I have a friend who went to veterinary school and another who is a vet tech and a mouse breeder and have consulted both of them on this issue, as I had similar symptoms. Together we have concluded that this "wasting illness" is possibly either Sendai Virus or a form of Murine Hepatitis Virus (MHV), both of which can be introduced from any number of wild rodents who share the same airspace, even briefly.
Both are diseases which have two forms: active (mostly juveniles) and inactive (adults), and both are transmitted primarily to and from young animals (under 6 weeks), although they do sometimes affect older animals as well. Both diseases may present with or without diarrhea, but the rapid "wasting" and dehydration is a characteristic feature.
There is no direct curative agent, but supportive treatment includes Baytril (enrofloxacin) in everybody's water for three or four weeks, with a period of at least 3 weeks (four or more is preferable) with absolutely no breedings and no new litters and no new mice brought into the same air space. This "down time" allows time for the virus to naturally disappear from the colony of adult animals without being passed on unknowingly to younger animals who will start the cycle over again.
As long as there are any new breedings, the viruses mentioned will continue to spread back and forth among mice, and it will pop up again by re-infecting some of the young animals, killing some animals and leaving others seemingly unscathed, as everybody here (including me) has seen!
If you do not have baytril on hand, the mice may still recover but you may lose more individuals to secondary bacterial or viral infections; the most important aspect to getting rid of the viruses is to cease breeding for at least three weeks with no new mice brought into the airspace, period. This is the only way to be certain that you are not perpetuating the virus in your colony, as it can be present in adults with no symptoms whatsoever and yet kill younger mice as they are weaned or shortly thereafter.
