Ichthyophthirius multifiliis (ICH) is a protozoan parasite infecting both freshwater fish and ornamental fish worldwide, and causes ichthyophthiriasis, also called white spot disease, inducing high morbidity and mortality in fish. There are no prophylactic treatments commercially available and repetitive treatments with auxiliary substances are needed to control the infection. Feed additives have been shown to decrease the susceptibility of rainbow trout towards infection by I. multifiliis in vivo. Evaluation took place of the effect of the feed additives recorded through parasitological investigation and the potential mechanisms, including immuno-stimulation, associated with the observed effects. The use of astaxanthin 100 mg kg-1 feed, garlic, oregano, thyme and coriander resulted in a significant lower infection burden at day 5, however, there was no indication of any effect of the feed additives at day 8. Garlic showed the highest in vitro killing capacity on theronts followed by oregano, thyme and astaxanthin. Coriander had no direct effect on the parasite. Oregano was the only feed additive resulting in a significantly increased lysozyme activity in serum of trout.