Canine Heartworm Treatment in Shelters

Treatment of canine heartworm infection is a major challenge for shelters. The long treatment period coupled with the welfare concerns for increased length of stay in the shelter, the cost of daily care during the longer stay, the expense of melarsomine, and lack of a veterinarian on staff make this disease particularly challenging in shelter populations. The prolonged period of exercise restriction during HW treatment is of particular concern for treatment in the shelter as exercise may be the most practical form of enrichment for coping with the stressful environment.  For these reasons, treatment protocols for heartworm infection in shelter dogs may differ from the AHS best practices.

A 2010 survey of heartworm treatment protocols in southeastern shelters found the following variations:

• For shelters that performed heartworm testing, 39% treated all infected dogs, 4% treated some dogs, and 12% did not treat any dogs
• Municipal shelters were more likely to euthanize infected dogs or release them without treatment
• Municipal shelters were less likely to follow the AHS best practice treatment protocol than private nonprofit shelters
• 35% of shelters that treated heartworm-infected dogs used the 3-dose melarsomine protocol, 44% used the 2-dose melarsomine protocol, and 22% used the monthly heartworm preventive “slow-kill” protocol instead of melarsomine
• <50% of the shelters used doxycycline prior to adulticide treatment

A similar survey of shelter heartworm treatment practices in 2019 reported the following findings:

• 93% of the responding shelters treated at least some of their heartworm-infected dogs
• Municipal shelters were less likely to have treatment SOPs than private nonprofit shelters
• 43% of shelters that treated heartworm-infected dogs used the 3-dose melarsomine protocol, 38% used the 2-dose melarsomine protocol, and 17% used the monthly heartworm preventive “slow-kill” protocol instead of melarosmine
• 59% of treating shelters started infected dogs on macrocyclic lactone preventives
• 92% used doxycycline prior to melarsomine
• 71% gave prednisone with melarsomine injections
• 65% of the shelters housed dogs in foster homes during treatment or provided treatment for dogs after adoption
• 32% housed dogs in the shelter for the duration of treatment

These surveys revealed that shelters utilize a variety of heartworm treatment strategies ranging from full compliance with AHS guidelines to a complete lack of attention to heartworm infection. Many shelters develop their own protocols based on available resources and without the supervision of a veterinarian or support by evidence.

For both surveys, shelters reported that the biggest challenges for heartworm treatment were cost of treatment, long duration of treatment, housing during treatment, and managing activity restriction for ≥5 months during and after treatment without compromising the dog’s behavioral health and welfare. How to deal with these challenges is frequently discussed on shelter medicine practice listservs and discussion board and at continuing education conferences.

Solutions vary based on how shelter managers and veterinarians allocate their finite financial and staff resources to best serve a variety of competing needs for shelter operations and healthcare. One common solution is to to place the dog in a foster or foster-to-adopt home for treatment. In this safer and quieter environment, the foster administers the doxycycline for 30 days along with the monthly HW preventive, and the dog enjoys the welfare benefits of being in a home environment during exercise restriction. The foster brings the dog back to the shelter for each melarsomine injection, then takes them home for monitoring and continued exercise restriction.

Think about the shelters you have worked in – what were their approaches to treating heartworm-infected dogs?

• Was the policy to transfer infected dogs to a rescue for treatment instead of being done by the shelter?
• Were infected dogs adopted out with the understanding that the new owner would seek treatment by their veterinarian?
• If the shelter provided treatment, did they treat all infected dogs or only those considered most adoptable?
• Did dogs stay in the shelter for the duration of treatment or were they in foster homes or new adopter homes?
• If the shelter provided treatment, who paid for this?

Let’s review the ASV position paper on heartworm management in shelters. This statement provides recommendations to guide shelters in making decisions on treatment protocols, including choosing protocols based on available resources and options for providing treatment outside of the shelter.

As discussed by Dr. DiGangi, one modification of the AHS best practice protocol that many shelters have opted for is administration of 2 doses of melarsomine over a 24-hour period starting at Day 60.  This reduces cost and shortens the total treatment time by 4 weeks while maintaining a high kill efficacy for adult worms (90% efficacy compared to 98% with 3 doses of melarsomine). The  time  to a negative heartworm antigen status is also similar (≥90%) for 2-dose vs. 3-dose melarsomine treatment.

What other modifications can be considered for a shorter and less expensive course of heartworm treatment that is safe and effective?

There is a lack of consensus regarding the optimal doxycycline dosage regimen for heartworm treatment, with support for 10 mg/kg SID, 5 mg/kg BID, or 10 mg/kg BID.  The AHS recommended dose of 10 mg/kg BID X 30 days is more expensive, associated with GI side effects (nausea/vomiting), and is difficult for shelter staff to reliably administer at the proper dosing interval. A recent study at Toronto Humane Society investigated outcomes for HW-infected dogs administered doxycycline 10 mg/kg SID vs. 10 mg/kg BID for 30 days. In addition, the study evaluated outcomes for starting melarsomine injections much earlier in the treatment protocol. One group of dogs received the 1st melarsomine injection on Day 14 of doxycycline treatment, followed by the 2nd and 3rd doses one month later.  The other group of dogs received the 1st melarsomine dose on Day 60 (30 days after completing doxycycline), followed by the 2nd and 3rd doses one month later.  Since doxycycline reduces pulmonary inflammatory damage and risk for PTE following killing of adult heartworms, the primary outcome assessed was frequency of respiratory complications following melarsomine treatment. Here are the results:

• Dogs receiving doxycycline 10 mg/kg SID had fewer GI side effects than dogs receiving 10 mg/kg BID
• No difference in frequency of post-melarsomine respiratory complications between dogs that received doxycycline 10 mg/kg SID vs. 10 mg/kg BID
• No difference in frequency of post-melarsomine respiratory complications between dogs that started melarsomine on Day 14 vs. Day 60

This study provided support for using a lower dose of doxycycline (10 mg/kg SID) and initiating melarsomine after 14 days of doxycycline. Compared with the AHS best practice protocol, starting melarsomine injections on Day 14 shortens the treatment period by 6 weeks. This increases lifesaving capacity and improves quality of life for shelter dogs by reducing the duration of exercise restriction and length of stay in a shelter or foster home.  There are no studies evaluating outcomes of dogs receiving two doses of melarsomine on Days 14 and 15 of doxycycline treatment, but this would further decrease treatment time to 30 days instead of the 90 days for the AHS best practice protocol.

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, treatment with moxidectin-doxycycline (moxi-doxy) provides a life-saving alternative to no treatment at all. A recent 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. Based on current evidence, the most effective moxi-doxy treatment protocol is:

• Doxycycline 10 mg/kg PO daily for 28-30 days
• Topical (Advantage Multi) or oral (Simparica Trio) moxidectin monthly. Sustained release injectable moxidectin (ProHeart) can be used if monthly administration is unreliable or not feasible.
• Perform HW antigen test at 12 months. Studies assessing the killing efficacy of moxi-doxy found that 90%-98% were antigen-negative between 10 to 18 months.
• Repeat doxycycline if the HW antigen test is still positive at 12 months. Repeat HW antigen test at 6-month intervals.

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, welfare concerns can be addressed through housing in foster or foster-to-adopt homes until treatment is successfully completed.

A HW treatment app is available that includes all the options for HW management based on shelter resources, ranging from the AHS best practice guidelines to modifications of doxycyline and melarsomine administration and the moxi-doxy protocol. The app, called Decision-Making Considerations for Heartworm Management in Shelter Dogs, is a tool that incorporates evidence-based medical, behavioral, and operational perspectives into canine heartworm disease management policies and protocols for shelter populations.  This app works best for cell phones and can be downloaded here:  www.ShelterHWTool.com

What are some of the canine heartworm treatment policies and protocols in Florida shelters?

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Miami Dade Animal Services is one of the largest municipal shelters in the US with an annual intake of 30,000 animals. They treat more than 300 dogs each year for HW infection, including dogs in foster homes or adoption homes and dogs staying in the shelter. Review their current HW Treatment Protocol. Note the doxycycline dose, use of ProHeart injectable moxidectin, the number of melarsomine injections, and when the melarsomine injections are administered. The treatment timeline is 31 days!!