Specific and natural antibody response of cod juveniles vaccinated against Vibrio anguillarum S. Gudmundsdo ´ ttir * , B. Magnado ´ttir, B. Bjo ¨ rnsdo ´ttir, H. A ´ rnado ´ ttir, B.K. Gudmundsdo ´ ttir Department of Fish Diseases, Institute for Experimental Pathology, University of Iceland, Keldur v/Vesturlandsveg,112 Reykjavı ´k, Iceland article info Article history: Received 30 July 2008 Received in revised form 19 September 2008 Accepted 26 September 2008 Available online 14 October 2008 Keywords: Cod Fry Vaccinations Vibrio anguillarum Antibody response abstract The purpose of the present study was to study specific and natural antibody levels in individual cod juveniles before and after being vaccinated against Vibrio anguillarum. Different vaccine preparations and vaccination regimes, i.e. bathing, dipping, i.p. injection or combination of treatments were employed and the performance of different groups to bath challenge by the bacterium tested. Antibody responses to V. anguillarum antigens in groups vaccinated by bathing and/or dipping were negligible, while responses were observed in i.p. injected fish. Fish receiving i.p. injection in addition to bathing, showed significant antibody response. Both groups showed increased levels of natural antibodies while levels were low in other groups. Fish bathed or dipped showed higher mortality when challenged than untreated fish, while fish that received a second vaccination showed the best protection. It was not ascertained whether there is a long term difference between the effects of immersion versus i.p. injection as a booster method. Levels of antibodies against V. anguillarum antigens or natural antibodies in groups with the lowest mortalities show that neither could have been used to predict protection given by the vaccines tested. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Cod has for centuries been one of the most important species for fishing communities around the North Atlantic [1]. Today, with catches rapidly declining, cod is probably the marine species with the highest potential for sustainable cold water aquaculture [2]. Intensive rearing of fish, suffers from outbreaks of diseases. In cultivated cod, vibriosis, caused by Vibrio anguillarum, is one of the major bacterial diseases [3,4]. Vibriosis in cod is often associated with serotypes O2a and O2b [5,6] while serotypes O1 and O2a are most commonly isolated from salmonids. Vaccinations are the most effective way to avoid bacterial diseases and different fish species may need different vaccines, vaccination methods and vaccination schemes [7,8]. Cod has been successfully vaccinated against vibriosis and recently, vaccines adopted for use in cod, containing serotypes O1, O2a and O2b, have become available commercially. Vaccine trials have included cod of various sizes, using bathing, immersion and injection and different challenge methods [9–12]. Several studies of antibodies and antibody responses in cod have been carried out during the last two decades. Cod blood has high concentration of IgM in serum that increases by influence of environmental factors such as temperature [13] and size [14]. Cod mounts specific response to a limited number of antigens following infections and immunizations and typically individuals in a group show a wide range of responses [10,15,16]. Studies on cod immu- noglobulins were reviewed recently and the authors concluded that the explanation for the lack of specificity does not appear to be due to deficiencies in their structure, organization, diversity or expression [17]. Natural antibodies are important in the first line of defence against infections [18]. They react to common antigens of patho- gens and various self-antigens. They are present in sera of normal, non-immunized individuals, where they are proposed to form a link between innate and adaptive immunity [19]. Studies on natural antibodies have been carried out in a number of fish species [20] and high levels of natural or non-specific antibodies have been reported in cod [13,14]. These antibodies reacted to a variety of antigens and the strongest response was towards haptenated bovine serum albumin or TNP–BSA. Subsequently it was shown that i.p. injections with Freund’s complete adjuvant (FCA) led to increased activity against this antigen [16]. In a vaccination trial with V. anguillarum in adult cod some of the pre-immune sera contained antibodies that bound to antigens in the bacterial membrane [9] and all non-immunized control fish had antibodies that reacted with V. anguillarum in another trial [10]. A study in goldfish has shown that natural antibodies can influence results of vaccination [21]. * Corresponding author. Tel.: þ354 5855100; fax: þ354 5673914. E-mail address: siggag@hi.is (S. Gudmundsdo ´ ttir). Contents lists available at ScienceDirect Fish & Shellfish Immunology journal homepage: www.elsevier.com/locate/fsi 1050-4648/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fsi.2008.09.017 Fish & Shellfish Immunology 26 (2009) 619–624