Welcome, @montyg.
According to Mayo Clinic's information (https://www.mayoclinic.org/diseases-conditions/hypereosinophilic-syndrome/symptoms-causes/syc-20352854)
"Some varieties of hypereosinophilic syndrome tend to run in families. Other types have been associated with certain types of cancers, infections or other health problems."

It sounds like HES in your case may be related to an allergic reaction. Do I have that right? Can you elaborate?