ORIGINAL ARTICLE Effectiveness of egg yolk immunoglobulin against the intracellular salmonid pathogen Piscirickettsia salmonis C. Oliver 1 , K. Valenzuela 1 , H. Silva 1 , R.E. Haro 1 , M. Cort es 1 , R. Sandoval 1 , J.P. Pontigo 1 , C.  Alvarez 1 , J.E. Figueroa 1,2 , R. Avenda ~ no-Herrera 2,3,4 , J.M. Troncoso 5 and A.J. Y a ~ nez 1,2 1 Instituto de Bioqu  ımica y Microbiolog  ıa, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile 2 Interdisciplinary Center for Aquaculture Research (INCAR), Concepci  on, Chile 3 Laboratorio de Patolog  ıa de Organismos Acu aticos y Biotecnolog  ıa Acu  ıcola, Departamento de Ciencias Biol  ogicas, Facultad de Ciencias Biol ogicas, Universidad Andr es Bello, Vi ~ na del Mar, Chile 4 Centro de Investigaci  on Marina Quintay (CIMARQ), Quintay, Chile 5 EWOS Innovation Chile, Calbuco, Chile Keywords chicken egg yolk, growth inhibition, IgY, immunoglobulin, Piscirickettsia salmonis, piscirickettsiosis. Correspondence Alejandro J. Y a~ nez, Universidad Austral de Chile, Campus Isla Teja, Valdivia, Chile. E-mail: ayanez@uach.cl 2015/0568: received 26 March 2015, revised 14 May 2015 and accepted 18 May 2015 doi:10.1111/jam.12857 Abstract Aims: To produce and characterize egg yolk immunoglobulin (IgY) against the fish intracellular pathogen Piscirickettsia salmonis as well as to evaluate the antibacterial activity of IgY in vitro and the availability in the serum of fish immunized orally. Methods and Results: Specific IgY was produced by immunizing hens with P. salmonis proteins. The IgY was obtained from egg yolks using the ammonium sulphate precipitation method and it was characterized by SDS- PAGE, Western-blot and ELISA, demonstrating that anti-P. salmonis IgY strongly reacted specifically against P. salmonis proteins. In an in vitro neutralization assay, IgY inhibited the growth of P. salmonis in liquid medium at concentrations ranging from 128 to 256 lg ml 1 in a dose-dependent manner. Interestingly, IgY against P. salmonis also generates a strong protective effect on the infection of P. salmonis in salmon head kidney-1 cells. In addition, the bacteriostatic function of IgY appears to result possibly from agglutination by the interaction of IgY with surface components of the pathogen. Finally, to confirm this IgY as an alternative for salmonid treatment, Atlantic salmon (Salmo salar) specimens were orally inoculated with IgY. The analysis of the sera demonstrates that IgY was effectively transported by fish intestine and that this immunoglobulins maintains its properties and recognizes several proteins of P. salmonis up to 12 h after inoculation of IgY against P. salmonis. Conclusions: Specific IgY effectively inhibited the growth of P. salmonis and this immunoglobulin can be released in the Atlantic salmon sera when administered orally to fish. Significance and Impact of the Study: We propose that this specific IgY against this fastidious micro-organism could be a useful strategy for the treatment of piscirickettsiosis. Introduction Piscirickettsia salmonis, a facultative intracellular Gram- negative, is the etiologic agent of piscirickettsiosis, which causes significant mortality and economic losses in fish farming in Chile. This pathogen was originally identified in southern Chile in 1989, isolated from farmed Coho salmon (Oncorhynchus kisutch) (Bravo and Campos 1989; Branson and Diaz-Mu~ noz 1991). After this first report, piscirickettsiosis was described in Norway (Olsen et al. Journal of Applied Microbiology 119, 365--376 © 2015 The Society for Applied Microbiology 365 Journal of Applied Microbiology ISSN 1364-5072