The poisson story is only a distraction to hide the real culpit.
Let suppose that someone by accident or real intention added some poisson to the food, what happens? It will depend on the amount of poisson.

A large amount of poison will kill the pet in few minutes and it will affect all the pets that consumed the food, this is something that didn't happened.

A medium amount of poisson will not kill the pet, but the pet will have all the evidences that is something wrong with him, so it will be a flood of people going to the veterinares with their pets.
This also was nothing that happened.

It only remains a little amount of poisson. A little amount does nothing, but it has a cumulative effect. The effects will only appear after sometime of the use of the poissoned food and also will depend on the pet, someone can be more resistant or inmune and another more sensible.
As I said before, the poisson in this case to produce some notable effect it must be consumed for a long period and this creates a big problem in the poisson theory:
It requires that not only one shipment was poissoned by accident, it requires that all shipments and production was poissoned for a long period of time!!!!

In resume it looks more as some responsable chemical used was added or its amount increased at least one year ago,  produced a cummulative effect and when the amount of cases present at veterinaries increased in a very big proportion raised the alarm signal that something must be wrong with the food.

Stupidity also evolves!