Feline Heartworm Disease--The Hard Part
ACVIM 2008
Danielle Russ, LVT, BS, BA, AS
Virginia Beach, VA, USA

What is HARD?

Heartworm Associated Respiratory Disease (HARD) is a feline respiratory disorder secondary to previous or current heartworm infection (HWI). Historically, heartworm disease (HWD) was regarded as a canine problem, and the cat was considered to be an atypical host. Infection rates in cats are certainly much lower, but recent studies have shown that the consequences of HWI and HWD are far more severe in cats than dogs. The impact of even a small worm burden can be substantial, and permanent lung damage can occur prior to full maturation of the heartworms (Dirofilaria immitis).

What Does a HARD Patient Look Like?

The clinical signs of HARD mimic those of primary pulmonary diseases, and include coughing, wheezing, tachypnea, and dyspnea. On initial presentation, it may be impossible to distinguish a cat with HARD from a patient with asthma, pulmonary parasitism, diffuse bacterial pneumonia or fungal disease. Some patients may manifest additional signs more commonly attributable to HWD (e.g., vomiting, or cardiac complications) which should put this differential high on the list of possibilities. However, the majority of cats with HARD present with respiratory signs only; if HWD/HWI is not considered in these patients the correct diagnosis will be missed.

Making it Easier to Identify HARD

The first step in understanding the impact of HARD on your patient population is to recognize the prevalence of HWD in your geographical location. The expected incidence of infection is 5-20% of that seen in dogs, and feline HWD is more likely to occur in warmer climates. Essentially, if HWD is a concern for your canine patients, then it is probable that your feline patients are also at risk.

Maturation of the L3 stage of D. immitis is dependent on the weather, so there is a natural seasonality to HWI, with transmission occurring during the late spring and summer months. Clinical symptoms of HWD may develop as early as 70-90 days post infection, so it is reasonable to see these patients in the late fall/early winter months.

Naturally, the risk of HWD is higher in outdoor cats. However indoor cats are not safe from infection, and in fact a third of cats diagnosed with HWD at NCSU were apparently kept indoors.

Antibody and Antigen Testing

The currently available tests identify both feline antibodies and worm antigens (from adult females). A positive antibody test indicates exposure, not infection. A positive antigen test confirms the presence of adult worms, but may overlook single sex or low number infection. False negative results are not uncommon.

Any and every combination of results may be present in patients with HARD.


Thoracic Radiography

Radiographic abnormalities reported in felines with HARD include focal or diffuse infiltrates (interstitial, broncho-interstitial, or alveolar), perivascular densities, and atelectasis. Classic changes suggesting HWD (including enlargement of the caudal pulmonary arteries) may also be present, but can be subtle and easily overlooked. If a cat is known to have HWI, significant radiographic abnormalities may not be apparent initially, but may occur later.


Echocardiography is a sensitive way to detect adult heartworms in cats. A distinct "double-lined echodensity" is imaged in the main pulmonary artery. Two particular limitations to this testing method are it requires a high level of expertise and immature worms may be missed due to their size.


Clinical presentation


Chronic or cyclical respiratory signs

Heartworm prevention (monthly)

Prednisone (1-2 mg/kg PO q 48-72 hrs)

Acute respiratory distress


Dexamethasone (1 mg/kg IV/IM) OR
Prednisolone sodium succinate (50-100 mg/cat IV)

Aminophylline (6.6 mg/kg IM q 12 hrs) OR
Theophylline ER (10 mg/kg PO) OR
Terbutaline (0.01 mg/kg SC)

Heartworm prevention (monthly)

Asymptomatic--Antigen positive

Heartworm prevention (monthly)

Asymptomatic--Antibody positive

Heartworm prevention (monthly)

Asymptomatic--Antigen and antibody negative

Heartworm prevention (monthly)

Prognosis for Cats with HARD

The respiratory signs can often be managed successfully with anti-inflammatories. However, worm removal and adulticide therapy are both associated with severe morbidity in cats, and most cats are simply supported in the hope that they will outlive the worms; this is generally thought to take 2 years. Clients must be warned of the consequences of HWD in cats, including sudden death, neurological complications and thromboembolism.

What's Next?

Despite many advances in feline heartworm disease over the last decade, most of the data has been acquired from cats in shelters or those presented to universities for evaluation of active disease. There is a substantial lack of serological data from pet felines nationally, and periodic testing of healthy pet cats is still not routine.

In 2007, the American Heartworm Society listed three key reasons for heartworm testing cats:

 To establish an etiologic diagnosis in those individuals that, based on other clinical evidence, are suspected of being infected

 To monitor the clinical course of those that have already been diagnosed with feline HWD

 To establish a baseline reference prior to initiating chemoprophylaxis


1.  American Heartworm Society Web site. 2007 Guidelines for the Diagnosis, Prevention, and Management of Heartworm (Dirofilaria immitis) Infection in Cats. Available at http://www.heartwormsociety.org/. Accessed February 1, 2008.

2.  Atkins CE. Understanding Feline Heartworm Disease. Proceedings of the 79th Annual Western Veterinary Conference. 2007.

3.  Atkins CE, DeFrancesco TC, Coats J, et al. Heartworm infection in cats: 50 cases (1985-1997) Jour Vet Med Assoc 2000; 217:355-358.

4.  Atkins CE, Ryan WG. CVT Update: Diagnosis and Prevention of Heartworm Disease in Cats. CVT XIII, 2000. pp 782-7.

5.  Blagburn BL, Dillon, AR. Feline heartworm disease: Solving the puzzle. Supplement to Veterinary Medicine, March 2007.

Speaker Information
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Danielle Russ, LVT, BS, BA, AS
Brydelle Farm
Saluda, VA

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