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The Realities of Heartworm Disease


Heartworm disease is found in many areas of the world, including the United States. It is a serious disease that can be fatal, even with treatment. It is caused by worms that can grow up to a foot long, and live in the heart, lungs, and associated blood vessels of affected animals. Over time, the worms will cause severe lung disease, heart failure, and damage to other organs, such as the kidneys. Dogs and other species such as wolves, coyotes, and foxes are the primary hosts of heartworms. Since coyotes and foxes are often found in urban and suburban areas, they serve as a potential reservoir of infection for pet dogs. Cats can also be infected by heartworms, though they are affected differently than dogs.

Unlike many other common parasites, heartworms require two separate hosts to reach maturity. Immature heartworms are carried between dogs by mosquitoes. When an infected mosquito bites a dog, the larval stages are left on the skin of the dog and enter the dog’s body through the wound left by the mosquito. It takes 6 months for the larvae to mature into adult heartworms if untreated. Adult worms typically live from 5 to 7 years in dogs and 2 to 3 years in cats. As female worms reproduce, the number of worms will increase with time, then there will be more worms to spread the disease to other animals if an infected animal is not treated.

Even though Minnesota’s climate does not support a year-round population of mosquitoes, we are seeing more cases of heartworm disease in the last several years. There are likely many reasons for this. Dogs travel with their owners more now than ever before, and many will join their owners in warmer climates during the winter months. Local animal shelters often will bring in dogs and cats from the south, where many shelters cannot accommodate them. Dogs are typically heartworm tested prior to transport, but due to the life cycle of the heartworm, there is a window where dogs may test negative but still have heartworms. Heartworm in these dogs may go undetected for several months or more, allowing them to serve as a source of infection for other dogs. Still another cause is inadequate prevention. Many pet owners believe that heartworm prevention only needs to be given during the summer months, leaving their pet vulnerable during the late spring and early fall. While mosquitoes may not be as numerous during those times, they are still active.

Heartworm disease is diagnosed through a blood test, and can be easily prevented in dogs by administration of monthly preventive medication, such as Heartgard Plus. Dogs typically enjoy the chewable, flavored medication and most will readily eat it. We recommend administering preventive medications year-round for a few reasons. The medication will protect against the common intestinal parasites roundworms and hookworms. Year-round administration will insure dogs are protected if we have an early spring or warm fall weather, and will also protect “snowbird” dogs that travel with their owners.  It will also help prevent resistance to the preventative.

Treatment of heartworm disease is possible, but is much more expensive and risky than the monthly preventive medication, and requires strict rest for several months during the treatment period, which is typically not easy with young, active dogs.

Has your dog been heartworm tested in the last year? If not, please contact us to discuss testing and protecting your dog against this serious but preventable disease.