July 2019

Heartworm Disease (Dirofilaria immitis) Causing Death in a Cat

By Dr. Sarah Schneider

Background: Dirofilaria immitis (commonly known as Heartworm) is an intravascular parasite of carnivores spread by a mosquito vector. Heartworm disease can affect both dogs and cats. Although cats are naturally more resistant, the prevalence of feline heartworm disease is between 5-20% of that in dogs in the same area. Feline heartworm disease differs from canine disease is several respects: in particular, cats often have more severe disease with a lower worm burden and sudden death.

History: A 10-year-old male neutered, indoor/outdoor cat was laterally recumbent and gasping on the drive-way of his home. On presentation, he was in respiratory distress and open-mouth breathing. Diffuse crackles were auscultated and increased opacity was observed in all lung fields radiographs. The patient was placed on oxygen and administered furosemide, but died shortly thereafter. A dog in the household had previously died from heartworm disease.

Adult D. immitis worms within the caudal vena cava of a cat.
Figure 1. Adult D. immitis worms within the caudal vena cava of a cat.
Multiple D. immitis worms within the right ventricle and extending into both pulmonary arteries of a cat.
Figure 2. Multiple D. immitis worms within the right ventricle and extending into both pulmonary arteries of a cat.

Necropsy findings: The thoracic cavity contained ~4 mL of fluid. The caudal vena cava was mildly distended and contained three worms (Figure 1) consistent with Dirofilaria. Additional worms partially obstructed the right atria, right ventricle, and the pulmonary arteries (Figure 2). The pulmonary artery endothelium was mildly roughened. Approximately 60% of the lungs were mottled dark red, rubbery, and did not collapse. On cross section, the lung had multifocal to coalescing firm, irregular, tan, nodules (0.1 cm to 1.6 cm diameter) and several of the larger pulmonary arteries were thickened. The abdominal cavity contained ~10 mL of clear, serosanguinous fluid and all liver lobes were firm with rounded margins and a moderately increased reticular pattern (congestion).

Discussion: Similar to dogs, heartworm disease in cats is caused by Dirofilaria immitis. Cats are typically more resistant to heartworm infections since the majority of immature worms die once they enter the pulmonary artery, resulting in a typically lower worm burden. Despite this, cats are susceptible to pulmonary thromboembolism, and vascular and pulmonary inflammation (associated with both adult and juvenile parasites) acute right-sided heart failure and anaphylaxis. Most cats with heartworm disease die due to acute right heart failure or pulmonary thromboembolism, which typically results in acute, progressive respiratory distress.

The high worm burden in this animal is unusual and was likely influenced by close proximity to a dog with untreated heartworm disease. The frequency of mosquitoes positive for D. immitis increases dramatically with exposure to a single heartworm positive animal significantly increasing the risk to animals sharing mosquito exposure.

Prophylactic treatment remains the best means of preventing heartworm disease in both dogs and cats. Prophylaxis not only protects the individual, but reduces risk to other animals as well. Prophylactic treatment is especially important for cats as immiticides cannot be used for treatment and they are more prone to sudden death when infected.


  1. Lee, A.C.Y., Atkins, C.E. (2010) Understanding Feline Heartworm Infection: Disease, Diagnosis, and Treatment. Topics in Companion Animal Medicine;25(4):224-230.
  2. Levy JK, Snyder PS, Taveres LM, Hooks JL, Pegelow MJ, Slater MR, et al. (2003) Prevalence and Risk Factors for Heartworm Infection in Cats From Northern Florida. Journal of the American Animal Hospital Association; 39:533–7.
  3. Mckay T, Bianco T, Rhodes L, Barnett S. (2013) Prevalence ofDirofilaria immitis(Nematoda: Filarioidea) in Mosquitoes From Northeast Arkansas, the United States. Journal of Medical Entomology; 50:871–8.

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