Heartworm in Dogs: Signs, Symptoms & Treatment

Three Minutes
Mar 26, 2024

Heartworms are a life-threatening parasite that can damage a dog’s arteries, heart, and lungs.1 But recognizing the signs of heartworm in dogs can be difficult. That’s why prevention is so important. Let’s take a closer look at heartworm symptoms, causes, and treatment. Then, we’ll explore how pet insurance could help cover the cost of treating or preventing heartworm in dogs.

How Do Dogs Get Heartworms?

Heartworms are parasites that are typically transmitted through mosquito bites. When a mosquito bites a dog or other animal already infected with heartworm, it picks up the larvae (known as microfilaria) that are present in the animal’s bloodstream. Then, the next time the infected mosquito bites an animal, the microfilaria get deposited into the circulatory system of the new host. These larvae get dispersed throughout the body and mature over a period of 6 months. Once heartworm larvae have matured, they can remain present in the host for up to 7 years.1

Tracking mosquito bites is already difficult enough, and there’s no way of knowing if a mosquito is carrying heartworm larvae. That’s why prevention is so important.

A MetLife Pet Policy May Help Cover Heartworm Costs

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Which Dogs Are Most at Risk?

It’s tempting to think that heartworm is only a problem in regions with a heavy mosquito population. However, heartworm disease has been recorded in all 50 states — even in northern states where the weather is colder.1

Pets who spend a lot of time outdoors are typically at a higher risk of infection, but that doesn’t mean indoor pets are completely safe. Mosquitoes can easily get inside your home and target your dog.1

Also, shelter animals tend to be at a higher risk for heartworm since they’ve often spent time outdoors without vet care and preventive medication.2 If you adopt a dog from a shelter, be sure to have them tested and put on a prevention plan as soon as possible. (Note: It can be dangerous for dogs already infected with heartworms to be put on preventive medication. Have your dog checked by a vet first and always follow their recommendations.)3 If you adopt a dog with heartworm, be prepared to give them the treatment they need.

Heartworm Symptoms

Early heartworm disease often doesn’t present symptoms, which is why it can be so difficult to diagnose.1 As the larvae mature, symptoms will become more apparent.

What are the first signs of heartworms in dogs?

Early infection is virtually undetectable. This is the time between the initial mosquito bite and the worms reproducing. It’s not until the worms mature in the heart and lungs that your dog may begin to exhibit:1

Symptoms of mid-stage heartworms in dogs

This is when the mature heartworms begin to clog your dog’s blood vessels, heart, and lungs. At this point, heartworm symptoms become more obvious:1

Symptoms of late-stage heartworms in dogs

This is the most dangerous stage of heartworm disease in dogs because the abundance of worms causes organ failure. Blood flow is blocked by the sheer volume of worms in the heart and circulatory system, a condition known as caval syndrome. Hallmarks of this condition include:1

  • Pale gums
  • Blood in urine
  • Dark urine color
  • Difficulty breathing

If dogs at this stage don’t receive immediate veterinary care, the prognosis is often fatal. Surgery may be required to remove the mass of heartworms from the heart and lungs.1

Heartworm Prevention For Dogs

The good news is heartworm is entirely preventable. Furthermore, taking steps to prevent heartworms isn’t just the safe option, it’s also the most cost-effective one. A monthly prescription for heartworm tablets can range from $6 – $18 per month.4 Annual medications that only need to be given once every 6 or 12 months tend to max out at $150 per 6 months or $350 per year.4 Even at the higher end, that comes out to less than $30 per month.

Diagnosing Heartworm in Dogs

If your dog is exhibiting symptoms of heartworms, bring them to the vet even if they’re already on preventive medication. Vets can use an antigen blood test to check for proteins produced by the heartworms themselves, although these can only be detected 5 months after the initial infestation.3 After 6 months, vets can also test for the presence of the microfilariae in your dog’s blood.3 An X-ray may also be recommended to image the mass of worms in your dog’s heart or lungs.3

When to test for heartworm in dogs

The American Heartworm Society advises pet owners to get their pets tested for heartworms every 12 months and give them a heartworm preventative each month of the year.1 It’s generally recommended not to test dogs for heartworms until they are 7 months old.3

It’s important to test before starting your pup on a heartworm preventative. If your dog already has an adult heartworm infestation, the preventative could kill off the worms. While this might sound like a good thing, the downside is it can trigger systemic shock in your dog, which could endanger their life.3

Treatment for Heartworms in Dogs

Even after it progresses, heartworm disease in dogs is treatable. It can get expensive, however, and may take months before the infection clears up completely.

During the first 2 months, your vet may begin by putting your dog on a regiment of steroids and an antibiotic known as doxycycline. These help reduce inflammation and weaken the heartworms. Combined, these medications can cost nearly $200.4

Preventative medication may also be administered for the first 30 days. This is intended to get rid of any larvae lingering in your dog’s body. A waiting period of another 30 days follows, after which your dog will begin treatment to kill the adult worms. This involves a series of injections of a drug called melarsomine, which can run you between $500 and $1,500 per dose.4,5 Additional rounds of steroid treatment may also be recommended during this stage.4

What is the recovery time for a dog infected with heartworms?

Most cases of heartworm clear up after 1 – 3 months of treatment. A dog’s life expectancy after heartworm treatment is generally favorable, so long as they respond well to the treatment and no permanent damage is caused.

Melarsomine as a treatment is known to have side effects, however, including:6

  • Most common: Pain and swelling at the injection site
  • Less common: Lethargy, vomiting, loss of appetite, depression
  • Rare: Diarrhea, coughing up blood, excessive panting, death

Your vet will monitor your dog’s progress for these side effects or any other complications. Additional steps, such as limiting exercise, can help mitigate the likelihood of complications and may reduce overall recovery time.5

Pet Insurance Could Help You Prevent Heartworm

The cost of heartworm prevention is minimal when compared to the high price tag for treatment and recovery from this dangerous parasite. Protecting your dog also saves them from having to go through the trauma and pain of extensive treatment.

While you protect your dog, let MetLife Pet Insurance help protect your wallet. A standard dog insurance policy could reimburse you for the cost of heartworm treatment, while our Preventive Care add-on can help cover the cost of parasite prevention.7

Learn more about how our insurance coverage works. Then, fetch a free quote to see exactly how much you could save while keeping your pup safe!

Help Protect Your Pup from Major Illnesses

**As with any insurance policy, coverage may vary. Review our coverage and exclusions.

1 “Heartworm in Dogs,” American Heartworm Society,

2 “Managing Heartworm Disease in Shelter Animals,” American Heartworm Society,

3 “Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease,” U.S. Food & Drug Administration, 2022,

4 “Overview of Cost of Heartworm Treatment,” PetMD, 2020,

5 “Heartworm Positive Dogs,” American Heartworm Society,

6 “Melarsomine for dogs: Uses, Dosage & Side Effects,” Wedgewood Pharmacy,

7 For IAIC policies, optional Preventive Care coverage is based on a Schedule of Benefits. For MetGen policies, optional Preventive Care coverage is included in the annual limit.

Coverage issued by Metropolitan General Insurance Company (“MetGen”), a Rhode Island insurance company, headquartered at 700 Quaker Lane, Warwick, RI 02886, and Independence American Insurance Company (“IAIC”), a Delaware insurance company, headquartered at 11333 N Scottsdale Rd, Ste 160, Scottsdale, AZ 85454. Coverage subject to restrictions, exclusions and limitations and application is subject to underwriting. See policy or contact MetLife Pet Insurance Solutions LLC (“MetLife Pet”) for details. MetLife Pet is the policy administrator. It may operate under an alternate or fictitious name in certain jurisdictions, including MetLife Pet Insurance Services LLC (New York and Minnesota) and MetLife Pet Insurance Solutions Agency LLC (Illinois).

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