Yes, flea collars can make cats sick. They can even kill cats if a dog or puppy flea collar is placed on a cat. Never put a dog flea collar on a cat and never use dog flea treatments on cats if you think it will save money and is convenient.
Flea collars for cats can harm cats too. They can cause hair loss due to flea collar dermatitis which is a reaction to the insecticide. There will be local inflammation and itching. It may spread and it may lead to toxicity (poisoning your cat) because the skin is broken with open sores.
The problem of dog flea collars killing cats may be due to the insecticide used; Diazinon otherwise known as Dimpylate. This was a nerve gas in the Second World War. It is hazardous to children and other humans too as they handle their pets and sleep with them. Symptoms of Diazinon poisoning include: vomiting, nausea, blurred vision and dizziness.
The whole area of using insecticides on cats is fraught with dangers I believe. Please read a document prepared by the UK’s Veterinary Medicines Directorate for information about spot on treatment containing Permethrin, another poisonous chemical:
You know the manufacturers say you should not get spot on insecticide treatments like Frontline on your hands. So why are we putting it on the skin of our cats?
I have never considered a flea collar as a way of managing fleas. They are too toxic as far as I am concerned. I don’t even use a local spot treatment. I flea comb my cat regularly and keep my home clean and uncluttered. It surprises me that some cat owners totally miss the damage done by flea collars. The go to their vet with their cat in a terrible state. Surely they are too late to observe the injury. A failure of cat guadianship. It happens quite a lot.
My thanks to (1) Cat Owners Home Veterinary Handbook and (2) Cats Protection.