Intramuscular injection of DNA vaccines in fish.

The DNA-based immunization technology has only been applied to fish very recently. Though a preliminary study showing reporter gene expression in fish muscles was published in 1991 (1), the first demonstration of an immune response to plasmid-encoded antigen was not reported until five years later (2). Thus, relatively little is known about the administration methods, immunological responses, and protective efficacy of DNA vaccines in aquatic animals. In some instances, results obtained with other classes of vertebrates (mammals and birds) can be applied directly to fish, but because of physiological, immunological and structural differences between these animals, this is not necessarily true. For example, it was shown recently that short specific DNA sequences (CpG motifs), in a particular nucleotide context, act as immunostimulants (3,4). No study has been reported yet assessing these immunostimulatory sequences in fish, but it would be unlikely that the same sequences are effective in all animal species (Weeratna et al., this volume).