Heartworm prevention: "Oops, I missed a dose!"

(Photo courtesy of the American Heartwom Society)

Q. My client says she missed a dose of her dog's heartworm preventive. What now?

Dr. Clarke AtkinsA. While heartworm preventives are highly effective and convenient for clients to give, compliance persists as a problem in veterinary practices. Even the most conscientious client can miss a dose now and then, while other clients have much longer lapses.

Unfortunately, there's no one-size-fits-all answer to this question. Instead, veterinarians should consider the following questions:

• What is the prevalence of heartworm in your client's geographic region?

• Has the pet traveled with the owner? If so, what is the prevalence of heartworm in the area they visited?

• What preventive is being used?

• When and where did the lapse occur?

• How many doses of preventive were missed?

The reason for these questions is that time and place have a great impact on heartworm risk. The risk, for instance, is much less when a monthly preventive is missed one time in Ohio in February than when a three-month lapse occurs at that same location in the summer. Likewise, just a two-week lapse in summer can result in infection in the Mississippi Delta.

Macrocyclic lactones provide a safety net know as “reach-back” or “retroactive efficacy,” when given continuously for at least 12 months. The length of the reach-back varies by product, with all products proven to be about 95% efficacious against nonresistant strains in the laboratory when given for at least 12 consecutive months after lapse. This protective benefit can be useful but should not be relied upon as part of routine heartworm prevention.

Another important compound is doxycycline, which can be administered as monotherapy at 10 mg/kg twice daily for 30 days to kill L3 and L4 larvae-it even kills immature adults that have escaped or will escape macrocyclic lactone prevention.1

Recommendations for treatment lapses

• If the lapse is one month or less, reinstitute the preventive and conduct a heartworm test at the next scheduled visit if the visit occurs more than seven months from the current date. In highly endemic areas, consider adding doxycycline therapy for one month. If a dog is receiving imidacloprid-moxidectin, a one-month lapse will likely not be problematic, provided the preventive had been given for at least four months continuously before the lapse.

• If the lapse is two months or longer, reinstitute the preventive immediately and consider adding doxycycline for one month.

• If the lapse is more than seven months, perform an antigen test and consider adding doxycycline to the macrocyclic lactone therapy for one month. For such protracted lapses, imidacloprid-moxidectin has been shown to have superior reach-back efficacy with doxycycline, when given continuously for 13 months post-lapse.2

In all instances, preventive therapy should be administered on a year-round basis, both for the animal's protection and to help ensure improved compliance in the future.

References

1. McCall JW, Kramer L, Genchi C, et al. Effects of doxycycline on early infections if Dirofilaria immitis in dogs. Vet Parasitol 2011;176:361-367.

2. Chandrashekar R, Beall MJ, Saucier J, et al. Experimental Dirofilaria immitis infection in dogs: effects of doxycycline and Advantage Multi administration on immature adult parasites. Vet Parasitol 2014;206:93-98.

This question and others are the focus of a new series of 15-minute recorded talks designed to offer practical information on heartworm prevention, diagnosis and treatment. The “Eye on Heartworm” videos, which are made available by the American Heartworm Society, can be found at https://www.heartwormsociety.org/veterinary-resources/veterinary-education/videos.