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May 21, 2019 – Heartworms (Dirofilaria immitis) were first identified in animals over 400 years ago, and for more than 100 years veterinarians have known that D. immitis causes heartworm disease in dogs. Despite advances in our knowledge of heartworm disease, including improved diagnostic tests and safer and more effective treatments, heartworm remains a major health threat for dogs.

Heartworm Lifecycle 101

After mating, adult female heartworms produce microfilariae (Mf) which are released into the dog’s circulation. These microfilariae are ingested by female mosquitoes during feeding.  The microfilariae mature in the mosquito and are transmitted to a dog when the mosquito feeds. The transmitted larva finish maturing in their canine host.

Adult heartworms eventually reach the blood vessels of the heart and lungs where they mate and produce new microfilaria. The microfilaria are found in the blood as early as six months post-infection, although in most dogs microfilaria are found seven-to-nine months post-infection. Adult heartworms have a life expectancy of 5 to 7 years.

Two factors influence larval molts and maturation: ambient temperature and the presence of Wolbachia pipientis, a bacterium that lives within the heartworm and is needed for normal maturation of heartworm larvae. The discovery of Wolbachia was an important factor in improving heartworm treatment efficacy and the successful elimination of D. immitis.

Understanding Heartworm Diagnostic Tests

Even among heartworm experts, recommendations regarding diagnostic testing vary. The current (2014) guidelines from the American Heartworm Society recommend antigen and microfilaria testing be performed annually on all dogs, regardless of geographic location. This combination of tests is the best method to identify heartworm infection in dogs, given the wide variety of   possible presentations.

Diagnostic tests for heartworm can be broken down into two main categories:

  • Primary diagnostic tests – microfilaria and antigen tests
  • Secondary diagnostic tests – imaging and clinical pathology

Primary testing for microfilaria includes wet-mount direct examination and the modified Knott test or millipore filter test. Looking for microfilaria is important for diagnostic purposes and to assess microfilaria burden. In addition, some heartworm-infected dogs can have antibody-antigen complex formation that results in false negative antigen tests. The presence of microfilaria can help confirm a diagnosis of heartworm infection in these patients.

Heartworm antigen tests are the most sensitive method for diagnosing canine heartworm disease. Antigen tests are almost 100% specific and identify dogs that are heartworm infected but do not have circulating microfilaria. The tests can detect as few as one female worm, but there are no antigen tests capable of detecting infections of only adult males.

Other helpful secondary diagnostic tests include imaging such as X-rays and heart ultrasound. Although both tests are incapable of definitively diagnosing heartworm infection, radiography and echocardiography are extremely useful for providing added support for a diagnosis of heartworm infection and assessing the severity of heartworm disease once it has been diagnosed.

Further diagnostic tests, such as a complete blood panel, biochemistry profile and urinalysis, are important for evaluating a patient prior to treatment and for detecting underlying diseases that could affect treatment. However, these tests are not intended, and should not be relied on, for making a diagnosis of heartworm disease. Rather, they should be used to support decision making by both veterinarian and pet owner.

Best Friends - Wolbachia and D. immitis

In the 1970s, scientists discovered bacteria living in the body of both adult and immature heartworms. It took another 20 years for scientists to understand that Wolbachia are not just passive passengers but play a crucial role in a worm’s survival.

Because of this co-dependency, the discovery of Wolbachia has changed the treatment of canine heartworm disease. The American Heartworm Society recommends that doxycycline, an antibiotic that kills Wolbachia, be given for one month prior to treatment for adult heartworms.

Antibiotic therapy directed against Wolbachia leads to decreases in all stages of immature heartworms and might decrease numbers of adult heartworms as well as reduce the potential for respiratory complications. Because Wolbachia also could contribute to a significant, and often dangerous, inflammatory response seen in some dogs, experts believe treating with doxycycline might decrease complications associated with heartworm disease.

Treatment concepts

Although living heartworms can trigger a significant immune response and damage to the blood vessels, dead and dying worms are even more destructive. Treatment of heartworm needs to be customized for each patient based on the presence or absence of clinical signs. However, we can simplify our understanding of heartworm treatments by breaking them down into three categories:

  • Adulticide treatment (including antibiotics against Wolbachia)
  • Microfilaricide treatment
  • Adjunct therapy

Melarsomine is the only adulticide approved by the FDA. Although other treatment regimens have been described, a three-dose protocol is recommended (2014) by the American Heartworm Society as the most effective drug for eliminating adult heartworms.

In contrast to older protocols, therapy aimed at killing microfilaria is initiated prior to adulticide therapy. Microfilarial burden must be determined prior to starting therapy, and in dogs with large numbers of microfilaria, pre-treatment with steroids and antihistamines is recommended.

The use of adjunctive therapy with prednisone and antihistamines, as well as supportive care measures, are dictated by pre-treatment diagnostic tests, clinical signs and worm burden.

One month after completion of adulticide therapy, dogs should resume appropriate heartworm chemoprophylaxis and be retested for microfilaria. Another antigen and microfilaria test should be performed six months after the last dose of melarsomine.

Although our treatment of heartworm disease in dogs has come a long way in the last 30 years prevention is still best. There are many choices when it comes to heartworm preventives, so it’s important to work with your veterinarian to decide which is best for your dog.

You can find more details and strategies for treating heartworm-infected dogs at the American Heartworm Society.