Author: Dr. Clarissa G. Noureddine, DVM, MS, MS
Heartworm treatment for dogs has not only changed over the years, it has also become more involved as we have developed an even greater understanding of the disease. In this case, ‘more involved’ is a good thing, because we are increasing the effectiveness and the safety of heartworm treatment.
The earliest treatment for adult heartworms was a product called thiacetarsamide – an arsenical that had a large potential for toxic effects on the animal being treated. In the 1990s, a newer product called melarsomine hydrochloride (also containing arsenic) came along that was safer, more effective, and more convenient to administer than thiacetarsamide. Now, melarsomine is the recommended drug for the treatment of adult heartworm infections in dogs.
Keep in mind that ‘safer than thiacetarsamide’ does not equal ‘safe’. Problems can still occur from melarsomine (e.g. injection site reactions, pain, fever, lethargy, decreased appetite, vomiting, salivation, allergic reactions). Problems can also still develop after treatment that can be attributed to the dying worms (e.g. coughing, lung congestion, pulmonary thromboembolism).
The American Heartworm Society (AHS) helps guide the veterinary profession in the management of heartworm disease. Periodically, the AHS updates their treatment guidelines for heartworm infections. The approach to heartworm treatment will certainly be dependent on the individual animal (e.g., Are they young or old? Do they have other health problems? How severe is the infection?), but the guidelines offer a wealth of information based on new research and historical understanding of the disease.
One of the more recent updates to the guidelines incorporates the use of the antibiotic doxycycline. This is actually a pretty interesting concept, so let’s break it down a little. Heartworms are a host organism to a bacteria called Wolbachia. The Wolbachia are endosymbionts. This means they live inside the heartworm. What’s fascinating about endosymbiosis is that it often means there is a mutually beneficial relationship between the host (heartworm) and the organism living within (Wolbachia). What’s even cooler? When the antibiotic doxycycline is administered to a heartworm positive animal, the doxycycline kills the Wolbachia. Since the heartworm is dependent at some level on the Wolbachia, the death of the Wolbachia can be detrimental to the survival of the heartworms. The Wolbachia is present in all stages of the heartworm life cycle, but it is known that the Wolbachia populations expand between the third and fourth larval stages. It is thought that if the Wolbachia expansion is prevented, then the heartworms may be unable to develop further.
If we delve into the complexities of Wolbachia further, we also find that the bacteria can be a contributor to the clinical signs that can develop in a heartworm positive dog as heartworms die. When heartworms die (either naturally or after being treated with melarsomine), surface proteins on the Wolbachia are released. These surface proteins trigger an immune response that can cause inflammation and partial blockage of blood vessels.
For these reasons, the American Heartworm Society recommends a heartworm treatment protocol that incorporates the use of doxycycline. The doxycycline is given prior to starting adulticide therapy to allow time for the death of the Wolbachia. In this way, the inflammation typically caused by the Wolbachia in the dying heartworms can be reduced, and the heartworm mass may also be reduced.
While it is exciting to see progress being made in the treatment of heartworm disease, it is so important to not lose focus on how easily it is to prevent the disease. Lawndale Veterinary Hospital and the American Heartworm Society recommend year-round heartworm protection for all dogs and cats. Preventing heartworms is so much easier, safer, and cheaper than having to deal with a heartworm infection. Is your pet due for heartworm prevention? Then let’s get them protected against heartworms right away!
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