Control of marine ectoparasites including Amyloodinium ocellatum Part 1
References for this article may be found here
Marine ectoparasites are a source of considerable losses both within the food and ornamental fish markets. Once established within a system, infections may become chronic, with periodic blooms of disease sweeping the system. Costly and wasteful for fish farmers, these infections can be heartbreaking for marine aquarium owners as they watch their pets succumb to these virulent pathogens. This series of articles will cover the general infection cycle of some parasites with an emphasis on the dinoflagellate Amyloodinium ocellatum, the cause of marine velvet disease. Pathogenesis, immunology, and current treatments will be covered, as will considerations for fish farms and options available to develop vaccination strategies to prevent these diseases.
As the world’s population continues to grow, so does the demand for fish, both as a food, as well as for entertainment as inhabitants of aquariums. Increasingly polluted and overharvested, natural waters will be hard pressed to meet demand. In response, emphasis on intensive aquaculture has been growing rapidly. Intensive aquaculture allows for the maximal production of aquatic organisms per unit of rearing volume by closely controlling the physical and biological parameters of the system. But, with increased density comes increased potential for rapid spread of disease within the system. A small stressor may decrease immunity in a fish such that it predisposes it to lethal infection by pathogens (13). Similarly, a minor failure in a contamination control system could spell disaster in a system running at the limits of its capacity. Traditional methods of controlling pathogens in such system have relied on chemotherapeutics and are becoming rapidly antiquated or impractical as antibiotics are being more tightly controlled and are often ineffective with the emergence of resistant strains. Other old-standby treatments such as formalin and copper sulfate, while effective, are being disallowed in the culture of food fishes due to potential adverse effects on human health. As such, newer methods are necessary. Vaccination against disease would be an ideal strategy, relying on the fishes’ own immune system to maintain health and productivity while decreasing reliance on chemotherapeutics and complex contamination control systems.
Of key importance in maintaining fish health is control of ectoparasitic pathogens. While viral and bacterial pathogens are at the forefront of human medical research, in aquaculture, they are often considered secondary diseases. Instead, it is the ectoparasites that are of primary concern. No current marine pathogens cause the massive morbidity and mortality associated with these diseases (3). Infections can spread rapidly throughout a facility, often causing massive loss of livestock within 24 hours of an outbreak occurring. Among the most virulent of these is the dinoflagellate Amyloodinium ocellatum, a highly infective parasite that when in the trophont stage attaches to the gills, fins and body of infected fishes (3). Effective treatments exist for this parasite, but the outcome is often poor due to the massive damage that can occur in the fish before signs of the disease are visible. A vaccine to prevent disease from occurring, or to limit the virulence of the organism would be highly desirable.