Flea and Parasite Prevention.

Most of our care guides are about making a cat's life nicer. This one is about not ending it by accident.

The pet aisle in a supermarket contains, on the same shelf, at the same price, in near-identical boxes, a product that will clear your cat's fleas and a product that will put her into seizures within hours. The difference is a word on the back. Nobody at the till will check. The Veterinary Poisons Information Service in London records dog flea spot-ons as the most common cause of fatal poisoning in UK cats, and it keeps happening because the box looks friendly and the price looks right.

So this guide does three things. It tells you what to never, ever put on a cat, and why. It explains why the cheap stuff mostly does not work any more even when it is safe. And it explains why the thing your vet prescribes is not a racket, but the only category of product that is still winning against the flea.

There will be jokes. Not in the next section.

Permethrin is a synthetic insecticide in the pyrethroid family. It is in most dog spot-on treatments, many household flea sprays, and a lot of garden products. Dogs tolerate it. People tolerate it. Cats cannot, and the reason is a gene.

Most mammals break down this kind of chemical in the liver using an enzyme called UGT1A6. In cats, the gene for that enzyme is broken; it has accumulated so many mutations over evolutionary time that it no longer produces a working enzyme at all. Cats evolved as pure meat-eaters and simply never needed to process the plant toxins that enzyme exists to deal with. That gap is thought to explain much of her sensitivity to permethrin. So when permethrin gets into a cat, there is no way out. It sits in the nervous system and holds the sodium channels in her nerves jammed open, so that every nerve impulse fires again, and again, and again.

A dog spot-on can be 45 to 65 percent permethrin. No safe amount has been set for cats, and a fraction of one dog pipette is enough to poison her.

How it happens. Almost never on purpose. The usual routes are:

• Buying the dog product because it is cheaper, or bigger, or the picture on the box was ambiguous.
• Splitting one large dog pipette between several cats to save money.
• Treating the dog, and then the cat grooming the wet patch on the dog's neck, sleeping curled against it, or lying on the sofa cushion where it rubbed off. The application site stays dangerous until it has completely dried.

If you have a dog and a cat, the cat needs to be kept away from the dog until the spot-on has fully dried, and the dog's bedding needs a wash. Or, better, ask your vet for a dog product that does not contain permethrin at all. They exist, they work, and they remove the problem entirely.

Symptoms usually start within a few hours of contact, though they can take up to about three days. They follow a pattern.

First: ear flicking, skin rippling along the back, twitching around the face, frantic grooming at one spot, and drooling.

Then: whole-body tremors, a wobbly drunken walk, and muscles that will not stop shaking.

Then: seizures. The constant muscle activity can push her temperature dangerously high, though some cats go cold instead. Either way, it is an emergency.

In one large UK study about one cat in ten died, most of them despite treatment. Treated promptly, with muscle relaxants and a lipid infusion that pulls the chemical out of the nerves, most cats recover. The sooner she gets to a vet, the better her chances.

What to do:

1. Phone the vet, or the out-of-hours emergency vet, immediately. Say the words "permethrin" and "cat". They will know.
2. Take the packaging with you.
3. Do not bath a cat who is already twitching or seizing. It sounds logical and it is dangerous: the stress, the cold, and the water around a fitting animal can kill her faster than the chemical. The vet will stop the tremors first, then wash her.
4. If you have caught it early, before any symptoms, and your vet tells you to wash her at home, use lukewarm water and ordinary washing-up liquid, which cuts the oily carrier the permethrin sits in. Not cold, which triggers shivering, not hot, which opens the skin and speeds absorption. Dry her thoroughly and keep her warm. Then still go to the vet.

There is no safe amount, no waiting to see, and no home remedy. This is an emergency from the moment you realise.

Next to the dog spot-ons, you will find the other dangerous shelf: the one with leaves on the label.

Tea tree oil. Eucalyptus. Citrus. Peppermint. Lavender. Pennyroyal. Sold as gentle, plant-based, chemical-free flea sprays, collars and drops, mostly to people who have been frightened off the chemical ones. The problem is that these are chemicals too, they are just chemicals made by a plant, and they are broken down in the liver by the same enzyme the cat does not have. The exact same missing gene that makes permethrin lethal makes concentrated essential oils lethal.

The signs are different but no better: a cat who goes quiet, cold and wobbly, then weak, then collapses, with liver damage that may not show until later. Tea tree is one of the commonest culprits, usually after a well-meaning owner has dabbed it on a flea bite.

There is also a category of sprays and powders that work by coating the flea in silicone so it cannot move. These are safe. They are also useless at anything except the adult flea currently on the cat, which, as the next section explains, is about five percent of the problem.

The word natural on a cat product is not a safety claim. It is a marketing one. The things that are proven safe for cats are, without exception, the ones that have been through a licensing process, and those come with a product licence number on the box, not a leaf.

Now the jokes. Or at least the fleas.

The flea iceberg. The fleas you can see on your cat are five percent of the fleas in your home. The other 95 percent are eggs, larvae and pupae in the carpet, the sofa, the skirting board gaps and her bed. A female flea starts laying within a day or two of arriving and produces up to 40 to 50 eggs a day at her peak, which are smooth and roll straight off the cat onto the floor. The larvae hate light and crawl downwards into the carpet pile, where they eat flea droppings, which is the kind of detail you did not need. Then they spin a sticky cocoon, pick up a camouflage of carpet fluff, and wait. For up to six months. For a vibration, a breath of carbon dioxide, a bit of warmth: anything that says a cat has walked past. Nothing you can spray gets through that cocoon. This is why a home can seem clear for weeks after treatment and then, apparently from nowhere, fill up with fleas again. It was never clear.

Fleas also carry things. The most common tapeworm in cats comes from swallowing a flea while grooming. Cat scratch disease travels by flea, and fleas are suspected of spreading feline infectious anaemia too. A flea allergy, where the cat reacts to the saliva, is one of the commonest skin diseases a vet sees.

Ticks. The sheep tick, which lives in long grass, woodland and increasingly in suburban parks, climbs a grass stem and waits for something warm to brush past. It can carry Lyme disease. Transmission takes 24 to 48 hours of attachment, so a product that kills ticks quickly matters more than one that kills them eventually.

Roundworm. Nearly every kitten is born with it, because the larvae pass from mother to kitten through the milk. Adults pick it up from soil, from prey, or from eating something that ate something. The eggs are shed in faeces and survive in soil for years, and they can infect people, where the larvae wander through tissue and occasionally end up in an eye. Children, who put things in their mouths, are the main risk.

Tapeworm. Two kinds: one from fleas, one from mice. An indoor cat with no fleas has effectively no tapeworm risk. An outdoor hunter has both.

Ear mites. Tiny, contagious, and responsible for the dark coffee-ground wax and the frantic head-shaking. Common in kittens and rescue cats, easy to treat, and included in most of the good combination products.

The cat-safe flea products you can buy without a prescription are, almost all of them, built on two chemicals from the 1990s: fipronil and imidacloprid. Fipronil is the one in the familiar green and white box. They were excellent. Thirty years ago.

Thirty years of use on every dog and cat in the country has done to fleas what thirty years of antibiotics did to bacteria. Flea populations in the UK have been found carrying a genetic change in the receptor fipronil targets, and many owners and vets report that the older products no longer clear an infestation the way they used to. The product still kills some fleas. It may no longer kill enough of them, fast enough.

There is a second problem. Fipronil is a spot-on that spreads across the skin in the natural oils and has to get everywhere by itself. By week three of the four-week dose, the concentration at the far end of the cat has dropped off, and the speed of kill slips past 24 hours. A female flea needs 24 to 36 hours on the cat before she starts laying. So in the last week or so of every dose, fleas are arriving, surviving long enough to lay, and refilling the carpet. You treat every month and you never get clear. It is not you. It is the chemistry.

None of this makes fipronil dangerous to a cat. It makes it a product that costs money and largely does not do the job, which is a different kind of failure.

The modern flea treatments are a class called isoxazolines. The names on the boxes are things like sarolaner, fluralaner and lotilaner, and your vet will have a favourite. They work on a receptor in the flea's nervous system that nothing older ever targeted, which means the resistance fleas have built up over thirty years counts for nothing. There is no documented field resistance to them in the UK. Yet.

The number that matters is speed of kill. The fastest of them kill over 95 percent of fleas within 8 to 12 hours of a flea landing on the cat, and they keep doing that for the full four to twelve weeks of the dose. A flea needs 24 to 36 hours to start laying. So no flea that lands on a treated cat ever lays an egg. The carpet reservoir stops being refilled, the pupae hatch into a cat they cannot survive on, and within a couple of months the home is actually, finally clear. No sprays, no foggers, no vacuuming the sofa in tears.

They are also absorbed into the cat rather than sitting on the skin, so there is no dead zone at the far end, no wearing off in week three, and nothing to rub onto the dog, the sofa or a toddler.

The second reason to use the vet is the combination spot-on. Most prescription products now pair an isoxazoline with a second ingredient, usually selamectin, moxidectin or eprinomectin, that handles roundworm, hookworm, lungworm and ear mites, and some add praziquantel for tapeworm. One pipette on the back of the neck, once a month, covers fleas, ticks, mites, lice and worms. If you have ever tried to get a worming tablet into a cat, you will understand that this alone is worth the consultation fee.

The margin of safety is wide. The isoxazolines have been tested at three to five times the label dose in cats, which is a sentence that cannot be written about anything on the other shelf. Rare neurological side effects, including seizures, are listed on the leaflets, which is one more reason they come through a vet.

The catch is that they are prescription only, which means a vet has to have seen your cat. That is not gatekeeping. It is the reason they are still allowed to be this strong.

Every licensed animal medicine in the UK carries a legal category on the packaging. Once you know what they mean, the shelf reads differently.

The logic is that a product is only allowed on the general sales shelf if it is safe for an untrained person to use. Which rules out anything strong enough to work well, and which, through a gap nobody intended, still allows a dog product that is fatal to cats to sit next to the cat products with no one at the till to say a word.

Dog permethrin products must carry a clear warning not to use them on cats, and many also show a crossed-out cat. Look for it, but do not rely on it: if a product is made for dogs, it is not for your cat, whatever the box shows.

The European parasite body ESCCAP, whose UK guidelines most vets follow, is clear that there is no single schedule. It depends on where your cat goes, what she eats, and who she lives with. Most UK cats are in the higher-risk groups, yet one study found about two thirds of them were wormed less often than recommended.

Why monthly for the last two. Roundworm takes around a month or more from being swallowed to producing eggs. If you worm every three months, there are two months in every three where adult worms are shedding eggs into your home, your garden and, through the litter tray, your hands. Worm every 30 days and the larvae are killed before they ever grow up. It is not overkill. It is arithmetic.

Indoor cats are not exempt. Lilou has never seen a garden and has had fleas twice, both times courtesy of a visitor's dog. An indoor cat needs fewer treatments, not none, and the first sign is usually a cat who suddenly cannot stop grooming the base of her tail.

This is the part nobody puts on the box.

Fipronil and imidacloprid are banned from farming in the UK and EU because of what they do to insects and the rivers insects live in. They are still legal on pets. Research from the University of Sussex found both chemicals in English rivers at levels often several times above the threshold that harms aquatic life, downstream of sewage works, and pointed to pet flea treatments as the likely main source. Specifically: washing your hands after applying a spot-on, bathing a treated animal, and treated fur going down the drain or into bird nests.

The prescription systemic products are absorbed into the cat and stay there. There is nothing on the coat to wash off, and nothing to rinse into the river. It is one more reason the newer products are the better choice, and it is the reason your vet may ask whether your cat actually needs a flea treatment this month rather than handing one over automatically. That question is not laziness. It is the current guidance.

If you do use a topical spot-on: do not bath the cat for a week afterwards, do not let her in the bath or shower, and bin the pipette rather than rinsing it.

1. Nothing made for a dog goes on a cat. Not a bit of it, not a diluted bit of it, not the leftover half. Check the box for the crossed-out cat.
2. Nothing with essential oils goes on a cat. Tea tree, eucalyptus, citrus, peppermint, lavender, pennyroyal. Natural is not safe.
3. If you treat a dog, isolate the cat until the spot-on has completely dried, and wash the dog's bedding. Or ask the vet for a non-permethrin dog product.
4. Weigh the cat. Every dose is by weight. A product for a 4 kg cat on a 6 kg cat does not work; a product for a 6 kg cat on a 2 kg kitten is an overdose.
5. Apply it where she cannot reach: the back of the neck, on the skin, not the fur. Keep her away from other cats until it dries, or they will groom it off each other.
6. Treat every animal in the house on the same day, or the untreated one reseeds the carpet.
7. Use the vet products. They are the only ones still ahead of the flea, the only ones that do worms at the same time, and the only ones that do not end up in a river.
8. If anything goes wrong, report it. The Veterinary Medicines Directorate takes reports about animal medicines directly from owners (search "report a veterinary medicine problem" on gov.uk), for a side effect, a poisoning, or a product that simply did not work. It is how warnings get strengthened and products get pulled. Your bad week can be someone else's warning label.

Parasite control fails on dates. The dose was due on the 14th, it was given on the 28th, the fleas had a fortnight, and now the carpet is full again. The Maleb's Tracker has a treatments log for exactly this: product, dose, date given, date next due, and a note of which cat it went on if there is more than one. Log the scratching too. A cat who starts grooming the base of her tail in week three of a dose is telling you the product is fading, and that is a conversation to have with the vet before the next one, not after.

This brand exists because a cat died in a way that was entirely avoidable. Permethrin poisoning is the other one. Both come down to a moment of checking before you act. Check the dryer. Check the box.

If you think your cat has been exposed to a dog flea product or an essential oil, do not wait for symptoms. Phone your vet or the nearest emergency vet now. The Animal PoisonLine (run by the Veterinary Poisons Information Service) is available to owners 24 hours a day on 01202 509000; it is a paid call. This guide is general information and is not a substitute for veterinary advice. All doses and products should be confirmed by your vet for your cat's weight and health.