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Veterinary Forum August 2008 (Vol 25, No 8)

Expert shares new protocol to manage heartworm signs

by Marie Rosenthal

    LAS VEGAS Sometimes it is better to manage canine heartworm rather than treat it. At the Western Veterinary Conference held here, Thomas Nelson, DVM, presented a protocol he uses for managing dogs with clinical signs of heartworm disease.

    "In certain situations, immediate adulticide treatment may not be the best option, and you may have to look at management," Nelson said, because dead worms result in thromboembolisms that can cause an increased inflammatory response and more problems than live worms. "We know that the presence of heartworms in the pulmonary arteries and heart cause inflammation, but the dog (and cat) can tolerate live worms fairly well because of compounds secreted by the worms that suppress the immune system," he explained.

    "When a dog has difficulty breathing, is running a fever, is not eating well and has lots of issues, the worst thing you can do, in my opinion, is kill more worms. That dog needs to have an opportunity for the lungs to quiet down," Nelson said, although he admitted that there are differences of opinion about this strategy.

    "There are people who think we need to get rid of [the heartworms] as soon as possible, but from a clinical standpoint, that dog needs to be stabilized before we treat it," added Nelson, who is surgical director and co-medical director of the Animal Medical Centers of NE Alabama in Anniston.

    Although the number of worms plays a role in the severity of heartworm disease, of greater importance is the activity level of the dog. Veterinarians do not see a lot of severe clinical disease in dogs that are "couch potatoes." Once the worms start to die, clinical signs are more prevalent in active dogs because activity increases the complications associated with thromboembolism. It is important to restrict exercise in these dogs, Nelson said.

    "Dead worms fragment and shower the capillaries in the lungs with small particles that block blood flow. Increased activity results in higher blood flow in the pulmonary vessels, causing capillaries to delaminate and rupture. The dog starts coughing up blood as a result of trying to pump blood through blocked blood vessels. If the dog is kept quiet, complications are significantly reduced," he said.

    Melarsomine is not effective against immature worms younger than 4 months of age. Two injections of melarsomine kill only about 90% of the worms, and the animal can remain antigen positive when it is retested.

    "If two injections kill only 90% of worms, that's important to know. The three-dose protocol kills 98%," Nelson said.

    Nelson prefers using the three-dose protocol, "because you killed half of the worms the first month and the other half the second month. The animal will tolerate it quite well."

    Thromboembolisms occur regardless of whether worms die naturally or from an adulticide. Prednisone is the best treatment for a thromboembolic event because the dead worm products, which no longer suppress the immune system, inflame both pulmonary arterioles and arteries. "When we talk about pulmonary thromboembolisms in heartworm treatment, it is as much an inflammatory issue as it is a blocking issue," Nelson explained.

    Veterinarians used to be taught not to use glucocorticosteroids and, instead, to use aspirin because of its antithromboembolic effects in humans. "As veterinarians tend to do, we adopted something that worked well on the human side. But these [canine] embolisms are actually worm products, and aspirin is contraindicated in heartworm treatment," Nelson warned.

    Doxycycline is now being added to the regimen for heartworm management, he said, because it reduces the number of Wolbachia endosymbionts and stops female worms from producing microfilariae. After female worms stop producing microfilariae, they tend to shrink, which contributes to reduction of the worm mass. "We know Wolbachia is involved in the disease process of other filarial diseases, such as onchocerciasis, and a recent publication by Dr. John McCall at the University of Georgia shows a therapeutic benefit of using doxycycline in heartworm treatments," Nelson explained.

    Nelson uses a multimodal approach for managing heartworm disease. "This is not a protocol that's been adopted or approved by any organization, but it is one that's being looked at. A lot of the universities are starting to try this protocol," he said, adding that he's been using it for 4 years.

    "When I diagnose a dog with heartworm disease, I place it on a macrocyclic lactone for the first 2 months before doing anything, as initially advocated by Drs. Matt Miller at Texas A&M University and Clarke Atkins at North Carolina State University. I personally use an ivermectin compound because more studies have been done on that particular compound. But any of the macrocyclic lactones would work," said Nelson, who added that this is an off-label use of a macrocyclic lactone.

    If milbemycin is used, pretreat the dog with diphenhydramine and dexa­methasone to avoid any potential reaction to the rapid die-off of microfilariae, he said. Treating the dog with a macrocyclic lactone for 2 months kills larvae younger than 2 months of age. The remaining worms will grow to adulthood after 2 months and can be killed with melarsomine. "A lot of the dogs I'm diagnosing are sick — they came in with clinical signs, so I'm giving a little opportunity for the lungs to quiet down and I'm allowing the immature worms to grow up to an age when I can kill them."

    Nelson also uses a high dose of doxycycline (5 to 10 mg/kg bid) during the first month to kill Wolbachia organisms, which live within the heartworm. By reducing the Wolbachia population, production of microfilariae within the worms also will decrease.

    Nelson uses the three-dose melarsomine protocol, as advocated by the American Heartworm Society in all dogs treated for heartworms. Worm burdens do not correlate to the severity of clinical signs, Nelson reminded, as most dogs with heartworm disease only have 12 to 14 worms per infection. "I treat all dogs as if they have advanced heartworm disease," he added.

    "With what we know about the inflammatory process while not knowing which dogs could have a problem, I put all [heartworm-infected dogs] on prednisone following the first melarsomine injection," Nelson said. He uses 1 mg/kg to start and tapers it weekly. One month later, he gives two injections of melarsomine again, followed by a reducing dose of prednisone.

    "Exercise restriction is mandatory for the entire treatment period, as this is the most important aspect of any heartworm treatment," emphasized Nelson.

    For more information:

    Nelson T. Heartworm: what's new? Presented at: The 80th Annual Western Veterinary Conference. Las Vegas, Nev.; Feb. 17-21, 2008.

    McCall JW, Genchi C, Kramer L, et al. Heartworm and Wolbachia. Therapeutic implications. Vet Parasitol 2008; accepted for publication.

    Dr. Nelson has conducted research on heartworm disease and presented sponsored lectures on behalf of various pharmaceutical companies. This presentation was sponsored by the Companion Animal Parasite Council (CAPC).

    NEXT: FORUM FIVE — CIV on the rise

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