Guidance for Heartworm Disease Management 

During the Adulticide Unavailability

Background

August 9, 2011: The American Heartworm Society became aware of a pending product 

unavailability of the only available heartworm adulticide product, IMMITICIDE®

, for an 

undetermined duration.

Because there are no other approved products available for killing adult heartworms, the 

American Heartworm Society Board and Scientific Committee has developed and approved 

the following management plan for heartworm positive dogs during this period of adulticide 

unavailability. While the unavailability persists, heartworm-positive dogs should be managed to 

achieve three primary goals: 

• Reduce potential pathology from the infection.

• Maintain the health of the dog until it can be appropriately treated.

• Prevent additional heartworm infection of the dog.

These goals may be achieved by strict adherence to the following:

• Limit the activity level of the dog to reduce pathology.

1,2

• Carefully place the non-protected dog on heartworm prevention.

3,4,5,6,7

• Administer doxycycline to reduce pathology and infective potential of heartworms.

8

The Management Plan:  Heartworm-Positive Dogs and Dogs Not Completing a 

Full Course of Adulticidal Therapy

1. Verify any positive antigen test by performing a second antigen test, sourced from a 

different manufacturer. If a dog is confirmed antigen positive or for dogs already initiated 

on, but not completed adulticidal therapy (due to product unavailability), a microfilariae 

test should be performed. In rare cases, the administration of heartworm preventives to 

microfilaremic dogs can result in shock-type reaction. For this reason professional observation 

is highly recommended.

• If microfilariae are detected, the dog should be pretreated with corticosteroids 

with or without antihistamines9

 and then administered a dose of heartworm 

preventive (macrocyclic lactone).10

  

o While all heartworm preventives affect microfilariae, the resulting immunologic 

reaction of the dog to the microfilariae can vary dramatically.

3,4,5,6,7

  Therefore dogs 

should be kept under clinical observation for at least 8 hours following the initial dose 

of heartworm preventive to allow rapid, appropriate medical treatment should a shock 

reaction occur.

o Anti-inflammatory pretreatment:  Dexamethasone at 0.25 mg/kg intravenously 

and diphenhydramine at 2.2 mg/kg intramuscularly, or 1 mg/kg of prednisolone orally 

1 hour before and 6 hours after administration of the first dose of preventive.

9

o Elimination of every single microfilaria is not the goal; the health, safety, and 

maintenance of the patient is the goal. o Microfilariae will likely persist following preventive dosing.

o If the positive dog is already on prevention, continue regular monthly oral or topical 

dosing or twice yearly injections of preventives.

• If the dog is negative for microfilariae, a heartworm preventive should be 

administered. 

2. Dogs should then be maintained continuously on heartworm preventive to limit 

further infection of the dog until the adulticide product is again available.

3. Dogs should also receive doxycycline at 10 mg/kg BID for 4 weeks. This dosing 

should be repeated quarterly until adulticide is available. (Dose may be reduced to 5 mg/kg 

BID if tolerance issues exist.)

• Doxycycline has been demonstrated to affect the viability of subsequent heartworm 

stages.

• Microfilariae will still be able to infect mosquitoes, but the infective larvae from these 

mosquitoes will be less capable of infecting another dog. 

• The doxycycline protocol would be 1 month on, 2 months off, 1 month on, 2 months off, 

etc. 

• The combination of macrocyclic lactone and doxycycline is proven to be more effective 

than macrocyclic lactones alone. Research studies have demonstrated  macrocyclic 

lactone/doxycycline combinations will:

o Shorten the life span, but not eliminate the adult worm infection.

o Lessen the pathology associated with worm death.

o Disrupt heartworm transmission.

4. Restrict ALL activity of the dog!  Limit ALL exercise!  

• The severity of heartworm disease is directly related to the activity level of the dog.

• As physical activity increases, pathology associated with adult heartworms increases. 

5. Any dogs that are symptomatic for heartworm infection should be treated medically 

to relieve signs of respiratory distress.  Surgical options should be weighed for dogs 

exhibiting cardiovascular compromise.

6. When adulticide product is again available: 

• Adult heartworms will likely persist in the dogs managed under this protocol.

• Nevertheless, dogs should be retested to revalidate the presence of an adult heartworm 

infection.

• If positive, the dog should be appropriately treated for adult heartworms with the 

approved adulticidal product.

REMEMBER THE GOALS:

• Reduce potential pathology from the infection.  

• Maintain the health of the dog until it can be appropriately treated.

• Prevent additional heartworm infection of the dog.

For further background information considered in development of this management protocol, 

refer to the American Heartworm Society’s Current Canine Guidelines under the Veterinary 

Resources at www.heartwormsociety.org.  This American Heartworm Society guidance statement is based upon the cumulated knowledge and 

expertise of the American Heartworm Society’s Board of Directors and AHS Scientific Committee experts, 

considering the latest information on heartworm disease, disease processes, known drug activity, and 

impact of these factors on disease management as of August 2011.

The American Heartworm Society needs your support. Please help us by

becoming a member so that we can continue to provide information and guidance to

the profession. Membership applications are available at www.heartwormsociety.org. 

IMMITICIDE®

 is a registered trademark of Merial Limited, Duluth, GA.

©2011, American Heartworm Society. Use with acknowledgment of the American Heartworm Society.

REFERENCES

1. Dillon AR, Brawner WR, Hanrahan L. Influence of number of parasites and exercise on the severity of 

heartworm disease in dogs. In: Soll MD, Knight DH (eds): Proceedings of the Heartworm Symposium 

’95. Batavia, IL: American Heartworm Society, 1995, p 113.

2. Fukami N, Hagio M, Okano S, et al. Influence of exercise on recovery of dogs following heartworm 

adulticide treatment with melarsomine, Recent Advances in Heartworm Disease: Symposium ‘98. 

Batavia, IL: American Heartworm Society, 1998, pp 225-227.

3. Lok JB, Knight DH, LaPaugh DA, Zhang Y. Kinetics of microfilaremia suppression in Dirofilaria immitisinfected dogs during and after a prophylactic regimen of milbemycin oxime.  Proceedings of the 

Heartworm Symposium ‘92. Batavia, IL: American Heartworm Society, 1992, pp 143-149. 

4. Courtney CH, Zeng QY, et al. The effect of chronic administration of milbemycin oxime and ivermectin 

on microfilaremias in heartworm-infected dogs. Recent Advances in Heartworm Disease: Symposium 

‘98. Batavia, IL: American Heartworm Society, 1998, pp 193-199.

5. Neer TM, Hoskins JD. Clinical experience with ivermectin used as a microfilaricide and for prophylaxis 

in the dog. Proceedings of the Heartworm Symposium ‘89. Batavia, IL: American Heartworm Society, 

1989, pp 95-97.

6. Blagburn BL, Paul AJ, et al. Safety of moxidectin canine SR (Sustained Release) injectable in 

ivermectin-sensitive collies and in naturally infected mongrel dogs. Recent Advances in Heartworm 

Disease: Symposium ‘01. Batavia, IL: American Heartworm Society, 2001, pp 159-163.

7. Dzimiankski MT, McCall JW, et al. The safety of selamectin in heartworm infected dogs and its effect on 

adult worms and microfilariae. Recent Advances in Heartworm Disease: Symposium ’01. Batavia, IL: 

American Heartworm Society, 2001. 

8. McCall JW, Genchi C, Kramer L, et al. Heartworm and Wolbachia: Therapeutic implications

Vet Parasitol. 2008;158:204-214 (Special issue: Heartworm Revisited - Selected papers presented at 

the 12th Triennial Heartworm Symposium 2007).

9. Bowman DD, Atkins CE. Heartworm biology, treatment, and control. Vet Clin North Am: Small Anim 

Pract. 2009;39(6):1127-1158. 

10. Nelson CT, McCall JW, Rubin SB, et al. Diagnosis, Prevention, and Management of Heartworm 

(Dirofilaria immitis) Infection in Dogs (2010), accessed on 5 Aug, 2011 @ http://heartwormsociety.

org/veterinary-resources/canine-guidelines.html.