When Cardiorespiratory Signs Mislead: A Practical Approach to Canine Heartworm Diagnosis

Canine heartworm disease is caused by the nematode Dirofilaria immitis, with adult worms primarily inhabiting the pulmonary arteries. Many infected dogs may remain clinically silent, while others develop nonspecific cardiorespiratory signs that overlap with several other diseases¹. For the practicing veterinarian, this makes heartworm disease an important differential diagnosis when evaluating dogs with cough, exercise intolerance, dyspnea, or unexplained tiredness^1,2. 

Clinical Signs: Look Beyond the Obvious^1,2 

The clinical findings from dogs evaluated at a cardiorespiratory care unit illustrate how easily heartworm disease can be overlooked. Cough was reported by owners in 80.95% (17/21) of dogs with a positive cough tracheal reflex, emphasizing the value of information obtained during history-taking. 

Exercise intolerance, however, was much less frequently recognized by owners. It was reported in only 27.3% (3/11) of infected dogs and 26.7% (4/15) of non-infected dogs, despite pulmonary pattern abnormalities being identified in 88.5% (23/26) of the evaluated dogs. This suggests that gradual reductions in exercise capacity may not always be apparent to owners. 

Cough should also be interpreted cautiously. Among the evaluated dogs, positive cough tracheal reflexes were actually more frequent in non-infected dogs (90.3%, 14/15) than in infected dogs (63.6%, 7/11). Cough may have extrapulmonary origins and can occur even when pulmonary changes are not evident. Therefore, cough alone should not be used to attribute clinical signs to heartworm infection. 

CBC and Thoracic Imaging: Useful, but Not Definitive 

A complete blood count remains useful when assessing a dog with tiredness or nonspecific systemic signs. It can identify anemia and other abnormalities that may provide an alternative explanation for reduced activity¹. However, hematological findings may be subtle in dogs with mild or moderate disease. 

Thoracic radiography can help evaluate the lung parenchyma, trachea, main bronchi, intrathoracic structures, and cardiac silhouette¹. In the evaluated dogs, bronchial and interstitial pulmonary patterns were common among both infected and non-infected animals. Therefore, these radiographic patterns may indicate cardiorespiratory involvement without necessarily identifying D. immitis as the underlying cause. 

Right-sided cardiac enlargement also requires careful interpretation. Right atrioventricular enlargement was identified in 36.4% of infected dogs, while right heart enlargement was also observed in 40% of non-infected dogs. Patient positioning can influence apparent cardiac size on radiographs, and similar findings may occur with other causes of cor pulmonale¹. 

Echocardiography: Valuable, but Know Its Limitations 

Echocardiography is valuable for identifying morphofunctional cardiac abnormalities. It can sometimes visualize D. immitis within the pulmonary artery or right heart chambers, but its sensitivity for detecting infection is limited^1,3. 

This was evident in the evaluated dogs, where echocardiographic detection of worms had a sensitivity of only 45.5%. Consequently, failure to visualize worms should not be interpreted as sufficient evidence against infection. Specific testing remains necessary, with antigen and microfilariae detection considered standard diagnostic aids for D. immitis infection¹. 

Practical Diagnostic Approach 

When a dog presents with cough, exercise intolerance, dyspnea, or tiredness, the clinical examination, CBC, radiography, and echocardiography can help characterize the underlying cardiorespiratory abnormalities. However, the findings from the evaluated dogs demonstrate that these assessments alone may not reliably determine whether D. immitis is responsible. 

A practical approach is therefore to: 

• Consider heartworm disease among the differential diagnoses when compatible cardiorespiratory signs are present. 

• Give careful attention to the owner's observations, particularly changes in coughing or activity. 

• Use CBC and imaging to identify associated abnormalities and alternative causes. 

• Avoid interpreting cough, pulmonary patterns, cardiac enlargement, or echocardiographic findings in isolation. 

• Include specific microfilariae and antigen testing when investigating D. immitis infection. 

The key clinical message is that cardiorespiratory abnormalities can indicate disease without identifying its etiology. Because canine heartworm disease may be asymptomatic and its clinical manifestations can resemble those of other conditions, specific diagnostic testing is essential when D. immitis is part of the differential diagnosis. 

References 

1. Lemos NM, Alberigi B, Labarthe N, Knacfuss FB, Baldani CD, da Silva MF. How does Dirofilaria immitis infection impact the health of dogs referred to cardiology care. Brazilian journal of veterinary medicine. 2022 Sep 20;44:e002622. https://pmc.ncbi.nlm.nih.gov/articles/PMC9511925/pdf/bjvm-44-e002622.pdf 

2. Keene BW, Atkins CE, Bonagura JD, Fox PR, Häggström J, Fuentes VL, Oyama MA, Rush JE, Stepien R, Uechi M. ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. Journal of veterinary internal medicine. 2019 May;33(3):1127-40. https://academic.oup.com/jvim/article-pdf/33/3/1127/66659642/jvim15488.pdf 

3. Venco L, Manzocchi S, Genchi M, Kramer LH. Heat treatment and false-positive heartworm antigen testing in ex vivo parasites and dogs naturally infected by Dirofilaria repens and Angiostrongylus vasorum. Parasites & vectors. 2017 Nov 9;10(Suppl 2):476. https://link.springer.com/content/pdf/10.1186/s13071-017-2444-6.pdf