We also advocate minimal use of heartworm medicines including Hartguard (sometimes spelled Heart Guard). Though vets will often tell you to use it every 30 days, it clearly says on the box that every 45 days is sufficient. Unless you live in Florida or the Rio Grande Valley area of Texas, it is not necessary to use year round. There are states in the northwestern part of the country where heartworms are practically nonexistent. It must be over 57 degrees F. for 45 days in a row in order for the heartworm nematode to complete its cycle in the mosquito before it can be transferred from a mosquito to a dog. If the temperature goes below that at anytime during the day or night, the cycle is broken and the 45 days must start over again. For most of the country, heartworms are only a danger to dogs for less than half the year. By keeping the amount of toxic chemicals that you are giving to a minimum, you are increasing your pet’s chance of having a long, healthy life.