Yes, and it can affect your ability to claim if you don't. Lassie's policy requires you to keep up with your pet's routine care as a condition of cover.
This includes:
An annual health and dental check-up
Vaccinations kept up to date following your vet's guidance. For dogs this must include distemper, hepatitis, parvovirus, and leptospirosis. For cats, feline infectious enteritis, feline leukaemia, and cat flu as a minimum.
Any illness that could have been prevented by vaccination won't be covered. More broadly, if your pet develops a condition that could have been avoided, or is made worse, because routine care wasn't kept up, Lassie may reduce or refuse a claim payout.
You also need to act promptly if your pet shows signs of illness or injury. The policy requires you to see a vet as soon as possible and follow their recommended treatment plan.
Delaying treatment that leads to further complications won't be covered.