Fleas: More Than an Annoyance
October 9, 2025
Karen Thomas, DVM
Since nursing kittens are slow to replace lost blood, flea infestations can be fatal. Fleas are also responsible for most skin problems in cats including flea allergy dermatitis, eosinophilic granulomas, miliary dermatitis, and rodent ulcers. All are frequently caused by a reaction to flea saliva. When fleas bite, they inject a tiny bit of saliva to keep the blood from clotting while the flea is feeding. While the amount injected is minute, when you add that each flea bites an average of 25 times an hour it does not take long for a significant amount of flea saliva to enter the body and cause reactions.
Once fed, the female flea will lay eggs-up to 50 per hour. In a few days the eggs will hatch into tiny worm-like flea larvae. They feed on flea feces which is basically processed blood deposited in the same area as the eggs. As they grow, the larvae shed their exoskeleton several times, then spin a cocoon, much like caterpillars turning into butterflies. Flea cocoons are impervious to all insecticides and can lay dormant for a long time. When they feel vibrations signaling a possible food source nearby, the adult fleas pop out of the cocoons to start the cycle over again. Thus, the fleas you see are only a small part of the flea population in your house.
Since cats are sensitive to many insecticides, care must be taken when attempting to eradicate fleas. Many things will impact the selection of flea products you may use. These include how many cats live in your house and whether they go outside, what other animals also live in the house such as dogs, birds, and pocket-pets such as rabbits, guinea pigs, etc. Another consideration is whether the population is stable or includes cats or dogs that move into and out of the house, such as a foster home or rescue situation.
In most cases the safest way to control fleas is to disrupt their life cycle with premises control. For the outside of the house, it is possible to treat the lawn with beneficial nematodes (worms that eat flea larvae). These are available from garden centers, DIY stores, and online resources. They only need to be applied once a season. For inside the house, you can purchase sprays and foggers that contain insect growth regulators (IGR) such as methoprene, also available from garden centers. These products are very safe if they do not also include adulticide. If there are a lot of carpets in the house, boric acid powder can be applied and worked into the carpet with a broom. This is usually an annual treatment. Care must be applied to follow all instructions as boric acid can be toxic if inhaled or ingested, but it is safe once worked into the carpet.
Two other products that can disrupt the life cycle are lufenuron and selamectin. Lufenuron was formerly available as an injection given by a veterinarian and lasted six months. As of this writing, as far as I know, the injectable is no longer available but oral lufenuron is available and is given once a month. One source is a web site called PuppyButts.com. Not only is this product very safe, but it has the added benefit of concentrating in the milk of nursing moms, thus treating the kittens as well. Lufenuron works by preventing the fleas from producing chitin, the exoskeleton. Without chitin, the embryos are unable to break out of the eggs and the larvae are unable to mature. As mammals do not have chitin in their bodies this product is completely safe.
Selamectin is a topical product that is applied once a month. The brand name is Revolution® but there are numerous generic versions. Unlike most topical insecticides, selamectin is absorbed through the skin and then deposited in the oil producing glands so it is available all over the animal. While it is labeled to kill adult fleas and ticks, it is poorly effective in that regard for cats. But it does kill flea eggs and larvae and, as a bonus, it also treats ear mites, round and hook worms, and prevents heartworms. Selamectin is the first product mentioned in this article that requires a prescription from your vet. It comes in many strengths for both dogs and cats, but it is all the same product, so the dog variety can be measured into smaller doses, thus saving money. Revolution® is also available in a “plus” formulation containing a stronger adult flea killer called sarolaner.
All the products listed so far will effectively eliminate fleas, but they can take months to do so. If your house is already infested with fleas, you will want to use something that kills adult fleas right away. A unique product in this category is nitenpyram, brand name Capstar®. This insecticide is an oral product that is extremely effective in killing adult fleas within hours of ingestion. However, it is only active for about 48 hours. Its best use is to treat any new cats or dogs coming into a home, but not for treating those already there because of its short duration of action. This product is readily available as the brand name, as well as a generic name with no prescription needed.
For adult flea killers there are several things to take into consideration. Pyrethrin, one of the earlier flea control products (supplied under the brand name “Adams” as a spray or shampoo) is seldom effective any more as fleas have developed resistance. The same holds true for Sevin®, available as premises control or topical powder. Topical products for dogs may contain permethrin which is toxic to cats. It is best not to use these products on dogs that may come in contact with cats. In fact, many products that treat ticks as well as fleas are not safe for cats. One good option for households with both dogs and cats is Bravecto®. It can safely prevent fleas from entering the house on dogs for three months with each dose.
There is only one commonly used oral flea killer for cats, Credelio®. This is a once monthly flea killer, and it is especially useful for cats that have reactions to topical preparations. Many of the available flea preparations contain a mixture of insecticides. There is a vast array of products and ingredients, including imidacloprid, esafoxolaner, eprinomectin, pyriproxyfen, s-methoprene, spinetoram, etofenprox, dinotefuran. All of these ingredients are for use once a month. Most are available over-the-counter if you do a lot of searching. One product, Bravecto® for cats, is labeled for 3 months. What is important in choosing one of these products is to read and follow label directions. Make sure the product is labeled for cats. Carefully follow directions regarding age and weight limitations, use during pregnancy and nursing, and application procedures. And be aware that topical products can cause skin reactions varying from mild itching to severe burns. Besides those precautions, the decision as to which product to use will mostly depend on cost and ease of application. It is wise to consider consulting a veterinarian initially until you decide on the product that is best for you. Special consideration must be given in situations where there are show cats that cannot afford to have skin reactions or even greasy spots where the product was applied.
Other flea control options such as baths, sprays, and collars are generally less effective and not very practical in multi-cat households. For very young kittens it may work to use a flea comb or to shampoo them and pick off fleas. Immediately put the fleas in alcohol to kill them. Be extremely careful of so called “natural” products. Some (such as lavender, peppermint, or neem oil) can be toxic while most others are simply not effective.
References:
https://www.cdc.gov/fleas/about/flea-lifecycles.html
https://www.fleascontrol.com/fleas-life-cycle/#google_vignette