Fleas don't always die off in the winter. They oftentimes will go into hiding--even in your home. While chances of an infestation on your pet might decrease in winter months, foregoing flea prevention can allow small numbers of fleas to multiply. This can be costly if an infestation occurs.

Protect your pets from fleas year-round. Fleas enjoy temperate weather with high humidity, so we often notice fleas most often in the summer. But once the days get shorter and the weather gets colder, we feel at ease and see fleas a lot less often. But just because some of the fleas go into hiding doesn't mean that they are gone. Fleas reproduce year-round where environmental conditions are adequate, especially in southern climates where even the winters are temperate.

They say "out of sight, out of mind", so it’s no surprise pet owners let up on flea medication during the winter months. But in actuality, even when your pet is infested, only 5 percent of fleas in an infestation will be on your pet.1 The rest will be in the pet's environment such that they are less visible to pet owners. But during the winter months, fleas can survive indoors where it's warm.
There is no flea season
Many pet owners opt to use flea medications only after the infestation occurs. They believe this will save them money they don’t need to spend on using preventatives like Advantage® II for dogs and cats. But sometimes preventing the problem will save you from a sudden and excessively large cost. Fleas can be on your pet without your knowledge.

If your pet does happen to get infested, you can face quite the financial burden. Flea infestations can often spread throughout a home and it can be costly to remove them from an infested area. Additionally, these fleas can transmit diseases to your pets that can be costly to treat. Aside from the costs, it can be a great deal of effort to have to treat fleas after the fact, combing through fur and checking furniture to ensure that the infestation has been totally eradicated.
There is no flea season
One of the leading brands of flea treatments for dogs is K9 Advantix. However, K9 Advantix contains an ingredient that makes it unsuitable for flea treatment on cats. If you are a cat owner, there are still a variety of flea treatments available. Among them are Seresto® and Advantage II®, both of which are flea preventives available in formulations for cats or dogs. For more information on flea preventives, you can look at this buyer’s guide and this ingredient guide.

Treatment is still an option for risk takers, but we can only see what kind of risk they’re taking if we compare the efficacy of flea repellents with a lack of treatment. Although it depends on the ingredients of your preventive product; studies have shown that flea medication with a particular amount of imidacloprid, one of the active ingredients in K9 Advantix®, is 98.43% effective over the 30-day period for which many topical flea preventives are labeled. Though the study used the original formula for K9 Advantix® rather than the formula for K9 Advantix® II, the formula still uses concentrations of imidicloprid and permethrin.2

Do not use K9 Advantix® II on cats.
K9 Advantix, Advantage and Seresto are registered trademarks of Bayer.

1Grace SF. Fleas. In: Norsworthy GD, Crystal MA, Grace SF, et al, eds. (2006). The Feline Patient. 3rd ed. Ames, IA: Blackwell Publishing; 106-107.
2Science Direct, Efficacy of a combination of imidacloprid 10%/permethrin 50% versus fipronil 10%/(s)-methoprene 12%, against ticks in naturally infected dogs