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Heartworm Prevention and Your Pet

Heartworm infection, caused by the organism Dirofilaria immitis, is spread by mosquitoes (at least 70 species of mosquitoes serve as intermediate hosts carrying these organisms). Because mosquitoes are so prominent here in Florida, it is essential dogs are treated with heartworm prevention to stop a life threatening heart condition.

Mosquitoes acquire the first stage larvae (also known as microfilaria) while feeding on an infected host. Development of these larvae occurs in the mosquito for 1-4 weeks depending on environmental temperatures before it becomes an infective stage. As the mosquito feeds on our beloved pets, these larvae migrate into the bite wound to fulfill the rest of their life cycle and cause the clinical signs associated with heart disease. When the larvae become young adult heartworms (approximately 2 months after the bite from the infected mosquito), they migrate through the dog’s tissues, arriving in the pulmonary arteries approximately 50 days later. Adult worms reside primarily in the pulmonary arteries of the caudal lung lobes, but can move into the right ventricle of the heart if the worm burden is high. Microfilaria (remember the juvenile stage) are then produced from these gravid adult female worms (6-7 months post-infection) to circulate in the bloodstream awaiting the bite of a mosquito to keep the reservoir of infection in the area.

Clinical signs associated with heartworm infection include coughing, exercise intolerance, unthriftiness, episodes of collapse and fluid build-up in the abdomen due to right-sided congestive heart failure. These signs may not be seen in sedentary/ inactive dogs. Heartworm disease can be grouped into four classes dependent on these clinical signs.

  • Class I: Asymptomatic to mild disease with no clinical or radiographic signs
  • Class II: Moderate disease with occasional coughing and mild to moderate exercise intolerance; Increased lung sounds, slight right ventricular enlargement of the heart on radiographs
  • Class III: Severe disease and anemia, weight loss, exercise intolerance, severe/persistent coughing, fluid in the abdomen and marked right ventricular hypertrophy of the heart
  • Class IV: Caval syndrome, usually fatal

Diagnosing heartworm infection is relatively simple at your veterinary clinic. All that is needed is a few drops of blood from your pet to run an antigenic detection test for microfilaria (the juvenile stage of heartworms). These tests tend to be carried out when our pets are approximately 1 year old (due to the life cycle of the organism). At least 90% of dogs harboring ≥3 adult female worms will test positive! Strong quick positive reactions correlate with high worm burdens. If your dog has been diagnosed with heartworms, a work up at your clinic is indicated to classify the stage of the heartworm disease and to initiate the type of necessary treatment. This work up may include blood work, urinalysis, fecal test and radiographs.

Heartworm disease is COMPLETELY PREVENTABLE with a monthly oral tablet or topical application. Prevention should begin around 8 weeks of age. Here at Clay County Humane Society, we carry both formulations:

  1. Advantage Multi topical application (to be applied on the back of the neck) and prevents not only heartworms, but also fleas and gastrointestinal worms.
  2. Tri-Heart/Heartgard oral tablet that prevents heartworms and gastrointestinal worms.

So why test your dog for heartworms every year? If infection is already present in our pets, shock reactions can occur in the face of high microfilarial numbers circulating in the bloodstream if we administer our monthly preventative tablet. So before initiating preventative heartworm treatment, dogs should be tested if they are 1 year of age. Annual testing is also recommended because overall owner compliance is approximately 50%! Try to avoid forgetting your pet’s monthly treatment by:

  1. Keeping a date easy to remember (i.e. the 1st of every month)
  2. Writing the date on a calendar
  3. Setting an alarm in your cell phone

Keep our family pets healthy in our humid, mosquito-friendly environment of Florida and get your dog tested for heartworm infection and on prevention!