I didn't even know it existed!
About a month ago, I noticed black flecks on Turk's white chin, but I hadn't seen it on Magic's chin and I hadn't noticed it on Diva... So I dismissed it as sloppy grooming and eating on Turk's part. He is just a little boy after all.
And then the day before yesterday Diva and Turk were snuggling on the couch and I noticed a little bald spot under Diva's chin! When I looked closer I realised she also had the little black flecks that Turk had! Then this morning I realised the bald spot had grown considerably in the last couple of days and I took her to the vet. She diagnosed feline acne and Diva was given a shot to stop the itching from spreading and getting worse, and her fur should start growing back with a week.
The digging that I've now done puts it down to allergies, plastic food dishes, stress or poor grooming. There are a couple of other things- like ringworm- that could present with similar symptoms but they've all been ruled out as there are no other lesions anywhere on any of the cats. They've all six been de-wormed now anyway.
I think it may be allergies as both Turk and Diva have the problem and none of the other cats do. They eat out of stainless steel bowls that are washed daily. They get brushed once a week and groom each other on a regular basis, and nothing has changed at home that could stress them. At the moment they are being fed Hills Science Diet (Optimal) and all seem okay, so I'm going to keep an eye on both of them to make sure the food isn't making them sick.
I Googled a couple of pictures so you can see what it looks like in the early stages. Its easily noticeable on a white or light coloured cat, and on a black cat you'll have to look a little closer to see the black flecks.
These are pictures of Diva's acne. The first picture is from when she was losing fur and had scratched herself raw. The last one is after her shot where she started healing. The acne is now almost all gone.
You can read more about it here and here.