Heartworm treatment drug remains in short supply

FDA must approve manufacturing facility, Merial reports

Revised: April 15, 2010
Published: April 15, 2010

By Jennifer Fiala

Regulatory approval by the U.S. Food and Drug Administration’s Center for Veterinary Medicine (FDA-CVM) stands between veterinarians and their access to the only drug licensed in the United States to treat adult heartworms in canine patients.

Supply lines of Merial’s Immiticide (melarsomine dihydrochloride) to U.S. practitioners hinge on the agency’s authorization of an overseas manufacturing site. The location and identity of that site have not been released.

According to Merial, the new manufacturer already produces the drug for European and international markets. Last December, Merial alerted practitioners to company plans to ration Immiticide due to “unforeseen technical difficulties.” With the product’s active ingredient no longer available from Merial’s U.S. manufacturer, the company implemented a restricted distribution program, one that permitted veterinarians treating only severe cases of heartworm disease to access the drug on a case-by-case basis.

At the time of the initial announcement, Merial expected that the shortage would soon be alleviated and veterinarians could once again stock the drug after the first quarter of this year. With that time period past, Merial has issued an updated letter to veterinarians concerning details about the delay and a new, potential manufacturing site. In it, Merial acknowledges that while it’s found a new supplier of melarsomine, gaining regulatory approval for the manufacturing facility will “take time” though company officials hope to “return to full supply of Immiticide as soon as possible.”

Officials at FDA-CVM would not provide the VIN News Service with more detailed information on the approval process, citing confidentiality constraints. Yet CVM spokeswoman Laura Alvey verified Merial's latest statement to practitioners concerning the overseas manufacturer.  

While Merial waits for the CVM's nod, the company "has instituted a temporary measure to extend the supply of Immiticide and continue to make it available to veterinarians,” the Merial letter states.

Translation: Like before, veterinarians can still buy Immiticide on a case-by-case basis by calling Merial Technical Solutions at (888) 637-4251, option 1. The difference is that the Immiticide they now will receive comes from the new supplier, per a temporary regulatory allowance that’s been made by the CVM.

A Merial technician must first screen a patient’s medical information before selling the drug, and that has some veterinarians losing patience. Others in a Veterinary Information Network (VIN) discussion expressed distress about not being able to properly treat patients and frustration about not knowing exactly what is happening in terms of the drug’s availability.

“Look, I've been a good boy, and I haven't pretended my patients have clinical signs when they don't,” writes Dr. Everett Mobley, a VIN member and practitioner in Missouri. “I haven't tried to order product for dogs that are apparent lack of efficacy cases. I've got 12 people breathing down my neck wanting their dogs treated. It would help if I could just get a straight answer from (Merial) on what the problem actually is.”

In an attempt to get more details, the VIN News Service queried Merial spokeswoman Natasha Mahanes, who did not immediately respond to the request. Dr. Michael McGuire, of Texas, reports on VIN that Merial’s latest letter to DVMs clashes with what he’s heard from his corporate sales representative, who reportedly relayed on April 6 that Merial ran out of the drug and that a timeline on future production was unknown.

Adding to the uncertainty, McGuire says he received a shipment of two boxes of Immiticide that apparently were on backorder the same day.

Florida practitioner Dr. Heather McCauley shares in his confusion: “What on Earth is the problem with this product being manufactured in the U.S.?” she writes on VIN. “... Is this an example of our increasingly bloated regulations hamstringing our ability to produce anything at all in this country?”

Whether that argument carries weight in this case remains in question. In the meantime, Dr. Tom Nelson, a past president of the American Heartworm Society and a practitioner at the Animal Medical Center in Anniston, Ala., recommends that practitioners follow a management plan designed in response to the Immiticide shortage.

Developed by the American Heartworm Society, the protocol centers on the use of a heartworm preventative plus doxycycline — a combination that has been shown to reduce pathology, the number of adult worms and the infective potential of microfilaria in canine patients.  

“Doxycyline therapy will eliminate Wolbachia, an intracellular bacterium found in pathogenic filarial nematodes, which contributes to the disease process," he says. "Removing the Wolbachia organisms weakens the worm and shortens the life span, but it’s not an ideal situation as it will take a year or more during which time the dog should be exercise restricted."

(Dirofilaria immitis is the pathogenic filarial nematode that causes heartworm disease in dogs and cats in the United States.)

Nelson adds Merial’s policy to ration Immiticide to only the most severe cases could present a problem. If possible, it's better to treat a dog before the symptoms become severe, he says. Dogs that are acutely ill with severe heartworm disease are likely suffering because a worm has recently died and fragments are blocking vessels and causing inflammation in the lungs.

The last thing a dog in that situation needs, Nelson says, is to be faced with more dead worm fragments.

“Dead worms cause more problems than live worms. You need to wait until that dog has stabilized before killing more worms with Immiticide,” he advises.

The American Heartworm Society’s eight-step protocol for veterinarians treating heartworm-positive patients can be found at


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