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 heartworm preventatives 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.

Immiticide (melarsomine dihydrocholoride), manufactured by Merial, is the only U.S.-licensed drug to treat adult heartworm infection in dogs. Some time ago, Merial acquired the only other approved adult heartworm treatment and ceased manufacturing it. Then pharmaceutical giant Sanofi-Aventis last summer announced a 100 percent acquisition of Merial. (See FindRetrievers article on this acquisition by clicking this link.)

On Dec. 1, Merial notified veterinarians that problems with Immiticide’s production are due to “unforeseen technical difficulties during a planned manufacturing site transfer.” Merial believes “with careful management of existing product, we should be able to meet the needs of most veterinary practices for treatment of heartworm disease in their patients.”

Merial is asking veterinarians who elect to use the drug to order only enough drug as necessary to provide immediate treatment to dogs infected with adult heartworms. It can now only be obtained 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.”

The Richmond SPCA reports about their experience with the Merial Customer Care line in a recent blog entry. They were recently denied the drug for one of their shelter dogs.”It became apparent to me once again last week how the odds can be so unfairly stacked against shelter animals..The magnitude of this dilemma hit home when I called about a dog we just transferred into our care named ‘Fred.’ Fred is a 2-year-old shepherd mix who is positive for heartworms…When I called Merial to see what I needed to do in order to purchase the amount of Immiticide that Fred would need…I was told that he was not a candidate for the current supply. In other words, Fred was not a critical enough case to warrant selling the Richmond SPCA the drug for his treatment.”

Many shelters are now at a loss of how to handle the current situation as they rarely allow clients to adopt out dogs with heartworms and don’t necessarily have enough space to kennel the heartworm-positive dogs for four months, while they wait for supplies of Immiticide. This may result in additional euthanasias for shelter animals who are being housed in shelters without a “no-kill” policy. For “no-kill” organizations, the other option is keeping dogs four or more months until Immicitide becomes available, treating them with the generic ivermectin, which kills baby heartworms and keeps the infection from getting more severe but doesn’t kill adult worms, which can live up to two years. That means heartworm-positive dogs will take up space in foster homes and shelters, keeping them from accepting new dogs.

But some vets are worried they won’t even be able to get Immiticide for “owned dogs” in their practices, let alone rescues. Dr. Patrick Hackett of Pellissippi Veterinary Hospital in West Knoxville, president of the board of Humane Society of the Tennessee Valley, said Merial has been “very strict” about who gets Immiticide.

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. 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.

This presents a very challenging scenario for dogs in the southern areas of the U.S. where there have been reports of Ivermectin resistant heartworms. Southern rural areas and outdoor dogs are more likely to have heartworms, which are spread by the mosquito bite. Up to two-thirds of pet owners whose dogs are diagnosed with heartworms in the South opt not to treat with Immiticide due to the expense, which can run up to $500-600 per dog including blood work, X-rays and time in the hospital. Some don’t treat at all, ultimately a death sentence for the dog, while others opt for the cheaper “slow-kill” method to rid the worms.

With the South’s warm weather making this a year-round problem and the increasing rate of heartworm infected dogs, why is the #1 heartworm preventative and the ONLY HEARTWORM treatment made by only one manufacturer in the good ol’ USA! With this situation, where is the research from our Southern Veterinary Colleges for an affordable treatment and effective preventative for heartworms?????