Two Merial drugs designed to combat heartworm disease are in short supply, with indications that the company is having problems with its separate manufacturing facilities.

The drugs affected are Immiticide (melarsomine dihydrochloride) and Heartgard (ivermectin) tablets.

With Merial’s more popular Heartgard chewables still widely available and other products containing ivermectin on the market, the temporary shortage — expected to resolve in 2011 — will likely be less problematic than the rationing of Immiticide, which is expected to be in short supply through the first quarter of 2010.

That's because Immiticide is the only drug licensed in the United States to treat adult heartworms in canine patients.

On Dec. 1, Merial notified veterinarians that problems with Immiticide's production are due to “unforeseen technical difficulties.” That means veterinarians who elect to use the drug can no longer order enough to stock.

Instead, they must buy it on a case-by-case basis by calling Merial Customer Care at (888) 637-4251. A Merial technician must first screen a patient’s medical information before selling the drug.

“We want to make sure we can supply the product for emergency cases and cases with the greatest need,” explains Natasha Joseph, a Merial spokeswoman.

In Merial’s ‘dear doctor’ letter, Vice President Dr. Zack Mills writes: “We realize that this limitation of Immiticide supply can create a potential challenge for veterinarians who need to provide heartworm adulticidal therapy. On behalf of Merial, I thank you for your patience and understanding.”

Dr. Skip Fix, a Veterinary Information Member (VIN) and practitioner in Houston, says he was able to stock up on Immiticide when he caught wind of Merial’s plan to ration it. That’s good, he says, because he sees a couple of dogs weekly in need of the drug.

“Obviously it’s important that veterinarians can treat their patients,” he says.

Others have lamented in a VIN discussion that Merial’s criteria for selling Immiticide is unclear. Company spokeswoman Joseph reiterates that such decisions are made on a case-by-case basis.

“It’s not a one-size-fits-all solution,” she says. “It’s a veterinarian-to-veterinarian discussion about the dog’s health and duration and seriousness of the infection. There aren’t specific parameters. Multiple factors are taken into consideration.”

That’s frustrating for veterinarians like Dr. Angela Ivey, who reportedly was told by a Merial technician that her patient wasn’t sick enough to receive Immiticide.

“(The dog) is only about two years old with no clinical signs, and the current supply had to be reserved for the most severe cases,” writes Ivey, director of veterinary medicine for the Richmond Society for the Prevention of Cruelty to Animals, in a VIN discussion. “Unfortunately, those will not be the cases likely to receive the limited shelter resources. ... In private practice, I preferred to treat heartworm-positive dogs before they presented with clinical signs.”

In an interview with the VIN News Service, Ivey adds that it's hard to adopt out dogs with heartworms; the shelter she works for traditionally doesn't do it. What's more, in shelters that don't have a no-kill policy and are crammed for space, a rationing of Immiticide might be a death sentence for dogs that test positive for heartworms.

To wait out Merial's rationing, some specialists are recommending a two- to three-month pre-adulticide period during which ivermectin is administered, with or without doxycycline. That’s for dogs with mild disease where there is not much pulmonary pathology, dogs that have only mild pulmonary changes and dogs in non-endemic areas, where the worm burden is likely small.

This potentially serves two purposes, explains Dr. Mark Rishniw, a boarded cardiologist and VIN consultant. It allows any larvae or immature adults to mature enough to be susceptible to Immiticide, and it weakens the adults, so they are theoretically more susceptible to the drug. (Success of that method has not been clinically demonstrated.)

“So if we opt to wait two to three months in these dogs while pre-treating, then we’ve likely waited through the shortage period,” he says.