Abstract
Canine heartworm infection, caused by the filarial parasite Dirofilaria immitis, represents a serious and expanding animal welfare concern that is expected to increase due to the effects of climate change and the COVID-19 pandemic. A body of evidence has emerged to support the use of a non-arsenical adulticide treatment protocol, using moxidectin and doxycycline to kill adult heartworms over a prolonged period. While a three-dose protocol using the arsenical drug melarsomine is currently the safest and most effective treatment for heartworm infection, this drug is not available in some countries and is inaccessible for many owners and animal shelters. Moxidectin-doxycycline (moxi-doxy) provides a viable alternative to no treatment at all, in cases where arsenical treatment is not possible. Based on current evidence, the most effective non-arsenical treatment regimen is doxycycline 10 mg/kg PO q 12 or 24 h for 28 days, combined with topical moxidectin at label dose. Moxidectin is repeated monthly until no antigen detected (NAD) status is confirmed. Sustained release injectable moxidectin, in combination with doxycycline, may provide an alternative in remote regions or in settings where significant compliance or accessibility concerns exist, but more studies are needed. In moxi-doxy protocols, doxycycline should be repeated annually until NAD. This review summarizes the safety and efficacy of moxi-doxy, addresses controversies surrounding this treatment approach, and provides detailed recommendations for treatment regimens and post-treatment testing.
Highlights
Current guidelines for the treatment of adult heartworm (Dirofilaria immitis) infections in dogs recommend 3 doses of the arsenical drug, melarsomine dihydrochloride, accompanied by doxycycline and a macrocyclic lactone (ML) [1,2,3]. This is considered to be the safest, most rapid and most efficacious treatment protocol. It is the only protocol recommended for treatment of severe heartworm disease (HWD) unless the disease is so severe that surgical intervention is warranted or life-threatening adverse effects to melarsomine and/or its parasiticidal results are anticipated [2, 4]
Results are available at the 21 day time point for dogs treated with different doses of doxycycline, allowing for some comparison between dosages, albeit from different studies [79, 85]
In 17 naturally infected dogs treated with doxycycline 10 mg/kg q 24 h for 21 days, Wolbachia DNA was detected in blood samples from 15/17 (88%) on treatment day 0, with a similar proportion positive at day 21 (14/17, 82%) [79]
Summary
Current guidelines for the treatment of adult heartworm (Dirofilaria immitis) infections in dogs recommend 3 doses of the arsenical drug, melarsomine dihydrochloride, accompanied by doxycycline and a macrocyclic lactone (ML) [1,2,3]. This is considered to be the safest, most rapid and most efficacious treatment protocol. No longer considered best practice, melarsomine is labeled for 2 doses, 24 h apart, in dogs with asymptomatic, mild or moderate HWD [2] This protocol is commonly used in animal. The 2-dose protocol (with no adjunctive treatment) killed ∼90% of adult worms, compared with 99% after 3 doses [6]
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