If you have a pet, especially if you have a dog, there are three insect "vectors" you need to worry about: fleas, ticks and mosquitos. If you have a cat that spends time outdoors, these vectors are of concern for them too.
Fleas cause itchiness, dermatitis and can be a source of tapeworms! They are very easily spread from pet to pet and lay lots of eggs. So a single treatment is not going to be enough to rid your pet of fleas once they have them.
Ticks are a source of Lyme disease, which most people have heard of, and for which there is a vaccine for dogs. However, they are also a source of some other tick borne diseases like Anaplasmosis and Canine Ehrlichiosis for which we do not have vaccines, and these diseases can be an even greater threat to your dog's health than Lyme is.
Luckily for dogs and cats, mosquito bites rarely cause the itchy welts for them that they do in humans. Dogs and cats are not as allergic to mosquito saliva as we are. However, unluckily for dogs and cats, they can contract heartworm disease from mosquito bites.
In the case of fleas and ticks, we try to prevent the diseases they carry by killing and/or repelling the fleas and ticks. In the case of mosquitos, while there are some flea/tick products which also repel mosquitos, we do not rely on them to prevent trasmitting heartworm. Instead we give a medication that would kill any heartworm larvae that may have been transmitted by a mosquito bite.
Another difference between that approach to fleas/ticks and the approach to mosquitos/heartworm is that the flea/tick medication, once it has been given, is typically going to last for a month (or up to three months depending on the product). Heartworm treatment typically only lasts a few hours. What it does is it kills any larvae in the pet that were obtained within the last month by a mosquito bite. So we give heartworm preventative monthly, not because it will last for a month, but because it can kill any heartworm that the pet was exposed to in the previous month. Once those heartworm larvae have been living in your pet for more than a month, they get harder to kill with preventative.
So when do you begin flea/tick/heartworm preventative, and when can you stop it? The answer depends on the weather and the temperature. If it's an early spring or an unusually warm fall/winter, it is easy to start too late or end too early giving the preventative medications that you need to keep your pet healthy. For that reason, we recommend giving these medications all year round.