In fact, I live in Hong Kong, but I didn’t know that leeches are present in Hong Kong until I searched for it on iNat a little while ago.
Now, I suppose that a subtropical climate (with actual dry winters) is less conducive to having a large number of leeches, which is why neither I nor anybody I know here notices any.
But Singapore has only 7 observations, and it seems that you don’t need leech socks in Singapore. It’s a very wet tropical place. So I do wonder what the difference is with places that do have enough leeches to bother people.
The lack of leech observations in Singapore may have a lot more to do with the behavior of people than leeches. Not many people in Singapore are going crawling around in the places leeches like.
The most fearsome pest in Singapore is sandfly and mosquitoes. There are cobras and blue ring octopus. You won’t meet these most venomous creatures in 10 trips that last a few hours per day on each trip. But for sandfly, you might get these on every trip to the mud flats or mangrove. All tourists and local naturalists move along pavements, or maintained tracks, there is rarely a leech that can jump on you from roadside plants. Singapore is highly urbanised. I last saw leech in the tubifex worms that I used to feed my aquarium fish.
There are medicinal leeches. These are domesticated ones. I don’t know which clinic is using it.
I guess if I go to observe some lotus ponds or to net some freshwater fish in some stagnant drains, there might be a chance to see some black colour leeches.