This invention relates to berberine alkaloids, formulations thereof, and their use in the prevention and/or treatment of infectious disease in animals, particularly Lawsonia infection. In particular, the invention relates to berberine alkaloids, formulations thereof, and their application as antimicrobial agents in the prevention and/or treatment of infectious disease including bacterial, viral, parasitic or fungal infections, and for improving feed conversion ratio in food-producing animals. Also described is the use of berberine alkaloids as feed preservatives, added to feed compositions, and used in combination with further agents suitable for treating infectious disease.