Heartworm Positive Dog

By January 10, 2012 July 14th, 2012 Interesting Cases

‘Lola’ is an approximately 4 year old, female mixed breed dog who was picked up by a shelter early this year.

As part of routine screening, ‘Lola’ had a heartworm antigen test, and her test came back positive! A heartworm antigen test is a very sensitive test that detects the presence of adult heartworms, so a positive result means heartworm infection! Although (fortunately!) Lola was not showing any clinical signs, treatment was recommended because untreated heartworm disease can lead to major complications, including death.

Following a heartworm antigen test, a filter test is run to check for the presence of microfilaria (heartworm larvae) in the bloodstream. The standard treatment for heartworm infection involves killing off the adult worms. Since the microfilaria will eventually mature and form adult heartworms, it is important for us to also provide treatment to kill off the microfilaria if we find them to be present. Lola’s filter test came back positive with a whopping 3 160 microfilaria per 1 ml of blood!!

Click Play to see a video of a “wet mount” of Lola’s blood containing microscopic microfilaria

In order to formulate a treatment plan, a positive heartworm patient needs to be classified into Class I (virtually no clinical signs or changes on x-rays), or Class II or III (moderate to severe clinical signs, and evidence of heart and lung damage on x-rays). Since Lola was not showing any clinical signs and her radiographs (x-rays) were normal, she was classified as Class I. Heartworm treatment consists of 2 or 3 injections (depending on disease severity) of Immiticide, a medication that works by killing off adult heartworms. Patients must be strictly rested for 4-6 weeks following treatment because of the risk of pulmonary thromboembolism (blood clot to the lungs) occurring secondary to the dying worms. Following Immiticide treatment, Lola was placed on Heartgard, a medication generally used as heartworm preventative that kills of the microfilaria (larva), preventing them from developing into adult heartworms.

Lola is doing very well post-treatment, and we will be following up with heartworm antigen tests in the months to come. She looks forward to finding her ‘forever home’ and remaining heartworm free for the rest of her days!

What is heartworm and how is it transmitted?

Heartworm is an internal parasite that affects dogs, and less commonly, cats. While many internal parasites of animals live in the gastrointestinal system and are detected in the stool, heartworm larvae live in the bloodstream, with adults living in the pulmonary (lung) blood vessels and the heart. It is transmitted from dog to dog by mosquitoes. A mosquito bites an infected dog, and ingests some larvae along with the blood meal. Over the next 3-4 weeks, the immature larvae develop into a stage capable of infecting another dog. The mosquito then bites an uninfected dog, transmits a small amount of saliva with the bite, and in turn, infects the dog with heartworm larvae. Over the next 5-7 months, the larvae develop into adult heartworms, and inhabit the heart and blood vessels.

What are the clinical signs of heartworm disease? How would I know if my pet had it?

Clinical signs of heartworm disease can vary greatly. Coughing and exercise intolerance (tiring easily while walking or running) occur due to damage to the lungs. Lethargy, cachexia (severe weight loss) and a swollen abdomen may occur secondary to heart failure. Your veterinarian may notice tachycardia (increased heart rate), hepatomegaly (an enlarged liver), pale mucous membranes (gums), weak pulses and/or increased lung sounds.

On the other hand, as was the case with Lola, a dog infected with heartworm may not show any clinical signs. A dog can have a moderately severe infestation and still act, exercise, and breathe normally. Although these dogs will eventually develop enough adult heartworms to demonstrate many of the signs described above, a routine screening test may show a positive result before any signs are seen by the owner. This is why yearly testing is so important, even if your dog is on regular heartworm prevention. While finding and treating heartworm disease early is important, preventing heartworm disease all together is, by far, the best option for your pet!