How often should I worm my cat or kitten?

Roundworms are the most common worms in cats. Adults grow up to about 10 cm long and live in the small intestine. This is also the worm that matters most: it is the most common, it can harm young kittens and it can pass to people.

Tapeworms are long, flat worms made of small segments. The flea tapeworm and the mouse tapeworm are the two cats carry most. There are others: hookworms in the gut can cause anaemia, and lungworm lives in the airways. In a European survey of 1990 cats, 10.6% had lungworm. Heartworm occurs in warm areas and is not found everywhere. It matters when a cat travels to, or comes from, such an area, so ask your vet before you travel.

Cats often carry more than one kind of worm at the same time, and one product does not cover every worm. Some work on roundworms only, others on tapeworms too.

Kittens most often get roundworm from their mother's milk. Larvae that sleep in her body wake up around birth and reach the milk. So even a well-wormed mother can infect her kittens.

Whether larvae also cross the placenta is disputed. ESCCAP says kittens cannot be infected before birth, while another source lists it. The milk route is the one all sources agree on.

Adult cats get roundworm by swallowing eggs from the environment, for example when they lick soil off their paws, or by eating prey such as mice and birds. The eggs are not infectious straight from the cat. They need a few weeks in the soil, and then can stay infectious for years.

Cats get tapeworms from fleas, when they swallow an infected flea while grooming, and from prey or from raw meat and offal. A cat cannot catch flea tapeworm by eating tapeworm eggs or directly from another pet. It must swallow an infected flea.

Lungworm comes from eating slugs and snails, or prey such as frogs, mice, birds and reptiles that carry the larvae. A cat that eats raw meat or offal, or hunts, has a higher risk of tapeworm and other worms.

An indoor cat is not safe. Eggs come in on shoes and mats, fleas come in with other pets or visitors, and a mouse can get in. In one German study about 20% of cats with roundworm were, according to their owners, indoor-only.

Often you cannot. Adult cats with roundworm or tapeworm usually show nothing, and worm eggs are too small to see. A healthy-looking cat can still carry worms and shed eggs.

In kittens, roundworm can show as slow growth, a dull coat, a round pot belly, vomiting or soft stool. A heavy burden can be dangerous for a young kitten.

Tapeworm shows as small segments that look like rice grains or cucumber seeds around the bottom, in the faeces or where your cat sleeps. Fresh segments move. A cat that drags its bottom may have tapeworm, but it can have other causes too, such as the anal glands.

Adult roundworms are seldom seen in stool or vomit, but when a long, pale, spaghetti-like worm turns up it is worth taking to the vet, wrapped in damp cotton wool.

For your cat: a heavy roundworm burden can stunt a kitten's growth, block the gut and, in the worst cases, be fatal. Hookworm can cause anaemia.

For people: roundworm eggs can be swallowed by accident and the larvae can wander through the body. Most infected people notice nothing, but in some, mostly young children, they cause fever, cough, belly pain, or damage to the liver or the eye. Children are at most risk from contaminated soil and sandpits, much more than from stroking the cat.

Flea tapeworm can reach people only by swallowing an infected flea. It is rare, mostly seen in small children, and treatable. Fox tapeworm is serious in people. ESCCAP says cats rarely shed its eggs and play a minor role, much less than dogs. A French public-health report, however, counts cats as possible sources. Where the fox tapeworm occurs in your region, ask your vet whether it matters for your cat.

Vets in Europe base their advice on ESCCAP, a group of parasitology experts. The guideline is built on risk: how often depends on how your cat lives, not on one number for every cat.

A kitten is wormed from 3 weeks old, every 2 weeks until 2 weeks after weaning, then monthly until about 6 months. The mother is treated at the same time. An adult cat that stays indoors with little contact with prey is wormed or tested once or twice a year. A cat that goes outside, hunts or eats raw meat or offal, at least 4 times a year. For cats that roam and hunt the guideline gives 4 to 12 times a year.

Once or twice a year, or even every 3 months, is not always enough for outdoor cats: eggs can start showing in the faeces again between treatments. If you cannot judge the risk, worm or test at least 4 times a year. In homes with young children (up to about 5 or 6 years), elderly people or people with weak immunity, monthly worming or testing is discussed.

Your vet knows your cat. A routine of twice a year for every cat, and trade tables of every 3 months, are simpler than the guideline.

Fleas and tapeworm are linked. A cat with fleas can have flea tapeworm, and with a flea infestation the tapeworm is treated once, as well as the fleas.

Tapeworm medicine alone does not solve it. If the fleas stay, the tapeworm comes back. That is why 'it keeps coming back' almost always means fleas are still around, on the cat and in the home.

Worm medicine does not kill fleas and flea medicine does not always kill worms. Some products do both, so ask your vet what each one covers. Do not stack different spot-ons yourself: a cat charity advises time between different spot-ons and different places on the neck, so they do not mix. Ask your vet how to combine them. Combing for fleas and flea dirt helps, because light infestations are easily missed. Washing bedding and vacuuming help too.

Remove faeces from the litter tray and the garden every day and put it in household waste, not the compost or the toilet. Fresh faeces is not yet infectious, so daily removal stops eggs from maturing.

Wash your hands after the litter tray, gardening and playing with your cat, and before children eat. Wash fruit and vegetables from the garden. Cover sandpits when they are not used.

Raw meat and offal can carry tapeworm cysts and roundworm larvae. Cooked or commercial food, or meat frozen for a week at minus 17 to minus 20 degrees Celsius, lowers the risk. Keep a note of every worming, with the date and product, and bring it to the vet.

Give fresh drinking water and do not let your cat drink from puddles. Limit access to prey, carcasses, and snails and slugs where you can.

Vets choose the product by the worm, your cat's age and weight, and how well it accepts it. Worming comes as tablets, pastes and spot-ons. If giving it at home does not work, the vet can do it at the practice.

Active ingredients vets commonly use include praziquantel (tapeworm), pyrantel, fenbendazole, milbemycin oxime and emodepside (roundworm). This is information, not a recommendation, and the right one differs per cat. Use only what the vet or the leaflet gives for your cat, because the amount depends on weight and product.

Worming is short: a treatment works for about 1 to 3 days and gives no lasting protection. A cat can be infected again the next day. Many products kill adult worms but not larvae in the tissues, so the vet may repeat treatment and check afterwards. Some medicines are not suitable for very young kittens, kittens under a set weight or pregnant and nursing cats.

A stool test is a good alternative to routine worming in many cats, if it is repeated as often as the worming would be. Stool from 3 days in a row gives the best chance. A negative result does not prove a cat is worm-free, and the test misses tapeworm, because the eggs sit inside the segments. For cats that hunt or eat raw meat, vets therefore advise worming by risk. A stool test at the first kitten check can find roundworm, and for adult cats a stool test at the yearly check is a good idea.