. The efficacy of a novel topical formulation of selamectin plus sarolaner (Revolution® Plus/Stronghold® Plus) in preventing the development of Dirofilaria immitis in cats. Vet Parasitol 2019;270:56-62.
4. Little SE, Hostetler JA, Thomas JE, et al. Moxidectin steady state prior to inoculation protects cats from subsequent, repeated infection with Dirofilaria immitis. Parasit Vectors 2015;8:107.
5. American Heartworm Society. Current feline guidelines for the prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in cats. Wilmington, DE: AHS; 2020. Available at: heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
6. American Heartworm Society. Current feline guidelines for the prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in dogs. Wilmington, DE: AHS; 2018. Available at heartwormsociety.org/images/pdf/2018-AHS-Canine-Guidelines.pdf.
7. Small MT, Atkins CE, Gordon SG, et al. (2008). Use of a nitinol gooseneck snare catheter for removal of adult Dirofilaria immitis in two cats. J Am Vet Med Assoc 2008;233(9):1441-1445.
8. Atkins CE, DeFranceso TC, Coats JR, et. al. Heartworm infection in cats: 50 cases (1985-1997). J Am Vet Med Assoc 2000;217(3):355-358.