The only heartworm adulticide approved by the U.S. Food and Drug Administration (FDA) for use in canines is temporarily unavailable, with no known re-release date.

News of the Immiticide (melarsomine dihydrochloride) shortage surfaced last week with an open letter to practitioners from drugmaker Merial that warns of an outage expected to “last several weeks to months.” The letter, dated Aug. 4, asked practitioners to help conserve Merial's dwindling Immiticide supplies by ordering product only to treat dogs with severe heartworm infestation. However, the notice of the shortage caused a run on remaining stock. The company's technical services department confirmed today that Merial is "officially out" of the drug.

“After mailing the Dear Doctor letter we experienced an unprecedented surge in Immiticide orders and depleted inventory much more rapidly than anticipated,” explains Natasha Mahanes, a Merial spokeswoman.

Immiticide availability has been shaky since early last year, when Merial, the animal health arm of Sanofi, reported that its U.S. supplier could no longer obtain the product’s active ingredient, and the FDA was hesitant to allow Merial’s overseas supplier to fill American orders. As a result, Merial stopped allowing veterinarians to simply order and stock the drug in an effort to conserve U.S. supplies and implemented a “restricted distribution program.” Translation: Veterinarians treating only severe cases of heartworm disease could access the drug on a case-by-case basis with approval from the company.

In an email exchange with the VIN News Service, Mahanes noted that Merial’s latest Immiticide supply problems are not tied to the troubles of 2010. Rather, it is “a new and separate manufacturing challenge," she writes.

“This situation is related to technical issues providing finished product to us. The finished product is made by a manufacturing company in the U.S.," she explains. "… We are working diligently to mitigate this situation, and there is a possibility that an alternate source of supply may be identified."

That’s encouraging news for veterinarians such as Dr. Skip Fix, a practitioner in Houston. With two boxes of Immiticide left in his clinic, Fix is pondering how best to ration it.

“At this moment I have a 50-pound dog that I’m going to treat,” he says. “There are five bottles to a box, and he’s going to need two-and-a half of them. We’re trying to find out from the shelter near us if they need the remainder for a small dog so we can use every last drop of this.

“This shortage could take a month, it could take forever,” Fix adds. “I usually have a couple heartworm cases going every week, so it is a concern."

Once his in-house supply runs dry, Fix plans on turning to a heartworm preventative and doxycyline to manage infestation — a protocol supported by the American Heartworm Society, slated soon to release updated guidance for practitioners on treating heartworm-positive dogs without Immiticide.

Dr. Tom Nelson, a past president of the American Heartworm Society and a practitioner at the Animal Medical Center in Anniston, Ala., notes that the use of doxycyline in combination with heartworm preventive has been shown to reduce pathology, the number of adult worms and the infective potential of microfilaria in canine patients.

"The shortage of Immiticide is unfortunate, but we're going to have to deal with it," he says.

Treating heartworm is tricky business. An infected dog, for example, must not vigorously exercise because an increase in heart rate raises the risk that fragments of dead worms in its blood vessels could cause thromboembolism — the formation of an embolism, in this case of dead worms — to the lungs, which usually presents as acute dyspnea. The condition is treatable with steroids but may prove fatal. Nelson notes that Immiticide quickly kills worms, thereby shortening the time that a dog's exercise must be restricted. 

Slow-kill therapies, such as the use of heartworm preventatives, potentially allow for greater opportunity in which worms might block blood vessels, leading to worsened pulmonary pathology.

"Exercise should be restricted during the duration of the treatment. We're normally talking about 12 to 18 months," Nelson says.