Metal pollution-induced immunomodulation in fish

The exact relationship between disease incidence in aquatic organisms and environmental pollution is not well defined. A number of mechanisms by which aquatic pollutants may act to increase disease incidence in fish have been speculated, many suggesting immunosuppression as a link in the etiology of disease among fishes in highly contaminated areas. This article will review the effects of metal pollutants on the immune responses of fish by examining in vitro and in vivo laboratory studies carried out since 1980. It will also describe how those alterations may be responsible for pollution-associated diseases in directly exposed fish. While a large number of environmental contaminants represent aquatic pollutants of concern (e.g. polycyclic aromatic hydrocarbons, chlorinated organics, and pesticides), heavy metals were selected as the pollutants for this review because of their: (a) prevalence in polluted aquatic environments; (b) immunotoxic potential in mammalian systems; (c) ability to induce tumors in exposed rodents; and (d) their overall toxicity in a variety of species. It can be concluded that a number of heavy metal pollutants shown to be immunotoxic in mammalian systems, including cadmium, chromium, copper, lead, manganese, nickel, and zic, also alter immunoregulatory functions in a variety of fish species. These alterations may ultimately lead to increased host susceptibility to infectious and malignant diseases in fish inhabiting heavy metal-contaminated waters.