This is what I have seen on our muskrats. Wee little tiny ones that you really have to look for to see.

Out In ND I've seen rats infested with those things that look like tiny shrimp. Never see them on live rats but on those dead drowned rats they are sometimes infested with them.

From page 127 ,just pulled this up;
Yes there are fleas on muskrats, especially if they are bank rats that live with other rodents in the dens, the rats get them as a secondary host from their bedding etc.
They can also get ..the beaver beetle Platypsyllus castoris because they go into beaver lodges and pick them up.

what Beav is referring to is Gammarus or fresh water shrimp

Ya I have had rats with those shrimp on them by the hundreds-when you skin them and give them a flick they fall all over the floor-when they die and dry up they turn pink in color.The perch and walleye love those shrimp! If you find a bunch of those when you ice fish the fish will be near by-guaranteed!

Everything and everybody has bugs living on them.
If you cant see or feel them they don't bother people.

Caught one the other day with a big leech on it.

Also they have both chewing and sucking lice, they are white snow flake size.
4 species of mites live on them as well