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 stud y was carried out in order to define the antigenicity, the immunogenicity o f Oestrus ovis ESP and the role of sheep immune response to ESP. Twenty-fou r six to eight month old female lambs were randomly allocated into two grou ps. The first one was immunised twice, four weeks apart, with excretory-sec retory products of Oestrus ovis third instar larvae (L3ESP) in complete the n incomplete Freund adjuvant. The second one served as a control, and recei ved two injections of PBS plus complete and incomplete Freund adjuvant. Fif teen and twenty-eight days after the second immunisation, animals of both g roups were experimentally challenged with O. ovis first instar larvae. Twel ve days after the second experimental challenge, the twenty-four lambs were necropsied. The total number of O. ovis larvae, their stages of developmen t, weights and sizes were recorded per animal and compared between the two groups. Establishment rates were very similar in both groups: 39% and 35% i n control and vaccinated groups respectively but the percentage of developi ng stages was higher in the control group (13%) than in the vaccinated grou p (6%). It was concluded that the L3ESP immunisation of sheep did not prote ct against larval establishment but provided an inhibitory effect on larval growth.