in 2008. ProHeart 6 remains on the market in many other countries, including Canada and Japan. Its sister product, ProHeart 12, is used extensively in Australia and Asia as a 12-month injectable preventive. It was approved for use in the United States by the FDA in July 2019. Topical treatments are available, as well. Advantage Multi (imidacloprid plus moxidectin) Topical Solution, uses moxidectin for control and prevention of roundworms, hookworms, heartworms, and whipworms, as well as imidacloprid to kill adult fleas. Selamectin (Revolution) is a topical preventive likewise administered monthly, and can also be used to control fleas, ticks, and mites. Preventive drugs are highly effective, and when regularly administered, have been shown to protect more than 99% of dogs and cats from heartworm. Most compromises in protection result from the failure to properly administer the drugs during seasonal transmission periods. In regions where the temperature is consistently above 14 °C (57 °F) year-round, a continuous prevention schedule is recommended. Due to newly emerging resistant strains of heartworms, which no macrocyclic lactone (heartworm prevention) can protect against, the American Heartworm Society recommends dogs be on a repellent and a heartworm preventive. The repellent, such as Vectra 3-D, keeps mosquitoes from feeding on the dog and transmitting the L3 stage worms. If a dog is bitten, the heartworm preventive takes over when administered. If a mosquito feeds on a heartworm positive dog on a repellent, they do not live long enough for the microfilaria they ingested to molt into the infective L3 larva. Vectra 3-D was tested using thousands of mosquitoes infected with the resistant heartworm strain JYD34. In the control group that was given only a placebo, every dog contracted heartworms. In the experimental group that was given only Vectra 3-D, two of eight dogs contracted heartworms and had an average of 1.5 adult worms each. In the experimental group given both heartworm prevention and Vectra 3-D, one dog was infected with L3 stage larvae that did not mature into adulthood due to the heartworm prevention. Using a repellent and a prevention is at least 95% effective. Ivermectin, even with lapses up to four months between doses, still provides 95% protection from adult worms. This period is called the reach-back effect. Since dogs are susceptible to heartworms, they should be tested annually before they start preventive treatment. Annual heartworm testing is highly recommended for pet owners who choose to use