The U.S. Food and Drug Administration (FDA) has announced a limited return of a reformulated heartworm prevention drug for dogs, ProHeart 6.

Manufactured by Fort Dodge Animal Health in Kansas, ProHeart 6 (moxidectin), had been withdrawn from the market in 2004 because of adverse reactions, including loss of appetite, lethargy, vomiting, seizures, difficulty walking, jaundice, bleeding disorders, allergies, convulsions and death.

The FDA approved its limited return to the U.S. veterinary market under a "risk minimization and restricted distribution program." The program is intended to educate veterinarians and pet owners regarding the possible risks associated with the use of ProHeart 6. Veterinarians who wish to purchase ProHeart 6 will be required to register with the company and participate in a Web-based training program before obtaining the product.

The return of ProHeart 6, an injectable drug, to the market is based on results of additional toxicological and pharmacologic studies by Fort Dodge Animal Health coupled with the low adverse reaction frequency in international markets.

The FDA says the manufacturer has improved the manufacturing specifications for ProHeart 6 to decrease the presence of solvent residues. Few adverse events have been reported with this reformulated product, the FDA said.

Dog owners who suspect their dog is experiencing an adverse reaction to ProHeart 6 should immediately contact their veterinarian. Veterinarians should contact Fort Dodge Animal Health to report any adverse events at (800) 533-8536.

Heartworm disease is a serious and potentially fatal condition for dogs. The parasite that causes heartworm disease is transmitted through the bite of a mosquito.