Veterinarian - San Diego
1540 Morena Boulevard
San Diego, CA 92110



By Heather Beeson of Morena Pet Hospital
September 27, 2012
Category: Uncategorized
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Our doctors and staff here at Morena Pet Hospital are very disheartened to report that we have had our first case of heartworm infection present to us this past month!

Max, a 4-year old male, neutered poodle mix, had just been adopted and presented to us for a routine new pet adoption exam, experiencing some debris in the ears, soft stool, and itchy skin.  After a thorough history and examination, some routine diagnostic panels were recommended.  The blood panel (which included a heartworm test) was the only indicator that Max was infected with heartworm--as outward signs are not usually apparent until the infection has progressed.  (This is why we recommend periodic lab-work for pets and why it is so often necessary!).

How does my pet get heartworm?

According to the American Heartworm Society, dogs are considered the definitive host for heartworms; however, heartworms may infect more than 30 species of animals, including cats and humans.  Heartworm disease is spread through mosquitoes; so when a mosquito carrying infective heartworm larvae bites a dog (or other pet), it transmits the infection.  The larvae migrate into the heart and surrounding blood vessels, growing into adult spaghetti-like strands of worms; left untreated, this disease causes heart failure and death.

What are some common symptoms of heartworm?

Heartworm disease can cause major damage to the lungs, heart, liver, and kidneys.  Clinical signs can range from very minimal symptoms to chronic infection and physiological changes.  According to the American Heartworm Society, a dog with a low number of adult worms present in the body that does not undergo strenuous exercise may never have apparent signs of heartworm infection.  Here is a chart of varying degrees of clinical signs as noted from the American Heartworm Society:

Clinical Signs Associated with Canine Heartworm Disease

Early Infection

No abnormal clinical signs observed

Mild Disease


Moderate Disease

Cough, exercise intolerance, abnormal lung sounds

Severe Disease

Cough, exercise intolerance, dyspnea (difficulty breathing), abnormal lung sounds, hepatomegaly (enlargement of the liver), syncope (temporary loss of consciousness due to poor blood flow to the brain), ascites (fluid accumulation in the abdominal cavity), abnormal heart sounds, death


How is heartworm diagnosed?

After Max tested positive for heartworm, Dirofilaria immitus, we repeated the test in-house.  Unfortunately for Max, the result was also positive and verified the initial diagnosis.  Our staff then proceeded to take chest x-rays to determine the severity of the heartworm disease and evaluate treatment potential.  We also sent out what is called a ‘microfilaria knotts’ test.  This tests for the presence of the offspring (microfilaria) of heartworms from a blood sample. 

Max’s microfilaria results came back negative.  How can this be?  Some dogs that test negative for microfilaria may have adult heartworms detected on the heartworm serology test (as is the case with Max). This can occur for a number of reasons:

·  The heartworms are still young and sexually immature.

·  There is infection with a single sex: all male or all female worms.

·  The dog's immune system is actively destroying the microfilaria as they are being produced.

·  The dog has been exposed to medications that happen to kill the microfilaria being produced, but has not had an affect on the adult worms or the medication has resulted in the infertility of the female adult worms.


The goals of any heartworm treatment are to improve the clinical condition of the pet and to eliminate all life stages of the heartworms (microfilariae, larval stages, juveniles and adults) with minimal post-treatment complications. 

The next step for Max is to undergo treatment with Heartguard (a heartworm preventative) and doxycycline (an antibiotic), both of which help to kill any migrating larvae and to decrease the size of the female worm to make the treatment with the adulticide less risky (complications can arise when treating with adulticide if the dead or dying worms start to block blood flow through the arteries to the lungs). 

In 30 days, Max will be rechecked by our doctors here and potentially begin Immiticide treatments.  Melarsomine dihydrochloride (Immiticide®, Merial)has demonstrated a higher level of effectiveness and safety than any other adult heartworm treatment previously available. The American Heartworm Society states that it is imperative to restrict exercise, excitement, and overheating from the time of diagnosis through the period of treatment and recovery, with the most extreme degree of exercise restriction recommended for the first 4 weeks following Immiticide administration.  After the last treatment phase, Max will be re-tested to ensure the adult heartworms have been eliminated and will be recommended to stay on heartworm preventative year round.

Take-Home Message:

Heartworm disease is a serious condition and is being reported with increased frequency in San Diego County due to the increased number of mosquitoes resulting from the wet weather. Our weather here allows for heartworm disease to be a concern all year (not just seasonally), thus dogs (and cats) should be on YEAR-ROUND prevention.

The treatment for heartworm disease is costly and bears some risk-when the adult worms are killed with treatment (the pet may suffer a major stroke, embolism, or other severe side effects, which may also be fatal). The best measure to take is PREVENTION, which consists of giving a monthly application to prevent any larva from developing into adult worms. Prior to starting heartworm preventative, a simple blood test is run to confirm heartworm infestation is not already present.  Testing every year is recommended for dogs on year-round preventative medication, as well as those pets that miss multiple doses of preventative.  

For information on preventative medication, please refer to our Pet Parasites article located in our Pet Care Library of our website.


American Heartworm Society