Heartworm in Dogs – The Dog Owners Guide

When it comes to unpleasant parasitic infections, heartworm is in a league of its own. Indeed the worm, Dirofilaria immitis, responsible for this infection competes for the dubious honor of being the “most pathogenic (deadly) parasitic worm affecting dogs.”

Perhaps you have a new puppy and are wondering about heartworm preventatives. Or maybe you’re considering skipping regularly screening or preventative treatment. Before you make any cost-cutting decisions, read this article.

Heartworms in Dogs – A Fact of Life

No dog should live life in a plastic bubble. Running in fields and playing in woodland exposes our canine companions to all manner of unwanted visitors. Picking up parasites is one sign a dog leads a full and active life, and is nothing for an owner to be ashamed of – however – heartworm is more than unpleasant, it can kill.

The Life and Loves of the Heartworm in Dogs

First, let’s introduce the heartworm.

  • Full name: Dirofilaria immitis  (nickname: Heartworm)
  • Location: Now found in every single US state, including Alaska
  • Lifestyle: A bit of a freeloader, this parasite is transported between hosts by mosquitoes
  • Ambition: To find a place in every dog’s heart
  • Hobbies: Ruining lives

In short, this small threadlike worm passes from dog to dog when blood-sucking mosquitoes feed. But this parasite’s name is nothing to do with being warm-hearted, and instead reflects the inappropriate place it sets up home.

Home is Where the Heart Is

As unwelcome as any squatter, heartworm does untold damage to its host. The trouble is, a dog’s heart pumps blood and worms get in the way. But this is only one side to Dirofilaria’s character:

  • The worms damage and inflame the walls of blood vessels
  • The physical presence of the worms causes turbulent blood flow
  • Like a blocked drain, Dirofilaria in the blood vessels increase the resistance to blood flow, which causes high blood pressure in lung circulation
  • Parasite waste products cause local areas of cell death
  • Parasites within the heart, damage the heart valves
  • To try and improve blood circulation the wall of the right heart thickens, this eventually leads to heart fatigue and then failure
  • The body’s immune system defends itself by producing antibodies against Dilofilaria, but these can cause kidney damage and protein leakage.
  • The dog may have a serious allergic reaction to the presence of worms

All in all, this is anything but a match made in heaven.

How Long Does It Take For Heartworm Symptoms To Show Up In A Dog Or Puppy?

This probably leaves you anxious to know what the signs of heartworm infection are, which we will come to shortly.

First, a dog under six months of age is too young to show signs of infection; Dirofilaria is a slow developer, and it takes at least this long to be troublesome. However, after about seven months no dog – regardless of age, breed, or gender, is safe from the problems caused by heartworm.

For the dog owner, one worrying aspect of heartworm is your dog may be infected but show no signs (hence the importance of blood tests).



Symptoms of Heartworm In Dogs

From infection to showing clinical signs takes at least six months, so once the dog starts to show signs the infection is well-established. The symptoms range from mild to severe, as outlined below:

–       Minimal signs: The dog has heartworm but outwardly is fine

–       Tire with minimal exercise:  Running after a ball leaves him weak, and breathless.

–       Breathing difficulties: Rapid shallow breathing at rest, or exaggerated chest movements.

–       Cough:  Dogs cough for lots of reasons, one of which is heartworm, and in this case the cough may be accompanied by blood.

–       Swollen belly:  The impaired circulation leads to fluid build-up in the tummy, and a pot-bellied appearance.

–       Weight loss, poor body condition: These dogs are too busy concentrating on breathing to eat, and lose weight.

–       Fainting:  The blood circulation is so poor the dog is prone to fainting.

–       Death:  From caval syndrome due to a sudden mass movement of worms into the right side chambers of the heart.

How Does My Veterinarian Make a Diagnosis of Heartworm in my Dog?

If you notice any suspicious signs, please get your dog checked by your veterinarian. They will complete a thorough physical examination and ask questions about the dog’s lifestyle.

Physical signs they might pick up that indicate a problem include:

  • Lung crackles as the dog breathes in and out
  • Jugular vein distension
  • Anemia

The veterinarian may then suggest further tests such as:

  • A chest x-ray: To look for shadows on the lung that are typical of heartworm
  • A heart ultrasound scan: To see the worms and check out any damage done to the heart
  • Blood tests: To find evidence of heartworm infection.

How Do the Blood Tests Work?

There are two types of diagnostic blood test. One looks for the physical presence of microfilaria (the immature, resting form of the Dilafilaria worm), whilst the other detects the body’s immune response to infection. Each test has pros and cons, but it is the immune response (antibody) test which is used for routine screening.

The reason the antibody test is so popular is that is can detect infection before the dog becomes ill – which is great news for successful treatment. However, like any test there may be the occasional false result.  Therefore, if your fit healthy dog throws up a positive result, before starting lengthy treatment your vet may suggest confirmation with an alternative test.

Treatment of Heartworms in Dogs: Getting Rid of the Heartache

For arguments sake, let’s assume your dog tested positive for heartworm (more on screening later). You are anxious to get him better as soon as possible, and yet the vet insists on waiting two months before giving a drug to kill the adult worms.

Why can’t he start adulticide treatment right away?

Whilst your instincts are screaming at you to get rid of the adult worms as soon as possible, starting therapy too soon is fraught with problems.  The reasons it’s wise to plan ahead are:

  • Bacterial contamination: Dirofilaria are not clean-living parasites and come with bacterial groupies. Killing the worms releases these bacteria, which poison the host’s blood. The way round this is to first clean up the Dirofilaria with a four week course of antibiotics.
  • Build Up the Patient: If the patient is already unwell, it pays to get them stronger so they are in a better place to cope with worms dying in their blood stream
  • TheInvincible” Stage: There is a stage in the heartworms’ life cycle, as the larvae grow and mature, when drugs can’t kill them. Delaying treatment allows all larvae to mature to a point where they are vulnerable to treatment.

A Typical Treatment Plan

Apart from this being a deeply unpleasant condition, heartworm is complicated to treat (hence the importance of prevention). Your dog may be prescribed so many pills they start to rattle, so here is the low-down on the rationale behind the therapy once a diagnosis is made.

(1) Starting a preventative medication

A dog that throws a positive blood test result has both adults and larval forms, which need different drugs to kill them. Preventatives work by killing immature larvae.

However, 2 – 4 months after infection the larvae reach an “invincible” stage where they are too old to be killed by preventatives, but too young for death by adulticide (drugs that kill the adult form).

To get around this, the veterinarian treats your dog with preventatives for two months. This prevents fresh infection and allows two-month old larvae time to mature and become sensitive to adulticides.

(2) Antibiotics

The dog takes four weeks of antibiotics from the tetracycline family to kill bacteria hitching a ride on the heartworm. This clean up stops the dog from getting septicaemia when the adult heartworms die.

(3) Address the Complications

Coping with dying adult worms places a big strain on a body, especially a damaged heart. It pays to support heart function and get the pet better placed to cope with dead worms in their circulation. The drugs commonly used are diuretics and cardiac medications.

(4) Steroids

When the two months are nearly up, some dogs take steroids, which help control inflammation and allergic reactions associated with dying worms.

(5) Last but Not Least: An Adulticide

You’ve done what the vet recommended, and waited two months for the invincible larvae to mature. Now it’s time to start an adulticide – a drug to kill adult worms. The treatment used is a drug called melarsomine (Immiticide), given by injection.

The first dose is given, followed by a second and third on days 30 and 31 after the first.

The Complications of Treatment

As if treatment wasn’t complicated enough, it’s potentially fraught with problems, such as:

–       Drug side effects (pain at injection site, vomiting, appetite loss)

–       Dying worms blocking blood vessels

–       Allergic reaction to dying worms

–       Heart failure from damage already done

Indeed, one potentially devastating consequence if dying worms lodge in a vital blood vessel, is the sudden death of the pet,

Prevention Of Heartworms In Dogs

There is no simple was for a dog to divorce themself from heartworm, which makes prevention better than cure. For once there is some good news because simple, effective preventative treatments are readily available.

Given monthly at home you can use oral or spot on products, or alternatively a six-monthly injection given by your veterinarian.

These products contain drugs from the ivermectin family.  Those you apply at home include Heartgard, Tri-hart, Sentinel, Revolution, Advantage Multi, and Triflexis.

Climate change means it’s no longer safe to assume there is a mosquito season, and it is essential to give a preventative all year round. However, you can also help your pet by draining any areas of standing water where mosquitoes could set up home. Reducing the mosquito population means reducing the background level of heartworm in your area.

Treating Heartworm in Puppies

Start treating your puppy before they turn eight weeks of age. If the puppy is older than this when he has the first dose, he needs a blood test six months down the line.

You also need to stay on top of dosing, because if you are two months late, it gives the “invincible” phase (remember those larvae aged 2 – 4 months that aren’t killed by preventatives) a chance to develop. If a couple of months slip by without treatment, then speak to your vet about testing your pet BEFORE restarting the preventative.

When Heartworm Treatment Is Used

Heartworm is a potentially deadly disease, but in the early stages it’s not all doom and gloom. When diagnosed on a routine blood test and if the dog has no physical symptoms, there’s every chance of successful treatment. However, a well-established infection where the dog is already sick has a higher complication rate and even after successful treatment the dog may be left with a “broken” heart.

Any dog on treatment for heartworm MUST rest and absolutely not go chasing after things. Geeing up his circulation increases the chances of worm fragments breaking off and lodging in small blood vessels and causing life-threatening embolisms.

Routine Screening For Heartworm

Routine screening should be part of every dog’s annual health check, because catching infection in the “silent” phase, greatly improves the dog’s chances of a cure.

True to form, heartworm is a bit of a rogue because its only 5 – 6 months after infection that antibody levels become detectable in the blood.

Hence, strays dogs with an unknown lungworm status are tested after 6 months to avoid false negative results. The same goes for young dogs. There’s no point testing before 7 months of age because even if they had heartworm, they might give an unreliable negative result and a false sense of security.

Dogs with an unknown history need testing at 6-month intervals until a pattern of negative results is established. Those dogs on regular preventative treatment should have a yearly test to check they remain clear.

Key Points for Dog Owners to Remember

  • Never skip a preventative treatment: Juvenile worms aged just 52 days are no longer killed by preventatives
  • Once infected it takes the body 6 months to generate detectable antibodies: Test taken before 6 months can come back as false negatives
  • Test dogs once a year, even those on preventatives: Occasional treatment failures happen, so early detection of infection is imperative.
  • Test dogs with unknown heartworm status every 6 months: Until a regular pattern of negative results is obtained
  • Dogs testing positive need careful assessment and a prolonged course of treatment


  • Guidelines for the diagnosis, prevention, and management of heartworm (Dirofilaria immitis) infection in dogs. Nelson, McCall, Rubin, Buzhardt, et al. Vet Parasitol 133, 255266
  • Rationale Use of Diagnostic Tests: Textbook of Veterinary Internal Medicine. Ettinger and Feldman. Publisher: Elsevier Saunders
  • Seasonal timing of heartworm chemoprophylaxis in the United States. Knifth and Lok. Proceedings of the heartworm symposium’95.
  • Recommended procedures for diagnosis, prevention, and management of heartworm (Dirofilaria immitis) infection in dogs. American Heartworm Society. Canine Pract 22.



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