Abstract: Several parasites responsible for mammalian diseases are dependent on cysteine proteases for various life-cycle functions. Inhibition or decreasing function of specific cysteine proteases can be useful in the treatment and/or prevention of these parasitic diseases including trichomoniasis, histomoniasis, coccidiosis, trypanosomiasis (trypamonosis) and cryptosporidiosis. Compounds of formula I of the invention are capable of treating and/or preventing the above-identified diseases in mammals, e.g. in avian species as Galliformes including chickens, turkeys, ducks, geese, grouse, guinea fowl, peafowl, quail, partridges, and pheasants, Falconiformes, Passiformes, Columbiformes (i.e. Columba livia domestica), Psittaciformes, and in domestic mammals such as cattle, sheep, goats, horses, pigs, camels, lamas, alpacas, cats and dogs.