Long term, low level exposure to toxic substances can weaken a pet's immune system and cause it to attack the healthy cells and organs in a pet's body. This is called autoimmune or immune-mediated disease. It can have many causes, but flea products, along with other toxic substances we put in and on our pets (like vaccines), are a leading concern.
There's an interesting article about flea products today in the ecologist.com, which describes the gradual nature of the harm that can be done by repeated use of flea and tick control products.
The problem is that ‘spot-on’ products may not harm or kill pets or humans with the initial exposure, or even after several exposures. Exposed in this way it takes longer for the negative effects – often due to and continual assault on a animal’s immune system - to reveal themselves. Vigilance is required because the health issues for flea and tick treatments are the same as with pesticide exposure to those living close to farmland, or those who use pesticides in their homes: small chronic exposures add up.
But because low dose exposures act like slow motion poison, the pesticide industry continues to mislead the public, and the government regulatory agencies, by insisting that there is no scientific proof showing a definite cause and effect link between pesticide exposure and illness in either pets or humans.
Read the full article here.
For my own pets, I do use flea products, but I use them sparingly. I treat--once--at the first sign of fleas, using a combination of spot products and a pill (Program) which work together; one kills the adult fleas that are living on my pets, and the other prevents adult and larval fleas in the environment from reproducing. I've found this to be an extremely effective, double edged approach that eliminates the need for repeated exposure to these toxins. Once I've treated my pets with these products, it takes about a month for all the adult and larval fleas to die off, Once they do, we are flea free, and we stay that way for several years. I don't treat again until the next time fleas appear.