What Are Heartworms?

Heartworms are bloodborne parasites that live in the heart, lungs and associated blood vessels of affected animals. They primarily infect dogs, cats and ferrets, but can also affect other mammals like raccoons, opossums and coyotes. 

What is the lifecycle of a heartworm?

The maturation of a heartworm relies strictly on two hosts: a mosquito and a host mammal.

1. Adult worms mate inside of the host, and the females release microscopic larvae called microfilaria into the infected animal’s bloodstream. 

2. When a mosquito bites the infected animal, it draws up and ingests microfilaria. Over the next 10-14 days, the microfilaria develop into “infective” larvae. Microfilaria that have not been consumed by a mosquito will not mature into adult heartworms.

3. When the mosquito carrying microfilaria feeds on another animal, the larvae are deposited into the animal’s bloodstream.

4. Infective microfilaria will then mature into adult heartworms over the course of 6-7 months, and primarily migrate to the pulmonary artery. In a heavy infection, adult heartworms will also take residence in the chambers of the heart and/or lungs.

5. Adult heartworms can live as long as 5-7 years without treatment, and may grow to be up to 1 foot long.

How are heartworms transmitted?

An animal infected with heartworms is not contagious. Heartworms can only be transmitted by the bite of a carrier mosquito. When a mosquito bites an infected animal, it draws up microscopic larval heartworms. These microfilaria then enter the bloodstream of other animals the mosquito feeds on, and mature into adult heartworms. 

Animals on heartworm prevention will not be infected if bitten by a carrier mosquito, and humans are not  affected by the bite of a carrier mosquito.

What are the repercussions of heartworm infection if a dog does not receive treatment?

Short Term:

1. Lethargy and loss of appetite: heartworms restrict the natural flow of blood, which can result in lethargy and loss of appetite.

2. Vascular inflammation: heartworms trigger the body’s immune response and can cause inflammation of blood vessels. 

3. Exercise and temperature intolerance: heartworm positive dogs will tire much faster, and are more likely to overheat.

4. Persistent cough: worm congestion in the lungs and pulmonary artery can cause a persistent, dry cough.

5. Vascular damage: irritation to the blood vessels, as well as the physical impact of adult heartworms filling and latching to the body, causes damage to the vascular system.

Long Term:

1. Pulmonary hypertension: narrowed blood vessels increase blood pressure, making it much harder for the heart to pump blood to the rest of the body.

2. Congestive heart failure: heart health declines from persistent high blood pressure. This causes enlargement of the heart, and the eventual failure of its right side.

3. Fluid retention: as heart function declines, fluid begins to leak and accumulate in the abdomen.

4. Organ damage: adult heartworms restrict flood flow to the body, and microfilaria can block small capillaries, leading to organ damage.

5. Caval syndrome: an emergency situation where a heavy worm burden blocks blood from returning to the heart, leading to labored breathing and potential organ failure.

How will my dog’s heartworms be treated?

In accordance with the recommendation of the American Heartworm Society, heartworm positive HLCR dogs will go through what is known as the “fast kill” treatment process. Dogs with very high energy needs or compounding health issues may be treated with the “slow kill” method, but this is uncommon. 

The “fast kill” process involves a veterinarian administering three immiticide - heartworm killing -  injections which will destroy any adult worms living in the lungs, pulmonary artery and heart. Dogs will be maintained on monthly heartworm prevention through this process to kill any microfilaria in their bloodstream, and should be continued on prevention for the duration of their lives to prevent future infection. 

HLCR will cover the medical expenses and procedures required for the dog to receive the three injections prescribed by the veterinarian, which may include: veterinary exams and overnight stays, chest x-rays, blood work, and medications like Doxycycline, Prednisone and Gabapentin. 

What does the heartworm treatment process look like?

1. The heartworm positive dog will be started on an oral heartworm prevention medication, and be maintained on this medication for the duration of their treatment.

2. The dog will go to a veterinarian for an initial exam. At this appointment, the doctor will check their vitals, perform a heartworm test to confirm a positive infection, possibly perform chest x-rays to look for visible damage to the heart or lungs, and prescribe a 14-30 day course of the antibiotic Doxycycline. Dogs demonstrating symptoms of a more severe infection, such as coughing, wheezing or lung congestion, may also be prescribed a tapering course of the steroid Prednisone.

3. Once the dog has completed the doxycycline, there will be a 30 to 60 day “rest period” before the first immiticide injection is administered. The dog may go for walks and play, but should be kept from any strenuous exercise that significantly elevates their heart rate.

4. The dog will return to the vet for their first immiticide injection 30 to 60 days after completing the doxycycline. They will:

   1. Be dropped off early in the morning on the first day.

   2. Receive their first injection midday.

   3. Stay the night at the clinic.

   4. Begin a tapering oral course of prednisone, and possibly be sent with sedative medications like gabapentin or trazodone.

   5. Go home after noon on the second day. 

5. The first injection is followed by a strict 30 day period of rest. The dog should not be taken for walks, needs to be kept calm and should not be allowed to elevate their heart rate.

6. Once this 30 day period is complete, the 2nd and 3rd injections will be administered two consecutive days in a row. The dog will:

   1. Be dropped off early in the morning on the first day.

   2. Receive their first injection midday.

   3. Stay the night at the clinic.

   4. Receive their second injection midday on the second day.

   5. Stay the night at the clinic.

   6. Go home after noon on the third day, once again with a course of prednisone and possibly sedative medications.

7. The final injection is followed by another strict 30 day period of rest. 

8. 4-6 months after the final injection, your dog will return to the vet for a heartworm test to confirm that the treatment was successful. 

Understanding the Drugs Associated with Heartworm Treatment

Heartworm Prevention

Heartworm prevention protects animals from heartworm infection by killing microfilaria in the animal’s bloodstream. Heartworm prevention medication shortens the lifespan of adult heartworms, but will not kill them outright. Although there are now injectable alternatives, most heartworm prevention is an oral pill, with different brands utilizing a broad range of drugs.

Melarsomine - Immiticide Injection

Melarsomine is an FDA approved, arsenic-based drug prescribed for the treatment and eradication of adult heartworms. It is the only drug that has been FDA approved for this purpose. Immiticide injections are spaced out over the course of three treatments to allow the dog’s body time to safely process the dying worms’ bodies.

Doxycycline

Doxycycline is an antibiotic that is prescribed to kill Wolbachia, a microscopic organism that lives in the reproductive system of adult heartworms. The destruction of Wolbachia serves three key purposes:

1. The removal of Wolbachia damages the heartworms’ reproductive systems and can make them infertile. 

2. Microfilaria deposited by heartworms that were treated with doxycycline typically do not develop into infected microfilaria when ingested by mosquitos.

3. Treatment with doxycycline increases the speed at which adult heartworms die. 

Prednisone

Prednisone is a steroid known to reduce inflammation and suppress immune response. It is typically administered as an injection before each immiticide treatment, in addition to being given orally in the weeks following. Prednisone serves three key purposes during heartworm treatment:

1. Dying microfilaria and adult heartworms release toxins into the bloodstream that trigger an immune response which causes tissue inflammation in the pulmonary artery and lungs. Prednisone suppresses this immune response to significantly reduce inflammation.

2. Prednisone’s immunoresponse qualities protect dogs against allergic or adverse reactions to the immiticide injections. 

3. Prednisone’s antiinflammatory properties help to prevent life threatening blockages in the lungs, caused when adult heartworms die and break off into the blood stream.

Gabapentin and Trazodone

Gabapentin and Trazodone are sedative drugs used to manage pain following immiticide injections, and help the dog remain calm during the post-injection recovery periods. 

How will HLCR assist with the treatment process?

The Highland Lakes Canine Rescue partners with several veterinary organizations offering low-cost heartworm treatment:

• Burnet Veterinary Clinic in Burnet, TX

• Hope Animal Clinic in Marble Falls, TX

• Emancipet, located in many large cities across TX

If adopters are able to transport the dog to and from treatment appointments at one of the clinics mentioned above, HLCR will assume the cost of the heartworm treatment. HLCR does not cover any services or medications outside of the standard for heartworm treatment and is not responsible for any other expenses, including unrelated treatment, procedures, vaccinations, items etc. charged in or outside the event of a coordinated heartworm disease injection.