Fleas, ticks and worms: the parasite basics

Fleas, ticks and worms are the three parasites almost every cat and dog will meet, and the sensible plan is to stop them before they arrive rather than fight them once they have. Each one causes more than the obvious itch: fleas carry tapeworm, ticks pass on disease, and some worms can infect people, especially children. Here is what each parasite does, how to spot it, and the prevention that actually works.

Fleas

Fleas cause relentless itching, and in sensitive animals a single bite can set off flea-allergy dermatitis, an over-reaction to flea saliva that leaves the skin raw, bald and infected from scratching. They do more damage than the scratching suggests. Fleas carry the tapeworm Dipylidium caninum, which a pet swallows when it bites at an itchy patch and eats an infected flea. In kittens and puppies a heavy infestation can drain enough blood to cause anaemia, which is genuinely dangerous in a small animal.

The trap with fleas is that what you see is a tiny fraction of what is there. The adult fleas jumping on your pet are only about 5% of the population. The other 95% are eggs, larvae and pupae sitting in carpet, bedding and the cracks of the floor, waiting to hatch. For every flea you spot, assume there are many more you cannot. That is why a quick spray on the cat never ends the problem, and why treating the home matters as much as treating the animal. To check, comb dark specks onto a damp white tissue: if they smear rusty red, that is flea dirt (digested blood), and you have fleas whether or not you saw one.

Ticks

Ticks are far more than a nuisance because they can pass on disease while they feed, including Lyme disease and several others depending on where you live. A tick attaches, buries its mouthparts in the skin and swells over a day or two, so the sooner you find and remove it, the lower the risk of infection. Run your hands over your dog after every walk through grass or woodland, paying attention to the ears, neck, armpits and between the toes. If you walk in long grass, the same check catches foxtail grass seeds too.

To remove a tick, use a proper tick tool or fine-tipped tweezers and grip as close to the skin as you can. Pull straight out with steady, even pressure. Do not twist, do not squeeze the tick's body, and never try to burn it off or smother it in oil, all of which make it more likely to regurgitate infected material into the bite. Wash the area afterwards and keep the tick in case your vet wants to identify it. See a vet if the mouthparts break off and stay in, if the site becomes red and swollen, or if your pet is off-colour, tired or lame in the weeks after a bite.

Worms

Worms are internal parasites that a pet picks up from the environment, prey, fleas or slugs, and they range from common to life-threatening. Roundworm and hookworm live in the gut and are passed through infected faeces and soil; puppies and kittens are often born with or nurse in roundworm. Tapeworm arrives by way of a swallowed flea, which ties worming and flea control together. The one many owners have never heard of is lungworm (Angiostrongylus vasorum), which a dog catches by eating the slugs and snails hiding on grass, in puddles or on a chewed toy, and it can cause bleeding problems and be fatal if untreated. In some regions heartworm, spread by mosquitoes, is a serious risk and needs its own preventive.

Some of these worms are zoonotic, meaning they infect people. Roundworm larvae from dog and cat faeces are a real risk to children, who can swallow eggs from contaminated soil or sandpits and, rarely, suffer damage to the eyes or organs. That is the honest reason regular worming is not optional: it protects the family as much as the pet. Because you usually cannot see worms until an infection is advanced, you treat on a schedule rather than waiting for signs.

Prevention

Prevention beats treatment because it is cheaper, safer and stops the disease and home infestation before they start. The core plan has a few parts. Use a vet-recommended preventive against fleas and ticks, whether spot-on, tablet or collar, and keep it up on schedule, most are monthly. In much of the world a mild winter means fleas survive indoors and ticks stay active, so year-round cover is often the right call; your vet will tell you whether your climate allows a seasonal break.

Worming runs on its own timetable and is more frequent for the young. Puppies and kittens are typically wormed every two weeks until around 12 weeks, then monthly to six months, because each round only kills the worms present that day and a new batch is always maturing. Adults are usually wormed several times a year, more often if they hunt or scavenge. Alongside the products, check the coat after every walk, and treat the home and all the pets in it together: clear one animal and leave the others, and the fleas simply move back in. Ask your vet for a product matched to the species, weight and region rather than guessing off a shelf, the same care you would take feeding a Beagle that scavenges everything on a walk.

The permethrin warning for cats

Never put a dog flea or tick product on a cat, because many contain permethrin, which is highly toxic to cats and can kill them. Cats lack the liver enzyme that breaks permethrin down, so it builds up and attacks the nervous system, causing drooling, twitching, muscle tremors, seizures and death. Using a smaller amount does not make it safe. The danger is not only direct application: a cat that grooms a recently treated dog, or curls up where the dog has been, can absorb enough to be poisoned, so keep the two apart for a couple of days after a permethrin dog treatment, or better, choose a dog product without it if you share a home with cats. If you think a cat has been exposed, whether you applied the wrong product or it rubbed against a treated dog, contact a vet straight away, because early treatment saves lives. The simple rule holds for every parasite product: ask your vet for the right one for the species, weight and region, and read the label before it goes anywhere near your pet.

Common questions

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