Immunisation of lambs with excretory secretory products of Oestrus ovis third instar larvae and subsequent experimental challenge.

Excretory-secretory products (ESP) of myiasis producing agents are involved in nutrition and development of larvae and are often immunogens. This study was carried out in order to define the antigenicity, the immunogenicity of Oestrus ovis ESP and the role of sheep immune response to ESP. Twenty-four six to eight month old female lambs were randomly allocated into two groups. The first one was immunised twice, four weeks apart, with excretory-secretory products of Oestrus ovis third instar larvae (L3ESP) in complete then incomplete Freund adjuvant. The second one served as a control, and received two injections of PBS plus complete and incomplete Freund adjuvant. Fifteen and twenty-eight days after the second immunisation, animals of both groups were experimentally challenged with O. ovis first instar larvae. Twelve days after the second experimental challenge, the twenty-four lambs were necropsied. The total number of O. ovis larvae, their stages of development, weights and sizes were recorded per animal and compared between the two groups. Establishment rates were very similar in both groups: 39% and 35% in control and vaccinated groups respectively but the percentage of developing stages was higher in the control group (13%) than in the vaccinated group (6%). It was concluded that the L3ESP immunisation of sheep did not protect against larval establishment but provided an inhibitory effect on larval growth.