Booster immersion vaccination using diluted Yersinia ruckeri bacterin confers protection against ERM in rainbow trout Jiwan Kumar Chettri a, ⁎, Rzgar Mohammad Jaafar a , Jakob Skov a , Per Walter Kania a , Inger Dalsgaard b , Kurt Buchmann a a Laboratory of Aquatic Pathobiology, Department of Veterinary Disease Biology, Faculty of Health and Medical Sciences, University of Copenhagen, Denmark b National Veterinary Institute, Technical University of Denmark, Denmark abstract article info Article history: Received 15 January 2015 Received in revised form 28 January 2015 Accepted 29 January 2015 Available online 7 February 2015 Keywords: Rainbow trout Vaccination Booster Yersinia ruckeri Antibody response A single immersion vaccination of rainbow trout fry using a Yersinia ruckeri bacterin confers immunity to reinfection but only for a shorter period. A longer protective period is needed in practical trout farming and we have shown that booster vaccination prolongs immunity. Due to economic considerations and management practices it is not possible to immersion vaccinate large trout (20–30 g) with the recommended high bacterin concentration. We here demonstrate that booster vaccination using dilutions of the bacterin (1:100, 1:1000 and 1:2000) with increased exposure time (1 h, 2 h) confers a higher and longer lasting immunity although a short term (30 s) booster in 1:10 bacterin proved superior. This has practical implications for fish farmers because this diluted booster vaccination over longer time can be applied when farmers handle and transport fish between ponds and farms. Therefore such a practice will not challenge farm management and economy. Increased antibody levels were recorded after challenge of vaccinated fish but not after immersion vaccination alone which suggests that immersion induces priming of memory cells. © 2015 Elsevier B.V. All rights reserved. 1. Introduction A single immersion vaccination against Yersinia ruckeri, causing enteric red mouth disease (ERM) has been a common practice for fry and fingerlings (body weight 5 g) in rainbow trout farming since early investigations showed the efficacy of this technique (Anderson et al., 1979; Cossarini-Dunier, 1986; Newman and Majnarich, 1982). The pro- tection developed following one immersion has a limited duration of less than six months (Buchmann et al., 2003; Deshmukh et al., 2013; Johnson et al., 1982). However, booster vaccination has been indicated to increase the protective level and to extend the protection period (Chettri et al., 2013) which suggests that trout farmers should provide their production animals with regular booster vaccination. Such opera- tions are on the other hand extremely costly when fish size has in- creased several times a few months later. Handling of these large fish (20–30 g) for immersion is impractical when using the standard 30 s exposure to 1:10 dilution of the bacterin. In addition, such a procedure would require very large amounts of vaccine which would challenge farm economy. Therefore it is worthwhile to investigate if larger fish can achieve protection if they receive a booster vaccination by immersion into additionally diluted vaccines but with an extended ex- posure time. This could be performed while fish are handled for other reasons such as transport in tanks between ponds or farms. Previous studies by Tatner and Horne (1985) and Tatner (1987) on a single im- mersion vaccination of brown trout and rainbow trout using a diluted vaccine indicated that short term immunity against Y. ruckeri could be obtained by trout if the exposure time was prolonged. We have there- fore investigated if a booster immersion, using a diluted bacterin, will extend the protection period in rainbow trout. Vaccine dilutions of 1:100, 1:1000 and 1:2000 were tested at exposure times of 1 h and 2 h and compared to the effects of a booster using 30 s immersion into the bacterin diluted 1:10. It is known that bacterial antigens during im- mersion of trout into a bacterin or live bacteria are taken up by different organs (such as gills, skin, lateral line) (Khimmakthong et al., 2013; Zapata et al., 1987) and transported to central immune organs. This leads to initiation of a protective immune response which in many cases includes the production of specific antibodies (Anderson et al., 1979; Deshmukh et al., 2013; Olesen, 1991; Siwicki and Dunier, 1993). In order to evaluate the strength and duration of the immunity we have performed challenge experiments at different time points postbooster vaccination and subsequently recorded survival and anti- body production in exposed trout. Challenge experiments were performed only with Y. ruckeri O1 biotype 2 because this is the domi- nating cause of ERM outbreaks in Danish trout farms. If this type of vac- cination using diluted vaccines with longer exposure time proofs to be effective then it will be worthwhile to introduce this form of booster Aquaculture 440 (2015) 1–5 ⁎ Corresponding author. Tel.: +45 35336728; fax: +45 35332742. E-mail address: jkc@sund.ku.dk (J.K. Chettri). http://dx.doi.org/10.1016/j.aquaculture.2015.01.027 0044-8486/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online