On December 1, 2009, Merial published an open letter to veterinarians, announcing a shortage of Immiticide (melarsomine dihydrochloride), the only drug licensed for use in treating heartworm infestations in dogs. The shortage is due to a manufacturing site transfer. The company expressed hope that the shortage will not persist beyond the first quarter of this year.
An apparently unrelated problem is responsible for Merial’s announced shortage of Heartgard (ivermectin) tablets, which may be unavailable until 2011. Heartgard prevents canine heartworm disease by eliminating the “tissue stage” of heartworm larvae for a month after infection.
Fortunately, Heartgard chewables and other ivermectin products (including products made by other manufacturers) remain available, so a shortage of the tablets is not cause for concern. The Immiticide shortage, however, has alarmed veterinarians and shelters (who see a lot of heartworm-positive dogs) across the country.
To repeat: Immiticide is the only drug licensed or used to treat adult heartworms in dogs, and Merial is the only company who makes this product. Because of the shortage, veterinarians can no longer order Immiticide from distributors, in order to prevent stockpiling.
Instead, veterinarians who have a heartworm-positive patient must contact Merial directly and provide details of their patient’s case. For now, Merial is selling the drug on a case-by-case basis, providing the drug only to the more severe cases, those dogs with clinical signs of heartworm disease. Dogs who test positive but have no clinical sign of disease will have to wait.
Safe, effective alternative
Fortunately, there is an alternative treatment for heartworm. As we discussed in “Update on Doxycycline and Heartworm Disease” (Whole Dog Journal August 2009), a combination of ivermectin (the active ingredient in Heartgard) and doxycycline (an antibiotic), weakens and sterilizes adult heartworms, eventually killing them. The time this takes depends on the age of the worms; the older the worms, the longer they take to die.
In addition, giving doxycycline and ivermectin prior to treatment with Immiticide lowers the risk of adverse reaction to worm death, making the treatment much safer. It also lessens the negative effects of the worms themselves, primarily due to doxycycline’s effect on Wolbachia, a parasite of heartworms (see “Parasites within Parasites,” Whole Dog Journal August 2006).
The American Heartworm Society (AHS) recently updated its guidelines for treatment of heartworm infection in dogs. It says, “Studies have shown that heartworm-positive dogs pretreated with ivermectin and doxycycline prior to receiving melarsomine (Immiticide) injections had less pulmonary pathology associated with the death of the heartworms. If doxycycline is incorporated into a heartworm treatment protocol it should be given before administration of melarsomine so the Wolbachia organisms and their metabolites are reduced or absent when the worms die and fragment. Doxycycline administered at 10mg/kg BID for four weeks has been shown to eliminate more than 90 percent of the Wolbachia organisms and the levels remain low for three to four months.”
For dogs who are not treated with Immiticide, the guidelines say, “the use of a monthly ivermectin-based heartworm preventive along with doxycycline could be considered. It has been reported that ivermectin and doxycycline administered periodically over 36 weeks resulted in a 78 percent reduction in adult worm numbers. Moreover, microfilariae from dogs treated with doxycycline that were ingested by mosquitoes developed into third-stage larvae that appeared to be normal in appearance and motility, but these larvae were not able to develop into adult worms, thus negating the risk of selecting for resistant strains. The administration of doxycycline at 10 mg/kg BID for a four-week period every three to four months should eliminate most Wolbachia organisms and not allow them to repopulate.”
While the AHS still recommends monthly use of heartworm preventatives in combination with doxycycline during treatment for heartworms, the studies reported above used standard heartworm preventative doses of ivermectin given weekly during the 36-week treatment period. They also pulsed doxycycline throughout the treatment period rather than just giving it every three to four months.
Based on the above, it may be best to give Heartgard (not Heartgard Plus) weekly until treatment with Immiticide is begun, or until the dog no longer tests positive for heartworms, if Immiticide treatment is not used. Giving Heartgard weekly (rather than monthly or every two weeks) is less important for dogs who will be treated with Immiticide than those relying on ivermectin and doxycycline alone to get rid of heartworms. (Note that weekly Heartgard is not recommended for dogs with the MDR1 gene mutation that causes sensitivity to ivermectin.)
Doxycycline should be given at the dosage level listed above for four weeks prior to starting Immiticide treatment. If treatment has not been completed within three to four months, doxycycline should be given again for four weeks. If Immiticide treatment is not done, treatment with doxycycline should be repeated every three to four months until the dog no longer tests positive for heartworms.
Ivermectin and doxycycline may seem a safer (though slower) alternative to Immiticide, even when the shortage is over. But heartworms cause damage as long as they are in the dog’s body, and the danger from the dying worms, while reduced by the use of doxycycline, exists as long as the worms are present. Immiticide following one month of treatment with doxycycline and ivermectin is still the treatment of choice for most dogs with heartworm disease. If Immiticide treatment is not available, or if you have a dog with early-stage heartworm disease, then long-term use of ivermectin with doxycycline is a reasonable alternative. – Mary Straus
For more information:
Merial Customer Service, merial.com, (888) 637-4251 (option 1)
heartwormsociety.org/veterinary-resources/guidelines-Can-HW-Disease.pdf
