Not a vet but I have a degree in Animal Science and been in the busiess 30 years.
My guess would be some new strain of parvovirus.
This is an arbovirus carried by Midges and mosquitoes much like Lacrosse Virus. The virus in the Bunyamera classification orthobunyamera virus. There are a bunch of nasty viruses in this family. This is a ss RNA virus whereas parvo is a ssDNA virus.