Chitosan gels release lower levels of substances when in acidic medium (Cojocariu et al., 2012). The low levels of EsEO released from the chitosan matrix could be due to crosslinking among chemical groups present in the chitosan matrix and the EsEO. The minor release does not invalidate the use of EncEs in controlling haemonchosis in sheep, given that factors related to body temperature, internal pressure and abomasal flora may also influence the release kinetics of the oil. Chitosan hydrogels also have the ability to respond to changes in external environmental factors such as pH and temperature (Sokker et al., 2009).