Cattle herds in Colorado and South Dakota are the latest to be infected in the H5N1 avian flu outbreak.
The investigators also cultured infectious H5N1 virus from milk and cattle samples taken from 2 Texas dairy farms.
The CDC is offering support for targeted programs and vaccine supplies to states, part of a plan to cut the risk of H5N1 reassortment with seasonal flu.
So far, humans who have been infected with H5N1 via contact with infected cows have also exhibited mild illness.
Michigan's outbreak in dairy cows is its second of the month and is the first in Van Buren County.
The UK assessment pointed to uncertain trends amid ongoing transmission, the possibility of undetected human cases, and potential for further evolution.
The woman, from Anhui province in eastern China, had recently visited a live poultry market.
The CDC said the latest patients have mild illness and have been offered antiviral treatment.
The state has now reported 49 outbreaks in dairy cows, nearly half of its licensed dairy farms.
Early results from blood sampling found no evidence of earlier undetected infection in Michigan dairy farm workers exposed to sick cows.
