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Heartworms are large worms, spread by mosquitoes, which live in the heart and large blood vessels of dogs. They cause severe heart/vessel damage, which leads to heart failure. They are found in many areas of the US, including Oregon. The northern part of the state (north of Eugene) has historically been very low-risk. Southern Oregon (south of Eugene) has been a higher risk area for years. However, in 2008 the northern Willamette Valley (Eugene to Portland) was declared to be endemic for heartworms. Any dog that can be bitten by mosquitoes is potentially at risk.

Heartworm disease can be successfully treated, but symptoms do not occur until after heart damage has developed. The treatment itself is expensive and can be risky, so our efforts are focused on prevention.


Ideally, we begin treating puppies with heartworm preventative once a month at the age of 8 weeks. At present we use the Heartgard Plus or Advantage Multi, because it kills most intestinal worms in addition to preventing heartworms. We are now using one of these medications as our routine puppy wormers and give it once monthly until the puppy reaches 6 months. At 6 months you can choose to continue monthly with the Heartgard Plus chewable tablets or Advantage Multi topical, or you can choose to begin an alternative medication (such as Trifexis, Revolution, etc.) to prevent heartworms. Prevention should be continued year-round, as mosquitoes can be found all months of the year in Oregon.


We use a blood test to check for the presence of heartworms. The test can only detect mature heartworms (worms at least 6 months old). This affects the timing of the test, as noted below:

Dogs started on preventatives as puppies, or older dogs that have spent their lives in low-risk areas, are tested after they have been on a heartworm preventative for six months. That way, we are testing for their heartworm status as of the time they began preventative medication. Thereafter, as long as they remain in relatively low risk areas, they are tested at two-year intervals.

Older dogs that have lived and/or traveled in high-risk areas are tested when heartworm medication begins and again 6 months later. Thereafter, they are also tested at 2-year intervals.

Any dog that routinely travels to very high-risk areas (the southern United States, the eastern seaboard, southern California, Mexico) should be tested yearly. In Oregon, the risk increases as you travel south and peaks in southern Oregon, especially near Grants Pass/Medford.

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