Once considered only a problem in the warm, humid South, heartworm is now an issue throughout the United States. Spread by the bite of a mosquito, adult heartworms settle inside the dog's heart and lungs and slowly strangle these organs until they cease to function properly.

Products designed for monthly administration, in the form of a pill or tablet, control heartworm disease by distributing active ingredients systemically throughout the dog's body. Since the heartworm is a nematode with larvae that reside in the dog's bloodstream, blood containing these active ingredients will stop the nematode's growth.

One ingredient that arrests the development of heartworm is the chemical selamectin, which disrupts the nematode's nervous system. "Selamectin works by affecting the third and fourth stages of the heartworm larvae," says Georgette Wilson, DVM, companion animal veterinarian for Pfizer. "This chemical has a 60-day reach back, which means it will kill heartworm that may have infected the dog as long as two months prior." Selamectin has also proven effective against sarcoptic mange, ear mites, hookworm, fleas and ticks.

Ivermectin is another active ingredient commonly used in the control of heartworm. Known for its safe usage in most dogs, ivermectin is effective in killing heartworm microfilia before it has a chance to develop into an adult heartworm. Although classified as a broad-based insecticide effective on various mites, it is not effective with fleas and ticks. Care should be taken not to combine ivermectin use with any flea or tick products that are also monoamine oxidase inhibitors. When used together, products in this chemical group can create sedation and adverse neurological effects.

Because uncontrolled heartworm infection can lead to fatal heartworm disease, it's vital that dog owners treat their
pets with one of the several products available through veterinarians to combat this condition.