When you’re looking to add a canine companion to your household, seeing the phrase “heartworm-positive dog” on an adoption profile might give you pause. You should know that although heartworm can be fatal, it is very treatable if caught early. And even more importantly, your veterinarian can prescribe preventatives that are highly effective in stopping your dog from getting heartworm in the first place.

What is heartworm and how is heartworm transmitted?

“Heartworm” describes a condition where parasites live and reproduce in the host’s heart. The heartworms themselves can be up to a foot long and are passed by mosquitoes, who carry the larvae from dog to dog.

Where is heartworm most common?

Mosquitoes thrive in hot, humid environments, and heartworm is rampant in the southern U.S. According to the American Heartworm Society, Mississippi, Louisiana, South Carolina, Arkansas and Alabama are the states with highest heartworm infection rates.

Unfortunately, living in a cold area won’t protect your dog from heartworm. The disease has been diagnosed in all 50 states, and some veterinary epidemiologists believe that climate change is a factor in the rise of heartworm across the U.S.

What are the symptoms of heartworm?

Immediately after infection, a dog might not show many symptoms. But as the parasites multiply, you might see the following signs:

  • Mild persistent cough
  • Reluctance to exercise
  • Fatigue after moderate activity
  • Decreased appetite
  • Weight loss
  • Excess fluid in abdomen

Late-stage heartworm symptoms tend to be more severe. An untreated heartworm infection can damage a dog’s heart, arteries, lungs and other organs and can lead to congestive heart failure and death.

How do you treat heartworm?

Fortunately, multiple drugs can kill heartworms and treatment has a high success rate. Heartworm treatment typically uses melarsomine and/or ivermectin. Some dogs will also receive an antibiotic.

  • Melarsomine (often prescribed by its brand name, immiticide) injections treat adult worms and late-stage larvae.
  • Ivermectin targets immature larvae and microfilaria (the parasites’ earliest life stage). Most heartworm preventatives use ivermectin; it also helps prevent spread.
  • An antibiotic kills bacteria living within the heartworms.

Timing is key when treating heartworm. The larvae and worms exist in varying stages in your dog’s body, and your veterinarian will want to ensure that you eradicate every stage. Heartworm treatment often takes several months.

What to expect during heartworm treatment

Caring for a dog with heartworm requires a two- to four-month exercise restriction while the worms are dying. If dogs get too excited or rambunctious during this stage, they could experience a massive worm die-off. In some cases, this event can lead to a pulmonary embolism (blood clot in the lungs) or anaphylaxis, both of which can be life-threatening.

Cage rest might help give your pup the best chance of recovery during treatment for heartworm. Your veterinarian will provide guidelines, such as how long your dog should be crated and how often they can have a bathroom break. Enrichment toys and plenty of quiet, calm attention can help the days pass more quickly.

How do you prevent heartworm?

The best defense is a good offense. Your dog should get an annual blood test and receive a heartworm preventative on a consistent basis. Although heartworm infections are less common during the winter, they’re not impossible—that’s why the American Heartworm Society recommends year-round preventatives.

Monthly heartworm preventatives are available as chewable treats or as part of some topical flea-prevention medications. Your veterinarian might also give your dog an injection that lasts for six to 12 months.

Can cats get heartworm?

In most cases, a cat’s immune system will destroy heartworms before they become a problem. But there is no heartworm treatment for cats, so they are not fully immune. And even a seemingly small number of heartworms can be dangerous. That’s why some veterinarians recommend heartworm preventatives for cats.