Given the question i would answer like this
Feeding on human blood
The feeding on human blood requirement can be met. The size should also be ok.
They could use animals as a means of transport without infecting the host (or do they?). So humans can stay "primary host" for (zombification by) the leeches. Much like toxoplasma gondii
"...is capable of infecting virtually all warm-blooded animals, but
felids ... are the only known definitive hosts in
which the parasite can undergo sexual reproduction..."
They could alter the hosts behaviour by secreting enzymes (as leeches do) and a bunch of other stuff (like compounds from an endosymbiotic bacteria?) into the bloodstream. This wouldn't allow a leech to gain full control over a human (probably no leech controlled movements), but it could be able to "adjust" the behaviour in a way supporting it's needs (as Henry Taylor mentioned).
Another way could be secreting superleech salvia containing ingredients triggering a higher hormone output, while another ingredient inhibits the hormons resumption.
As to the stereotypical zombie and his zombification.
The leeches would need to ensure the hosts survival as they feed from blood and as soon as the human is dead blood will become a limited resource and the heart stops beating. Sure one leech won’t need that much blood, but no new blood and a bunch of blood-sucking super leeches will render the host useless (after some time). The leeches would need a way to prevent this but since these are superleeches they may have found a way to kill their host maintaining "basic" functions. Like heartbeat or some other way to let the blood flow.
A zombies mindset doesn’t include a rotten body.
This (more optical) feature/requirement of a zombie could be obtained by the leeches fecies, which (might) include a wide variety of microorganisms feeding on/poisoning living tissue. Nonetheless, this would also render the host useless and can't be in the leeches interest (unless they found a way to maintain a dead body functioning).
And one last to adress reproduction, leeches are hermaphrodites, and since these specific leeches are superleeches, they could either still need a second leech to reproduce or simply fertilize themselves. The eggs could be released into the hosts bloodstream (think of metastases in cancer) and hatch somewhere in the body (superleeches hatch superfast). Some of the leeches might release their eggs into the hosts saliva so every bite would be infectious.
open questions to me
- how do they approach their host
- how do they infect their host primarily? (infecting one host who infects other humans or tsunamiwaves of leeches swapping over cities)
- are those zombies the stereotypical rotten living dead?
3.1 if they are living dead,
how do the leeches maintain the dead bodies functions? (this would require them to heavily modify the hosts body)
3.2 if they are not,
what makes them to zombies and not just humans behaving strange
- do those zombies eat because they are "hungry" or are they on a killing spree as long as the body lasts?
super(zombificating)leeches could be possible but there are still a lot of questions to answer to make this work
and the five minute zombification requirement, superleeches have to be superfast (with whatever they do), otherwise this won’t work
imho leeches as hosts for a zombificating bacteria or virus would make more sense.
nonetheless i like dem little leeches....i thought there were five of 'em not just one..grahhh