Why is it so difficult to get rid of fleas?

Anyone who has fought fleas at home knows the pattern: the bites stop for a week or two, then start again. That is not bad luck. Most of a flea problem is out of sight, in stages of the life cycle that shrug off sprays and wait patiently for the right moment to hatch.

01The flea life cycle

Fleas have four stages: egg, larva, pupa and adult. Each one needs dealing with in a different way, and that is the root of the difficulty.

• Eggs. A female flea starts laying soon after her first blood meal and keeps laying every day. The eggs are smooth and do not stick to fur, so they drop off wherever the pet walks, sleeps and sits.
• Larvae. The eggs hatch into tiny larvae that avoid light and burrow deep into carpet pile, between floorboards, under sofa cushions and into pet bedding. They feed on debris and on flea dirt, the dried blood left by adult fleas.
• Pupae. Each larva spins a sticky cocoon that picks up dust and fibres and is very hard to see. Inside it, the flea is protected from most insecticides and is not easily lifted by a vacuum cleaner.
• Adults. The adult waits in the cocoon until it senses a host nearby, through vibration, warmth and the carbon dioxide in breath, then emerges and jumps on to feed.

In a warm house the whole cycle can take just a few weeks. In cooler conditions it slows down, and pupae can sit dormant for months. That is why people moving into a house that has stood empty are sometimes met by a burst of hungry fleas: the vibration of footsteps wakes them all at once.

02Why DIY treatments often fall short

• Treating the pet alone kills the adults on the animal but leaves the eggs, larvae and pupae in the house to keep hatching.
• Sprays and foggers often kill the adults they reach but do not get deep enough into carpets and gaps to reach the larvae, and they do little to the pupae.
• Stopping too soon is the most common mistake. The bites ease off, the treatment stops, and then the next wave of pupae hatches.
• Missing a pet keeps the cycle going. If one animal in the house is treated and another is not, the untreated one carries on feeding fleas.

03Signs that fleas are the problem

• Pets scratching, biting or licking themselves more than usual.
• Tiny black specks in pet fur or bedding that turn reddish-brown on a damp white tissue. That is flea dirt.
• Small, very itchy red bites on people, usually around the ankles and lower legs.
• Small dark insects that jump when disturbed, often seen against white socks or a pale floor.

You do not need a pet to get fleas. They can be left behind by a previous occupant's pet, or brought in from birds' nests, hedgehogs and other wildlife close to the house.

04How to break the cycle

• Treat every pet in the household with a vet-recommended product, at the same time, and keep it up as your vet advises. Ask your vet about worming too, as pets can pick up tapeworm by swallowing fleas while grooming.
• Vacuum thoroughly and often, including under furniture, along skirting boards, sofa cushions and the car if the pet travels in it. Vibration helps coax adults out of their cocoons where a treatment can reach them. Empty the vacuum into a sealed bag in an outside bin.
• Wash pet bedding, throws and cushion covers on a hot wash.
• Treat the home so that the larvae and newly hatched adults are killed as they appear.
• Expect a few fleas afterwards. It is normal to see fleas for a couple of weeks after a home treatment, as pupae that were protected in their cocoons hatch and meet the treated surfaces. Keep vacuuming during this time, and do not shampoo or steam-clean carpets until you have been told it is fine to do so.