Apparently Lepto 4 is because more dogs are coming in from other countries.
My current pair, One came from Spain on Lepto 2, boosted the following year Lepto 2, then boosted last year Lepto 4 (ok, I admit, I didn't notice!)
My other dog, born in this country, she started on Lepto 4, then last and this year, boosted Lepto 4.
Now, when I took them for anal glands (I'll do many things for my dogs, anal glands NOT being one of them) the nurse said to me, "we're On Lepto 4 now, so when they come in for their booster, they need to have 2 jabs as it's a new, so it's a primary and a secondary ".
So that got me thinking, got home, checked their vaccination records. UK born, as said, already On it, Spain born not, BUT she only had the booster not the restart.
I was at the vets this week for the UK booster, and asked the Vet, I said "the Vet who jabbed Spanish pup, if the nurse says the dog needs the primary AND secondary jab, WHY didn't the Vet tell me and treat her that way?" The vet I was speaking to said she didn't know.
We (me and dogs) are here at.home or rarely in car so she should be ok! Oh, when I paid the bill, £51 I was asked for. I said for what? The booster!
The joys of pet ownership!