Bacterial pathogens of marine fish

Abstract

Biological agents, particularly bacteria (Table 17.1) and viruses (Table 17.2), are recognized as the etiological agents of infectious diseases in a wide range of marine animal species. Although this chapter will focus on the bacterial disease of marine fish, it should be emphasized that marine invertebrates and plants may become infected with any of a wide range of bacteria and viruses. Examples include Vibrio harveyi (Liu & Lee, 1999) and baculovirus (Rodriguez et al., 2003), both of which have been responsible for epizootics of luminous vibriosis and white spot syndrome, respectively, in farmed penaeids, particularly in Asia and South America. Largescale outbreaks of disease among wild-fish populations have been attributed to a growing list of viruses, including lymphocystis (Lang et al., 1999). Considerably more attention has been devoted to viral diseases of farmed fish, with an example including the recently described infectious salmon anemia (ISA), which has affected Atlantic salmon production in Canada, Ireland, Norway, and Scotland (Miller & Cipriano, 2003). Of the bacterial fish pathogens, some genera, such as Vibrio, include many species that have been acknowledged to be long-established pathogens of marine fish. Other pathogens, e.g. Pasteurella skyensis (Birkbeck et al., 2002), are new to the literature. Although some bacteria constitute primary pathogens, many others are opportunists colonizing and causing disease in already damaged hosts (Austin & Austin, 1999). Overall, the study of fish diseases has concentrated on problems of aquaculture rather than of wild-fish populations. Yet, it is becoming increasingly apparent that disease is not necessarily caused by single species, but may involve synergistic interactions between two or more taxa. The isolation of an organism from clinically diseased fish does not necessarily imply that these have caused infection. Indeed, all too often Koch's Postulates are conveniently forgotten. Many pathogens, such as Eubacterium tarantellae, have been rarely considered after their initial recognition as causal agents of disease (Austin & Austin, 1999). Nevertheless, other pathogens have been studied in detail, and these will be considered here.