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Compendium July 2008 (Vol 30, No 7)

Dirofilaria immitis in Cats: Anatomy of a Disease

by C. Thomas Nelson, DVM

    CETEST This course is approved for 2.0 CE credits

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    It is now understood that wherever heartworm infection exists in the local canine population, it will also be found in the feline population. However, this does not mean that the parasite and resulting disease behave the same way in both species. For example, heartworms rarely reach the adult stage in cats, but they can cause respiratory sequelae nonetheless.

    Despite recent advances in the understanding and prevention of heartworm disease in dogs, the question remains: Is Dirofilaria immitis a significant health risk in cats? After speaking with practitioners across the country, I have found that there are definitely more skeptics than believers; indeed, I was once a skeptic myself. That changed in 1997, however, when I conducted a necropsy survey on shelter cats in southeast Texas and discovered that 10% of them had adult heartworms.1 I also learned that the heartworm infection rate in this area was greater than that for FIV or FeLV.1

    The past decade has seen major additions to the knowledge base for heartworm disease in cats (Box 1), and this information has been widely disseminated via trade magazines and journals. Nonetheless, fewer than 5% of cats in the United States are on a preventive regimen.2 Why? Because most veterinarians have never diagnosed a case of heartworm disease in a cat, and although practitioners may believe that the disease exists in highly endemic areas such as the Gulf Coast, they tend to discount it as a problem elsewhere.


    Heartworms were first re­ported in dogs in Italy in 16263 and in cats in Brazil in 1921.4 They were found in cats in the United States and the Philippines in 1922.5,6 Over the next 73 years, feline heartworm infections were noted in 30 US states and 15 other countries, reflecting the widening reach of canine infections.7 The increased incidence of heartworms in cats has led to further research into the pathogenesis, prevention, and diagnosis of heartworm disease; in 1998, 60% of the research presentations at the annual Heartworm Symposium focused on feline disease.


    A review of necropsy data and antigen serology results indicates that heartworms exist in cats wherever they are found in dogs. The infection rate for mature adult heartworms in cats, as determined by prevalence studies, ranges from 5% to 20% of the rate in the local dog population.7


    Female mosquitoes feeding on heartworm-infected dogs ingest circulating microfilariae. These microfilariae transform into first-stage (L1) larvae within hours of entering the mosquito. The larvae undergo two molts over the next 2 to 4 weeks, depending on the average ambient temperature, ultimately becoming infective (L3) larvae. The L3 larvae are deposited on the skin of a new host in the saliva of a feeding female mosquito. They enter through the bite wound into the subcutaneous tissue, where they molt to L4 larvae within a couple of days. The L4 larvae migrate into the subcutaneous adipose tissue and muscle over the next 2 months, ultimately undergoing a final molt to a juvenile worm stage and entering a peripheral vein.8 Historically, juvenile worms have been viewed as L5 larvae; however, because this stage does not undergo subsequent molts, it is more properly considered an immature or juvenile worm. This worm matures into an adult over the next several months.

    Immature worms in peripheral veins are carried in the bloodstream to and through the heart, arriving in the caudal pulmonary arteries at 75 to 90 days postinfection. By day 100, the juvenile worms are two inches long. In dogs, most juvenile worms mature into adults that can live for 5 to 7 years. In cats, most juvenile worms die shortly after arriving in the pulmonary arteries, initiating an inflammatory response. In a small percentage of cats, a few worms become mature adults that can live for 2 to 4 years9,10 (Figure 1).


    There are three stages of heartworm disease in cats. The first stage begins soon after the arrival of the juvenile worms in the caudal pulmonary arteries as an acute vascular and parenchymal inflammatory reaction to the presence and subsequent death of most of these worms. The most common clinical signs are coughing or dyspnea (64%) and intermittent vomiting unrelated to eating (38%), but 28% of cats are asymptomatic.11 The significant number of asymptomatic cats is probably a result of their characteristic sedentary lifestyle; exercise has been shown to be a major factor influencing the severity of heartworm disease in dogs.12

    Thoracic radiography may show a bronchointerstitial lung pattern and what appears to be an enlarged right caudal lobar artery (Figure 2). This apparent enlargement is a result of inflammatory infiltrates surrounding the vessel and producing a radiographic shadow. The bronchointerstitial lung pattern may be misinterpreted as allergic bronchitis or asthma, and glucocorticosteroid administration will improve the radiographic signs, further supporting the misdiagnosis.

    Evidence of this initial phase (i.e., radiographic signs of pulmonary parenchymal disease by 3 months postinfection) was reported by Donahoe et al13 in 1976. In a later study (1992), Holmes et al14 demonstrated vascular disease in the caudal lung lobes by day 75 postinfection, followed by interstitial and alveolar disease by day 90. The vascular lesions were described as severe muscular hypertrophy of the medium and small arteries (Figure 3). Browne et al15 found the same type of lesions in cats that were positive for heartworm antibodies but were determined to be free of adult heartworms by necropsy, providing evidence that cats need not have fully mature adult worms to have disease.

    A recent study by Dillon et al,16 the results of which were presented at the 2007 Heartworm Symposium, compared the pulmonary pathology of cats that were experimentally infected with 100 L3 larvae while receiving selamectin prevention, cats with abbreviated juvenile worm infections, and cats with adult heartworms (Figure 4). These infections reproduced the vascular lesions described by Holmes et al14 and Browne et al.15 In addition, the researchers noted lesions in the bronchioles and alveoli of the cats with abbreviated juvenile worm infections that were almost identical to those found in cats with adult worm infections (Figure 5 and Figure 6). Furthermore, despite the severity of the pulmonary lesions in the abbreviated juvenile infections, 50% of the cats were antibody negative 8 months postinfection. The cats that received preventive therapy had no histopathologic lesions.

    Findings from all of these studies provided additional evidence that pulmonary disease occurs in cats as a result of juvenile worm infections, even if the infections do not progress to the mature adult worm stage. This has led to the adoption of a new term to describe heartworm infections in cats, heartworm-associated respiratory disease (HARD).17

    In cats, if a juvenile worm matures to adulthood, the host immune response is suppressed and there may be some resolution of clinical signs. Two studies have demonstrated the ability of heartworms to suppress the activity of the pulmonary intravascular macrophage, the main component of the cat's reticuloendothelial system.18,19 When the worm dies, this down-regulation of the immune system ceases and the second stage of disease begins. The degenerating parasite causes an intense inflammatory reaction and thromboembolism, which can lead to catastrophic acute lung injury and sudden death. Sudden death has been reported in 10% to 20% of cats with mature adult heartworm infections.9-11 If the cat survives this stage, hyperplasia of type II alveolar cells replaces the normal type I cells, which may cause permanent lung injury.20 This can lead to chronic respiratory disease, the third stage of heartworm disease in cats.


    Although the existence of feline heartworm infection is now well established, veterinarians (and therefore cat owners) have been slow to recognize and react to the problem. Moreover, practitioners need to understand that, whereas infected cats seldom harbor adult worms, they can still be subject to serious respiratory consequences. Clearly, this is a widespread disease that can no longer be ignored.

    A companion article on diagnosis and management begins here.

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    Dr. Nelson discloses that he has received financial support from the American Heartworm Society, the Companion Animal Parasite Council, IDEXX Laboratories, Fort Dodge Animal Health, the KNOW Heartworms campaign, Merial, and Pfizer Animal Health.

    1. Nelson CT, Self TS. Incidence of Dirofilaria immitis in shelter cats in southeast Texas. In: Seward RL, ed. Recent Advances in Heartworm Disease: Symposium '98. Batavia, IL: American Heartworm Society, 1998:63-66.

    2. Pfizer Animal Health. Independent market research. 2005.

    3. Birago F. Trattato cinegenetico, ouero della caccia del Sig. Francesco Bitago, Signor di Metono, & di Siciano. Milan: G.B. Bidelli; 1626:58-60.

    4. Travassous LP. Notas helminthologicas. Brazil-Medico 1921;35:67.

    5. Riley WA. Dirofilaria immitis in the heart of a cat. J Parasitol 1922;9:48.

    6. Otto GF. Occurrence of the heartworm in unusual locations and in unusual hosts. In: Morgan HC, ed. Proceedings of the Heartworm Symposium '74. Bonner Springs, KS: VM Publishing; 1975:6-13.

    7. Ryan WG, Newcomb KM. Prevalence of feline heartworm disease—a global review. In: Soll MD, Knight DH, eds. Proceedings of the Heartworm Symposium '95. Batavia, IL: American Heartworm Society; 1995:79-86.

    8. Bowman DD. Dirofilaria. In: Bowman DD, Lynn RC, Eberhard ML, eds. Georgi's Parasitology for Veterinarians. 8th ed. St Louis: Saunders; 2003:216-222.

    9. Genchi C, Venco L, Ferrari N, et al. Feline heartworm (Dirofilaria immitis) infection: a statistical elaboration of the duration of the infection and life expectancy in asymptomatic cats. Vet Parasitol 2008. Accepted for publication.

    10. Venco L, Genchi C, Genchi M, et al. Clinical evolution and radiographic findings of feline heartworm infection in asymptomatic cats. Vet Parasitol 2008. Accepted for publication.

    11. Atkins CE, DeFrancesco TC, Coats JR, et al. Heartworm infection in cats: 50 cases (1985-1997). JAVMA 2000;217:355-358.

    12. Dillon AR, Brawner WR, Hanrahan L. Influence of number of parasites and exercise on the severity of heartworm disease in dogs. In: Soll MD, Knight DH, eds. Proceedings of the Heartworm Symposium '95. Batavia, IL: American Heartworm Society; 1995:113.

    13. Donahoe JM, Kneller SK, Lewis RE. Hematological and radiographic changes in cats after inoculation with infective larvae of Dirofilaria immitis. JAVMA 1976;168:413-417.

    14. Holmes RA, Clark JN, Casey HW, et al. Histopathologic and radiographic studies of the development of heartworm pulmonary vascular disease in experimentally infected cats. In: Soll MD, ed. Proceedings of the Heartworm Symposium '92. Batavia, IL: American Heartworm Society; 1992:81-89.

    15. Browne LE, Carter TD, Levy JK, et al. Pulmonary arterial disease in cats seropositive for Dirofilaria immitis but lacking adult heartworms in the heart and lungs. Am J Vet Res 2005;66:1544-1549.

    16. Dillon AR, Blagburn BL, Tilson DM, et al. Immature heartworm infection produces pulmonary parenchymal, airway, and vascular disease in cats. Presented at the 12th Triennial Heartworm Symposium, Washington, DC, July 2007.

    17. Nelson CT, Seward RL, McCall JW, et al. 2007 Guidelines for the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Cats. American Heartworm Society. Accessed May 2008 at www.heartwormsociety.org.

    18. Dillon AR, Warner A, Hudson J, et al. Role of PIM in inflammatory lung disease of cats and dogs. J Vet Intern Med 1996;10:162.

    19. Dillon AR, Warner AE, Brawner W, et al. Activity of pulmonary intravascular macrophages in cats and dogs with and without adult Dirofilaria immitis. Vet Parasitol 2008. Accepted for publication.

    20. Dillon AR, Warner AE, Molina RN. Pulmonary parenchymal changes in dogs and cats after experimental transplantation of dead Dirofilaria immitis. In: Soll MD, Knight DH, eds. Proceedings of the Heartworm Symposium '95. Batavia, IL: American Heartworm Society; 1995:97-101.

    References »

    NEXT: Dirofilaria immitis in Cats: Diagnosis and Management

    CETEST This course is approved for 2.0 CE credits

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    Did you know... Cats harbor inflammatory cells in their lungs and have a unique response to pulmonary pathogens, often resulting in an exaggerated reaction to pulmonary parasites such as heartworms. Read More

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