Clinical Challenge: The Coughing Cat

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When the veterinary team is presented with a coughing cat, there are many possible differentials, including feline asthma, bacterial pneumonia, viral infection, lungworm, congestive heart failure, chylothorax, pyothorax, heartworm disease, neoplasia, and a number of other conditions. Even after obtaining a thorough history and performing a physical examination, laboratory tests, and thoracic radiography, clinicians are often left to distinguish between 2 clinically similar conditions, feline asthma/bronchitis and heartworm disease, which can be challenging. Historically, many cases of heartworm disease have likely been misdiagnosed as feline asthma.^1,2

Obtaining an accurate diagnosis in a coughing cat typically requires a combination of diagnostic tests and is not always a straightforward process. Although a definitive diagnosis may not dramatically impact the patient’s short-term treatment, a correct diagnosis is essential to providing the owner with an accurate prognosis and making long-term management decisions.

Feline Respiratory Disease: Sadie’s Case

Sadie, a 5-year-old spayed, domestic shorthair cat, was presented for an annual wellness examination. She was obtained from a shelter as a kitten and has been an indoor-only cat since adoption. There were no other cats in Sadie’s home. She was up to date on rabies and FVRCP vaccination; she was not on parasite prevention or other medications. Sadie had an unremarkable medical history, with no significant prior illnesses.

Sadie had been doing well overall, according to her owner. The client does mention, however, that she had recently observed a few intermittent episodes of gagging, sometimes leading to vomiting. She attributed these episodes to hairballs and was not overly concerned about them.

On physical examination, Sadie was anxious but not aggressive. Her mucous membranes were pink and moist, with a capillary refill time of <2 sec. She had mild dental calculus and gingivitis. Heart rate was 180 bpm, with no murmurs or arrhythmia auscultated. Respiratory rate was 50 breaths per minute, and slight wheezing was noted. Abdominal palpation, peripheral lymph nodes, and the remainder of Sadie’s physical examination were within normal limits.

Feline asthma and bronchitis are recognized as 2 components of the same syndrome, frequently referred to as feline lower airway disease.³ Asthma/bronchitis and heartworm disease can produce nearly identical clinical signs, although heartworm disease can also cause vague and intermittent signs without any coughing.⁴ Therefore, either asthma/bronchitis or heartworm disease could be a cause of Sadie’s respiratory difficulties. Other less common causes for Sadie’s respiratory signs could include pneumonia, pulmonary neoplasia, and other thoracic diseases. Additional testing would be required to diagnose the cause of Sadie’s gagging, tachypnea, and wheezing.

Preliminary Diagnostic Testing: The Minimum Database

Based on physical examination findings and Sadie’s history, Sadie’s veterinarian recommended a CBC, serum chemistry profile, urinalysis, and thoracic radiography. No significant abnormalities were identified on CBC or serum chemistry, and urinalysis was within normal limits. However, thoracic radiography revealed a diffuse bronchointerstitial pattern.

The lack of visible pulmonary nodules makes pulmonary neoplasia less likely. Asthma/bronchitis and feline heartworm disease both are associated with a bronchointerstitial pulmonary pattern, and both have similar radiographic appearances,² meaning that radiographic findings cannot be used to distinguish these conditions.

Feline Heartworm Disease: Diagnostics

Sadie’s veterinarian elected to begin diagnostics by testing for heartworm infection because she was not on parasite prevention and because blood sample collection would be less stressful for Sadie as compared with bronchoalveolar lavage (BAL) to rule out feline asthma.

An in-house heartworm antigen test was performed using blood collected for Sadie’s CBC and was negative.

Twenty-five percent to 50% of heartworm-positive cats test negative on an antigen test.^5,6 Single-worm infections are common, and heartworm-acquired respiratory distress syndrome (HARDS) can occur in the absence of mature adult worms.^1,6 Necropsy studies of shelter cats have shown that as few as 50% to 70% of infected cats have an adult female worm present that could trigger a positive heartworm antigen test, as antigen tests only detect female worms.^4,6,7 Therefore, a negative heartworm antigen result should not rule out infection, and further testing is required. Infected cats are seldom microfilaremic, making microfilarial testing of limited utility.⁴ Cardiac ultrasonography and heartworm antibody testing can also help obtain a diagnosis of heartworm disease; however, cardiac ultrasonography can only be used in cases of adult worm infection.⁴ Further, an antibody test would be less stressful for Sadie and less expensive for Sadie’s owner and is therefore typically the next test after a negative heartworm antigen test. If a heartworm antibody test is negative, cardiac ultrasonography may be considered as a next step.

A feline heartworm antibody test was submitted to a reference laboratory. Results were received the following day and indicated the presence of antiheartworm antibodies. Although an antibody test does not distinguish between current infection and previous infection, a positive test in the presence of clinical signs and supportive radiographic findings is adequate to make a diagnosis.

Based on these findings, Sadie’s veterinarian made a diagnosis of heartworm-associated respiratory disease.

Next Steps for Sadie

There is no FDA-approved heartworm adulticide treatment for cats. In theory, adult heartworms can be removed surgically, but this is not feasible for most owners. Likewise, there is no practical treatment for HARDS, aside from limiting inflammation.

Managing adult heartworm disease and/or HARDS requires controlling inflammation within the airways. Prednisone is recommended for cats with clinical or radiographic signs of heartworm disease.⁴ Prednisone is typically started at 2 mg/kg/day and gradually tapered over a 2-week period to 0.5 mg/kg every 48 hours. Cats are typically maintained at this low dose for 2 weeks before discontinuing treatment.⁴

Sadie’s veterinarian also recommended regular recheck examinations, which may include thoracic radiography, until Sadie’s clinical signs resolved. There was no indication for regularly rechecking a heartworm antibody test, as it may remain positive for an undetermined amount of time.⁴

In addition, the veterinarian recommended starting Sadie on year-round heartworm prevention labeled as approved for use in heartworm-positive cats, such as Revolution Plus® (selamectin and sarolaner topical solution). Sadie was clearly at risk for exposure in her environment, and heartworm disease prevention would prevent her from developing heartworm disease.

Finally, Sadie’s veterinarian again confirmed that there were no other pets in the home. If Sadie’s owner had other pets, the veterinarian would recommend testing those pets. Although heartworm disease cannot be transmitted directly between pets, there may have been a shared exposure, including both Sadie and her housemates. In Sadie’s case, she was her owners’ only cat, and, therefore, no further testing was required.

What Can We Learn from Sadie’s Case?

Heartworm disease should not be overlooked in indoor cats. Many pet owners forego heartworm prevention in indoor cats because they do not feel their cat is at risk. Unfortunately, mosquitoes regularly enter homes, and heartworm disease can occur in cats that never set foot outdoors.

A retrospective study of 50 cats diagnosed with heartworm disease found that 27% of infected cats were indoor-only.⁸ This suggests that, although living indoors offers some protection against feline heartworm disease, indoor cats are still at risk and should receive heartworm prevention.

Heartworm disease should be considered any time a cat is showing signs of feline asthma/bronchitis, as these conditions are clinically indistinguishable. Assuming a diagnosis of feline asthma, without adequately ruling out feline heartworm disease, robs the veterinary team of a chance to make an accurate diagnosis and provide valuable information to clients, especially in cases when the cat may go on to experience further clinical signs or sudden death. Misdiagnosis could also result in a cat being subjected to unnecessary lifelong treatments.

Antigen tests alone are not sufficient for the diagnosis of heartworm disease in cats. Single-worm infections are common, and HARDS can occur in the absence of mature adult heartworms. Both an antigen and an antibody test (interpreted in light of the patient’s clinical signs) may be needed to make an accurate diagnosis. In some cases, even this dual approach may miss heartworm infections, as heartworm infection cannot be entirely ruled out in cats with a negative antigen and antibody test.

Although heartworm disease prevention is often emphasized in dogs, feline owners may be less aware of the need for prevention. Unlike dogs, cats do not have a feasible treatment option for heartworm disease. Therefore, it is especially important to prevent infection by using an effective heartworm disease preventive, ideally one labeled for use in heartworm-positive cats in case a pet cat is undiagnosed.

Revolution Plus (selamectin and sarolaner topical solution) prevents heartworm disease, treats and controls hookworm and roundworm infections and tick and ear mite infestations, kills fleas, and prevents and treats flea infestations. In an experimental study using a macrocyclic lactone-resistant strain of Dirofilaria immitis, selamectin (6 mg/kg) was 100% effective at preventing heartworm disease in cats when administered at 0, 28, and 56 days postexposure.⁹

IMPORTANT SAFETY INFORMATION

The safe use of Revolution Plus has not been established in kittens less than 8 weeks old or in breeding, pregnant or lactating cats. Reported side effects in clinical trials included lethargy and anorexia. Use with caution in cats with a history of neurologic disorders. Sarolaner, one of the ingredients in Revolution Plus, is a member of the isoxazoline class, which has been associated with adverse reactions such as tremors, ataxia, and seizures. Reactions have occurred in cats with or without a history of neurologic disorders. In humans, Revolution Plus may be irritating to skin and eyes. See full prescribing information.

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