Animals, or people for that matter, can develop allergies at any time. Either a critter, or person, is born with the allergy, or they will develop it after a peroiod of exposure to the substance.

For example I used to be able to walk barefoot, in shorts, through poison ivy, not wash my legs and never get a rash. Now I can pick up a rash from a clients dog if it's been around the plant.

It's possible that because of the fleas it can cause some problems. I highly doubt the flea meds, of any type, could cause a corn allergy. Flea bites can cause an allergy all their own called flea dermatitis, which would look identical to a contact dermatitis, just with a different cause.

It's possible that your dogs got a few flea bites that caused that reaction (it can take just one bite from a flea to cause this reaction in a sensitive animal). From there, their systems were just hypersensitive to other things that their bodies had been fighting off up until that point. The other scenario is it was just flea dermatitis and you happen to change the food at the same time their bodies were recovering from the flea bites, thus it would seem that corn was the problem not fleas. I can't prove either theory and I'm no dermatalogist, but either would be a likely scenario.

A true food allergy trial goes like this. Change food to one without the most likely ingredient causing the problem, but change nothing else with the food, treats, environment, laundry etc. Each trial goes on for 2-3 months. Once the suspected ingredient is narrowed down, a true test is to re-introduce that ingredient to the diet. If a reaction re-occurs then in fact is it a true food allergy, if not then there is a different ingredient, or something else causing the problem. Although most clients decline that last part, prefering to just use the new diet food as long as things are working.

Since your dogs are all related, they all were exposed to the fleas, they may all have the same allergy and the same thing may have set them all off at the same time. This is when it's a bummer that dogs can't talk.

There are allergy tests that can be done on dogs, the accuracy isn't perfect, but gives a ballpark idea of possible causes. I do believe they have to be off all medication for allergies first before being tested, don't remember the time span on that though.

Hope that was of some help to you.