An Unexpected Case Of Heartworm

First-person report by Dolores Roeder, DVM, our electronic vet, exclusively at

It was the last thing her owners were expecting; the news was devastating. Seven year old Tiffany, the apple of her owner's eyes, was positive for heartworm. Tiffany, the 9.5 pound Lhasa who spent her days on her owner's laps and her nights on their bed had contracted a potentially life-threatening parasite. They needed to take action immediately, starting with understanding what they were up against.

News About Animals

Heartworm is just that; a worm that lives in the heart and pulmonary (lung) vessels. Heartworm is spread by mosquito, so even a dog who spends 100% of her time indoors is still susceptible to exposure.

Basically, a mosquito bites a dog that has circulating microfilaria (the offspring of the heartworm.) As the mosquito ingests a bloodmeal, it ingests these live microfilaria. There is a maturation phase that takes place in the intestine of the mosquito where the microfilaria develop into the larval form. When the mosquito bites the next dog, these larval forms are injected into the new host. Over the next several months, these larvae undergo additional molts and travel throughout the new host's body, eventually making their way to their ultimate destination: the heart.

Adult heartworm can range in size from two to fourteen inches and their physical presence in the heart can lead to congestive heart failure within months to years, depending on the number of worms present.

Heartworm is a preventative disease. Prescription medications can kill the infective larval forms before they have a chance to develop into adults. Several products are available in tablet or liquid form. Dosing, depending on the product chosen, may be daily or monthly. The choice of medications will be influenced by the prescribing veterinarian's knowledge of the individual patient and the patient's risk of exposure. It is ABSOLUTELY NECESSARY that any dog who has not been on continuous heartworm preventative be blood tested before starting on preventative medication as there is a risk of circulatory collapse and death if a dog with circulating microfilaria is put on certain heartworm preventatives.

Heartworm can be eliminated from the body by treating with intravenous or intramuscular arsenic compounds. The treatment is usually expensive and carries significant risks; the goal is to kill the worms without killing the dog. PREVENTION IS THE KEY!!!

Tiffany's owners were vigilant about making sure that she was given (and swallowed!!!) her heartworm preventative. In retrospect, it appears that she contracted heartworm because she was underdosed with preventative. The preventatives are mrketed for different weight classes, and at a critical time during last year, Tiffany put on just enough weight to put her over the cutoff. The result is that Tiffany must now undergo the heartworm treatment.

On the upside, Tiffany was diagnosed heartworm positive by the bloodtest conducted during her annual exam. At this time there are no clinical signs of heart or lung damage. She appears to be in good health and is a good candidate for the treatment. Her case is a good illustration of why annual bloodtests are so critical; had her owners declined the test, their first indication that Tiffany was heartworm positive may have been because she had advanced into congestive heart failure.

It will be six months or so before Tiffany finishes all the steps needed to rid her body of the worms. By this time next year, she is expected to be back on monthly preventative, but on a higher, more appropriate dose. Tiffany's owners and veterinarian have learned an expensive lesson.

By Dolores Roeder, D.V.M.,
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