This guest post was written by Linda Jacobson, BVSc, MMedVet(Med), PhD, of the Toronto Humane Society, who co-authored a recent review article on an accessible, affordable protocol for treating heartworm infection in dogs when standard treatment with melarsomine is not practical or possible. She outlines their findings and recommendations here.
At the 2019 AHS Symposium, an industry veterinarian told me that, in his opinion, dogs with heartworm infection, whose owners could not afford melarsomine, should have insect repellant applied – no other treatment. (“Just leave them to die?” “Yes, so long as they are not transmitting infection.” He didn’t say it, but the rest of that sentence seemed to be “to our dogs.”)
In our new review, we have instead argued an accessible care case for a non-arsenical alternative treatment protocol. The review outlines recent research findings, the major objections to this protocol, and why it is justifiable in circumstances where melarsomine is not a viable treatment option.
Toronto Humane’s heartworm journey began when we launched our rescue transport program in 2012 and began to receive dogs with heartworm infection. In some cases, we knew in advance that they were positive; in others, they had tested negative at the sending shelter but positive upon arrival; in yet others, the sending location did not have the resources to test.
A few years later, we started to work with Dr. Tammy Hornak, a veterinarian who attended vet school in the southern US. In addition to running her busy practice, Tammy has held regular outreach clinics in Indigenous communities in southwest Ontario for more than a decade. Heartworm was thought to be rare up here in the snowbelt, but Tammy discovered hyper-endemic foci in some Indigenous communities. These communities lack access to veterinary care and heartworm preventives.
At THS, we have the resources to treat heartworm in shelter dogs using the AHS-recommended 3-dose melarsomine protocol, which is considered the safest and most effective treatment in most cases. However, at the outreach clinics, where prevalence typically reaches double digits, this is rarely an option. Most clients can’t afford melarsomine, and many live a long distance from the nearest veterinary clinic. In the process of exploring a way to study and publish the field results using alternative protocols, I found myself getting deeper and deeper into the alternative adulticide (so-called “slow kill”) literature. This began to look like an opportunity. A body of evidence had emerged , that had not yet been summarized and integrated in one place. Dr. Brian DiGangi and I set about doing just that.
The goal was to provide a go-to resource for veterinarians, shelters and rescues that would address questions and controversies around alternative adulticide treatment and lay out practical information in an easy-to-access format (spoiler alert: Table 6, Figure 2 and Table 4). The combined evidence shows that, when teatment with melarsomine is not practical or feasible, topical moxidectin and doxycycline (“moxi-doxy”) is the most effective alternative adulticide combination. (Though not addressed in the review, an important point for shelters is that the popular 2-dose melarsomine protocol should always be preceded by doxycycline.)
While the slower efficacy of moxi-doxy may result in progression of pulmonary pathology in affected dogs, respiratory complications were uncommon in all studies, and serious complications were very rare. An extended period of exercise restriction is problematic. However, the data showed that significant respiratory complications remained rare, even when exercise restriction was moderate or absent. Microfilarial resistance appears to be uncommon, and the inclusion of doxycycline reduces this risk by affecting embryogenesis and transmission. The microfilaricidal effects of moxidectin prevent transmission to other dogs, even in cases where the adult infection is slow to clear.
We’ve provided treatment and repeat testing recommendations to help promote consistent, cost-effective approaches. Protocols using higher and more frequent drug doses and more frequent and expensive testing did not produce better results. For example, doxycycline can be used at 10mg/kg doxycycline once daily for 28-30 days, once a year. Topical moxidectin is effective when applied once a month, as per the label recommendation. Repeat antigen testing can reasonably be done after 12 months (although in some cases a negative antigen test may be obtained after 6 months of therapy) and heat treatment offers no benefit for earlier detection of antigen negative status.
For shelters, melarsomine remains the preferred treatment because of its safety and efficacy and because the treatment course is reliably shorter. However, if resource constraints or medical considerations warrant it, and as long as welfare aspects can be addressed through foster or foster-to-adopt pathways, moxi-doxy is a reasonable alternative.
Jacobson Linda S., DiGangi Brian A. An Accessible Alternative to Melarsomine: “Moxi-Doxy” for Treatment of Adult Heartworm Infection in Dogs, Front. Vet. Sci., 27 July 2021 | https://doi.org/10.3389/fvets.2021.702018
About the Author
Dr. Linda Jacobson obtained her veterinary degree in Pretoria, South Africa, in 1986 and subsequently completed a residency in small animal internal medicine and a PhD on the disease mechanisms of canine babesiosis, a tick-borne disease. She completed the University of Florida Online Graduate Certificate in Shelter Medicine in 2015. She currently serves on the Standard of Care Working Group for the College of Veterinarians of Ontario and is President of the Ontario Shelter Medicine Association. She joined Toronto Humane Society in 2010, and spent many years working primarily with cats. She is currently Senior Manager: Shelter Medicine Advancement, focusing on research and education that improves the lives of homeless animals and those lacking access to veterinary care. Her interests include infectious diseases, animal hoarding, and gardening for biodiversity.