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Veterinarian Technician January 2010 (Vol 31, No 1)

Parasites 101: Feline Heartworm-Associated Respiratory Disease

by Byron Blagburn, MS, PhD

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    Heartworm infections in dogs were first described in 1626.1 However, feline heartworm infections were not documented in the United States until 1922.2 Although canine and feline infections are caused by the same parasite, Dirofilaria immitis, there are few similarities between the infections in dogs and cats (TABLE 1). The prevalence of feline heartworm infection continues to increase, likely including more than 16 countries and well over 30 US states.3 Because of increased awareness and improved diagnostic methods, feline heartworm infection (and disease) caused by adult heartworms is now diagnosed with increasing frequency in veterinary hospitals.4 The spectrum of clinical signs in infected cats includes coughing, dyspnea, vomiting, and sudden death. However, up to 28% of infected cats may remain asymptomatic.5 The latter is likely the result of the characteristic inactive lifestyle of most pet cats. Interestingly, given the potential for heartworms to cause severe disease or death in cats, some have estimated that less than 5% of cats in the United States receive a heartworm preventive medication.6

    Initially, feline heartworm disease was thought to be due to the presence and/or subsequent death of mature heartworms in the heart or lungs of cats. However, recent research suggests that premature death of immature heartworms in cats can evoke severe and potentially fatal pulmonary reactions.7,8 Moreover, these infections cannot be easily diagnosed using antigen tests, antibody tests, or radiography because of the absence of adult female heartworms, potentially waning antibody levels, and/or the similarity of lung lesions to those associated with other feline pulmonary diseases.7,8 A. R. Dillon and I recently reproduced this condition in experimentally infected cats and coined the term heartworm-associated respiratory disease and the acronym HARD to characterize the potential for cats to present with heartworm disease that is not due to adult worms.7 HARD poses an additional difficulty in that no specific therapy is known.

    Cats harbor inflammatory cells in their lungs and have a unique response to pulmonary pathogens, often resulting in an exaggerated reaction to pulmonary parasites and other infectious agents. Based on cases of feline heartworm infection that we had seen, Dillon and I speculated that many immature heartworms that arrive in cats’ lungs at 70 to 90 days after infection are killed and do not survive to the adult heartworm stage.7,8 The intense inflammation caused by the death of these worms could result in coughing and difficult breathing that would not likely be recognized as heartworm infection by veterinarians or veterinary technicians. This was previously referred to as the 3-month disease cycle because it occurred just 90 days (3 months) after exposure to heartworm-infected mosquitoes.9

    To reproduce the 3-month disease cycle in cats, Dillon and I experimentally infected cats with infective larvae of D. immitis and treated them strategically with ivermectin to eliminate 70- to 90-day-old immature heartworms (i.e., abbreviated infections).7,8 We observed that treated cats developed signs of pulmonary disease similar to those seen in cats with adult heartworms or the 3-month disease cycle. Signs included coughing, dyspnea, and, in one case, even death. Lung lesions consisted of arteritis (including occlusive vessel hypertrophy), bronchitis, bronchiolitis, and interstitial lung disease (alveolitis; FIGURE 1 and FIGURE 2), which were similar to lesions in cats with infections that were not abbreviated. In some cats, we also observed lesions that were not accompanied by clinical signs. Because of the absence of adult heartworms, none of these cats was positive for heartworm antigen. Surprisingly, many cats became negative for antibodies to heartworms within 8 months after experimental infection (5 months after initial treatment to eliminate immature worms). At the end of the study (16 months after infections and 13 months after elimination of immature heartworms), all of the cats tested negative for heartworm-specific antibodies. Radiographs of the lungs of cats with abbreviated infections were similar to radiographs of cats with classic heartworm disease and other pulmonary diseases, such as feline asthma. Because these signs and lesions were caused by the death of immature heartworms, we coined the term HARD to characterize this unique respiratory infection.

    In a study of heartworm-infected stray cats, Browne et al10 demonstrated a correlation between antigen, antibody, and necropsy results that were similar to those of our HARD study. They also correlated these test results with lung lesions, concluding that cats can have lung lesions consistent with heartworm infection but without serologic or necropsy confirmation of heartworm infection.9

    Our HARD study and the Browne et al study should alert veterinarians and veterinary technicians that heartworm disease in cats can be caused by immature heartworms and that diagnosis of these infections can be challenging. It is important to note that specific therapies for HARD are unknown at this time. Cats with HARD can only be treated symptomatically with oxygen, glucocorticoids, bronchodilators, fluid therapy, and thermal support. Even with adequate supportive therapy, 10% to 20% of cats with symptomatic heartworm infections will likely die from respiratory complications.11 Given the potential complications of adult heartworm infections or HARD in cats, I strongly recommend administering broad-spectrum heartworm preventive products to cats with potential exposure to heartworm-infected mosquitoes.

    Downloadable PDF

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

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

    3. Ryan WG, Newcomb KM. Prevalence of feline heartworm disease: a global review. In: Soll MD, Knight DH, eds. Proc Heartworm Symp 1995:79-86.

    4. Lister AL, Atwell RB. Feline heartworm disease: a clinical review. J Feline Med Surg 2008;10:137-144.

    5. Nelson T. Dirofilaria immitis in cats: anatomy of a disease. Compend Contin Educ Pract Vet 2008;30(3):382-389.

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

    7. Blagburn BL, Dillon AR. Feline heartworm disease: solving the puzzle. Vet Med 2007;(suppl):7-14.

    8. Dillon AR, Blagburn BL, Tilson DM, et al. Immature heartworm infection produces pulmonary parenchymal, airway, and vascular disease in cats. Proc 12th Annu Heartworm Symp 2007.

    9. Dillon R. Feline heartworm disease: assessing the danger for owners. In: DVM Best Practices. DVM 2003;34(suppl):23-26.

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

    11. 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 symptomatic cats. Vet Parasitol 2008;158:177-182.

    References »

    NEXT: Tech Tips (January 2010)

    CETEST This course is approved for 1.0 CE credits

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