Yep, no ESD components, there's even positions for them right by the connector, see the USB pair coming off it, CDM/CDP.
ESD causes quite severe currents through whatever it's sunk to, very briefly. On the order of 15A peak, with a half-height duration of 50ns or thereabouts. It's not much energy, at least to something macroscopic like us (it hurts enough to be annoying..), but the peak power is incredible, and for the tiny (micron-sized) structures on a chip, it's easily fatal.
And something like a mouse, ought to be designed to be fully insulated (but there's enough things on this one, I'm not sure, maybe there's exposed metal parts near some of those buttons?). But it seems you exceeded that insulation level. It must've been a stonking great spark indeed. It might've exceeded the voltage it was rated to (typically these things are tested up to 15kV). Which doesn't bode well.
The main chip seems to be a Freescale (now NXP) MC9S08JM16 microcontroller:
https://www.nxp.com/docs/en/data-sheet/MC9S08JM16.pdfIt's only rated to 2kV ESD, and definitely needs external components to provide protection to this kind of level. (Probably they figured the enclosure would give enough insulation, so left off anything extra.) Very likely it's shit the bed, at least the USB pair if not additional internal damage (like shorting the supply pins together). Can go around the device with a multimeter on "diode test" and see which pins are diode-like (probably good) and shorted (or low resistance, and how low at that).
If someone's dumped its firmware, it's possible to get a replacement part, program it, and keep on using the same rest of the mouse. This has some caveats:
1. They probably read-protected the chip, so it will require some hacking to dump it. Often an exploit is required. This is non-trivial, and likely requires clever programming (maybe the USB port, programming port, or other pins are vulnerable to unexpected codes) or special hardware (such as a tool to momentarily drop power to the MCU, as it's executing a critical section, confusing it and causing it to pass the check).
2. Programming it, requires some hardware and software to connect to the chip itself. Typically a programmer adapter, drivers for it, and a software tool to send the file to it. Sometimes very simple hardware is required (some chips can be programmed with a generic serial adapter). They were nice enough to provide the manufacturer's recommended programming port, by the looks of it (J2, see datasheet page 329).
3. New from scratch: it's always a possibility, if the original firmware isn't available, to write it new. Knowledge of the pin functions, opto sensor, and device hardware (well, that's one of two datasheets down at least) is required, and simply a lot of time to bring it all together (100hr+?). It looks like there are one or a few open-source mouse designs, maybe one can be ported to the MCU, that would save a few hours digging through hardware specs. In any case, needless to say this would be prohibitively expensive, unless you happen to have the exact skill set to do it yourself -- in which case, still, you're far better off just getting the replacement unit. Alas.
And, it's still possible that your chip has its memory intact, but the physical issues caused by ESD damage may prevent accessing it, besides the above reasons.
Oh hah, the sensor actually exists, brand name Avago even:
http://www1.futureelectronics.com/doc/AVAGO%20TECHNOLOGIES/ADNS-9500.pdfso that's the datasheets. And the other chip in the corner is just an ordinary EEPROM, nothing special there.
Tim