What you don't know about heartworm treatment

July 26, 2019
Natalie Stilwell, DVM, MS, PhD

Dr. Natalie Stilwell provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting. In addition to her DVM obtained from Auburn University, she holds a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida.

You've got the fundamentals of heartworm disease down, with a good grasp of the diagnostics, but treatment can be tricky. Here are the tactics recommended by veterinary experts, plus less extensive and less expensive alternatives.

Regardless of where you practice, odds are you've treated a pet with heartworm disease this year. Early diagnosis and early elimination of heartworms with adulticidal therapy give affected dogs the best chance at a healthy life, but heartworm infection has consequences, and damage can be life-long.

The American Heartworm Society (AHS) offers heartworm disease treatment guidelines, consisting of combination therapy with melarsomine, doxycycline and a macrocyclic lactone. Brian Herrin, DVM, PhD, an assistant professor at Kansas State University College of Veterinary Medicine, recently delved into those recommendations in a class at Fetch dvm360 conference, including the aspects most likely to confuse practitioners.

Melarsomine

Melarsomine is the only FDA-approved adulticide therapy for heartworm disease. The AHS-recommended protocol, which is considered safer and more effective than alternative protocols, consists of an initial injection followed one month later by two injections spaced 24 hours apart. If a significant inflammatory reaction or embolic disease occurs after the initial dose, then it is acceptable to wait longer than a month for the final two injections. However, Dr. Herrin emphasized that the final two injections must be administered closely together because they offer a synergistic effect.

Doxycycline

Worm die-off can incite a massive host inflammatory reaction, which is why a combination protocol including doxycycline is “the safest way to treat” canine heartworm cases, according to Dr. Herrin. By killing Wohlbachia spp, the endosymbiont within the microfilaria, doxycycline appears to lessen both heartworm-associated pathology and D. immitis infectivity. Because the release of Wohlbachia is what triggers the host's inflammatory response during worm die-off, pretreating with doxycycline before administering adulticide therapy can reduce inflammation significantly. Also, once microfilariae lose the endosymbiont, they lose transmissibility if taken up by a mosquito. Therefore, doxycycline renders the microfilariae noninfectious and removes the patient as a reservoir for the population-an added public health benefit.

The AHS recommends administering doxycycline for 28 days, then waiting a month before giving the first melarsomine injection. Many parasitologists and clinicians now choose to remove the gap between treatments, as waiting may decrease client compliance.

Macrocyclic lactone preventive

Use of a monthly preventive is also recommended throughout the treatment course to manage circulating microfilariae and remove the dog as a reservoir of infection for other animals. Preventives do not kill microfilariae, but rather prevent the microfilariae from secreting certain proteins that help them hide from the host's immune system.

Dr. Herrin advised that all of the currently available preventive options, including ivermectin, milbemycin oxime, moxidectin and selamectin, are safe to use. Microfilaremic dogs may experience an inflammatory reaction after dosing, and those patients with significant microfilaremia should be pretreated with glucocorticoids and antihistamines before receiving preventive.

Exercise restriction

Proper communication upon heartworm diagnosis is crucial, Dr. Herrin explained, because it's important for clients to understand that treatment is lengthy, may be painful and requires absolute adherence to exercise restriction. Because exercise increases the risk for thromboembolism, strict cage rest is recommended beginning with the first melarsomine injection and extending until six to eight weeks after the final injection. Leash walks are acceptable, but dogs should never be allowed to run. (Check out these tips on helping your dog beat boredom while confined during treatment.)

Treatment alternatives

Some clients may find the AHS-recommended protocol either too extensive or too costly. However, most alternative therapies are not acceptable. Reducing the melarsomine protocol to two doses is considered unsafe; “slow-kill” methods using long-term macrocyclic lactone therapy do not effectively kill adult heartworms and may contribute to the development of resistant heartworm subpopulations.1,2

If owners can't afford treatment at the time of diagnosis, Dr. Herrin recommended delaying melarsomine therapy but still initiating treatment with both a preventive and doxycycline. This will prevent further infection and remove the dog as a heartworm reservoir. Upon declining melarsomine treatment, owners must understand that an untreated dog is at high risk for pulmonary thromboembolism and that exercise intolerance will inevitably develop as the disease progresses.

A recent study suggests that combined use of a topical moxidectin+imidacloprid product (Advantage Multi, Bayer) and doxycycline may be an effective adulticidal treatment, particularly for early infections.3 The study demonstrated that 30 days of doxycycline at 10 mg/kg followed by 10 months of moxidectin+imidacloprid cleared infection in nonclinical dogs with stage 1 heartworm disease. This therapy is not meant to replace the AHS-recommended adulticide regimen but may serve as an option when the client cannot afford recommended treatment or the patient is not healthy enough to receive an adulticide.

References

1. Geary TG, Bourguinat C, Prichard RK. Evidence for macrocyclic lactone anthelmintic resistance in Dirofilaria immitis. Topics Companion Anim Med 2011;26:186-192.

2. Bowman DD. Heartworms, macrocyclic lactones, and the specter of resistance to prevention in the United States. Parasit Vectors 2012;5:138.

3. Savadelis MD, Ohmes CM, Hostetler JA, et al. Assessment of parasitological findings in heartworm-infected beagles treated with Advantage Multi® for dogs (10% imidacloprid+2.5% moxidectin) and doxycycline. Parasit Vectors 2017;10:245.

Dr. Stilwell provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting. In addition to her DVM obtained from Auburn University, she holds a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida.