Heartworm Disease

What is it?

Heartworm disease impacts many pets worldwide, including those in the United States (US). While this disease is severe, and can even be fatal, it is preventable. It is considered to be the most common vector-borne disease in the US with 1-12.5% average occurrence in dogs¹¹, with as high as 48.8% in highly endemic areas in the Gulf Coast after natural disasters, such as hurricane Katrenia.¹² Fortunately for cats and ferrets this disease is far less common, but infection is of higher consequence.  Heartworm disease is caused by the filarial nematode Dirofilaria immitis (heartworm) that lives in the heart, lungs, and blood vessels of affected pets.¹ It primarily affects dogs, cats, and ferrets but has also been identified in other mammal species, rarely humans.² The main wildlife affected by this disease is wild canids like coyotes, wolves, and foxes.³

How is it contracted?

A pet becomes infected after being bitten by a mosquito that has contracted the microfilaria (baby microscopic worms) from an infected host animal that has then developed into larvae within the mosquito. When infected, the larvae are deposited on the pet's skin surface and enter the animal's system through the mosquito bite wound. Once in a host animal, the larvae finally mature adult heartworms over the next six months. These adult worms can live for up to seven years in dogs or three years in cats.²

Signs

Early in the infection, the pet may show no signs. Symptoms develop after adult heartworms are present. The signs will vary by species.

Dogs may develop a mild persistent cough, fatigue, or reluctance to exercise. As the disease progresses with a significant burden of worms, the pet can show signs of heart failure and develop similar symptoms, in addition to fluid buildup in the abdomen. In rare cases, they may experience acute cardiac collapse from large amounts of worms blocking blood flow. At this severe stage of sudden pale gums, labored breathing, and dark-colored urine. Without immediate surgical removal, these pets will not survive.⁴

Cats can exhibit subtle signs or severe ones. Some symptoms include asthma-like attacks, coughing, periodic vomiting, or lack of appetite.⁵ Unfortunately, the most common sign is sudden death or collapse.

Ferrets are similar to dogs but develop more rapidly due to their small size. Just one heartworm in a ferret can produce severe symptoms that can be deadly.⁶

Prevention

No preventative is 100% effective. There is some drug resistance developing in heartworms, and a risk of missing subtherapeutic doses being given to a pet.⁸ A severe and potentially fatal reaction can also happen when preventives are given to an animal with adult heartworms. Because of these risks, your vet will require a yearly test to ensure that your pet has not accidentally become infected with heartworms over the year. This is also why it requires a prescription. For dogs, the annual test will test for adult heartworms; however, cats and ferrets are unlikely to have adult heartworms present so an antibody test will also be done that detects the larval heartworms. Additionally, an echocardiogram may be recommended to check for heartworms in Ferrets due to the extremely low detection rates with the tests.  

Prevention must be used regularly and consistently since all effective heartworm medications work by eliminating the heartworm parasite's larval stages. Preventatives are available in multiple formats, varying from oral, topical, or injectable products used every 1, 6, to 12 months.⁹ It takes as few as 51 days for heartworm larvae to molt into a juvenile/immature adult stage, which preventives cannot eliminate.¹⁰ Because of this, heartworms must be stopped before they reach adulthood. Being late or missing a preventative dose can allow larvae to reach the adult stage.

Conclusion

Heartworm disease has been diagnosed in all fifty states but is most prevalent in the southeastern United States.⁷ Factors that play into its spread, like the relocation of animals, mosquitos, and climate variations as resulted in most areas of the United States recommending yearly testing and monthly prevention for control of the potentially fatal disease.

References

1. Litster AL, Atwell RB. Feline heartworm disease: A clinical review. Journal of Feline Medicine and Surgery. 2008;10(2):137-144. doi:1016/j.jfms.2007.09.007
2. McCall JW, Genchi C, Kramer LH, Guerrero J, Venco L. Heartworm disease in animals and humans Parasitol., 66 (2008), pp. 193-285
3. Aguirre, A. A. (2009). Wild canids as sentinels of ecological health: a conservation medicine perspective. Parasites & vectors, 2(1), 1-8.
4. Hoch H, Strickland K. Canine and feline dirofilariasis: life cycle, pathophysiology, and diagnosis. Compend Contin Educ Vet. 2008 Mar;30(3):133-40; quiz 141. PMID: 18409140.
5. Dillon R. Feline dirofilariasis, Veterinary Clinics of North America: Small Animal Practice14 (6), 1984, 1185–1199.

1. Miller WR et al. Dirofilariasis in a ferret J Am Vet Med Assoc (1982)
2. Noack S, et al. Heartworm disease – overview, intervention, and industry perspective Int. J. Parasitol. Drugs Drug Resist. (2021)

1. Ledesma N, Harrington L. Mosquito vectors of dog heartworm in the United States: vector status and factors influencing transmission efficiency Companion Anim. Med., 26 (2011), pp. 178-185
2. Optimizing heartworm diagnosis in dogs using multiple test combinations. https://parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-021-04715-4
3. Society, H. (1970, September 21). Heartworm basics. American Heartworm Society. Retrieved January 4, 2023, from https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics
4. Lee AC, Montgomery SP, Theis JH, Blagburn BL, Eberhard ML. Public health issues concerning the widespread distribution of canine heartworm disease. Trends Parasitol. 2010;26:168–73.
5. Levy JK, Lappin MR, Glaser AL, Birkenheuer AJ, Anderson TC, Edinboro CH. Prevalence of infectious diseases in cats and dogs rescued following Hurricane Katrina. J Am Vet Med Assoc. 2011;238:311–7.

 

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About the Author

Dr. Katy Miller works as the Director of Veterinary Services at BSM Partners. She earned her veterinary degree at Ross University and completed her clinical year at Louisiana State University. She previously served for 11 years as the Director of Dog and Cat Health and Nutrition for Mud Bay where she earned multiple certifications and specialized in pet food nutriton, prior to which she practiced general and emergency medicine for seven years. She is also a competitive three-day eventer, licensed falconer, and claims only two (Golden and Mini Doxie) of their nine dogs.