Treatment considerations for asymptomatic cats In many cats, heartworm infection is completely asymptomatic. In a cat with radiographic evidence of heartworm disease and no clinical signs, the American Heartworm Society currently recommends avoiding any sort of treatment and instead monitoring the cat carefully (American Heartworm Society, 2014). Cats that are infected with heartworms, but not showing any signs of pulmonary or cardiac disease, should receive a regular
monitoring including a heartworm antigen test, a heartworm antibody test, and thoracic radiographs every 6-12 months. As long as the patient’s radiographs are showing signs of improvement, or at least not showing evidence of worsening, no further treatment is necessary (American Heartworm Society, 2014). If a positive antigen test turns negative with monitoring, it can be assumed that the risk of clinical disease in the cat has markedly decreased (American Heartworm Society, 2014).
PROGNOSIS
The prognosis for cats with heartworm disease is guarded. Although approximately 80% of naturally infected cats will self-cure, 20% of natural infections will result in sudden death (ESDA, 2017). In many cases, the first indication that a cat has heartworm disease is when the cat presents to the veterinarian with respiratory signs. Cats that survive this initial veterinary visit have a better prognosis than those mentioned above; the
severity of disease and response to treatment is a significant predictor of prognosis. Put in other terms, the prognosis for cats with feline heartworm disease is approximately equivalent to the prognosis for cats with hypertrophic cardiomyopathy. In a study of 50 cats with naturally acquired feline heartworm disease, the median survival for affected cats was approximately 1.5 years with a range of zero days to over ten years (Stannard, 2015).
PREVENTION
Heartworm prevention is an essential component of veterinary preventive care for all cats, including indoor cats. While many owners feel that their indoor cats are not at risk of heartworm disease given a reduced risk of mosquito exposure, approximately 25% of cats diagnosed with heartworm disease are indoor cats (Atkins, DeFrancesco, Coats, Sidley, & Keane, 2000). Feline heartworm preventives typically contain a macrocyclic lactone that is administered once monthly. These products are effective against L3-L4 heartworm larvae; they are not effective against adult heartworms. Heartworm prevention should be administered year-round, for optimal prevention of heartworms (as well as intestinal parasites). Feline heartworm preventives recommended by the American Heartworm Society include the following: ● Ivermectin , administered at 24 µg/kg once monthly. ● Milbemycin oxime , administered at 2.0 mg/kg once monthly. ● Moxidectin , administered at 1.0 mg/kg once monthly. ● Selamectin , administered at 6 mg/kg once monthly (American Heartworm Society, 2014). Feline studies, performed to assess the efficacy of heartworm prevention, have confirmed that regular monthly heartworm prevention plays a valuable role in the prevention of heartworm disease. In one study, cats were experimentally infected with L3 larvae and treated with selamectin 28 days post-infection and monthly thereafter. Although these cats did go on to test positive on heartworm antibody tests, indicating exposure to the L3 larvae, they did not go on to develop adult heartworm infections. In another study, it was shown that doses of selamectin 32 and 2 days before experimental infection with Presenting complaint and history Sadie, a 4-year-old, spayed female domestic shorthaired cat, is presented to the veterinary clinic for coughing. Sadie’s owner is worried about hairballs. The owner states that Sadie often coughs as if she is trying to bring up a hairball but is unable to do so. The owner has never seen Sadie produce any sputum or vomit during these coughing episodes. Sadie’s medical history is otherwise unremarkable. She is up to date on all of her vaccines. Her owner does not administer flea or heartworm prevention, because Sadie is an indoor cat and lives in an apartment. Sadie’s owner reports that Sadie is eating and drinking well, using the litterbox normally, and otherwise doing well at home. No change in her appetite, thirst, urination, or behavior has been noted.
L3 heartworm larvae prevented the development of HARD (Colleran, 2017). While not required, it is considered beneficial to perform a heartworm antigen or antibody test before starting a cat on monthly heartworm prevention. Testing can be beneficial in establishing a baseline for the cat’s heartworm status, as long as the limitations of heartworm testing are understood by the client and the veterinary team. In the event that a client declines heartworm testing, however, there is no contraindication to beginning an untested cat on monthly heartworm prevention. Microfilaremia in cats is extremely uncommon; therefore, the risk of a heartworm- infected cat experiencing a serious reaction to heartworm prevention is very uncommon (American Heartworm Society, 2014). Traditionally, heartworm prevention has focused only on macrocyclic lactones. Exposure was considered inevitable, but macrocyclic lactones were considered valuable in killing the heartworm larvae as they were injected into the patient. With recent increases in heartworm disease transmission leading to concern about the possibility of resistance, heartworm experts are now beginning to promote the idea of “multimodal therapy” for heartworm prevention, at least in dogs (American Heartworm Society, 2016). In multimodal therapy, the emphasis shifts from solely macrocyclic lactones to the idea of using macrocyclic lactones in conjunction with a mosquito repellent (American Heartworm Society, 2016). While this approach is not currently receiving much discussion in treating cats, it is being discussed in treating dogs and is worth being aware of in case it becomes more practical for cats. Physical examination On physical exam, Sadie was bright, alert, and responsive. Her body condition score was 3/5 and her coat was smooth and shiny. Normal ambulation was observed in the exam room. Auscultation of the heart and lungs was challenging because Sadie purred through much of the exam. No cough was elicited on palpation of the trachea. Eyes, ears, nose, and throat were apparently normal, with the exception of mild dental calculus. Abdominal palpation was within normal limits, peripheral lymph nodes were within normal limits, and no abnormalities were noted on neurologic or musculoskeletal exam. Sadie’s weight was 9.6 lbs. Her temperature was 101.6°F.
CASE STUDY: HEARTWORM-ASSOCIATED RESPIRATORY DISEASE
Page 55
Book Code: VFL1524
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