Most of a flea infestation lives in the home, not on the cat, so the cat, other pets and the house are all treated. Dog flea products containing permethrin can kill cats.
Guidance issued in 2026 asks owners to use spot-on treatments carefully, because two common ingredients are harming river insects.
Fleas, ticks and worms are the common parasites of cats in Britain. All can be controlled, but the products involved are medicines, and some of them have been found to harm wildlife when they reach rivers, so how and how often they are used matters.
The cat flea is the species found on most cats and dogs. Adult fleas live on the animal and feed on its blood, but they are a small part of the population: the eggs fall off into carpets, bedding and gaps between floorboards, where the larvae develop and the pupae can lie dormant for months. In centrally heated homes fleas breed all year.
Signs include scratching, over-grooming, scabs along the back and neck, and black specks in the coat that turn reddish-brown on damp white paper. Some cats are allergic to flea saliva and react severely to a few bites. A heavy infestation can make a kitten anaemic, and fleas carry tapeworm.
Treatments come as spot-on liquids applied to the skin at the back of the neck, tablets, injections and collars. Products prescribed or recommended by a vet are generally more effective than those sold in supermarkets, and the dose depends on the cat's weight. Every cat and dog in the household is treated at the same time.
The home needs attention too: vacuuming floors and furniture thoroughly, washing pet bedding at 60°C and, in an established infestation, using a household flea spray on carpets and skirting boards according to the label. Because pupae keep hatching, it can take two to three months to clear a house.
Flea products made for dogs must never be used on cats. Many contain permethrin, which is highly poisonous to cats and can kill; a cat can also be poisoned by grooming or sleeping against a dog that has just been treated.
Two insecticides widely used in spot-on treatments, fipronil and imidacloprid, have been detected by the Environment Agency in waterways across the United Kingdom at levels that can harm aquatic insects. They reach rivers when treated pets swim or are bathed, and through household drains.
In May 2026 the Veterinary Medicines Directorate published guidance for owners using these products:
The Directorate was also reviewing in 2026 whether these products should be sold only with professional advice. Veterinary associations recommend that parasite treatment is based on the individual animal's risk and not given routinely to every pet all year round; a vet can advise on what a particular cat needs.
Ticks are picked up in long grass, woodland and moorland, mainly from spring to autumn, and attach around the head, neck and ears. They are removed with a tick hook, slid under the body and twisted, so that the mouthparts come away. Squeezing, burning or smothering a tick increases the chance of infection. Cats are less often affected by tick-borne diseases than dogs or people, but the bite site can become infected.
Roundworms are very common in kittens, which are infected through their mother's milk. Adult cats pick them up from soil and from prey. The eggs passed in faeces can infect people, particularly young children, which is one reason for clearing up cat faeces from gardens and covering sandpits.
Tapeworms are caught by swallowing infected fleas during grooming and by eating rodents and birds. Segments resembling grains of rice may be seen around the tail or in bedding.
Many infected cats show no signs. Heavy burdens cause weight loss, a pot belly in kittens, diarrhoea and a dull coat.
Kittens are wormed frequently in their first months, on a schedule set by the vet. Adult cats are commonly treated every three months, more often if they hunt and less often if they live indoors and have no fleas. Worming products do not prevent reinfection, so flea control and worming go together.
Washing hands after handling litter and before eating, wearing gloves for gardening and keeping children's sandpits covered reduce the small risk these parasites pose to people. More on tray hygiene is given under litter trays.