Mathijs Theelen is a Specialist in Equine Internal Medicine (Dipl. ECEIM). Half of his time he is working in the clinic treating horses with different medical problems. He is coordinator of the Foal Intensive Care Unit. In his clinical work he focusses mainly on neonatology, gastroenterology and hepatology.

Besides his clinical work he is also involved in teaching of veterinary students. He is also coordinator of the Skillslab of the Department of Equine Sciences.

In collaboration with the Department of Biomolecular Health Sciences (Clinical Infectiology) of Utrecht University Mathijs is also working on his PhD project. His research focusses on antimicrobial resistance and antimicrobial susceptibility patterns of bacteria isolated from foals and adult horses and the gut microbiome of horses. Part of his research is conducted at the University of California, Davis, USA.