The Case of the Poison Toad (Hank the Cowdog #83) by John R. Erickson

It's time for the ranch's annual Fourth of July picnic, and what begins as a happy occasion soon takes a sinister turn when Drover reports that a wart-spreading toad is endangering the occupants of the ranch house! And that's not all... Soon after that near-disaster, he discovers that a couple of town thugs - Buster and Muggs - have come carousing, threatening the success of the entire picnic. Will Hank be able to single-handedly rid his ranch of this sudden influx of invaders, or is it possible that he'll receive help from an entirely unexpected quarter?