Kin Kind outdoor shield spray and shampoo for topical(preventative and treatment).
Earth MD outdoor shield powder for preventative to add to food.
I definitely recommend Avoiding the medication from vet and pet stores as they are all chemicals and can cause seizures and tremors and other neurological issues and death.
Examples of those -Nexguard, Spectra.
The food we fe has garlic in it so that helps somewhat, but when I do take them out for extended time in the yard... I spray a mix of neem and coconut oil on them