There is a lot of talk on the Internet about killing cat fleas by drowning and using soap. I’d like to discuss these options in this article.
You can drown fleas if you force them underwater and hold them there for 24 hoursA. Yes, it is a very long time and impractical. Fleas do not sink into water because there are too light and they have a waxy coating over their exoskeleton. They float on the surface of the water. If you left a flea floating on water in a bowl they would eventually die of starvation but they wouldn’t drown. They can’t jump out because they are on water. FYI – (1) if fleas were the size of humans they’d jump almost 1,000 feet into the air. Jumping is their method of transportation. They can’t fly and (2) fleas are flat so they can more easily navigate through fur.
If you force a flea underwater for 20 minutes it would not kill them. A flea submerged for up to 12 hours will appear dead, but they will revive in about 60 minutes. Submerging them for 20 hours probably won’t kill them either. They would revive themselves over a longer period; 4 to 5 hours. Therefore drowning per se is not a viable option to kill fleas.
Perhaps drowning is discussed a lot because cat owners flea comb their cat and dunk the flea comb into water. Or they bathe their cat with soap and water. If you dunk a flea comb with fleas on it into water the fleas will probably float on the surface and if they are left there, as mentioned, they will eventually starve to death. Perhaps cat owners chuck the water down a toilet which might drown them eventually but they may survive the experience.
A good number of people swear by soap as a means to kill fleas. There are two aspects to this story. Firstly, washing your cat with household soap and water is not a great idea because the soap will dry out the cat’s skin by removing all the oils. You can use flea treatment shampoo but beware. It contains insecticides which can be dangerous unless you use it cautiously. For example, Adams Plus Flea & Tick Shampoo with Precor contains: Methoprene; Pyrethrins; Piperonyl Butoxide; N-Octyl Bicycloheptene Dicarboximide.
But the point is that soap may remove the waxy outer coating on the flea’s exoskeleton. This may cause a flea to die of dehydration, but how long does it take? I cannot find an answer to the question at this time. Knowing the hardiness of the cat flea I would suggest that it would take a long time to die through dehydration. Therefore soap as a means to kill a cat flea is an inefficient method.
Insecticides or physical killing
Proper use of spot-on insecticides is probably the best way to kill fleas on a cat. Great care needs to be taken and you must never use a dog flea treatment on a cat unless you want to kill your cat. You have to remove fleas from the home in parallel to removing them from your cat otherwise you’ll be going in circles. We all know that by now. Eliminating a bad home infestation of fleas requires a lot of effort and commitment.
One issue is that cat owners are reluctant to use insecticides which I understand and they want to use a home treatment which is safer and cheaper. If you are flea combing your cat and removing one or two fleas around the neck area at a time then the preferred method in my opinion as to kill them physically by crushing their exoskeleton with the nail of your thumb. You can do this by squashing them against the flea comb if it is rigid enough and made of metal and hard plastic. This may not appeal to many!
Source A: Forensic Entomology: An Introduction By Dorothy Gennard; John Wiley & Sons, Apr 30, 2013; section 4.3 which reflects a 1985 work by Simpson K. (Journal of the New York Entomological Society 76: 253-265.