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When Heartworms Migrate to the Heart in Dogs

Andrew R. Moorhead, DVM, MS, PhD, DACVM (Parasitology), University of Georgia


November/December 2021

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In the Literature

Romano AE, Saunders AB, Gordon SG, Wesselowski S. Intracardiac heartworms in dogs: clinical and echocardiographic characteristics in 72 cases (2010-2019). J Vet Intern Med. 2021;35(1):88-97.


Intracardiac heartworms in dogs have been associated with caval syndrome. Further exploration is needed to, however, determine the exact percentage of cases that progress to caval syndrome, associated clinical signs, and echocardiographic parameters. Not all dogs with intracardiac heartworms have clinical signs, including hemoglobinuria, which is one of the hallmark signs of caval syndrome.

The aim of this retrospective study was to determine clinical and echocardiographic characteristics of intracardiac heartworm cases. Records from a veterinary teaching hospital from May 2010 to September 2019 were searched to identify dogs that were referred for caval syndrome, underwent heartworm extraction, or showed intracardiac heartworms on echocardiography.

Seventy-two dogs were included in the study and were divided into cases with low worm burden (a few worms; estimated, <5) and high worm burden (more than a few worms and filling the right atrium). A majority of cases (81%) had high worm burden; of these patients, 75% were small-breed dogs. This bias could be due to breed popularity, specifically Chihuahuas, but the authors postulated it could represent a clinically relevant phenomenon. 

Pulmonary hypertension, assessed via echocardiography, was likely in 93% of cases, possibly secondary to vessel blockage, and pigmenturia was detected in 43% of the likely pulmonary hypertension cases. Pigmenturia, anemia, and bilirubinuria were significantly more common in dogs with a high worm burden. Although only 25% of dogs met the criteria for caval syndrome, extraction of worms was performed in 65% of cases. This study emphasizes the importance of echocardiography for identifying intracardiac heartworms and determining worm burden, especially in small-breed dogs. If worm burden is high, regardless of whether the patient has caval syndrome, extraction of worms may be advantageous.


Key pearls to put into practice:


Echocardiography is key for diagnosing intracardiac heartworms and determining worm burden, especially in small-breed dogs.


Dogs with intracardiac heartworms have a high likelihood of pulmonary hypertension, although diagnosis can be confounded by the presence of a large number of worms.



Presence of intracardiac heartworms does not necessarily indicate caval syndrome; however, the patient may still benefit from extraction of worms.

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