I read this pretty recently (late 2017 to early 2018) and have been trying to find it again, with absolutely no success. Details I remember:
- city has an annual festival/carnival thing, where at a ceremony (maybe a concert/show?) volunteers are called up from the crowd and a hollowed-out animal head is placed over their own like a mask
- set roughly in the present day, possibly in England
- protagonist is the girlfriend of one of the volunteers
- after the festival, the people who wore the heads have check-ups at a lab where the heads get put back on them for some kind of testing
- eventually, three of the people disappear, along with their corresponding animal heads, and it’s revealed that they had been breaking into the lab and illicitly putting the heads on
- the protagonist tries to track down her boyfriend by talking to a woman connected to one of the other animal-head-people
- the heads, when removed, have some kind of worm/larva things inside them, and leave sucker-like marks on the person’s skin underneath
That’s about all the details I can think of—does this ring a bell for anyone?