Heartworm Disease in Dogs

Overview

Canine heartworm disease (HWD) is a common condition in many regions of the world.  It is caused by the filarial (threadlike) worm Dirofilaria immitis that lives in the pulmonary arteries (blood vessels leading from the heart to the lungs) in most infected dogs.  The name “heartworm” is a bit misleading because only in very heavy infections do the worms actually reside in the heart itself.

Nonetheless, the presence of these worms causes strain to the heart and an intense reaction in the blood vessels, resulting in problems for the dog. D. immitis is injected into the dog by infected mosquitoes.  For this reason, HWD is more common in tropical regions of the world, where the conditions of warmth and moisture are ideal for mosquitoes to live.

Predispositions for HWD include living outdoors.  The most important predisposing factor is failure to receive heartworm preventative medication.  All dogs living in an area where heartworm disease exists are at risk, even if they live entirely indoors.

Transmission occurs when a mosquito bites an infected dog and ingests heartworm larvae (baby heartworms), which live in the bloodstream.  When the insect bites another dog or cat, some of the larvae are injected under the skin.  The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.

Impact on the pet is variable.  Dogs can be without symptoms if the infection is light or has occurred recently.  In some cases HWD can cause severe debilitation and eventually may be fatal.

Because heartworm disease is easily preventable, it has become a less common diagnosis in pets that receive regular veterinary care.

What To Watch For

Symptoms don’t usually develop until damage has already occurred to the heart.  Dogs can have a wide range of symptoms, with some dogs being completely asymptomatic (no symptoms at all).  Symptoms usually occur because of heart failure.  These include:

• Coughing
• Coughing up blood (hemoptysis)
• Heavy breathing
• Unwillingness to exercise
• Signs of right sided congestive heart failure, which include fluid distention of the belly, pulsation of the jugular veins in the neck when the dog is sitting or standing and heavy breathing.

Diagnosis

Blood tests are used to establish the diagnosis:

• Microfilaria screen:  This test detects the baby worms (microfilaria) circulating in the bloodstream (produced by the adult heartworms living in the pulmonary arteries) by looking for microfilaria with a microscope.

• Heartworm serology:  This test checks for proteins in the bloodstream of the dog that are produced by the heartworms.  These tests are very sensitive and accurate.

Once diagnosed, to stage the heartworm disease:

• Complete physical examination.  A physical examination will detect signs of disease or may suggest a co-existing disease condition.

• Laboratory evaluation.  A CBC, biochemical profile and urinalysis may be run as a general health screen, and to assess stability of patient for treatment.

Treatment

Killing of the adult heartworms (adulticide therapy)

• Melarsomine (Immiticide®):  This drug is given by injection deep into the muscles of the back as either a two-day series (one injection per day, two consecutive days) or, in patients with more severe disease, as a split protocol (one injection, followed one month later with two consecutive injections one day apart for a total of three injections).  Immiticide® is currently the most commonly used type of treatment.

Home Care

• Strict exercise restriction.  THIS IS THE MOST IMPORTANT PART OF THE TREATMENT PROCESS.  Dogs who have been treated for heartworm disease should receive the absolute minimum of exercise for four weeks following the treatment.  Ideally, cage rest should be provided.

Exercise restriction is so important, because as the drugs are killing the worms in the arteries, the worms can break off and travel to block parts of the blood vessels (pulmonary thromboemsolism or PTE).  PTE results in obstruction of the blood flow to parts of the lung (pulmonary infarction).  Keeping the dog quiet allows the body time to slowly break down and absorb the dying worms.

• Watching for complications.  The major complication to treatment is PTE.  PTE is associated with signs of fever, heavy or fast breathing and coughing.  Observation of any of theses clinical signs would necessitate a visit to your veterinarian or local veterinary emergency center.  In its worst form, PTE can result in sudden death.  Back pain is often noted as a minor complication secondary to the Immiticide® injections into the muscles of the back.

Selecting the Right Heartworm Meds for Your Dog

Heartworm Meds

The best way to choose the correct heartworm prevention product for your pet is to get the advice of your veterinarian.  Your pet’s risk of disease based on your pets lifestyle and region in which you live and your pets’ overall health will influence your veterinarian’s product selection.

The very best and safest products on the market are prescription products that are only available from your veterinarian or by prescription from your veterinarian.  Below we will discuss some features to consider in the different products as well as some ideal features.

Features to Consider

• DVM approved or recommended
• Size/dosage
• Effect (do you want more than one affect e.g. Fleas/ticks/heartworms)
• Presentation: spot-on and tablet
• Safety
• Efficacy
• Ingredients
• Cost
• Guarantee
• Frequency of administration

Desirable Features

The ideal product is easy to give or apply, safe, effective, and available at a reasonable cost.  The product should be the proper size/dosage for your pet.  It should cover the problem you are trying to prevent.  You may choose a product that does more than one thing e.g. one that prevents both fleas and heartworms.  You may also want to consider what type of product is easiest to give e.g. is a pill or topical easier for your dog? Indicate your preference to your veterinarian and they will help you select the best, safest, and most effective product in the presentation style you prefer.  It should also come with a guarantee providing the product is given based on veterinarian or manufacturer recommendations.  Monthly dosing is the common frequency of most products.

Ideal Choice

• Choose a product specifically recommended for your dog.  Make sure the product is the correct size for the weight of your dog.  Some of the top products on the market include:

Heartworm Prevention:

• Interceptor® - Prevents heartworm disease and protects against roundworms, hookworms in dogs and cats and certain whipworms in dogs.  It is a tablet formulation to be given at a monthly frequency.

• Heartgard Plus® - prevents heartworm disease and protects against roundworms and hookworms.  It is a tablet formulation to be given at a monthly frequency.

Controls Fleas and Heartworms

• Sentinel® - Prevents heartworm disease, protects against hookworms and whipworms and prevents flea infestation.  It is a tablet formulation to be given at a monthly frequency.

Controls Fleas, Ticks, and Heartworms

• Revolution® - Prevents heartworm disease, kills fleas, controls and treats ear mites and the deer tick infestations.  It is a liquid application to be given at a monthly frequency.

Safety Tips

• Always read label instructions before administering to your pet
• Review age and weight of your pet application or dosage
• Give all medications are directed, for example if your veterinarian recommends year round treatment, follow their instructions.
• Products labeled for another animal types should not be given to another pet (NEVER give your cat products labeled for your dog).