I did this on the Wistar mouse diabetic model, and I administered rats in the prevention group on the first day and the treatment group's mice from the fifteenth day of the disease.I would like to answer.
YES it actually is very possible for plant extracts to be effective in the management of diseases and not in their prevention. my reason is that most of the secondary metabolites implicated with these therapeutic activities in plants are produced in response to say injury in plants to fight pathogenic attack as such it is not surprising that they tend to do the neutralizing job more than the preventive. i too have tested that on albino rats treated with a plant extract which shows ability to neutralize certain snakes venom by administering the extract before envenoming and vice versa both the ability of such plants extract shows much more neutralizing activity when administered after envenoming.