Fighting fleas once they're present can take many approaches. It's best to start off conservatively, depending on how bad it is. Check with your vet first, though, especially if you have a puppy or a kitten, if your pet is pregnant, if your pet is in poor health or has a flea bite allergy, and if you have young children.
If you find only one or a few fleas on your pet, if you don't see fleas but you find very little flea dirt (or if you're not sure it's flea dirt), and if your pets are in good health, you might be able to get things under control with a few simple steps if you act quickly.
Give your dog or cat a bath in regular, gentle pet shampoo. Fleas drown easily, and a good lather can snag them before they can escape (flea shampoos, sprays, and powder have questionable value, and research has shown some of them to be hazardous). Lather every part of the animal, being careful around the face and avoiding eyes and ears, and rinse well. Use damp paper towels to wipe around eyes and ears, wrap in a plastic bag, and throw away. Best bet is to bathe the animal outdoors if possible (in warm weather, a garden hose attached to an indoor utility sink works great) but, if that's not an option, scrub the bathroom or kitchen thoroughly afterward in case there were any eggs or larvae that got shaken off.
Don't return the pet to his usual quarters before they're cleaned. Get other family members involved, if you can, and while you're bathing the pet, have them vacuum the room where your pet spends most of his time. Remove the pet's bed, if he has one, and replace it with clean blankets or towels or whatever works for your pet. The idea is to not return your freshly-bathed pet to an area where there may be fleas.
Clean the entire house thoroughly, and use the vacuum cleaner and washer and dryer like it's a new hobby. Vacuum and mop daily, and wash and change your pet's bedding daily or every other day if possible. Vacuum upholstery and pay close attention to any area in the house that your pet visits. Old sheets and blankets are easily-washable coverings for pet beds or, if your pet is small, towels or old pillow cases work well. Examine your pet every day, and examine the bedding. If, after a few weeks you don't see any sign of fleas (if the presence of fleas was questionable or you only found minimal evidence), you can cut back to twice weekly or weekly rigorous washing and cleaning. If you're still finding fleas or flea dirt, if it gets worse, and if you have a pet with poor health who may suffer more than others from flea bites and blood loss, you'll have to take more aggressive measures right away.
You may be wondering: why not just use a monthly application of a "spot-on" treatment such as Frontline or Advantage? If you live in an area where fleas are common, if your dog is frequently around other dogs or animals with fleas, or if your pet has an allergy to flea bites (many do) and he's frequently exposed, then it's probably a good idea. In many cases, though, there's just no need, and the fewer chemicals your pet is exposed to (including flea collars, which aren't much help), the better.
In the event careful bathing and housecleaning doesn't prevent a flea infestation, you'll have to take things a step further. Your best bet is to call your vet and take his or her advice. He or she may suggest a combination of a spot-on treatment for at least several months or longer and an oral medication, such as Capstar. Your vet may also recommend that you bring in a stool sample, since fleas can transmit tapeworms and other diseases.
As for the house, you'll have to continue with the frequent cleaning, and you may need to steam clean carpets and upholstery or even remove area rugs and cover furniture with sheets until the problem is resolved. You may also need to take measures in your yard, if you have one. Here are some great, natural approaches to consider for safe, effective flea control.
If all natural methods fail despite your very best efforts, you may need to consider using insecticides ("bombing" or spraying areas of the house, for example), but this is best done with the advice of an expert, and it doesn't always work the first time around. Your veterinarian may suggest various products or even refer you to a pest control company, but this should be a last resort and approached with caution and in combination with regular, intensive housecleaning.
Your best bet to keep fleas out of your house and away from your animals is prevention or early intervention. There is no one, easy way to get rid of fleas once they've set up camp, but understanding that it requires vigilance and time is the key to success.
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If you found this article helpful, you might also want to learn about ticks.
For more info: Fleas on dogs and cats and what you can do about them
Natural flea treatments for dogs and cats
Fleas and flea control (contains good photos of flea dirt)