Fleas survived spot-on treatment? What to do next

An expired spot-on is almost certainly why the fleas survived — the active ingredient degrades over time and simply won't reach the concentration needed to kill them. Before we sort your dog, it's worth knowing that 95% of fleas aren't on her at all — they're in your carpets, bedding, and soft furnishings — so tackling both the dog and the home is the only way to actually get on top of this. To point you to the right product: what's her approximate weight, how heavy does the infestation seem right now, and have you done anything to treat the house yet?

Got it — 30kg, active infestation, and a household spray that hasn't shifted it. Most cheap sprays don't contain an insect growth regulator, which means they kill adult fleas but leave eggs and larvae untouched, so the cycle just keeps going. For her, what product have you been using, and do you know when it expired? That'll help me understand whether we need a completely different active ingredient or just a fresh, in-date version of something that would work.

Two years out of date explains everything — the efficacy drops off significantly after expiry and supermarket own-brands often use older active ingredients like permethrin anyway, which many flea populations have built up resistance to. For a 30kg dog with an active infestation, I'd go straight to Frontline Plus for Large Dogs. It combines fipronil with S-methoprene, the second ingredient being an insect growth regulator that stops eggs and larvae developing — so it tackles the full life cycle on her, not just the adults. It's well within your dog's weight range and it's one of the most consistently reliable prescription-strength options you can buy without a vet visit. For your home, you genuinely need a spray with both an adulticide and an IGR — Indorex Defence Household Flea Spray is the gold standard and one can covers 270 square metres. Vacuum everything first to stimulate hatching, spray all soft surfaces, and wash her bedding at 60°C. Do both on the same day for best results.

The previous treatment failed because it was degraded, not necessarily because the fleas were resistant to it — so fipronil in a full-strength, in-date product is very likely to work. Frontline Plus uses a completely different mechanism to permethrin-based supermarket products: fipronil disrupts the flea's central nervous system in a way that permethrin doesn't, and the S-methoprene component means even if an adult jumps back on her from the environment, it can't reproduce. That said, if after three to four weeks you're still seeing live fleas on her despite treating the home properly, that would be the point to speak to your vet about a prescription-only oral product like NexGard or Bravecto, which work via a different pathway again. But start with Frontline Plus and a proper home treatment — the vast majority of cases like yours resolve with that combination.

Frontline Plus starts killing adult fleas within 24 hours of application. However — and this is important — you will likely still see fleas jumping onto her for two to four weeks after treatment, because pre-existing eggs in your home will continue hatching. That's normal and doesn't mean the product isn't working. Those newly hatched fleas will die when they contact her coat. The IGR in both the Frontline Plus and the Indorex spray is what breaks the cycle over that period. If you're still seeing a significant number of live fleas on her after four weeks with the home treated, that's when to reassess. But with a thorough same-day approach — apply Frontline Plus, spray every room with Indorex, vacuum daily for two weeks — most people see a dramatic reduction within ten to fourteen days.