Little Topsie weighs but 6kg. No problem, the active ingredients in Seresto collars for small dogs will bring no harm to dogs weighing less than 8kg, as long as they are over 7 weeks old. Seresto collars kill all fleas within a day and ticks within 48 hours after it has been attached to your dog. Thereafter you can expect no more flea or tick infestation. It also protects against biting lice.