Page 777 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

736 SECTION | X Avian and Aquatic Toxicology

VetBooks.ir Chlorine free Cu and copper monohydroxide bind to a biotic
ligand on the organism’s surface. Death occurs when a
Water is not considered safe for fish if a measurable level
critical amount of the total biotic ligand sites are
of chlorine (Cl 2 ) is present, not to be confused with chlo-
2 attached to copper. Multiple studies demonstrate that
ride ions (Cl ). Morbidity can occur at 0.02 ppm Cl 2 and
gill rapidly accumulates Cu following the onset of
mortality at 0.04 ppm and tap water can contain 2 ppm
waterborne exposure, and the accumulation of Cu results
Cl 2 (Hadfield et al., 2007). Chlorine, chloramines, and
in the disturbance of multiple physiological processes.
other chlorine compounds are used as disinfection agents
The USEPA BLM model does not consider differences
in municipal water and in aquaculture to disinfect ponds
in sensitivity due to size of the fish and ambient water
and tanks. Chlorine gas added to water forms several
altering the physiology of the fish (de Polo and
compounds (hypochlorous acid, hydrochloric acid, and
Scrimshaw, 2012).
hypochlorite), with the concentration of dissociated ions
The uptake of copper ions across the gill of fish is
depending on the pH of the water. Chloramines are also 21
dependent on many parameters, including levels of Ca
formed by the reaction of Cl 2 with NH 3 in water. 21
and magnesium ions (Mg ) in the water. There is gener-
Chlorine dioxide is used as a water disinfection agent
ally a small margin of safety for many aquatic species for
and is reduced to chlorite. Chlorine dioxide is approxi- 21
copper ions. Cupric ions (Cu ) disrupt the ATP-
mately 16 times more toxic to fish than is chlorite. For
dependent sodium/potassium pump located in the gill
rainbow trout, safe levels for chlorine dioxide appear to 1
chloride cells. This allows increased efflux of Na .
be approximately 0.2 ppm and approximately 3 ppm for 21
Cupric ions also replace Ca at the tight junctions,
chlorite. The toxicity of Cl 2 residues is variable with tem- 1 1
resulting in an efflux of Na . The net loss of Na results
perature changes. Residual Cl 2 in the water is generally
in disruption of osmoregulation and cardiovascular col-
oxidative and causes irritation and damage to the gills.
lapse. Copper ions are neurotoxic in fish, disrupting the
The acute gill lesion is necrosis of gill epithelium occur-
olfactory and mechanosensory-neuromast systems (Linbo
ring at higher Cl 2 concentrations, whereas the subacute
et al., 2006; Sommers et al., 2016). Peripheral olfactory
and chronic lesions are gill epithelial hypertrophy and
function is inhibited at levels as low as 5 μg copper/L
hyperplasia. Gill lesions not only cause hypoxia but also
ambient water, and loss of neuromast sensory cells occurs
affect the acid base homeostasis of fish. In this regard,
at concentrations greater than 20 μg/L. Exposure to cop-
gill damage in fish is akin to a mammal suffering nephri-
per ions also decreases immune function. Copper levels
tis with concurrent pneumonia
can be difficult to interpret because pH, carbonate ions,
and dissolved organic carbon are interactive in forming
21
METALS unavailable forms of copper, and Mg 21 and Ca compete
for copper uptake by the fish. Changes in these para-
There are numerous metals that can find their way into meters can cause a 60-fold difference in the lethal toxicity
aquatic systems. Contaminated surface water can be a of Cu. Decreasing water pH increases the toxicity, and
serious threat. Water conducting systems and aquacul- 100 times increase in toxicity can occur with each unit
ture equipment can also be a source of metals. Some decrease in pH. Warm-water fish are more tolerant of
plastics also contain metals and should not use in aquatic copper than are cold-water fish.
systems.
Copper Methylmercury
Copper (Cu) is an essential trace nutrient for fish. There Methylmercury (meHg) is a concern with human foods
are many sources of copper ions in aquaculture systems. from aquatic sources. The primary source of meHg in
Runoff water from lands receiving swine and poultry aquaculture is the use of fish byproducts in feedstuffs.
manure containing Cu from dietary copper sulfate can Methylmercury is also formed by biota in the benthic
be an important source. Copper compounds are com- region of the aquatic system, and it is biomagnified in the
monly used in aquaculture as algaecides and as treat- food web. In fish, meHg is bioaccumulated in skeletal
ments for parasites and copper piping may be used in muscle (80% of body burden in Salmo salar)(Berntssen
closed systems. In freshwater aquatic systems, Cu exists et al., 2004). Approximately 23% of the dietary meHg
in complexes with organic matter, other chemicals and and approximately 6% of dietary inorganic mercury are
is weakly associated with water molecules (USEPA, absorbed by fish. Fish fed diets containing 5 and 10 ppm
2007). These factors can affect the bioavailability of Cu meHg for 4 months had 1.1 and 3.1 ppm of meHg in mus-
in ambient water. USEPA uses the biotic ligand model cle (freeze-dried), respectively. The threshold toxic level
(BLM) to assess Cu toxicity. The model assumes that for Atlantic salmon is estimated at 0.5 ppm meHg in diet.