bumps on skin--pig pox?

We have 7 show pigs for 4H in our pens, pigs are breaking out one at a time with small red dots that turn into bumps.  The bumps are not scabbing over they kind of look like mosquito bites but aren't.  The problem started in one pig, moved to two more within a day or two, then a 3rd (this one has the most amount of bumps) a few days later, now our blue butt broke out in the little red bumps today.  There are still 2 pigs that have not shown signs yet.

We put new sand in our pens last year and our pigs all got the pig pox.  Before we got pigs this year we disinfected the pens 3 times hoping not to have this problem again.

I looked up pig pox online and the pictures do not really match what we have....not scabbing over, they just appear to be bumps.  Doesn't appear to be mites because the bumps started off on back and are all over body (not just behind ears or under leg "pits").
We have given them each shots of Dex and lincomed 300.
Does anyone out there have anymore info on what we need to do if it IS pig pox?
OR, what else could it be?

Have you treated the sand for fleas?

How bad of a fly or mostiqo problem do you have?