Are We Doing Enough to Prevent Heartworm Infections?by Clarke E. Atkins, DVM, Dipl. ACVIM (Internal Medicine/Cardiology) - College of Vet Med, NC State University, Raleigh, NC
Most Americans love their pets and consider them family members. Likewise, most clients would not knowingly expose a pet to infection with a potentially fatal disease. Yet millions of owners who fail to protect their pets from heartworm infection are doing exactly that.
Substantial Infection Risk
An unacceptable number of pets are infected annually with Dirofilaria immitis. According to the American Heartworm Society, more than 244,000 dogs and nearly 3,100 cats tested positive for heartworm infection in 2001.1 These numbers most likely represent the iceberg’s tip and point to our role in helping clients to understand the prevalence and serious associated risks of heartworm disease and the importance of complying with prescribed prevention methods. According to a 2003 survey conducted by Novartis Animal Health US, Inc., three-fourths of surveyed veterinarians recommend year-round heartworm prevention for dogs. Unfortunately, only half of surveyed pet owners report following their veterinarians’ recommendations.2
Year-round Heartworm Prevention Recommendations
The Companion Animal Parasite Council (CAPC), which laid out parasite protection guidelines earlier this year, advocates year-round heartworm prevention for cats and dogs. This recommendation is based on parasite and vector characteristics and human behavior:
- Heartworm transmission occurs throughout the year in portions of the United States.
- Mosquito presence and ability to transmit heartworm microfilariae are often unpredictable, making it impossible to pinpoint potential transmission seasons.
- More pets are traveling with their owners, often to and from heartworm-endemic areas during transmission season.
- Year-round prevention may help improve client compliance and efficacy of preventatives.
Heartworm Prevention for Cats?
While the rationale is somewhat less compelling than it is for dogs, year-round prevention is advised for cats living in endemic areas. When making decisions about heartworm prophylaxis in cats, we must consider heartworm infection prevalence relative to other feline diseases, severity of clinical signs, the lack of an approved treatment protocol and the guarded prognosis for infected cats. Infection with only one adult heartworm may prove fatal, and immature infections may produce signs of respiratory disease.
Studies of heartworm infection in southeastern U.S. cats obtained from shelters indicate a prevalence of 2.5 to 14 percent. This approximates or exceeds prevalence of feline leukemia virus and feline immunodeficiency virus infections. A 1998 nationwide antibody survey of more than 2,000 largely asymptomatic cats revealed an exposure prevalence of nearly 12 percent.3
The fact that many cats are “indoor” cats offers a false sense of security. Spending time outdoors does put cats at greater risk for heartworm infection. However, over one-fourth of the cats diagnosed with heartworm disease at North Carolina State University were solely indoor cats, according to their owners.4
The consequences of feline heartworm disease are potentially dire, and no clear therapeutic solution exists. I do not advocate adulticidal therapy for cats due to the inherent risks, lack of clear benefits, lack of an approved drug and the short life span of heartworms in cats. Surgical heartworm removal has been successful and minimizes risk of thromboemboli. This technique, however, is technically challenging, requires fluoroscopy and may be associated with an unacceptable mortality rate. Despite these limitations, I do believe surgical removal of heartworms holds promise.
The prognosis for naturally infected cats is similar to that for hypertrophic cardiomyopathy, the most benign feline heart disease.5 Median survival time for heartworm-infected cats is 1.5 years for all cats and four years for those surviving the first 24 hours.4 Young age (less than 3 years); gender; heartworm-antigen-positive ELISA test result; and presence of dyspnea, cough or echocardiographically identifiable heartworms did not appear to affect survival.
Avoid Infection and Broken Hearts
Heartworm prevalence is significant in enough areas that dog and cat owners should be informed about its prevention and strongly encouraged to follow year-round prevention recommendations. Several heartworm preventatives are available for use in dogs and cats. With a range of formulations and delivery mechanisms offered by these products, individual client needs can easily be met. Like so many diseases, heartworm disease is easier and less expensive to prevent than to treat. Today, no dog or cat should suffer from heartworm infection.
1 American Heartworm Society Web site [press release on the Internet]. Batavia (IL): American Heartworm Society; © 2004 [cited 2004 Aug 17]. Available from http://www.heartwormsociety.org/MediaRelease.htm
2 Data on file, Novartis Animal Health US, Inc. Veterinarian and pet owner survey: Parasite management attitudes and behaviors, August 2003.
3 Miller MW, Atkins CE, Stemme K, et al. Prevalence of exposure to Dirofilaria immitis in cats in multiple areas of the United States. In: Seward RL, ed. Recent Advances in Heartworm Disease: Symposium ’98. Proceedings of the Heartworm Symposium; 1998 May 1–3; Tampa, FL. Batavia (IL): American Heartworm Society. p 161–6.
4 Atkins CE, DeFrancesco TC, Coats JR, et al. Heartworm infection in cats: 50 cases (1985–1997). J Am Vet Med Assoc 2000 Aug 1;217(3):355–8.
5 Atkins C. Prognosis in feline heartworm infection: Comparison to other cardiovascular disease. In: Seward RL, ed. Recent Advances in Heartworm Disease: Symposium ’01. Proceedings of the Heartworm Symposium; 2001 Apr 20–22; San Antonio, TX. Batavia (IL): American Heartworm Society. p 41–4.