Heartworm preventive resistance and Immiticide shortage (text only)

Concern has been ongoing about possible heartworm resistance to macrocyclic lactone preventives. The Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) created a joint statement regarding heartworm resistance, prevention, testing, and treatment, and the key recommendations for veterinarians are presented here. To see a photo gallery version of this article, click here.

• Keep in mind that, when used properly, macrocyclic lactones remain effective as a prophylactic agent in most dogs.
• Follow the product label dose and frequency of administration directions, and do not concurrently administer multiple products.
• Maintain heartworm preventive administration year-round, don’t abandon it.
• Follow CAPC and AHS testing guidelines, and continue to stress the need to clients for annual heartworm testing of their pets.

If you diagnose heartworm disease:

• Use stage-specific medical management based on the classification of heartworm disease.
• Avoid "soft kill" heartworm treatment strategies-don’t use preventives to try to "treat" heartworm-positive dogs.

However, now that Immiticide is scarce because of technical issues in the manufacturing plant, the AHS has also developed a management plan to help you help heartworm-positive dogs and dogs that haven’t completed a full course of adulticidal treatment.

The three primary goals of the management plan are to

• Reduce potential pathology from heartworm infection.
• Maintain the health of a heartworm-positive dog until it can be appropriately treated.
• Prevent additional heartworm infection in the dog.

These goals can be achieved by

• Limiting an infected dog’s activity level
• Administering a heartworm preventive cautiously to a nonprotected dog and monitoring its response
• Giving doxycycline

The specific management steps include:

1. Verify the dog’s heartworm status with a second antigen test (from a different manufacturer) and perform a microfilariae test in antigen-positive dogs or dogs that have started but not completed adulticidal therapy (because of product unavailability).

2. Pretreat microfilariae-positive dogs with corticosteroids with or without an antihistamine, administer the initial dose of a heartworm preventive (macrocyclic lactone), and clinically observe the dogs for at least eight hours. Treat appropriately should a shock reaction occur.

3. Administer a heartworm preventive to microfilariae-negative dogs.

4. Maintain administration of an oral, topical, or injectable heartworm preventive on the regular dosing schedule to limit further infection until Immiticide is available.

5. Administer doxycycline on a one-month-on/two-months-off schedule.

6. Restrict ALL of the dog’s activities, and limit ALL exercise throughout the management period.

7. Medically treat symptomatic heartworm infection to relieve respiratory distress, weighing surgical options in dogs showing signs of cardiovascular compromise.

Adult heartworms will likely still be present in dogs managed with this protocol, but recheck a dog’s heartworm status to reconfirm adult heartworm infection before administering adulticide treatment once Immiticide is again available.

Resources

Read more about the CAPC-AHS statement on heartworm resistance here, and see the statement here.

Read more about the Immiticide shortage here.

Read about possible Immiticide importation from abroad here.

And read the full AHS guidance document on managing heartworm disease during adulticide unavailability here.