Sjaak received his Doctor of Veterinary Medicine degree from the Utrecht University in the Netherlands in 1989 and obtained his PhD at the same university in 1997. The title of his thesis was Infectious bronchitis virus in broilers, detection and transmission.

Sjaak has held a permanent position at Royal GD (former GD Animal Health), Deventer, the Netherlands since 1989. He started as poultry veterinarian, his current position at GD is senior researcher. Since 2019, Sjaak combines the position at GD with the position as Professor Integral Poultry Health at the Veterinary Faculty of the Utrecht University.

He has been actively involved in applied research related to the diagnosis and control of poultry diseases, especially viral diseases as IBV, IBDV, NDV, and AI. His work involves the quality of the serological tests at GD, including validation, accreditation, ISO 17025 and 17043. He has published over 70 peer-reviewed papers and has contributed to several books including ‘Poultry Diseases’ and the OIE Terrestrial Manual.

Since 2006, he is one of the associate editors of Avian Pathology. In 2007, Sjaak was one of the founding members of the European College for Poultry Veterinary Science, for which he now serves as Post President. In 2013, he was inducted as an inaugural member of the Hall of Honour of WVPA. He has been co-organising several international symposia and European COST-Actions. At present he serves as a member of the COGEM, a scientific advisory board for the Dutch government regarding Genetic Modification.