Epithelial tissue covers exterior and interior surfaces of the body of most living animals and is the first line of defense against parasites, viruses and bacteria.
Strong, healthy epithelial tissue or skin integrity is especially important in fish because they are in constant direct contact with water containing viruses, bacteria and parasites.
Weak epithelium may allow pathogens to invade a fish and accumulate in its organs, causing infection and inflammation that in turn may lead to lower growth rates and ultimately death.
Factors that Reduce Epithelial Integrity
Impaired skin integrity is common in farmed fish. These fish are exposed to a variety of mechanical stressors, including transportation, grading, vaccination and possibly to infectious agents that may cause skin damage and wounds. Additionally, environmental changes may affect skin morphology and integrity; for example, when farmed Atlantic salmon are transferred to seawater. Fortunately, zinc is long recognized as an essential factor in modulating wound healing in several species, including humans and fish. Zinc is essential to all stages of the wound healing process: coagulation or hemostasis, inﬂammation, proliferation and remodeling of epithelial tissues. When a fish sustains a wound or skin lesion, it’s important that healing occurs quickly. If wounds are slow to heal, this increases the potential for secondary infections as bacteria and viruses penetrate the skin.
Stress factors can also play a role in reducing skin integrity and weakening of epithelial tissue. Aside from environmental stressors and pathogens, anti-nutritional factors in feed ingredients are reported to negatively affect intestine microbial populations, microvilli morphology and intestinal enterocyte integrity, all of which may alter intestinal health resulting in impaired nutrient digestibility and intestinal immunity. In fact, anti-nutritional factors like saponins are pro-inflammatory with the potential to elicit enteritis in fish and compromise their growth and health.
Poor Epithelial Integrity Can Leave Fish Susceptible to Gill and Eye Damage
The fish gill is a multifunctional organ involved in gas exchange, osmoregulation, acid-base balance, ammonia excretion and immune defense, among other functions. Fish pathogens readily spread in water, and the thin respiratory epithelium of the gills represents an obvious point of entry for pathogens. The gill surface is estimated to be 0.1–0.4 m2/kg body weight, thus representing the largest organ-specific surface interacting with the external milieu. Gill epithelium is a complex tissue comprising multiple cell types, including mucous cells, and accounts for 50% of the body’s surface area. Maintaining gill epithelial tissue integrity is therefore of utmost importance to fish health.
Cataracts, a disease of the eye lens, are a common problem in farmed salmonids and are a function of damaged epithelial tissue. In vertebrates, the lens is a non-vascular tissue derived from the eye surface ectoderm: a simple epithelial structure. When evaluating essential nutrients, zinc deficiency has been found to cause cataracts. Fortunately, zinc supplementation has proven effective in correcting this problem in species such as rainbow trout and salmon.
Performance Trace Minerals for Improved Skin and Gut Health
The first step toward healthy skin integrity is nutritional optimization. One way to help fish better respond to skin integrity challenges is to supplement their feed with Zinpro Performance Minerals®. Zinc, manganese and copper are essential for improving wound healing. These trace minerals improve epithelial tissue integrity through maintenance of cell division, protein synthesis and antioxidant activity to remove superoxide radicals.
Partial or complete replacement of inorganic trace minerals with Zinpro Performance Minerals (ZPM) has been shown to increase the number of gut goblet cells in European sea bass by 25% (Fig. 1A) The number of goblet cells in skin was increased by 61% in those same fish (Fig. 1B) As part of the mucosal immune system, goblet cells play an important role in protecting fish against pathogens. The entire body surface of fish (gills, intestine and skin) is covered by mucus, which is one of the primary immune barriers against the invasion of pathogens.
Zinpro Performance Minerals are proven to be more effective than inorganic minerals in reducing skin lesions in Atlantic salmon after infestation with Caligus, indicating enhanced barrier defense mechanisms against pathogens. Availa®Zn supplemented at half the level of inorganic zinc was significantly more efficient in reducing Caligus abundance in salmon (Fig. 2) The abundance of Caligus per fish fed Availa-Zn was reduced by nearly 10%.
Interestingly, skin score evaluation indicated that fish fed Availa-Zn had a more desirable level of skin integrity than fish fed diets containing inorganic zinc (Fig. 3) In this study, the best performance and health results were found with a 20% fish meal diet supplemented with 60 ppm zinc as Availa-Zn. However, with progressive replacement of fish meal by plant protein sources, the requirement for essential trace mineral supplementation is expected to increase in level and metabolic availability.
For more details about Zinpro research, download the Essential Trace Minerals for Exceptional Performance brochure. To learn more about formulating your aquaculture nutrition plan with performance trace minerals, contact your Zinpro representative today.