The question arises as to whether heartworm prophylaxis is warranted for cats because they are not the natural host and because the incidence is low. Necropsy studies of feline heartworm infection in the Southeast have yielded a prevalence of 2.5% to 14%, with a median of 7%. When considering the question of institution of prophylaxis, it is worth considering that this prevalence approximates or even exceeds that of feline leukemia virus (FeLV) and feline immunodeficiency virus (FTV) infections. A 1998 nationwide antibody survey of over 2000 largely asymptomatic cats revealed an exposure prevalence of nearly 12%. It is also noteworthy that, based on owners’ information, nearly one third of cats diagnosed with heartworm disease at NCSU were housed solely indoors. Lastly, the consequences of feline heartworm disease are potentially dire, with no clear therapeutic solutions. Therefore the author advocates preventative therapy in cats in endemic areas. Three drugs with FDA approval are marketed for use in cats (Table Comparison of Spectra ofMacrolides Currently in Use in Cats). Ivermectin is provided in a chewable formulation, milbemycin as a flavored tablet, and selamectin (a broad-spectrum parasiticide) comes in a topical formulation. The spectrum and the formulation of these products varies; hence the clients’ individual needs are easily met in most cases (Table Comparison of Spectra ofMacrolides Currently in Use in Cats).
Comparison of Spectra ofMacrolides Currently in Use in Cats
|Drug||Heartwotm prevention||Hookworms||Whipworms||Roundworms||Tapeworms||Fleas & Eccs||Ticks||Sarcoptes||Ear Mites|
|Milbemycin (flavored tablet)||+||+||+|
Because the vast majority of cats are amicrofilaremic, microfilaricidal therapy is unnecessary in this species. The use of arsenical adulticides is problematic. Thiacetarsemide (sodium caparsolate), if available, poses risks even in normal cats. Turner, Lees, and Brown reported death due to pulmonary edema and respiratory failure in 3 of 14 normal cats given thiacetarsemide (2.2 mg/kg twice over 24 hours). Dillon and colleagues could not confirm this acute pulmonary reaction in 12 normal cats receiving thiacetarsemide, but one cat did die after the final injection. More importantly, a significant, though unquantified, percentage of cats with HW1 develop pulmonary thromboembolism (PTE) after adulticidal therapy. This occurs several days to 1 week after therapy and is often fatal. In 50 cats with HWI, seen at NCSU, 11 received thiac-etarsemide. There was no significant difference in survival between those receiving thiacetarsemide and those receiving symptomatic therapy.
Data on melarsomine in experimental (transplanted) heartworm infection in cats are limited and contradictory. Although an abstract report exists in which one injection (2.5 mg/kg; one half the recommended canine dose) of melarsomine was used in experimentally infected cats without treatment-related mortality, the worm burdens after treatment were not significantly different from those found in untreated control cats. Diarrhea and heart murmurs were frequently noted in treated cats. A second abstract report, using either the standard canine protocol (2.5 mg/kg twice over 24 hours) or the split dose (one injection, followed by two injections, 24 hours apart, in 1 month) described in posts, gave more favorable results. The standard treatment and split-dose regimens resulted in 79% and 86% reduction in worm burdens, respectively, and there were no adverse reactions. Although promising, these unpublished data need to be interpreted with caution because the transplanted worms were young (<8 months old and more susceptible), and the control cats experienced a 53% worm mortality (average worm burden was reduced by 53% by the act of transplantation). Additionally, the clinical experience in naturally infected cats has been generally unfavorable, with an unacceptable mortality. Because of the inherent risk, lack of clear benefit, and the short life expectancy of heartworms in this species, this author does not advocate adulticidal therapy in cats. Surgical removal of heartworms has been successful and is attractive because it minimizes the risk of thromboemboli. The mortality seen in the only published case series was, unfortunately, unacceptable (two of five cats). This procedure may hold promise for the future, however.
Cats with heartworm infection should be placed on a monthly preventa-tive and short-term corticosteroid therapy (prednisone at 1 to 2 mg/kg every 48 hours, three times a day) used to manage respiratory signs. If signs recur, alternate-day steroid therapy (at the lowest dose that controls signs) can be continued indefinitely. For embolic emergencies, oxygen, corticosteroids (dexamethasone at 1 mg/kg intravenously or intramuscularly, or prednisolone sodium succinate at 50 to 100 mg intravenously/cat), and bronchodilators (aminophylline at 6.6 mg/kg intramuscularly every 12 hours, theophylline sustained release at 25 mg/kg orally, or terbutaline at 0.01 mg/kg subcutaneously) may be used. Bronchodilators have logic, based on the ability of agents, such as the xanthines (aminophylline and theophylline), to improve function of fatigued respiratory muscles. In addition, the finding of hyperinflation of lung fields may indicate bronchoconstriction, a condition for which bronchodilation would be indicated. Nevertheless, this author does not routinely use bronchodilators in feline HWD.
The use of aspirin has been questioned because vascular changes associated with HW1 consume platelets, increasing their turnover rate and effectually diminishing the antithrom-botic effects of the drug. Conventional doses of aspirin did not prevent angiographically detected vascular lesions. Doses of aspirin necessary to produce even limited histologic benefit approached the toxic range. Despite this, because therapeutic options are limited, at conventional doses (80 mg orally, every 72 hours), aspirin is generally harmless, inexpensive, and convenient. Because the quoted studies were based on relatively insensitive estimates of platelet function and pulmonary arterial disease (thereby possibly missing subtle benefits), the author continues to advocate aspirin for cats with HWI. Aspirin is not prescribed with concurrent corticosteroid therapy. Management of other signs of heartworm disease in cats is largely symptomatic.